Symposium EQ18—Emerging Materials for Quantum Information

NIST is using atomically precise fabrication to develop devices for use in quantum information processing. We are using hydrogen-based scanning probe lithography for deterministic placement of individual dopant atoms with atomically aligned gates to fabricate single/few atom transistors, few-donor/quantum dot devices for spin manipulation, and arrayed few-donor devices for analog quantum simulation.

In this talk I will discuss the analog quantum simulation of an extended Hubbard model using arrays of few atom structures in silicon. These atomic clusters form the sites of a Hubbard model array in the strong interaction regime, where we vary the tunnel coupling with atomic precision between nearby dots from a weakly to strongly tunnel coupled regime.
Using an extended Hubbard model we explore the impact of site-by-site disorder on charge occupation and the Hubbard band structure. Numerical simulations of the model reveal the nature of charge distributions and magnetic correlations for different parameter sets. We quantify the electron addition energy spectrum through Coulomb blockade and charge stability analysis and demonstrate tuning of the array’s energy spectrum using gates. We map the Hamiltonian parameters to the physical system to tune the charge occupation, the spatial distribution of the eigenstates, localization/delocalization transition, and Hubbard band structure. We identify resonant and inelastic conductance paths and extract coherent tunnel coupling strength.

Details of the STM-fabrication will be described as well as quantum transport measurements from several arrays of few atom clusters ranging from 1x2 double dots to a multi-gate 3×3 dot array to explore the rich Hubbard physics of quantum dot-arrays.

The ability to place both n-type and p-type dopants relative to each other with atomic precision is crucial to the application of Atomically Precise Advanced Manufacturing (APAM) beyond quantum computing. While PH₃ and AsH₃are highly selective precursors for donor dopants [1-3], the direct equivalents for B and Al are unstable. However, diborane has been used to create a p-n junction[4], and halide precursors, BCl₃ and AlCl₃ are proving promising for acceptor dopants[5].
In this work, we have developed an APAM process for the sequential placement of P and B using PH₃ and BCl₃. First, Hydrogen Depassivation Lithography is used to create patterns on a H:Si(001) substrate. PH₃ is dosed onto the sample and adsorbs selectively into these patterns. An incorporation anneal then drives the adsorbed P into the top layer of the substrate. The sample is repassivated with atomic H before returning to the STM for the second dopant step. A similar process of patterning, dosing with BCl₃ and incorporation completes the bipolar device. The P regions are patterned, dosed and incorporated first, as the incorporation anneal temperature for PH₃ is higher than for BCl₃.
Alignment marks on the sample allow rapid relocation of the device area, and we have developed a high-speed dI/dV imaging method[6] to locate surface or buried dopants. The dI/dV imaging provides much higher contrast of the dopants than the topographic image, so that dopant patches can be relocated quickly in the STM using low-resolution, large-area scans. It also allows for precise alignment of new patterns to existing dopant patches. After both dopants have been incorporated, the surface is repassivated, and the presence of both dopants is confirmed in the STM. The dopant device can then be buried in epitaxial silicon. After burial, dI/dV imaging is then used to confirm the location of the bipolar device for electrode deposition.
1:Fuechsle, M. et al. A Single-Atom Transistor. Nat Nano 2012, 7, 242–246.
2: Wang, X. et al. Atomic-Scale Control of Tunneling in Donor-Based Devices. Commun. Phys. 2020, 3, 82.
3: Stock, T. J. Z.; Warschkow, O.; Constantinou, P. C.; Li, J.; Fearn, S.; Crane, E.; Hofmann, E. V. S.; Kölker, A.; McKenzie, D. R.; Schofield, S. R.; et al. Atomic-Scale Patterning of Arsenic in Silicon by Scanning Tunneling Microscopy. ACS Nano 2020, 14, 3316–3327.
4: Škeren, T. et al. Bipolar Device Fabrication Using a Scanning Tunnelling Microscope. Nat. Electron. 2020, 3, 524–530.
5: Dwyer, K. J. et al. Area-selective deposition and B delta-doping of Si(100) with BCl₃ ; Radue, M. S. et al. AlCl₃-Dosed Si(100)-2x1: Adsorbates, Chlorinated Al Chains, and Incorporated Al. arxiv (2021)
6: Alemansour, H. et al. High Signal-to-Noise Ratio Differential Conductance Spectroscopy. J. Vac. Sci. Technol. B2021, 39, 010601.

Holes in compressively strained germanium quantum wells is an emerging spin qubit system to overcome the challenges associated with valley degeneracy in silicon. The local electrostatic potential is critical to control the qubit state, and through the deformation potential there is a need to understand the local strain environment. Nanometer-scale experimental assessment of the strain in the quantum well is lacking due to the high spatial resolution and nearly single atom detectability required. Using combined Transmission Electron Microscopy and Atom Probe Tomography, strain variations are herein shown to be experimentally accessible in Ge quantum wells grown via MBE on SiGe buffer layers. Using 4-D STEM, anisotropic strain in the Ge layer was mapped in real space to better than 1 nm and lattice parameters measured to better than 0.1 Å in multiple crystallographic directions. The Ge quantum well was found to be tetragonally strained in defect-free regions and partially relaxed near threading dislocations. Combining TEM with APT, the SiGe/Ge internal interface structure was mapped in 3-D and shown to have sub-nm interfacial roughness. Individual ⁷³Ge atoms with non-zero nuclear spins are uniquely identified within the Ge quantum well in order to provide another metric that quantum wells can be characterized in order to explore potential interactions between nuclear and hole spins.

A current bottleneck for the development of semiconductor-based qubits is the need for high fidelity simulations that can assist in the device design process and minimize expensive and labor-intensive fabrication cycles. This need is largely unmet – there are very few publicly or commercially available simulation tools that are well suited for this task and, as devices get more complex and time consuming to fabricate, the need only continues to grow.

UCLA has developed simulation capabilities over the last several years to meet this need. We have developed and implemented grid-based software components that can be combined to create very accurate simulations of localized quantum mechanical systems. These software components support the construction of simulations that can be used to model everything from the properties of semiconductor heterostructures to the behavior of multiple interacting electrons.
This talk will describe how our simulation tools, which model realistic devices features including gate geometries and dielectric layers, can be used to design SiGe heterostructures and devices for Si-based qubits. We will show that full configuration interaction (FCI) methods can be used to generate highly accurate energy spectra for multi-electron and coupled few-electron quantum dots including important quantities like the exchange interaction between electrons in nearest neighbor dots including the effects of magnetic fields and valley physics. The exchange interaction is the foundation of most two qubit gates in semiconductor dots and being able to predict its behavior as a function of gate geometry and bias is critically important to optimizing the design of qubit devices.

Gate-defined Si/SiO₂ based single electron devices (SEDs) are a promising platform to implement spin qubits and a quantum standard of the Ampere. Noise can limit the performance and integrability of devices for each of these applications. One of the measures of low-frequency charge noise control is known as charge offset drift. The device stability at low frequencies is usually attributed to the presence of defects which are influenced by materials choices, fabrication processes and device design. Poly-Si gated silicon SEDs are known to have smaller charge offset drift (0.01e) than the metallic gates SEDs as observed in the charge offset drift measurement of Al gates SEDs (0.15e). The reactivity of the Al can produce defective AlO_x formation as the inter-gate dielectric as well as underneath the gate, increasing charge offset drift and degrading device performance. However, Pd gates which are much less reactive than Al, may induce fewer defects while preserving the benefits of metal-gated devices such as simplified fabrication, reduced parasitic resistances, and reduced grain size. To assess the impact on stability of Pd gates. we fabricate and measure SEDs with a combination of Ti/Pd and poly-Si gates with thermally grown SiO₂ as the inter-gate dielectric. This choice preserves a high level of tunability and functionality in the devices provided by overlapping gate design, while also avoiding defects associated with a deposited inter-gate dielectric. Our latest measurement results and the implications for device performance will be discussed.

Like the classical bit that launched the modern electronic age, the quantum bit or qubit lies at the heart of the ongoing quantum information revolution that is expected to transform science and society in previously unimaginable ways. Although solid-state defects such as the diamond-NV centers in three-dimensional hosts have been demonstrated as promising qubits, atomically-thin two-dimensional (2D) materials offer an exciting new paradigm for qubit fabrication and operation at room temperature. Using data-driven approaches and a symmetry-based hypothesis for qubit discovery, we show that neutral anion-antisite defects in six transition metal dichalcogenide (TMD) monolayers as viable spin qubits. We show how the optical transitions and triplet-singlet intersystem crossing paths in the qubit provide a complete cycle for initialization, manipulation, and readout of the qubit. The operational principles of the qubit are illustrated through an in-depth discussion of the antisite qubit in WS₂. Our study opens a new pathway for creating spin-qubits and multi-qubit platforms for quantum information technologies based on defects in 2D solid-state systems.

Material losses limit the performance of superconducting qubit processors. Recent advances in the fabrication techniques and materials science have reduced the majority of decoherence sources such as quasiparticles, magnetic vortices, and radiation, leaving two-level system (TLS) defects as the most prominent source of loss. Due to its high transition temperature and critical field, niobium is one of the common materials used to achieve highly coherent devices. Conventional niobium deposition methods, such as DC magnetron sputtering can result in a damaged substrate-metal interface during the deposition due to the presence of high energy argon ions. To eliminate this source of damage, here we report on a refined growth technique for extremely low-loss niobium thin films using an electron-beam evaporator. The films are deposited in a ultra-high vacuum pressure environment (< 5 nTorr) with deposition rates as low as 1.2 nm/min. The resulted films exhibit a superconducting transition temperature of 9.2 K and residual-resistivity ratio as high of 5.1 for a film with a thickness of 200 nm, proving the absence of any trace of impurities, despite the low deposition rate. To further study the sources of loss in our films, we fabricated the films into coplanar waveguide (CPW) resonators to extract the TLS loss tangent of the niobium using microwave measurements. Notably, we utilized a CPW gap of 2 μm to maximize coupling to the TLSs at the interfaces, and thereby gain a more reliable estimate of the defects in the films. Our results show that after passivating the niobium surface using a buffered oxide etch, we achieve TLS loss tangents as low as 1×10^-7, well below the previous limits.

The quality factor of superconducting coplanar waveguide resonators correlates strongly with the coherence properties of planar superconducting qubits. Recent works have shown that the microwave loss of niobium microwave resonators is dominated by an oxide layer of approximately 5 nm thick at the metal-air interface. Although the microwave loss can be reduced by removing the oxide layer using buffered oxide etch, the oxide regrows in a time frame of several hours. Our work demonstrates a novel method of preventing the regrowth of surface oxide by passivating the metal surface using nitrogen plasma. We will present data of surface and microwave characterizations of resonators passivated using this method.

Superconducting quantum circuits are a promising hardware implementation for large-scale quantum computing, but materials losses continue to significantly limit scalability and single-qubit performance. These cryogenic microwave materials losses can thus be characterized by coherence measurements of qubits and resonators. However, experimental factors such as thermalization, packaging, and microwave electronics have significant effects on these coherence measurements as well. In this talk, I outline a roadmap towards reproducible measurements of qubits and resonators and their further application to the investigation and characterization of materials losses.

NIST is using atomically precise fabrication to develop devices for use in quantum information processing. We are using hydrogen-based scanning probe lithography for deterministic placement of individual dopant atoms with atomically aligned gates to fabricate single/few atom transistors, few-donor/quantum dot devices for spin manipulation, and arrayed few-donor devices for analog quantum simulation.

In this talk I will discuss the analog quantum simulation of an extended Hubbard model using arrays of few atom structures in silicon. These atomic clusters form the sites of a Hubbard model array in the strong interaction regime, where we vary the tunnel coupling with atomic precision between nearby dots from a weakly to strongly tunnel coupled regime.
Using an extended Hubbard model we explore the impact of site-by-site disorder on charge occupation and the Hubbard band structure. Numerical simulations of the model reveal the nature of charge distributions and magnetic correlations for different parameter sets. We quantify the electron addition energy spectrum through Coulomb blockade and charge stability analysis and demonstrate tuning of the array’s energy spectrum using gates. We map the Hamiltonian parameters to the physical system to tune the charge occupation, the spatial distribution of the eigenstates, localization/delocalization transition, and Hubbard band structure. We identify resonant and inelastic conductance paths and extract coherent tunnel coupling strength.

Details of the STM-fabrication will be described as well as quantum transport measurements from several arrays of few atom clusters ranging from 1x2 double dots to a multi-gate 3×3 dot array to explore the rich Hubbard physics of quantum dot-arrays.

The relaxation time T₁ of superconducting qubits has increased by orders of magnitude over the last two decades. To test whether quasiparticles are a significant contributor to the loss, we have repeatedly measured T₁ in Al/AlO_x/Al transmons with electrodes with different superconducting gaps. In one device, the first layer electrode was formed by deposition of nominally pure aluminum, while the counter-electrode was formed by deposition of oxygen-doped aluminum. The superconducting energy gap of the electrode depends on the grain size, which depends on the oxygen doping as well as the layer thickness. The device showed relatively large fluctuations in T₁, varying from about 100 μs to just over 300 μs at 20 mK. In other transmons, we formed the first layer electrode by deposition of oxygen-doped aluminum, while the counter-electrode was formed by deposition of nominally pure aluminum. These devices showed a similar range of large fluctuating T₁ values, as well as a maximum T₁ over 300 μs. Measurements of the T₁ of these transmons as a function of temperature shows similar temperature dependence, with the relaxation time at high temperature dropping due to the thermally-generated quasiparticles.

Current implementations of superconducting qubits have short decoherence times attributed, in part, to losses to two level systems. These two level systems reside in dielectric materials and at interfaces in the device. Josephson junctions made of (Nb,Ti)N superconductors and AlN junction barrier grown via molecular beam epitaxy on sapphire have the potential to eliminate the amorphous material hosting two level systems and thus potentially improve qubit coherence times. Here, we utilize 4-D scanning transmission electron microscopy (STEM) to determine nanometer-scale changes in crystallographic structure, cation ordering, and orientation of the superconductors in a NbTiN / 2 nm AlN / NbTiN device stack grown on c-sapphire. We find that the NbTiN is rock salt structured and highly oriented towards <111> but with rotations about that axis corresponding to step edges in the c-sapphire. The AlN structure, orientation, roughness and thickness were investigated using correlative 4-D STEM and Atom Probe Tomography (APT). Under these growth conditions, the AlN can exhibit island growth on the lower NbTiN, leading to nm scale changes in thickness and shorts across the barrier junction. As a result, the top NbTiN has randomly oriented crystallographic structure. APT was also utilized to determine local changes in point defect chemistry at the internal interfaces. Changes in the nitrogen stoichiometry are observed near the AlN barrier, leading to negatively charged vacancies that may be a source of decoherence.

There are an increasing number of circuit architectures that put optical modes on-chip with superconducting microwave electronics, such as microwave-to-optical transducers which could help link quantum networks.[1, 2] However, such devices are frequently limited by absorption of the optical field in the superconducting electronics. This loss leads to a reduction in the quality factor of the optics, and may also induce quasi-particle generation which reduces the performance of the superconducting material. Although fine-tuning the geometry of the circuit can decrease optical absorption in the superconductor, the absorption could be further reduced if the electronics were made of a superconductor which was transparent to visible light.

In this work, we study sodium-doped ITO as a potential transparent superconductor. ITO, which has previously been studied as a transparent conductor with a low carrier concentration and a high mobility, has recently been shown to superconduct when electrochemically doped via intercalation of sodium ions.[3] However, the doping is also associated with a visible change in the optical properties of the material that has not previously been quantitatively measured. We develop an electrochemical cell for doping ITO and study how the process affects its electrical and optical properties. We use ITO on different substrates, including glass and silicon, with different doping levels set by intercalation time. We measure how doping changes the optical properties of the ITO using ellipsometry and spectrophotometry across the visible spectrum and near infrared. We also measure the room temperature resistivity and superconducting transition temperature, and their relationship to the optical properties. Based on these measurements, we evaluate the suitability of sodium-doped ITO as a transparent superconductor.

[1] Optica 7.12 (2020), pp. 1714–1720
[2] Optica 7.12 (2020), pp. 1737-1745
[3] Appl. Phys. Lett. 101, 252603 (2012)

Qubits on solid state devices could potentially provide the rapid control necessary for developing scalable quantum information processors. Materials innovation and design breakthroughs have increased the functionality and coherence of qubits substantially over the past two decades. Here we show by improving the interface between InAs and Al, one can reliably fabricate voltage-controlled Josephson junction field effect transistors (JJ-FET) to be used in tunable qubits, resonators, and couplers. We find that band gap engineering is crucial in realizing a two-dimensional electron gas near the surface. In addition, we show that coupling between the semiconductor layer and the superconducting contacts can affect qubit properties.
We analyze materials systems with varied growth conditions on the pseudomorphic buffer layer and quantum well active region for the purposes of making JJ-FET devices. In samples with a high indium content graded buffer layer, we find that surface roughnesses decrease drastically with the graded buffer layer growth temperature. We also find that samples with isotropic roughnesses along the〈110〉and〈1-10〉directions show a higher mobility than those with anisotropic roughnesses. We study the effects of the quantum well growth temperature over a range of 445°C to 465°C on 2DEG mobility. We grow a thin 1-6 nm In_0.81Al_0.19As capping layer in order to discern the effects of surface scattering on the quantum well and measure density and mobility as a function of the In_0.81Al_0.19As layer thickness. We compare the measured densities to those calculated by solving the Schrödinger and Poisson equations self-consistently, where the presence of a thin In_0.81Al_0.19As top layer causes the density and mobility increase, due to the decreasing conduction band energy of InAs and to separation from surface scattering sites.
We test these structures in coplanar waveguide (CPW) resonator devices using microwave transmission measurements, we study the quality factor on each device at high and low power and extract TLS losses in each materials combination. We also present anharmonicity and coupling strengths measured from one and two-photon absorption in a quantum two-level system engineered in a fabricated JJ-FET.

Material losses limit the performance of superconducting qubit processors. Recent advances in the fabrication techniques and materials science have reduced the majority of decoherence sources such as quasiparticles, magnetic vortices, and radiation, leaving two-level system (TLS) defects as the most prominent source of loss. Due to its high transition temperature and critical field, niobium is one of the common materials used to achieve highly coherent devices. Conventional niobium deposition methods, such as DC magnetron sputtering can result in a damaged substrate-metal interface during the deposition due to the presence of high energy argon ions. To eliminate this source of damage, here we report on a refined growth technique for extremely low-loss niobium thin films using an electron-beam evaporator. The films are deposited in a ultra-high vacuum pressure environment (< 5 nTorr) with deposition rates as low as 1.2 nm/min. The resulted films exhibit a superconducting transition temperature of 9.2 K and residual-resistivity ratio as high of 5.1 for a film with a thickness of 200 nm, proving the absence of any trace of impurities, despite the low deposition rate. To further study the sources of loss in our films, we fabricated the films into coplanar waveguide (CPW) resonators to extract the TLS loss tangent of the niobium using microwave measurements. Notably, we utilized a CPW gap of 2 μm to maximize coupling to the TLSs at the interfaces, and thereby gain a more reliable estimate of the defects in the films. Our results show that after passivating the niobium surface using a buffered oxide etch, we achieve TLS loss tangents as low as 1×10^-7, well below the previous limits.

The quality factor of superconducting coplanar waveguide resonators correlates strongly with the coherence properties of planar superconducting qubits. Recent works have shown that the microwave loss of niobium microwave resonators is dominated by an oxide layer of approximately 5 nm thick at the metal-air interface. Although the microwave loss can be reduced by removing the oxide layer using buffered oxide etch, the oxide regrows in a time frame of several hours. Our work demonstrates a novel method of preventing the regrowth of surface oxide by passivating the metal surface using nitrogen plasma. We will present data of surface and microwave characterizations of resonators passivated using this method.

Recently the drive to understand and control the order parameter characterizing the collective state of electrons in quantum heterostructures has attracted great interest. New physical behavior can emerge that is absent in the isolated constituent materials. With regards to superconductivity this has opened a whole new area of investigation in the form of topological superconductivity. Topological superconductivity are expected to host exotic quasi-particle excitations including Majorana bound states which hold promise for fault-tolerant quantum computing. Recent theory predicts in superconductor-semiconductor devices, Majorana bound states can emerge and are accompanied by a topological phase transition. We study epitaxial Al/InAs Josephson junctions in presence of Zeeman field where we find a transition between trivial and topological superconductivity. We observe a minimum of the critical current at the topological transition, indicating a closing and reopening of the superconducting gap induced in InAs, with increasing magnetic field. By embedding the Josephson junction in a phase-sensitive loop geometry, we measure a π-jump in the superconducting phase across the junction when the system is driven through the topological transition. These findings reveal a versatile two-dimensional platform based on epitaxial superconductors and semicondcutors.

Even though many advances have been made in the field of quantum computing, Majorana fermions, the exotic mystery of topological qubits, have yet to be conclusively identified and are thus still waiting to be explored.^[1] Materials science is still the bottleneck of this progress; high-quality semiconductor nanowires (NWs) in a precise configuration are required, as well as a deeper understanding of their structural, electrical, and transport properties. Previously, selective area growth of horizontal In-based NW networks by molecular beam epitaxy has been proposed for these systems due to the scalability of the process, inherent properties of the material, and the cleaner environment of the machine.^[2,3]Growing NWs on defect-free GaAs templates or nanomembranes (NMs) results in high crystal quality, eliminating undesired impurity incorporation coming from the substrate. As a result, these NWs show field-effect gating, quantum coherence, and induced superconductivity.^[3,4] Moreover, the carrier concentration of the In(Ga)As NW branches can be enhanced by quasi-remotely doping the (111)B GaAs NMs. This results in the observation of weak anti-localization, an indication of strong spin-orbit interaction (SOI), and enhanced coherence length and mean free path.^[5] However, the maximum achievable In concentration on these NMs is 50%, and Si dopants accumulate at the NW/NM interface contributing to the donor interface scattering. Thus, advancements are still necessary to achieve the optimal material design for efficient quantum transport.
In this study, we address several challenges related to the design, growth, and exploitation of horizontal NWs grown epitaxially on NMs. First, we explore the crystal orientation of the GaAs substrate as a design parameter for changing the morphology of the In(Ga)As NWs. Since growths on (111)B, (100), and (110) are all governed by both thermodynamics and kinetics, the GaAs NMs have different morphologies and facets depending on the surface energy. Consequently, the NW geometry and the amount of In incorporation in the wire change depending on the substrate orientation. We demonstrate that growth on (100) substrates results in NWs that grow conformally around a low-aspect-ratio NM. These nanowires can be nearly pure InAs and exhibit only a narrow-intermixed region. For growth on (110) substrates, the NWs have a trapezoidal cross-section with a moderate In concentration. Subsequently, we explore the possibility to modify the morphology of the NMs by adding a small amount of Sb during the epitaxial growth (less than 5 at%). Sb acts as a “morphactant” in the system, favoring the formation of a flat top facet. This provides a smoother growth surface for the InAs NW, helping to decrease the scattering at the NM-NW interface. We also investigate new routes to suppress Si dopant diffusion in remotely-doped structures by two different approaches: (i) suppressing dopant mobility during the growth by decreasing the temperature and (ii) introducing a few-nm thick trapping layer after the doped layer growth. Dopant displacement was characterized by atom probe tomography. Finally, we investigate the magnetotransport properties of some of the quasi-remotely doped NWs. By fabricating a dual gated architecture at each side of the NW, we aim to keep carrier density constant while changing the electric field across two gates. This novel device design would make it possible to tune the SOI present in these NWs.

References
[1] P. Yu et al., Nat. Phys. 2021, 17, 482.
[2] M. Friedl et al.,Nano Lett. 2018, 18, 2666.
[3] F. Krizek et al.,Phys. Rev. Mater. 2018, 2, 093401.
[4] S. Vaitiekenas et al.,Phys. Rev. Lett. 2018, 121
[5] M. Friedl et al., Nano Lett. 2020, 20, 3577.

Acknowledgments: Funding from Swiss National Science Foundation, NCCR QSIT

Superconductor-semiconductor hybrids are platforms for exploring the rich phenomena that can emerge from effective p-wave superconductivity. Spin-orbit coupling, combined with the proximity effect, causes the two-dimensional semiconductor to inherit p±ip intraband pairing, and application of magnetic field can then result in transitions to the normal state, partial Bogoliubov Fermi surfaces, or topological phases with Majorana modes. Experimentally probing the hybrid superconductor-semiconductor interface, however, is challenging due to the shunting effect of the conventional superconductor. Consequently, the nature of induced pairing remains an open question.

I will discuss the use of planar superconducting resonators to probe induced superconductivity in a two dimensional Al-InAs hybrid system. We have observed a strong, anisotropic suppression of superfluid density and enhanced dissipation driven by magnetic field, which cannot be accounted for by the depairing theory of an $s$-wave superconductor. These observations are explained by a picture of independent intraband p±ip superconductors giving way to partial Bogoliubov Fermi surfaces. Within this picture, we provide the first characterization of key properties of the hybrid superconducting system, with implications for routes to Majorana physics. In addition, this work raises questions surrounding the stability of Bogoliubov Fermi surfaces.

The manipulation of Majorana zero modes in networks of semiconductor nanowires is a promising route to quantum computing. A central challenge to this approach is to achieve control of both nanowire morphology and network topology. We have previously demonstrated wafer-scale growth of high mobility branched InAs nanowires (NWs) supported on GaAs nanomembranes (NMs) by molecular beam epitaxy (MBE).^[1] Here we address the challenge of controlling the NW morphology and dopant incorporation simultaneously. Modulation of growth temperature and III-As ratios can be used to determine the nanowire morphology by controlling anisotropic growth in these non-planar heterostructures. However, the incorporation kinetics of the dopants are generally facet-dependent, such that the ideal growth conditions for self-assembled NWs may not match the conditions needed to achieve the ideal doping profile. We have used atom probe tomography (APT)^[1] to make 3D compositional measurements at the atomic scale to understand the relationship between nanowire faceting and dopant incorporation under a range of growth conditions.

In our initial work, atom probe tomography analysis found that Si dopants accumulate at the top of the (111) facet of InGaAs nanowires,^[3] which will produce scattering from the ionized donors. Follow-up work was successful in introducing Si below the NW/NM interface, but with strong facet dependent doping and with incomplete control of morphology. The talk will describe strategies to mitigate facet dependent doping while maintaining precise dopant positioning beneath a well-defined channel. We visualized buried growth interfaces by introducing dilute Al marker layers during growth, providing new insights into of how faceting varies with temperature and III-As ratios in a continuous manner. Si diffusion can be suppressed by doping at reduced temperatures, leading to dopant profiles that are less diffuse both along the growth axis and along the growth interface. However, the growth interface shape changes as the temperature is increased for the definition of the InAs nanowire grown. The precise determination of the nanowire morphology and dopant distribution by atom probe tomography lays an essential foundation to understanding the origin of transport properties, and provides new strategies to mitigate Si segregation and facet-dependent doping.

References:
[1] Friedl, M., Cerveny, K., et al. Remote doping of scalable nanowire branches. Nano Letters 2020, 20, 3577.
[2] Chang, A. and Lauhon, L., Atom probe tomography of nanoscale architectures in functional materials for electronic and photonic applications. Current Opinion in Solid State and Materials Science 2018, 22, 171.
[3] Friedl, M., et al. Template-assisted scalable nanowire networks. Nano Letters 2018, 18, 2666.

α-Sn and SnGe alloys have recently attracted much attention as a new family of topological quantum materials [1,2]. Depending on strain, they can be topological insulators or topological Dirac semimetals (TDS), the latter being a 3D counterpart of graphene with the Dirac cone protected by crystal symmetry and spin-orbit coupling. Recently, theoretical predictions have also shown that SnGe alloys with >40% Sn composition can be a TDS under compressive strain [2]. If verified experimentally, the wide range of SnGe alloys could greatly enrich the family of 3D TDS since only a few of them have been discovered so far [1,3,4]. They are also naturally compatible with Si-based integrated circuits as alloys based on Group IV elements. However, bulk α-Sn is stable only at <13^oC. High Sn composition SnGe alloys have been rarely studied experimentally since they are well beyond the equilibrium solubility limit [5]. Moreover, the integration of α-Sn on silicon has also been hindered by a large lattice mismatch, and epitaxial α-Sn is limited to InSb substrates so far. To address these challenges, we demonstrate that ~4 at.% Ge-doped α-Sn microdots can be grown on native oxide on Si substrate via a reversed phase transformation from β to α-Sn at 300-500°C, seeded by Ge-rich GeSn nanoclusters in the as-deposited materials instead of heteroepitaxy from the Si substrate. This result is exactly opposite to the bulk phase diagram that says α-Sn cannot be formed at >13^oC. It also shows that Ge-doped α-Sn can be stabilized at elevated temperatures without the help of coherent epitaxial interface as in the common case of α-Sn growth on InSb substrates. The size of α-Sn dots reaches up to 280 nm in diameter and 150 nm in thickness, ~10x larger than the upper size limit for α-Sn formation reported before. Raman spectroscopy, X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM) have been used to confirm the formation and determined the Ge composition in α-Sn. A compressive strain of ~-0.6% is introduced to Ge-doped α-Sn, which makes it one of the few candidates for 3D TDS according to theoretical predictions. This work presents the first instance of α-Sn grown at relatively high temperature on Si without epitaxy, breaking through the long-existing constraints in substrate selection, size/thickness limit, and thermal stability of α-Sn growth towards topological quantum material integration on Si. These TDS microdots could be coupled through graphene towards quantum device applications [6]. Furthermore, since we found that Ge-rich GeSn nanoclusters can effectively seed the growth of Sn-rich α-SnGe in this study, this process could potentially be scaled up using selective epitaxial growth on GeSn virtual substrates on Si for high-quality quantum device integration.

Reference
[1] Xu, et al. Phys. Rev. Lett. 118, 146402 (2017).
[2] Lan, et al. Phys. Rev. B 95.20, 201201 (2017).
[3] Wang, et al. Nature materials 15, 38–42 (2016).
[4] Liu, et al. Science 343, 864–867 (2014).
[5] Predel, B. Ge-Sn (Germanium-Tin) Landolt-Börnstein - Group IV Physical Chemistry 5F(Ga-Gd – Hf-Zr) (1996) https://materials.springer.com/lb/docs/sm_lbs_978-3-540-44996-6_1506
[6] Wu, et al. Advanced Materials 32.2, 1905632 (2020).

α-Sn and SnGe alloys have recently attracted much attention as a new family of topological quantum materials [1,2]. Depending on strain, they can be topological insulators or topological Dirac semimetals (TDS), the latter being a 3D counterpart of graphene with the Dirac cone protected by crystal symmetry and spin-orbit coupling. Recently, theoretical predictions have also shown that SnGe alloys with >40% Sn composition can be a TDS under compressive strain [2]. If verified experimentally, the wide range of SnGe alloys could greatly enrich the family of 3D TDS since only a few of them have been discovered so far [1,3,4]. They are also naturally compatible with Si-based integrated circuits as alloys based on Group IV elements. However, bulk α-Sn is stable only at <13^oC. High Sn composition SnGe alloys have been rarely studied experimentally since they are well beyond the equilibrium solubility limit [5]. Moreover, the integration of α-Sn on silicon has also been hindered by a large lattice mismatch, and epitaxial α-Sn is limited to InSb substrates so far. To address these challenges, we demonstrate that ~4 at.% Ge-doped α-Sn microdots can be grown on native oxide on Si substrate via a reversed phase transformation from β to α-Sn at 300-500°C, seeded by Ge-rich GeSn nanoclusters in the as-deposited materials instead of heteroepitaxy from the Si substrate. This result is exactly opposite to the bulk phase diagram that says α-Sn cannot be formed at >13^oC. It also shows that Ge-doped α-Sn can be stabilized at elevated temperatures without the help of coherent epitaxial interface as in the common case of α-Sn growth on InSb substrates. The size of α-Sn dots reaches up to 280 nm in diameter and 150 nm in thickness, ~10x larger than the upper size limit for α-Sn formation reported before. Raman spectroscopy, X-ray diffraction (XRD), Energy-dispersive X-ray spectroscopy (EDS), and high-resolution transmission electron microscopy (HRTEM) have been used to confirm the formation and determined the Ge composition in α-Sn. A compressive strain of ~-0.6% is introduced to Ge-doped α-Sn, which makes it one of the few candidates for 3D TDS according to theoretical predictions. This work presents the first instance of α-Sn grown at relatively high temperature on Si without epitaxy, breaking through the long-existing constraints in substrate selection, size/thickness limit, and thermal stability of α-Sn growth towards topological quantum material integration on Si. These TDS microdots could be coupled through graphene towards quantum device applications [6]. Furthermore, since we found that Ge-rich GeSn nanoclusters can effectively seed the growth of Sn-rich α-SnGe in this study, this process could potentially be scaled up using selective epitaxial growth on GeSn virtual substrates on Si for high-quality quantum device integration.

Reference
[1] Xu, et al. Phys. Rev. Lett. 118, 146402 (2017).
[2] Lan, et al. Phys. Rev. B 95.20, 201201 (2017).
[3] Wang, et al. Nature materials 15, 38–42 (2016).
[4] Liu, et al. Science 343, 864–867 (2014).
[5] Predel, B. Ge-Sn (Germanium-Tin) Landolt-Börnstein - Group IV Physical Chemistry 5F(Ga-Gd – Hf-Zr) (1996) https://materials.springer.com/lb/docs/sm_lbs_978-3-540-44996-6_1506
[6] Wu, et al. Advanced Materials 32.2, 1905632 (2020).

Topological insulators are a class of quantum materials that have gaps in the bulk but metallic edge or surface states, which are protected by time reversal symmetry. Long-range magnetic order in a topological insulator can break the time reversal symmetry, open an exchange gap in the surface states, thereby leading to exotic topological quantum phenomena. Just like ordinary materials, defects are also omnipresent in topological insulators. The concentration of defects are low; few parts per million for vacancies to few percent for dopants. This makes first principles simulations of defects at realistic concentrations extremely challenging due to the large cell sizes required. We use the Locally Self Consistent Multiple Scattering (LSMS) code to solve the Kohn-Sham equations at realistic concentrations of disorder in the intrinsic magnetic topological insulator MnSb₂Te₄. We present the role of disorder such as doping and site-mixing on the magnetic ground state of the intrinsic magnetic topological insulator MnSb₂Te₄. Our results show magnetism can be tuned by carefully controlling the concentration of defects in MnSb₂Te₄.

Acknowledgements:
This research was funded in part by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division and it used resources of the Oak Ridge Leadership Computing Facility, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.

New processing method for synthesizing a topological insulator semiconductor material BSTS (Bi-Sb-Te-Se2) system as a single crystal has been designed by computational method to lower the process temperature relatively which has not been done before. This research study is based on CALPHAD approach (Thermocalc) to design processing parameters followed by a series of characterization techniques such as XRD- TGA/DSC- XRF- EDS- XPS and TEM. The semiconductor system contains three phases hexagonal, rhombohedral, and second rhombohedral crystal structures which are presented in the phase field of the theoretical pseudo-binary phase diagrams. The semiconductor has been theoretically evaluated based on Hume-Rothery rules with junction to CALPHAD approach. This method yielded significant low-temperature processing of 350 degrees Celcius. The findings from both the theoretical and the experimental results converged to give an explanation for the BSTS single crystal system mechanics that can be useful for researchers to apply the method not only on BSTS but can be applied on their targeted system as well.

At the interface of InAs/GaSb, the broken gap alignment leads to an anti-crossing between the InAs s-like conduction levels and the GaSb p-like valence levels. The hybridization of these levels cause a topological gap to open. The observation of the quantum anomalous Hall effect (QAHE) is expected to occur if the s-p ordering is inverted (non-trivial) for one spin species and trivial for the other, which can be achieved through the introduction of magnetism into system. In this work, we characterize a series of Mn doped InAs/GaSb type-II quantum wells. We show the successful growth of 15nm InAs/ 10nm GaSb quantum wells using transition electron microscopy (TEM). Using SQUID magnetometry, we find the samples host a paramagnetic order. Robust SdH oscillations and Hall quantization between 1.5K and 50K are observed in magneto-transport. We perform magneto-optics up to 15T to measure the cyclotron mass. We report a discrepancy between the carrier mass calculated from SdH oscillations and cyclotron resonance. The manifestation of paramagnetism in this system makes it an ideal platform for measuring the magnetic exchange interaction.

Like the classical bit that launched the modern electronic age, the quantum bit or qubit lies at the heart of the ongoing quantum information revolution that is expected to transform science and society in previously unimaginable ways. Although solid-state defects such as the diamond-NV centers in three-dimensional hosts have been demonstrated as promising qubits, atomically-thin two-dimensional (2D) materials offer an exciting new paradigm for qubit fabrication and operation at room temperature. Using data-driven approaches and a symmetry-based hypothesis for qubit discovery, we show that neutral anion-antisite defects in six transition metal dichalcogenide (TMD) monolayers as viable spin qubits. We show how the optical transitions and triplet-singlet intersystem crossing paths in the qubit provide a complete cycle for initialization, manipulation, and readout of the qubit. The operational principles of the qubit are illustrated through an in-depth discussion of the antisite qubit in WS₂. Our study opens a new pathway for creating spin-qubits and multi-qubit platforms for quantum information technologies based on defects in 2D solid-state systems.

Spins bound to electronic dopant atoms in silicon are promising qubits since they interact weakly with their environment, resulting in exceptional coherence times of the order of seconds. Characterising and controlling their properties and interactions is essential to fulfil the prerequisite for accurate quantum state manipulation needed to build scalable quantum computers. An extra challenge in silicon originates from the indirect bandgap that leads to a spatial modulation of the qubit’s wavefunction at the atomic scale that is incommensurate with the host lattice. This presentation will cover innovative methods to probe the quantum states encoded of individual and interacting atom qubits in silicon, including single electron-tunnelling in nanoscale transistors, scanning tunnelling microscopy to directly image the wave-function at the atomic scale and radio frequency reflectometry. Together with the development of atomistic simulations of realistic devices, these methods bring new insights to the understanding of the impact of the valley degree of freedom in silicon to accelerate the engineering of atom-based quantum processors.

We have developed a strategy to implant near-surface donor atoms into silicon that form the components of quantum computer devices employing the donor nuclear and electron spins. Our devices address the challenge of bridging the internal donor quantum states to read-out circuitry and hence the classical observer. One and two atom devices can be engineered by implanting a small number of ions, subject to Poisson statistical distributions, then finding optimally located donors by sweeping surface control gates to induce electron exchange with an adjacent single electron transistor. Enriched 28-Si substrates, depleted in the nuclear spin ½ 29-Si isotope, give long donor nuclear and electron spin coherence times. For large scale devices we have developed a strategy integrating a plasma ion source, which produces donor ions with random arrival times, together with an active substrate that signals the implantation of each ion. The ion implantation site is defined by a nanostencil incorporated in an AFM cantilever that also allows for non-destructive, in-situ imaging of location markers to align the implanted donors to sub-100 nm precision. The signals are generated from the e-h pairs induced in the substrate using high sensitivity readout electronics which has also provided insights into the physics of the ion stopping process for near-surface doping. We have developed the capability to implant molecules of 31-PFx (x=1,2,3,4), Sb isotopes and 209-Bi all within a mean 20 nm of the silicon surface. In the case of PFx molecules, the F bystander ions generate e-h pairs, then diffuse away during post-implantation annealing for donor activation. Signals from the ion-induced electron hole pairs also show that discrete ion impacts suffer much lower levels of charge recombination in the substrate compared to continuous ion irradiation. In parallel we have demonstrated a method to locally deplete the non-zero nuclear spin isotope 29-Si to 250 ppm using high fluence ion implantation. Electron Spin Resonance measurements of near-surface 31-P donors material with 29-Si depleted to 3000 ppm with this method produced a T2-Hahn electron coherence time of 285 µs which is commensurate with previous work on enriched silicon. Also, the signal amplitude as a function of time showed a single component exponential decay without the need for a third order term imposed by residual 29-Si. For a large-scale device, this would potentially allow both 29-Si depletion and donor array construction in the same ion implantation system.

Acknowledgements: This work is supported by the Australian Research Council Centre of Excellence for Quantum Computation and Communication Technology (CQC2T, CE170100012) and the US Army Research Office (Contract no. W911NF-17-1-0200). We acknowledge the AFAiiR node of the NCRIS Heavy Ion Capability for access to ion-implantation facilities at EME, ANU, Silicon Quantum Computing for access to the electron spin resonance infrastructure, the Surface Analysis Laboratory, SSEAU, MWAC, UNSW for SIMS, and the contribution of collaborators in CQC2T including A.M. Jakob, D. Holmes, B.C. Johnson, J.C McCallum, A. Morello, S. Rogge.

Electron spins in semiconductor quantum dots are a leading platform for quantum computing because they have extremely long coherence times and are compatible with advanced semiconductor manufacturing techniques. However, most two-qubit gates between spins rely on electric fields to manipulate the wavefunctions of the electrons. Spurious electric-field noise, or charge noise, can cause decoherence of multi-spin systems and is a significant obstacle to large-scale quantum computing with electron spins. I will describe what we know about charge noise in semiconductor quantum dots and potential strategies to mitigate its effects.

Spin qubits that are hosted in electrically-controlled, silicon quantum dots are promising candidates for implementing quantum technology. Due to their small size, relatively long coherence times and their compatibility with the semiconductor industry they offer great promises, both for qubit performance and for scaling. To pave the road towards large-scale quantum computing, making use of common CMOS fabrication techniques, like optical lithography and chemical-mechanical polishing is key. Spin qubit devices to date, however, still rely on the flexibility of e-beam lithography and mostly lift-off techniques.

Here, we present the first qubits in isotopically enriched 28 Si-MOS fabricated entirely with optical lithography in an industrial 300 mm process line. These devices are fully fabricated with optical lithography and chemical-mechanical polishing techniques for patterning, compatible with state-of-the-art fabrication. This leads to an extremely high nanofabrication yield across the full wafer. Furthermore, due to the large number of devices characterized, we can study correlations between room-temperature and low-temperature device behavior. In these devices, we demonstrate well-controlled single and double quantum dots in the multi-electron regime, which is also reproducible across the wafer for several wafers. Moreover, we demonstrate charge sensing with a signal-to-noise ratio high enough for single shot readout. With this, we form high-quality qubits in the single-electron regime, comparable to qubits in academic devices, but with much better prospects for scaling.

Nitride superconductors are attractive materials for superconducting quantum circuits. This work investigates methods of growing an isolated phase of NbN_x to create a materials platform that can be used for an epitaxial Josephson junction (JJ) that may perform better than existing platforms by eliminating interface defects. NbN_x has been used for JJ fabrication due to its high critical temperature, along with its mechanical and chemical stability when compared to pure Nb JJ layers. However, our work has shown that when growing NbN_x with Plasma Assisted Molecular Beam Epitaxy (PAMBE), several distinct crystallographic phases of the nitride are present, and isolation of an individual phase for JJ fabrication proved to be difficult.
Isolation of the rock salt δ-NbN phase is necessary for growing Nb_xTi_1-xN films where the (111) plane has the same atomic spacing as the (0001) plane of AlN. These lattice-matched, abrupt interfaces may reduce defects at the insulating barrier, reducing energy loss in Josephson junction devices. The composition of Nb_xTi_1-xN can be adjusted to the correct lattice constant for lattice matched growth. These transition metal nitrides are mechanically and chemically stable, with high critical temperatures, making them ideal for superconducting devices.
PAMBE grown NbN_x and Nb_xTi_1-xN films on Al₂O₃ (0001) are investigated regarding phase stabilization with varying substrate temperature, buffer layers, and alloyed composition. The rock salt phase of NbN_x is desired for heterostructure growth, and this phase is verified using high-resolution x-ray diffraction (HRXRD) and electron backscatter diffraction (EBSD). Atomic force microscopy (AFM) and electrical characterization of the normal and superconducting states are used to determine suitability for Nb_xTi_1-xN(111)||AlN(0001)||Nb_xTi_1-xN(111) and NbN_x(111)||AlN(0001)||NbN_x(111) Josephson junction heterostructures.
Samples were grown using a custom DCA M600 MBE system with a base pressure better than 1e-10 Torr. Samples were grown on 2” C-plane sapphire wafers, which were degreased and thermally cleaned before growth. A RF plasma generator is used for nitrogen incorporation, while Nb is deposited using an electron beam evaporator. Ti is deposited using a high-temp effusion cell. Nitrogen flux was set at 0.6 sccm at 500W, and Nb flux was kept between 0.5E17 and 3.5E17 m^-2s^-1. Substrate temps during growth were set at between 800°C and 1000°C by thermocouple.
δ-NbN grows predominantly at 1000°C with our growth technique, though phases γ-Nb₄N₃ and ε-NbN are still present in our films. To isolate the rock salt phase further, we investigate the effects of TiN buffer layers, along with alloying NbN_x with Ti. We have also observed that Ti incorporation in NbN_x films at lower temperatures increases the rock salt phase stability. For δ-NbN, XRD and EBSD results show uniform, single phase growth, while AFM results show a smooth, striated surface with a roughness of < 1nm, ideal for JJ fabrication. Internal quality factors of these thin films are investigated by way of superconducting coplanar microwave resonator measurements.

Quantum computers are an inevitable need for energy efficiency in large scale computation and absolute information security demanded in both military and civil applications. Among the proposed platforms, superconducting qubit-based processors are leading candidates because of its scalability, gate speed, and microwave control. However, the realization of feasible quantum processors is inhibited prominently by the decoherence processes emerging from materials-inherent defects such as suboxides on Nb based superconducting qubits. In recent work, microwave losses scaling inversely with NbO_x thickness in Nb resonator devices has been demonstrated indicating the impact of the suboxides on coherence time. DFT calculations by Griffin et. al., shown that sub-stoichiometric Nb₂O₅ has paramagnetic phase that can be a source of the decoherence. In this work we investigated impact of Nb₂O₅ on coherence time by selectively eliminating Nb₂O₅ via oxygen scavenging by TiN deposited on resonators by atomic layer deposition. In comparison to Nb resonators with native oxide, TiN passivated resonators shows traces of Nb₂O₅ while NbO and NbO₂ persist on the Nb according to photoemission spectroscopy. Controlled selective reduction of Nb₂O₅ to NbO on Nb resonators hence mitigation of the associated defects with TiN passivation suggest a viable step towards development of the large-scale fault tolerant quantum processor.

Solid state quantum information technologies are highly sensitive to material defects. In particular, charge centers at oxide interfaces have been shown to cause charge noise in electron-spin quantum dot qubits and negatively impact device yield, long-term stability, qubit coherence, and measurement fidelity. Semiconductor spin qubits are typically fabricated using standard metal-oxide-semiconductor gates to accumulate or deplete the two-dimensional electrons regions and localize single electrons in a quantum dot. These gates require an oxide layer, giving rise to two material-oxide interfaces that may contain charge defects that adversely affect the qubit performance. As such, thorough characterization of defects in an oxide layer interface is crucial for optimizing the behavior of quantum information devices. Several measurement techniques using operational qubits have already been established, however these techniques are limited in bandwidth and require long measurement times to fully characterize the charge noise spectrum of a single device. In this work we develop superconductive coplanar waveguide (CPW) resonators as a potentially much faster, scalable, and less expensive alternative charge-noise characterization technique to quantify the quality of oxides. To calibrate our technique, we present measurements of two different oxides, a high-quality gate oxide and an oxide with intentional defects.

Funding Acknowledgment: SNL is managed and operated by NTESS under DOE NNSA contract DENA0003525

Single crystal silicon, super-saturation doped with aluminum, has the promise of realizing superconductivity within a semiconductor with a transition temperature above 4 K, a substantial increase over similar demonstrations with boron and gallium, but is yet to be realized. In this talk, the process of saturation doping a clean silicon surface, overgrowing it with an encapsulating silicon layer and electrically activating the dopants is presented. Emphasis will be put on the determination of the surface-saturation doping density, found to be 3.4 x 10¹⁴ atoms/cm², which involves comparative measurements from scanning tunneling microscopy (STM), secondary ion mass spectroscopy (SIMS) and atom probe tomography (APT). Additional discussion on the electrical activation efficiency, low temperature transport modelling and prospects for increase the stable dopant atom density will be presented.

Superconducting qubits have emerged as a potentially foundational platform technology for addressing complex computational problems deemed intractable with classical computing. Despite recent advances enabling multiqubit designs that exhibit coherence lifetimes on the order of hundreds of μs, material quality and interfacial structures continue to curb device performance. When niobium is deployed as the superconducting material, two-level system defects in the thin film and adjacent dielectric regions introduce stochastic noise and dissipate electromagnetic energy at the cryogenic operating temperatures. In this study, through a combination of correlative electron microscopy imaging and diffraction methods and secondary ion mass spectrometry, we conduct a detailed investigation to disentangle the role specific fabrication procedures play in introducing such dissipation mechanisms in these complex systems. From these measurements, we find that during film deposition, oxide and silicide layers form at metal/substrate and metal/air interfaces and can reduce coherence times. Additionally, we observe that impurity species such as resistive niobium hydrides and carbides are incorporated within the niobium layer through the subsequent lithographic patterning steps, which impact the superconducting properties of the thin film. Armed with these findings, we seek to intelligently fabricate superconducting qubits and extend coherence times.

Nb/Al bilayers are of great interest for applications such as X-ray detectors or Josephson junctions where Al enables high-quality tunnel barrier fabrication and can be used to modify the Nb superconducting state. To investigate the evolution of the superconducting bilayer states with Al thickness, we fabricated a series of Nb/Al/AlOx/normal metal tunnel junctions with different Al thicknesses and measured the tunnel junction properties. Tunneling density of states spectra are presented in the regime of T_{c, Nb} > T > T_{c, Al} and reveal larger zero-bias conductance than can be explained by a change in superconducting transition temperature with Al thickness alone. We model the data using the Blonder-Tinkham-Klapwijk (BTK) theory and discuss trends in BTK barrier strength, superconducting energy gap, and Dynes broadening as a function of Al film thickness and temperature.

Superconducting quantum circuits are a promising hardware implementation for large-scale quantum computing, but materials losses continue to significantly limit scalability and single-qubit performance. These cryogenic microwave materials losses can thus be characterized by coherence measurements of qubits and resonators. However, experimental factors such as thermalization, packaging, and microwave electronics have significant effects on these coherence measurements as well. In this talk, I outline a roadmap towards reproducible measurements of qubits and resonators and their further application to the investigation and characterization of materials losses.

Surface electrode ion traps permit the individual storage, manipulation, and optical addressing of atomic ions for applications such as highly sensitive sensors, atomic clocks, and quantum computing, to name a few. For these applications to be successful, one must have precise control over the quantum state of the ion, which includes its motional state during entangling operations. Therefore, the ion must be isolated from environmental perturbations like electric field noise which uncontrollably excite the motion of ions. Here, we study the influence of materials used in the fabrication of surface electrode ion traps and how they contribute to electric field noise that resonantly couples to the motional (secular) frequency of a trapped ion. In addition, we will also show experimental data that compares motional heating at different frequencies and with different capacitive grounding methods.

Single-walled carbon nanotubes (SWCNTs) doped with sp3 defects are a promising class of optoelectronic materials with bright tunable photoluminescence and demonstrated single-photon emission capability. Recently, we have shown that doping of (6,5) SWCNT via diazonium reaction results in an isolated electron spin with bright photoemission in the near-infrared range. In order to understand the spin-orbit coupling (SOC) and the spin decoherence mechanism in this material, we apply density functional theory (DFT) simulations, complemented by experiment, on a variety of sp3 defects attached to (6,5) SWCNT. We determine that doping the SWCNT with a heavy atom does not necessarily result in spin-orbit splitting in the near-gap states (a measure of SOC in the excited state) if there is no overlap between the unpaired spin and the metal’s atomic orbitals; this overlap can be enforced by choosing a defect with a direct sp3 carbon-metal bond. Furthermore, the strength of spin-orbit splitting increases with reduction of the metal-tube separation or larger intrinsic SOC on the heavy atom. Additionally, we simulate dephasing by considering spin-bath interactions as the source of decoherence. We apply the cluster correlation expansion method with a pure-dephasing spin Hamiltonian to doped SWCNT in presence of water molecules at near 0 K and estimate the magnitude of T2.

We report [https://arxiv.org/abs/2107.13518] experimental detection and control of Schrodinger’s Cat like macroscopically large, quantum coherent state of a two-component Bose-Einstein condensate of spatially indirect electron-hole pairs or excitons using a resonant tunneling diode of III-V Semiconductors. This provide access to millions of excitons as qubits to allow efficient, fault-tolerant quantum computation. In this work, we measure phase coherent periodic oscillations in photo generated capacitance as a function of applied voltage bias and light intensity over a macroscopically large area. Periodic presence and absence of splitting of excitonic peaks in the optical spectra measured by photocapacitance point towards tunneling induced variations in capacitive coupling between the quantum well and quantum dots. Observation of negative ‘quantum capacitance’ due to screening of charge carriers by the quantum well indicate Coulomb correlations of interacting excitons in the plane of the sample. We also establish that coherent resonant tunneling in this well-dot heterostructure restricts the available momentum space of the charge carriers within this quantum well. Consequently, the electric polarization vector of the associated indirect excitons collective orients along the direction of applied bias and these excitons undergo Bose-Einstein condensation below ~100 K. Generation of interference beats in photocapacitance oscillation even with incoherent white light further confirm the presence of stable, long range spatial correlation among these indirect excitons. We finally demonstrate collective Rabi oscillations of these macroscopically large, ‘multipartite’, two-level, coupled and uncoupled quantum states of excitonic condensate as qubits. Therefore, our study not only brings the physics and technology of Bose-Einstein condensation within the reaches of semiconductor chips, but also opens up experimental investigations of the fundamentals of quantum physics using similar techniques.
Operational temperatures of such two-component excitonic BEC can be raised further with more densely packed, ordered array of QDs and/or using materials having larger excitonic binding energies. However, fabrications of single crystals of 0D-2D heterostructures using 2D materials (e.g. transition metal di-chalcogenides, oxides, perovskites etc.) having higher excitonic binding energies are still an open challenge for semiconductor optoelectronics. As of now, these 0D-2D heterostructures can already be scaled up for mass production of miniaturized, portable quantum optoelectronics devices using the existing III-V and/or Nitride based semiconductor fabrication technologies.

The prospect of building quantum circuits using advanced semiconductor manufacturing techniques position quantum dots as an attractive platform for quantum information processing. Initial demonstrations of one and two-qubit logic have been performed in gallium arsenide and later silicon. However, until recently, interconnecting larger spin qubit systems has remained a challenge.

Over the past years, hole states in strained germanium quantum wells have emerged as a host for spin qubits. These states have favourable properties for defining extended spin qubit arrays. The small effective mass relaxes constraints on lithography, the low degree of disorder enables reproducible quantum dots, the lack of a valley degeneracy ensures an well-defined qubit state and the strong spin-orbit coupling allows for local and electrical qubit control.

Within four years time, this platform [1] has rapidly evolved from materials growth to supporting multi-qubit logic [2]. In this talk, I will discuss our group's efforts on the development of this system, starting from material growth and characterization [3] to our recent results on operating a highly-connected two-dimensional qubit array [4]. We implement qubit logic all electrically and the exchange interaction can be pulses independently to freely program one, two, three, or four qubit gates. Furthermore, we show that we can extend the quantum coherence by several orders of magnitude by implementing dynamical decoupling. All these techniques are combined to perform a quantum circuit that generates a four-qubit Greenberger-Horne-Zeilinger state, showcasing coherent operation of all four qubits together. This positions strained germanium as a unique material for quantum applications. I will furthermore discuss strategies, challenges, and opportunities in scaling these systems up as a step towards the realization of scalable qubit tiles for fault-tolerant quantum processors.

[1] Scappucci, G. et al. The germanium quantum information route. Nature Reviews Materials (2020);
[2] Hendrickx N.W. et al. Fast two-qubit logic with holes in germanium. Nature 577 487-491 (2020);
[3] Sammak, A. et al. Shallow and Undoped Germanium Quantum Wells: A Playground for Spin and Hybrid Quantum Technology. Advanced Functional Materials 29 14 (2019);
[4] Hendrickx N.W. et al. A four-qubit germanium quantum processor. Nature 591 580-585 (2021).

Semiconductor quantum dots are one of the most promising platforms for quantum computing due to their nanofabrication capability for scaling-up. Recent technical advances have enabled high-fidelity one- [1,2] and two-qubit [3] gates for spins in quantum dots. The next crucial step toward scaling up is the demonstration of genuinely multipartite entanglement, an essential resource for quantum error correction. This requires at least three qubits with the full capabilities of control and measurement, a lot further than individual qubit manipulation. In this talk, we first show individual initialization, measurement, and universal manipulation of three electron spins in our Si/SiGe triple quantum dot device. Second, we show generation and characterization of a three-qubit Greenberger–Horne–Zeilinger state, a type of maximally entangled state that is usable to implement a bit-flip or phase-flip quantum error correcting code. We obtain a state fidelity of 88.0 % by quantum state tomography, which witnesses a genuine three-qubit entanglement. We anticipate that our results will enable exploration of multi-spin correlations and demonstration of multi-qubit algorithms in scalable silicon-based quantum computing devices.

[1] J. Yoneda et al., Nat. Nanotechnol. 13, 102–106 (2018).
[2] C. H. Yang et al., Nat. Electron. 2, 151–158 (2019).
[3] W. Huang et al., Nature 569, 532–536 (2019).

The design and development of charge stable, plasma oxidized Al/AlO_x/Al single-electron transistors (SETs) for use as charge sensors in single metal layer gated silicon MOS quantum dots will be presented, extending an integrated architecture that enables fabricating both devices simultaneously. In previous work, we achieved long-term charge offset stability (best ΔQ₀ = 0.13 e ± 0.01 e over 7.6 days) by using plasma oxidized AlO_x as tunnel barriers in Al/AlO_x/Al SETs [1], and persistent charge sensing (SNR > 5) of controllable quantum dots on single metal gate layer silicon MOS quantum dot devices [2]. By calculation, the required minimum coupling capacitance between these two devices is approximately 2.8 aF for the AlO_x SET to detect the charge motion on the MOS quantum dot. Here we will present the design of streamlined fabrication process of fabricating Al/AlO_x/Al SETs and MOS quantum dots simultaneously, preliminary modeling and calculations for charge sensing feasibility of vertically and laterally aligned configurations. This integrated architecture is expected to be useful as a diagnostic qubit platform for simplifying the process flow, testing gate materials and tunnel barrier materials, and detecting oxide interface defects, etc.

References:
[1] Hong, Yanxue, et al. "Reduction of charge offset drift using plasma oxidized aluminum in SETs." Scientific reports 10.1 (2020): 1-9.
[2] Hong, Yanxue, et al. "Developing single-layer metal-oxide-semiconductor quantum dots for diagnostic qubits." Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena 39.1 (2021): 012204.

Among promising materials for the realization of future quantum processors, silicon has several assets. A silicon quantum bit (qubit) device can be performed with an industry-standard fabrication process and allows a co-integration with classical control hardware [1]. However, an excellent control of carrier transfer in the channel of the transistor is required, ruling out any electrical active defects, which could act as recombination centers. Such an optimization of the device necessitates a fine characterization of the channel to evaluate the purity of the material.
In this work, we investigate the presence of defects in the channel of a fully depleted silicon-on-insulator (FDSOI) transistor designed for qubit applications. A buried oxide of 145 nm formed on a silicon substrate is covered by an 11 nm thick unintentionally doped silicon channel. A SiO₂/HfSiON bilayer and a TiN metallic electrode compose the gate. P-doped source and drain contact regions were realized by implantation. A Si₃N₄ layer is used to passivate uncovered silicon areas. To enlarge the capacitance of the gate and allow measurements, 111 cells were connected in parallel.
The ON state of the transistor is achieved by applying a negative voltage to the gate. Source and drain electrodes were connected together and voltage pulses were applied to the gate contact to perform capacitance deep level transient spectroscopy (DLTS) measurements between 77 K and 350 K. A depleted section of the channel extends beneath the gate and towards the source and drain regions when applying a positive voltage to the gate contact. By tuning a positive reverse bias and negative voltage pulses, we were able to probe the channel and localize the electrical active defects responsible for the DLTS signal.
Three hole traps, labelled H1, H2 and H3, were detected by DLTS at respectively 0.54 eV, 0.57 eV and 0.65 eV above the valence band edge. The apparent capture cross sections of these levels are respectively 7.9x10^-16 cm², 2.4x10^-14 cm² and 3.3x10^-12 cm². H1 and H2 are donor-like states whereas H3 is an acceptor-like state.
Electrical simulations using Synopsys demonstrated that the DLTS probed zone is nearby gate edges and extends towards the source and drain contacts when the gate voltage changes from 0 V to 1 V. According to this feature, all traps have been localized very close to the source and drain implanted zones. The origin of these traps is likely related to damages produced during the formation of the p-doped regions and are assigned to bulk [2] and Si/SiO₂ interface defects [3].
In the temperature range 77 K – 350 K, no traps have been detected in the active region, i.e. below the gate contact. Finally, this investigation highlights the good quality of the channel material and the ability of the studied FDSOI transistor to work as a qubit device.
[1] R. Maurand et al., Nat. Commun. 7, 13575 (2016)
[2] E. Fretwurst et al., Nucl. Instr. and Meth. in Phys. Res. A 388, 356 (1997)
[3] E. Simoen et al., Phys. Status Solidi C 13, 718 (2016)

²⁸Si/SiGe heterostructures provide a compelling host material for a scalable quantum computer,due to the long coherence times of electron spin qubits and compatibility with industry. Here we study ²⁸Si/SiGe heterostructures characterized by different roughness of the critical Si/SiGe quantum well interface to understand the energy splitting of the lowest lying conduction valleys (valley splitting). To improve control of valley splitting in ²⁸Si/SiGe, we implement a feedback loop for materials stack engineering which includes several elements. We use atom probe tomography to provide atomic 3D reconstruction of the material stack and statistical understanding of compositional variations at the Si/SiGe interface over nanoscale dimensions relevant for qubits in quantum dots. We feed the resulting data into a tight binding model to compute the valley splitting in real quantum wells and dots with varying quantum well thickness, Si/SiGe interface width, and interface chemical roughness. We complete the feedback loop by comparing the obtained simulation results with the valley splitting measured in heterostructure field effect transistors and quantum dots, making use of cryomultiplexer technology for achieving statistical significant metrics with high-throughput. We envision that such a feedback cycle - consisting of atomic understanding of the critical material stack interfaces fed into device modelling and high-throughput cryogenic measurements- may help to engineer optimal stacks for large and controlled values of valley splitting in Si/SiGe.

Magnetic topological insulators and semimetals have a variety of properties that make them attractive for applications, including spintronics and quantum computation, but very few high-quality candidate materials are known. In this paper, we use systematic high-throughput density functional theory calculations to identify magnetic topological materials from the 40000 three-dimensional materials in the JARVIS-DFT database. First, we screen materials with net magnetic moment >0.5μB and spin-orbit spillage (SOS) >0.25, resulting in 25 insulating and 564 metallic candidates. The SOS acts as a signature of spin-orbit-induced band-inversion. Then we carry out calculations of Wannier charge centers, Chern numbers, anomalous Hall conductivities, surface band structures, and Fermi surfaces to determine interesting topological characteristics of the screened compounds. We also train machine learning models for predicting the spillages, band gaps, and magnetic moments of new compounds, to further accelerate the screening process. We experimentally synthesize and characterize a few candidate materials to support our theoretical predictions.

Frustrated magnets wherein complex interplay between emergent degrees of freedom, macroscopic ground state degeneracy and competing magnetic interactions can lead to correlated quantum phenomena with exotic excitations highly relevant for potential technological applications. We present thermodynamic, and muon spin relaxation (μSR) measurements accompanied by density functional theory (DFT) results on the double-perovskite Ba₂MnTeO₆ in which Mn<span style="font-size:10.8333px"> </span>ions constitute a perfect triangular lattice. Magnetic susceptibility and specific heat data show the presence of a phase transition at T_N = 21 K. The magnetic specific heat result infers the presence of antiferromagnetic magnon excitations with a gap of 1.4 K below T_N. μSR reveals the presence of static internal magnetic field below T_N and short-range spin correlations high above T_N. Furthermore, critical slowing-down of spin dynamics at T_N and the persistence of spin dynamics even in the magnetically ordered state have been detected by μSR. Three-dimensional magnetic ordering in this frustrated magnet is attributed to the presence of a weak ferromagnetic inter-layer interaction.

Bound states of electric charge and magnetic monopole are anticipated to behave as anyons, i.e. 2D quantum states obeying fractional statistics. Image magnetic monopoles can be induced in topological systems with broken time-reversal symmetry, such as quantum Hall (QH) and quantum anomalous Hall (QAH) insulators, using a point charge located above the surface. In our experiments with the QAH insulator, charge-monopole composite is produced by the electric field of scanning tunneling microscope (STM) tip, which confines the current vortex of monopole inside ~ 1 nm radius quantum dot. Our studies are performed using magnetic topological insulator MnBi₂Te₄ whose surface is modified by STM-lithography to prepare room temperature QAH state displaying unusually high Chern number. We found a rich spectrum of many-body effects and nonlinear behavior manifested in the trapping of tunneling electrons by the current vortex and interaction of monopole with the Mn spin subsystem. High-Chern-number charge-monopole composites are anticipated to be non-Abelian anyons with potential applications in fault-tolerant topological quantum computing.

We have developed a new simple broadband ormicrowave spectroscopy technique for µm-scale flakes in which, unlike existing techniques, µm-scale flakes effect orders-of-magnitude (>30 dB) changes in the transmissivity without requiring sophisticated sample preparation or measurement techniques. Broadband microwave spectroscopy is a sensitive probe of the physics of material systems which has been used to study novel physics of mm-scale thin film material systems, such as ordered electron phases , superconductors , and metal-insulator transitions . In recent years, interesting materials have emerged which are only available as µm-scale flakes, and whose novel physics might be better understood through broadband microwave spectroscopy; examples include twisted bilayer graphene [4], 2D materials in which many-body phases are observed [5], and artificial lattices for analog quantum simulations [6]. With most current GHz broadband spectroscopy techniques, the signal for smaller (µm-scale) flakes is too weak to be useful [1, 2, 3].

In the new technique, a CPW containing a series gap in the signal line over the flake is fabricated on the chip, and the transmission spectrum is measured. The spectra are then normalized, we can immediately see spectral features, and we can also extract the complex conductivity. We present results from proof-of-principle experiments conducted for µm-scale P⁺ monolayer regions encapsulated in Si, whose conductivity spectrum is expected to be constant in frequency for microwave frequencies. These results show a broadband change in the transmissivity of more than 40 dB, indicating the ability to perform sensitive contact-free conductivity spectroscopy on µm-scale flakes. We then show through theory and simulations that one will be able to qualitatively measure spectral features of interest, and quantitatively deduce the frequency-dependent complex conductivity. These experimental and theoretical results show that this technique could make profound contributions to the characterization of 2D materials in the coming years.

References
[1] L. W. Engel, et al. Solid State Comms. 104 167 (1997)
[2] Y. Oamura et al. Phys. Rev. Lett. 122 057202 (2019)
[3] K. Steinberg et al. Phys. Rev. B 77 214517 (2008)
[4] Y. Cao, et al. Nature 556 43 (2018)
[5] S. Chen et al. Phys. Rev. Lett. 122. 026802 (2019)
[6] J. Salfi et al. Nat. Comms.7 1 (2016)

Atomically precise phosphorus-doped silicon nanostructures that incorporate few-atom arrays are attractive architectures for the solid-state implementation of quantum computing technologies and quantum simulations. These devices readily integrate with traditional electronic devices and can be fabricated to control critical properties, such as tunnel couplings and on-site charging energies. Furthermore, single-electron transport and quantum control of the electronic state is possible using atomically aligned gates, source, and drain contacts. Significantly, few-atom devices arranged in any number of different geometries are ideal for exploring a variety of many-body physics effects. This talk presents atomistic calculations and a detailed analysis of the many-electron states and transport properties of devices with two and four P atoms. In particular, we investigate mappings of these systems into the extended range Fermi-Hubbard model and study how the model parameters vary with the array's geometry and electron number. Our approach consists of self-consistent solutions to the Schroedinger-Poisson equations and incorporates the effect of the silicon matrix, electron-electron interactions, and corrections due to exchange and correlation. We also carry out transport calculations beyond the mean-field treatment by using the non-equilibrium Green functions based on the extended Fermi-Hubbard many-electron eigenstates to calculate Coulomb blockade diagrams. By comparing the predictions made with the Fermi-Hubbard model with the results of our detailed atomistic calculations for different dopant geometries, we show when exchange and correlation effects and coupling to higher energy orbitals missing in the Fermi-Hubbard model become important in predicting the stability diagrams of these devices.

Topological insulators are a class of quantum materials that have gaps in the bulk but metallic edge or surface states, which are protected by time reversal symmetry. Long-range magnetic order in a topological insulator can break the time reversal symmetry, open an exchange gap in the surface states, thereby leading to exotic topological quantum phenomena. Just like ordinary materials, defects are also omnipresent in topological insulators. The concentration of defects are low; few parts per million for vacancies to few percent for dopants. This makes first principles simulations of defects at realistic concentrations extremely challenging due to the large cell sizes required. We use the Locally Self Consistent Multiple Scattering (LSMS) code to solve the Kohn-Sham equations at realistic concentrations of disorder in the intrinsic magnetic topological insulator MnSb₂Te₄. We present the role of disorder such as doping and site-mixing on the magnetic ground state of the intrinsic magnetic topological insulator MnSb₂Te₄. Our results show magnetism can be tuned by carefully controlling the concentration of defects in MnSb₂Te₄.

Acknowledgements:
This research was funded in part by the U. S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division and it used resources of the Oak Ridge Leadership Computing Facility, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC05-00OR22725.

The last decade has witnessed substantial progress in probing topological states of matter whose wavefuntions possess a distinct topological invariant signature barring adiabatic deformation from a trivial phase to a non-trivial phase [1]. The search for topological materials is of profound scientific significance and will have applications for spintronics, low-dissipation electronics and quantum computing. Particularly, one dimensional (1D) topological superconductor systems, which are predicted to host localized Majorana bound states (MBSs), can be leveraged for fault-tolerant and scalable topological quantum computing. However, realizing 1D topological superconductors is challenging in synthesis of topological nanowires with sufficiently good sample quality, which can reveal and control the topological properties. Careful studies on precise control of the morphology and dimensions of topological nanowires along the axial and radial directions remain sparse.

In this work, we developed a reliable vapor−liquid−solid (VLS) route to synthesize narrow SnTe nanowires with diameters under 100 nm using Au−Sn−Te alloy nanoparticles as growth catalysts with high yield [2]. Experimentally, this was achieved by performing CVD growths twice. In the first growth, gold nanoparticles react with SnTe vapor to form alloy nanoparticles. In the second growth, the alloy nanoparticles catalyze the growth of narrow SnTe nanowires. The average diameter of the SnTe nanowires grown using alloy nanoparticles is 85 nm, in contrast to the 240 nm obtained using gold nanoparticles. Transport measurements showed a diameter-dependent residual resistance ratio and magnetoresistance.

We also observed that the ferroelectric transition temperature of SnTe nanowires is diameter-dependent, whose trend suggests that the carrier density decreases for narrower nanowires, likely due to the removal of bulk electron carriers. In situ cryogenic transmission electron microscopy (cryo-TEM) experiments down to 25 K confirmed the expected structural transition from the room-temperature cubic phase to the low-temperature rhombohedral phase for these SnTe nanowires. Electron diffraction data and TEM images at 25 K show there are two crystallographic orientations for the low-temperature rhombohedra domains in these SnTe nanowires. Simulation results suggest that both 109 and 71° domain walls are possible to explain our observations. The narrow SnTe nanowires are important to study the nature of the superconductivity induced in these nanowires, which was recently suggested to have a pairing symmetry of s ± is via Josephson junction measurements [3]. They also serve as a model system for the investigation of possible multiband superconductivity, Majorana bound states, and the interplay between ferroelectricity and topological orders.

1. Qi, X.-L. and S.-C. Zhang, Topological insulators and superconductors. Reviews of Modern Physics, 2011. 83(4): p. 1057-1110.
2. Liu, P., et al., Synthesis of narrow SnTe nanowires using alloy nanoparticles. ACS Applied Electronic Materials, 2021. 3(1): p. 184-191.
3. Trimble, C.J., et al., Josephson detection of time-reversal symmetry broken superconductivity in SnTe nanowires. npj Quantum Materials, 2021. 6(1): p. 61.

SnTe is identified to be a topological crystalline insulator, whose surface states are protected by crystal mirror symmetry. It has been recently found that by In doping, Sn_1-xIn_xTe (SIT) can be superconducting and also maintains its topological surface states. However, the non-trivial character of superconductivity in SIT remains debated. Most previous research of SIT was focused on single crystals. The growth and analysis of SIT thin films are necessary to search for boundary Majorana states. Here, we investigated the MBE growth of Sn_1-xIn_xTe(x=0.05-0.3) thin films(~100nm) on BaF₂ (111), and characterized them by X-ray diffraction , Transmission electron microscopy and magneto-transport measurement. A superconducting transition is consistently observed in films, and the critical temperature T_c increases as we increase the In concentration up to 3.5K. Hall measurements show that In is more likely to act as an electron donor instead of an acceptor as reported in earlier works on single crystal. The normal state is shown to host coexisting weak localization (WL) and weak antilocalization (WAL), analyzed using a 2D Hikami Larkin Nagaoka model. The WL only occurs in superconducting samples, indicating that electrons from trivial spin-orbit split states may play a dominant role in the superconducting state. Close to but above T_c, a signature of Maki-Thompson superconducting fluctuations is also observed. Overall, our findings motivate further experiments to find the origin of a possible spin-orbit splitting and study its impact on the pairing symmetry of Sn_1-xIn_xTe. Work supported by NSF-DMR-1905277.

Semiconductor heterostructures based on III-V materials such as InAs are candidate systems for realizing a topological quantum computer based on Majorana zero modes due to their high mobility, strong spin-orbit interaction, and ability to be proximitized by a nearby superconductor. While each of these components can be realized alone, combining them into a single device exhibiting all effects simultaneously and reproducibly remains a significant engineering challenge. In this talk, I will discuss materials processing that limits mobility in InAs 2DEG quantum wells during device fabrication, and discuss strategies for increasing this mobility by repairing the semiconductor-dielectric interface. I will also discuss the effect that these fabrication processes have on the spin orbit coupling within these heterostructures, and pathways to maximizing both mobility and spin orbit in a device.

We present progress on the synthesis of semimetal Cd₃As₂ by metal-organic chemical-vapor deposition (MOCVD). Specifically, we have optimized the growth conditions needed to obtain technologically useful growth rates and acceptable thin-film microstructures, with our studies evaluating the effects of varying the temperature, pressure, carrier-gas type, and group-V:II molar flow ratio for MOCVD of Cd₃As₂ when performed using dimethylcadmium and tertiarybutylarsine precursors. In the course of the optimization studies, exploratory Cd₃As₂ growths have been carried out on GaSb substrates, InAs substrates, strain-relaxed InAs buffer layers grown on GaSb substrates, and near-lattice-matched AlGaAsSb-alloy buffer layers grown on GaSb substrates. Notably, the InAs-terminated substrate surfaces yield desirable, nearly epitaxial results, while the pure GaSb surfaces instead yield laterally nonuniform, poor-quality, polycrystalline results. For the films presenting epitaxially on InAs, two different growth regimes are seen that depend strongly on the V:II ratio. For Cd₃As₂ thin films grown at a lower V:II ratio near 1.7, extensive microstructural studies have been performed by using multiple advanced imaging microscopies and x-ray diffraction (XRD) modalities. The as-grown films are 5-75 nm in thickness and consist of highly oriented, coalesced polycrystals with lateral domain widths of 30-80 nm. The most optimized films are smooth and specular, exhibiting a surface roughness as low as 1.0 nm rms. Under cross-sectional imaging, the Cd₃As₂-InAs heterointerface appears smooth and abrupt at a lower film thickness, ~30 nm, but becomes quite irregular as the average thickness increases to ~55 nm. The films are strain-relaxed with a residual biaxial tensile strain (e_xx = +0.0010) that opposes the initially compressive lattice-mismatch strain of Cd₃As₂ coherent on InAs (e_xx = -0.042). For more recent Cd₃As₂ thin-film growths done at a higher V:II ratio near 8.0, there appears to be a substantially improved thin-film microstructure, with thickness fringes emerging in both XRD 2theta-omega scans and x-ray reflectivity scans, which are absent for the growths at lower V:II ratio. Further microstructural characterization of the higher-V:II-ratio films remains as work in progress. Importantly, phase-identification studies find a thin-film crystal structure consistent with the P4₂/nbc space group for growth at the lower V:II ratio; in contrast, the space group appears to change to either I4₁cd or I4₁/acd for growth at the higher V:II ratio. These observed space groups place MOCVD-grown Cd₃As₂ among the Dirac semimetals of substantial interest for topological quantum materials studies. Our presentation will highlight selected results from the above-described research to grow Cd₃As₂ thin films by MOCVD, with a focus on growth-parameter trends, microstructural characterization, and electronic properties.

Supported by the Laboratory Directed Research and Development program at Sandia National Laboratories, a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525.

The study of unconventional superconducting states remains at the forefront of condensed matter physics research, driven in large part by the desire to understand the superconducting ground states of novel quantum materials. One of the defining features of any superconducting state is its collective mode spectrum, which encodes the dynamics of the order parameter. We will discuss how one can leverage the collective mode spectrum of unconventional superconducting states to gain new insights into their order parameter symmetry, making use of existing experimental probes [1,2]. In particular, we will discuss the existence and possible detection of a unique class of collective modes which can be used to identify and characterize time-reversal symmetry breaking superconductors [3].

References
[1] R Shimano & N Tsuji, Annu. Rev. Condens. Matter Phys. 11, 103 (2020)
[2] L Schwarz, et al., Nat. Comm. 11 287 (2020)
[3] N Poniatowski, et al., arXiv:2103.05641

At the interface of InAs/GaSb, the broken gap alignment leads to an anti-crossing between the InAs s-like conduction levels and the GaSb p-like valence levels. The hybridization of these levels cause a topological gap to open. The observation of the quantum anomalous Hall effect (QAHE) is expected to occur if the s-p ordering is inverted (non-trivial) for one spin species and trivial for the other, which can be achieved through the introduction of magnetism into system. In this work, we characterize a series of Mn doped InAs/GaSb type-II quantum wells. We show the successful growth of 15nm InAs/ 10nm GaSb quantum wells using transition electron microscopy (TEM). Using SQUID magnetometry, we find the samples host a paramagnetic order. Robust SdH oscillations and Hall quantization between 1.5K and 50K are observed in magneto-transport. We perform magneto-optics up to 15T to measure the cyclotron mass. We report a discrepancy between the carrier mass calculated from SdH oscillations and cyclotron resonance. The manifestation of paramagnetism in this system makes it an ideal platform for measuring the magnetic exchange interaction.