Symposium EQ14—Materials and Devices for Controlling Quantum-Coherent Spin Dynamics

The emergence of coherent magnons as a potential transduction channel for coupling remote spin qubits has generated intense interest in both the theoretical limits for these strategies and the practical barriers to their implementation. Here, we present recent progress on both fronts enabled by the emergence of the molecule-based coordination compound vanadium tetracyanoethylene (V[TCNE]_x), an ultra-low loss ferrimagnet with magnetic excitations whose quality factor, Q, exceeds 8,000 (corresponding to a linewidth of 0.51 Oe at 9.86 GHz) and whose Gilbert damping (alpha = 4 x 10^-5) is competitive with highly polished spheres of yttrium iron garnet (YIG). In contrast to other molecule-based materials V[TCNE]_x exhibits a Curie temperature of over 600 K with robust room temperature hysteresis with sharp switching to full saturation. Further, since V[TCNE]_x is grown via chemical vapor deposition (CVD) at 50 C it can be conformally deposited as a thin film on a wide variety of substrates and patterned using electron-beam lithography without introducing additional loss [1]. We will discuss how this combination of properties, coupled with an extremely low saturation magnetization (M_s = 100 Oe), allows access to a regime of magnetization dynamics that is theoretically predicted to yield cooperativities in excess of 10 for spin-magnon coupling in realistic device geometries (e.g., micron-scale magnon resonators) [2]. Further, we will present studies of the temperature dependent linewidth that show a modest peak at a temperature of 10 K and a recovery of room-temperature values at 5 K [3]. This behavior is consistent with models of coherent scattering from two level fluctuators (TLF) developed in YIG, but with a sensitivity to TLFs that is roughly 6 times lower. These results indicate the potential for the lowest damping measured in a magnetic system when temperatures are further reduced into the milliKelvin regime. Finally, we will present the results of combined theory-experiment study of the electronic structure of V[TCNE]_x that provides quantitative agreement between density functional theory (DFT) calculations and electron energy loss spectroscopy (EELS) at both the band edge and for core-level atomic transitions. This study reveals a distinction between apical and planar TCNE for octrahedrally coordinated V²⁺, and provide insight into the aging mechanisms of this material. Taken together, these results demonstrate the unique application potential of V[TCNE]_x for quantum information technologies and provide clear directions for further development and study.

[1] “Low-damping ferromagnetic resonance in electron-beam patterned, high- Q vanadium tetracyanoethylene magnon cavities” A. Franson, N. Zhu, S. Kurfman, M. Chilcote, D. R. Candido, K. S. Buchanan, M. E. Flatté, H. X. Tang, and E. Johnston-Halperin, APL Mater. 7, 121113 (2019).
[2] “Predicted strong coupling of solid-state spins via a single magnon mode” D. Candido, G. Fuchs, E. Johnston-Halperin, and M.E. Flatté, Materials for Quantum Technology 1(1), 011001 (2020).
[3] “Exploring a quantum-information-relevant magnonic material: Ultralow damping at low temperature in the organic ferrimagnet V[TCNE]_x” H. Yusuf1, M. Chilcote, D. R. Candido, S. Kurfman, D. S. Cormode, Y. Lu, M. E. Flatté, and E. Johnston-Halperin, AVS Quantum Sci. 3, 026801 (2021).

The moderate bulk perpendicular magnetic anisotropy (PMA, K_u ≈ 1 MJ/m³) and low Gilbert damping (α < 0.01) make L1₀-FePd a strong candidate for energy-efficient, dense spintronic devices with non-volatility down to 10 nm pitch or even lower. Existing applications subject spintronic devices to a wide temperature operating range from –55°C to 150°C. In order to better address the technological viability of FePd based devices, it is of utmost importance to evaluate the anisotropy strength and Gilbert damping of L1₀-FePd at elevated temperatures. In this work, we examine the impacts of buffer layers (e.g., Cr/Pt, Cr/Ru, and Cr/Rh) on the Gilbert damping and PMA of FePd at elevated temperatures (up to 150°C) using the ultrafast, laser-based time-resolved magneto-optical Kerr effect (TR-MOKE) metrology. Our initial data suggest Cr/Ru has a weaker spin-pumping contribution to α of the FePd layer at RT compared with Cr/Pt and Cr/Rh. However, the Gilbert damping of Cr/Ru/FePd increases faster with temperature compared with that of Cr/Pt/FePd and Cr/Rh/FePd. The buffer/seed layers also affect the reduction of effective anisotropy field (H_k,eff) at elevated temperatures, which is presumably correlated with the L1₀ ordering of the FePd layers.

Vanadium tetracyanoethylene (V[TCNE]_x; x≈2) is an organic-based ferrimagnet with low magnetic damping and compatible with a wide variety of substrates. These properties make it an attractive candidate in the field of coherent magnonics. However, similar to many organic-based materials, V[TCNE]_x is air sensitive. While encapsulation extends its lifetime in ambient conditions from hours to weeks, its aging mechanism is not well understood. Here we use confocal microscopy, microfocused Raman spectroscopy, ferromagnetic resonance, and magnetometry to study V[TCNE]_x. We identify key Raman peaks in V[TCNE]_x with C=C, C≡N vibrational modes, which are consistent with ab initio theory. We correlate changes in magnetic properties with changes in Raman intensity and in photoluminescence. These results enable optical characterization of local V[TCNE]_x film quality, which is invaluable when local magnetic characterization is not possible. We also find similar changes can be induced by high intensity laser illumination. We utilize this method for laser patterning V[TCNE]_x by selectively removing magnetism and study the damping, anisotropy, and spin wave modes of patterned structures.

Over the past two decades there has been growing interest in organic magnetic materials, due to their potential applications in the field of magnonics and spintronics.[1,2,3,4] Vanadium tetracyanoethylene, V(TCNE)₂, is a room temperature ferrimagnetic semiconductor with a T_c ~ 600 K [5], which has very low loss ferromagnetic resonance and spin-wave propagation [1,2]. Previous first principles calculations of the electronic structure have indicated an indirect band gap substantially larger (0.8 eV) than experimentally inferred (0.5 eV) [6,7]. The study of Ref. 6 used a local-orbital basis with B3LYP hybrid functional. Here we explore the electronic structure using a plane-wave code VASP [8,9,10] with Perdew-Burke-Ernzerhof (PBE) functionals with Hubbard model corrections, and hybrid functional Heyd-Scuseria-Ernzerhof (HSE06). We confirm that the structure of VTCNE has a triclinic unit cell with each V atom surrounded by 6 organic ligands, as found in Ref. 6. However, in contrast to the previous study we found a direct band gap of 0.4 eV located at a different k-point. This band gap better agrees with the experimental inference from the conductivity activation energy [7]. We also studied magnetic anisotropy, optical properties, elastic properties of V(TCNE)₂ and other metal TCNE materials with other metal substitutes. Further studies also involve magnetic dynamics, magnetoelastic properties, and oxidized or doped V(TCNE)₂ systems.

We acknowledge funding from NSF Grant No. DMR-1808742.

[1] H. Yu, M. Harberts, R. Adur, Y. Lu, P. C. Hammel, E. Johnston-Halperin, and A. J. Epstein, Appl. Phys. Lett. 105, 012407 (2014).
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[4] A. Franson, N. Zhu, S. Kurfman, M. Chilcote, D. R. Candido, K. S. Buchanan, M. E. Flatte, H. X. Tang, and E. Johnston-Halperin, APL Materials 7, 121113 (2019).
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Spin-wave excitations may become highly localized near the edges or surfaces of a magnetic material. Confinement to a small mode volume enhances the magnetic oscillations associated with a single magnon, and thus the local fringe fields that can couple to, e.g., localized spin qubits. This enhancement is critical for achieving high cooperativity spin-magnon transduction and high gate to decoherence ratio spin-spin coupling[1,2]. Localized modes at the edges of a finite magnetic structure originate from internal inhomogeneous demagnetization field at the boundaries[3-6]. For this reason, the edge modes are dependent on the shape of the surfaces and it is possible for the edge modes in non-trivial structures to have unusual properties. Furthermore the nature of these modes may depend strongly on the magnetization of the bar, and low-moment materials have not been extensively explored.

Here we present the spin wave edge modes in a micron-scale magnetic bar of a low-moment material that has tapered edges, with magnetic field in-plane, perpendicular to the bar axis. We used the micromagnetic modeling program MUMAX3[7] to simulate the spin wave excitations. The magnetic material was a low loss semiconducting organic ferrimagnet, V[TCNE] that has a saturation magnetization M_s=7560 A/m, an exchange stiffness constant A_ex=2.2×10^-15 J/m, a Gilbert damping constant α=4×10^-5 and a gyromagnetic ratio $2.7×10¹⁰ Hz/T [8]. The results show that it is possible to excite highly-localized corner modes with resonance frequencies above the uniform mode, in contrast to the well-known edge modes of perpendicular-wall bars which have frequencies below the uniform mode. We also performed analytic calculations and obtained similar results based on a formalism for inhomogeneous systems[9]. Our work extends the understanding of spin wave edge dynamics in non-trivial structures and suggests potential applications of these new corner modes with very small mode volume to quantum information science.

*We acknowledge funding from NSF Grant No. DMR-1808742.

[1] D. R. Candido, G. D. Fuchs, E. Johnston-Halperin and M. E. Flatté, Mater. Quantum Technol. 1, 011001 (2020).
[2] M. Fukami, D. R. Candido, D. D. Awschalom and M. E. Flatté, arXiv:2101.09220v [quant-ph], (2021).
[3] J. Jorzick et al., Phys. Rev. Lett. 88, 047240 (2002).
[4] B. B. Maranville, et al., J. Appl. Phys. 99, 08C703 (2006).
[5] R. C. McMichael and B. B. Maranville, Phys. Rev. B. 74, 024424 (2006).
[6] J. Jersch et al., Appl. Phys. Lett. 97, 152502 (2010).
[7] Vansteenkiste et al., AIP Adv. 4, 107133 (2014).
[8] A. Franson, et al., AIP Adv. 7, 121113 (2019).
[9] B. A. Kalinikos and A. N. Slavin, J. Phys. C: Solid State Phys. 19, 7013 (1986).

Spin Hall effect (SHE) driven spin-transfer torque facilitates an efficient manipulation of magnetization with flowing charge current. In aspect of experimental technique, electronic measurement methodologies have widely been used for SHE-driven spin-transfer torque analysis, e.g. Hall measurements, spin transfer ferromagnetic resonance, and so on. However, electronics methods usually detect final static states of the magnetization vector of the ferromagnetic layer, and it is difficult to observe detailed dynamic motion during ultrafast precessional process. Here, we demonstrate a time-resolved technique for pulsed electric current and following pulsed spin torque. We investigate the magnetization precession dynamics, driven by pulsed spin current, by using time-resolved magneto-optical Kerr effect with femtosecond laser. A low-temperature-grown GaAs-based photo-switch can convert an optical pulse to charge current pulse that results in pulsed spin-orbit torque (SOT). We argue that this technique is useful for investigating the current-driven SOT effect in ultrafast time-domain regime.

We present [1] a quantitative theory of the suppression of the optical linewidth due to charge
fluctuation noise in a p–n diode, recently observed in Ref. 2. We connect the local electric field with the voltage
across the diode, allowing for the identification of the defect depth from the experimental threshold voltage.
Furthermore, we show that an accurate description of the decoherence of such spin centers requires a
complete spin–1 formalism that yields a bi-exponential decoherence process, and predict how reduced charge
fluctuation noise suppresses the spin center's decoherence rate.

The material is based on work supported by the U.S. Department of Energy, Office of Basic Energy Sciences,
under Award Number DE-SC0019250.

[1] Denis R. Candido and Michael E. Flatté, arXiv:2008.13289
[2] C. P. Anderson, A. Bourassa et al., Science 366, 1225 (2019).

Point defect qubit sensors in semiconductors have demonstrated their outstanding potential for new discoveries in material science and medicine. The sensitivity and the spatial resolution of these sensors are currently limited by the distance between the qubit and the target, the spin coherence, and the readout characteristics of the qubit. Here, we report on the peculiar magneto-optical properties and on demand creation of a novel point defect qubit on the surface of hexagonal boron nitride (hBN). The electronic structure of the studied point defect enables optical initialization and single-shot readout of the spin states, while the reported large transverse zero-field splitting gives rise to robust clock transitions between the spin states and elongated coherence time at zero magnetic field. The stress absorption capabilities of the microscopic structure enable further tailoring of the properties of the qubit. Our results thus provide an alternative for advancing nano-scale sensing and magnetic resonance imaging utilizing few layer hBN platforms.

Understanding the dynamics of a quantum bit's environment is essential for the realization of practical systems for quantum information processing and metrology. We use single nitrogen-vacancy (NV) centers in diamond to study the dynamics of a disordered spin ensemble at the diamond surface. Specifically, we tune the density of "dark" surface spins to interrogate their contribution to the decoherence of shallow NV center spin qubits. When the average surface spin spacing exceeds the NV center depth, we find that the surface spin contribution to the NV center free induction decay can be described by a stretched exponential with variable power n. We show that these observations are consistent with a model in which the spatial positions of the surface spins are fixed for each measurement, but some of them reconfigure between measurements. In particular, we observe a depth-dependent critical time associated with a dynamical transition from Gaussian (n=2) decay to n=2/3, and show that this transition arises from the competition between the small decay contributions of many distant spins and strong coupling to a few proximal spins at the surface. These observations demonstrate the potential of a local sensor for understanding complex systems and elucidate pathways for improving and controlling spin qubits at the surface.

Nuclear spins in solids show long coherence times and could therefore be employed as quantum memories in quantum processors or quantum networks. However, their good isolation from the environment also makes it challenging to prepare and measure (i.e., write and read) nuclear spins, or to coherently transfer information to and from electronic or photonic qubits. We have developed and modelled a method for the readout of a phosphorous donor nuclear spin by probing the transmission of a microwave resonator coupled to a silicon quantum dot-donor system subjected to a transverse magnetic field gradient. We have identified optimal readout points with strong signal contrast to facilitate the implementation of nuclear spin readout. Furthermore, we investigate the potential for achieving coherent excitation exchange between a nuclear spin qubit and microwave cavity photons. A different approach is possible for transition metal (TM) defects in silicon carbide (SiC) that represent another promising platform in quantum technology, especially because some TM defects emit in the optical spectrum into one of the telecom bands. We present a theory for the interaction of an active electron in the D-shell of a TM defect in SiC with the TM nuclear spin and derive the effective hyperfine tensor within the Kramers doublets formed by the spin-orbit coupling [2]. Based on our theory we discuss the possibility to exchange the nuclear and electron states with potential applications for nuclear spin manipulation and long-lived nuclear-spin based quantum memories.

[1] J. Mielke, J. R. Petta, and G. Burkard, accepted for publication in PRX Quantum [arxiv:2012.01322]
[2] B. Tissot and G. Burkard, arXiv: 2104.12351

Spins in solid state defects represent promising building blocks for quantum information applications. For these purposes, maintaining the desired quantum state of these spins is crucial, however it is generally not straightforward, as spins can interact with various degrees-of-freedom of the solid state host. In many cases, hyperfine interactions between the electronic spin of the defect and the nuclear spins of the host lattice dominate decoherence. In these scenarios, the cluster-correlation expansion (CCE) method [1,2] has proven to be a useful numerical tool in qualitatively and quantitatively capturing the effective spin coherence of the combined spin-bath system, and more recent generalizations have included population dynamics as well [3]. In this work, we discuss extending this generalized CCE method to capture the spin dynamics of two qubit spins coupled with a common bath of nuclear spins [4]. Using this framework, we evaluate the spin dynamics in a variety of defect qubit candidate systems, where we also consider different defect pair separations, magnetic field strengths, and pulse sequence schemes. We study the interplay of the two qubits and the role of the hyperfine spin bath and highlight particularly important quantum-dynamical processes involved in these various regimes.

1. Yang and Liu, PRB, 78, 085315 (2008)
2. Yang and Liu, PRB, 79, 115320 (2009)
3. Yang et al, Annals of Physics, 413, 168063 (2020)
4. Ciccarino and Narang, in preparation.

Solid-state defect centers have achieved various quantum functionalities, and further development of their functionalities as well as the expansion of viable qubit material options are critical issues in quantum information science [1-5]. The electron spin coherence time T₂, is one of the most critical material parameters for defect center qubits. Recently, cluster correlation expansion (CCE) calculations have emerged as a promising tool to simulate the T₂ of materials with dilute spinful nuclei e.g., diamond and SiC [6-10]. Here we show a surprising scaling relationship of T₂ on g-factor, spin quantum number, and the densities of electron and nuclear spins based on the CCE calculations, constituting a simple new tool for the exploration of qubit materials [11].
We calculate T₂ of the well-known qubit materials (e.g., diamond, SiC, and Si) with second-order CCE calculations which include pairwise transitions of the nuclear spins, and show the CCE result agrees well with the experimentally observed T₂. Especially, the nuclear spin baths are shown to decouple under external magnetic fields typically encountered in experiments (> 30 mT). For a dilute (< 10²² cm^-3) spinful nuclear bath with a single nuclear species, T₂ is proportional to the nuclear spin density n^-1.0, which reproduces the previous experimental reports. Furthermore, based on the CCE calculations, we calculate the coefficients of n^-1.0 for all stable nuclear spin species, and find that T₂ of electron spin with spin S = 1/2 and electron g-factor g_e = 1/2 is expressed by T₂ = 1.5×10¹⁸×|g|^-1.6I^-1.1n^-1.0 (seconds), where g is the nuclear spin g-factor and I the nuclear spin quantum number, which we further generalize to the case for the electron spin with S ≠ 1/2 or g_e ≠ 1/2, and show that CCE reproduces the experimentally reported T₂ on the known materials. This quantitative and algebraic expression for T₂ offers qubit materials exploration without extensive computations; for example, we obtain the predicted T₂ of over 12,000 qubit host candidates with bandgap > 1 eV, and show that there are more than 700 materials with T₂ > 1 ms. Among these, SiC stands out as the only non-chalcogenide [11]. This study will provide a guideline for material exploration and will be an important resource for understanding nuclear spin bath decoherence in lesser-known compounds.

We thank He Ma, Jaewook Lee, and Huijin Park for their help in cross-checking the CCE predictions. This work was supported in part by JSPS Kakenhi Nos. 19KK0130 and 20H02178, the cooperative research of RIEC, the EFRC Center for Novel Pathways to Quantum Coherence in Materials (NPQC) supported by DOE/BES, National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) Nos. 2018R1C1B6008980, 2018R1A4A1024157, and 2019M3E4A1078666, AFOSR FA9550-19-1-0358, the Center for Novel Pathways to Quantum Coherence in Materials, and an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences in collaboration with the U.S. Department of Energy, Office of Science, National Quantum Information Science Research Centers.

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Spin defect qubits in SiC are strong candidates for scalable quantum communication technology. It is possible to encode and manipulate quantum information in a qubit under optical excitation. However, the qubit properties are often compromised by charge-state instabilities and phonon coupling, which result in diminished spin-initialization fidelity and limited room-temperature operation. Engineering of stacking faults in SiC offers a material-based solution towards stabilizing the qubit charge state at room temperature [1]. In this talk, we theoretically investigate stacking fault-qubit complexes in SiC and demonstrate that the local environment within a stacking fault gives rise to distinct qubit configurations. We report zero-field splitting values above 60 MHz for these complexes, that can further improve the coherence protection scheme achieved for basally oriented qubits in SiC [2]. Further, the calculated magneto-optical and hyperfine parameters can assist in identification of previously unassigned PL signals. Our results demonstrate the quantum-well feature of stacking faults can be exploited to develop robust spin qubits in SiC.

[1] V. Ivády, J. Davidsson, A. L. Falk, P. V. Klimov, N. T. Son, D. D. Awschalom, I. A. Abrikosov, and A. Gali, Nature Commun. 10, 5607 (2019)
[2] K. C. Miao, J. P. Blanton, C. P. Anderson, A. Bourassa, A. L. Crook, G. Wolfowicz, H. Abe, T. Ohshima, D. D. Awschalom, Science 369, 1493 (2020)

Defects in SiC are of significant interest for quantum information and sensing, due to the combination of long spin coherence times, optical spin polarization, and industrial maturity of the material system. The ideal defect system should have long spin coherence times, homogeneous spin and optical transitions, bright emission, and efficient optical initialization and readout. We have characterized many of these spin and optical properties of the V2 silicon vacancy in 4H-SiC for both single defects and ensembles, showing quite promising results as well as some non-ideal behavior. This defect is notable as it is one of the few that shows spin coherence at room temperature and also has an S=3/2 spin system. For single Si vacancies, we have performed low temperature photoluminescence excitation spectroscopy of the spin-dependent optical transitions, showing two sharp lines with linewidths near the radiative lifetime limit, corresponding to the m_s=±1/2 and m_s=±3/2 spin states. Time resolved measurements show significant differences between these two transitions that we understand and theoretically model in terms of the intersystem crossing, which occurs at different rates for the two spin states and has implications for spin initialization and readout. Spin coherence in this system is strongly influenced by the presence of nuclear spins, despite the low natural abundance of isotopes with nonzero spin. The nuclear spin environment gives rise to inhomogeneous dephasing and strong effects on spin echo measurements, particularly at low magnetic fields. We have grown isotopically purified SiC with a very low abundance of ²⁹Si and ¹³C to strongly suppress the influence of nuclear spins. Our room temperature spin coherence measurements show large improvements in the coherence times, with particular improvement in the inhomogeneous dephasing time T₂^*, reaching 20 μs in the m_s=±1/2 basis. We consider the prospects of this system for quantum sensing and for an efficient spin-photon interface.

This work was supported by the U.S. Office of Naval Research, the OSD Quantum Sciences and Engineering Program, the Defense Threat Reduction Agency, and the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-mission 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.

Quantum memories capable of faithfully storing and recalling quantum states on-demand are powerful ingredients in building quantum networks and quantum information processors. As in conventional computing, key attributes of such memories are high storage density, long storage lifetimes and random access, or the ability to read from or write to an arbitrarily chosen register. However, achieving such random access with quantum memories in a dense, hardware-efficient manner remains a challenge, for example requiring dedicated cavities per qubit or pulsed field gradients.

In this talk I will introduce a variety of spin systems in different host materials that are particularly promising for the realisation of microwave quantum memories, including various donors in silicon and optical defects in YSO. I will present recent progress in extending the spin coherence lifetimes in such systems, which impacts the timescale over which the microwave quantum memory is able to faithfully store coherent states. I will also present a protocol that uses chirped pulses to encode qubits within the spin ensemble, offering both random access and naturally supporting dynamical decoupling to further enhance the memory lifetime. Finally, I will discuss recent progress on the implementation of this protocol using different spin systems.

Recent work exploring rare earth ions as qubits has demonstrated key milestones – such as high cyclicity and high Purcell factor in nanophotonic devices (Raha et al, Nature Comm., 2020) – needed for controlling qubits based on rare earth atoms in a solid-state host. However, the material platform (Er-doped bulk crystals) used in these key studies is difficult to scale and integrate with other semiconductor technology. This capability is necessary for the high qubit densities needed in some advanced quantum applications and thin film host materials that enable scalable deployment is a topic of active research. Here we report on our progress in this direction.

Using the synthesis technique of metal-organic molecular beam epitaxy (MOMBE), we report the growth and characterization of Er-doped TiO₂ thin films – both in single crystal TiO₂ (on r-sapphire and strontium titanate), as well as in polycrystalline films (on silicon). Using XRD and TEM we confirm the dependence of the TiO₂ phase (anatase or rutile) on growth substrate and temperature. The lifetime of the first optical excitation (of Er³⁺) in our films varies with film structure and thickness, ranging from 1 to 1.85 ms in our films. Through low temperature photoluminescence characterization, we obtained an inhomogeneous linewidth of 5.2 GHz (T=3.4 K), which is about an order of magnitude larger than the value of 460 MHz reported on bulk Er-doped TiO₂ crystals (Phenicie et al., ACS Nano, 2019). This linewidth is thermally broadened and should reduce further at lower temperatures. In addition, we find that the linewidth depends on the thickness, and presence of buffer or capping layers, which we describe by an empirical model. Further, measurement of spectral diffusion, which sets the upper limit on the value of T₂*, is shown to reduce with Er concentration. Altogether, our study explores and presents data that highlight the potential to engineer a solid-state qubit platform using thin film hosts.

Nanoscale quantum probes are a versatile platform for studying solid state and living systems in a non-invasive way. They can be embedded in a sample of interest and can thus provide in situ, nanoscale information, for example from inside thick biological structures such as cells. In this talk, I will present results on using Nitrogen-Vacancy centres (NVs) in diamond nanocrystals and ensembles of NVs in diamond nanobeams for local sensing of magnetic fields and temperature.

First, I will discuss the detection of NMR signals from multiple nuclear species in a (19 nm)³ volume using a versatile NV-NMR device inside a diamond nanocrystal of ~25 nm diameter. Within this volume, we are able to sense ~1000 molecules[1]. These promising devices are robust to photobleaching and environmental changes, have low toxicity, and an ability to measure multiple spin species with high specificity. While the spin concentration extracted from the measured NMR signal amplitude is subject to errors due to the geometric variability of nanodiamonds devices, this error can be corrected for using an in-situ calibration scheme based on exploiting the signal from a thin layer of reference nuclei on the diamond surface. We are also able to perform nanoscale measurements of temperature inside living cells, a key tool for investigating thermogenesis on a sub-cellular level.

Second, I will present a quantum sensor composed of a dense spin ensemble in a diamond nanobeam that operates as a collection of independent, unperturbed sensors working in parallel[2]. We experimentally demonstrate fault-tolerant decoupling of spin-spin interactions, achieving a five-fold enhancement of spin coherence time. This is made possible by a novel dynamical decoupling sequence that simultaneously suppresses disorder, interactions, and imperfections in control operations[3]. We utilize the extended coherence time to demonstrate an increase in magnetic field sensitivity of over 40% compared to conventional sensing protocols such as the XY8 sequence. These results demonstrate a significant enhancement by breaking the natural interaction limit of dense ensembles, and reveal the potential of tailored control in complex, interacting ensembles.

[1] Holzgrafe et al., Phys. Rev. Appl. 13, 044004 (2020)
[2] Zhou et al., Phys. Rev. X 10 031003 (2020)
[3] Choi et al., Phys. Rev. X 10 031002 (2020)

Detection of AC magnetic fields at the nanoscale is critical in applications ranging from fundamental physics to materials science. Isolated nitrogen-vacancy centers in diamond can achieve the desired spatial resolution with high sensitivity. Still, vector AC magnetometry currently relies on using different orientations of an ensemble of sensors, with degraded spatial resolution. Here I will present a novel protocol that exploits a single NV to reconstruct the vectorial components of an AC magnetic field, by tuning a continuous driving to distinct resonance conditions. As an experimental proof-of-principle, I’ll show how to map the spatial distribution of an AC field generated by a copper wire on the surface of the diamond.
The proposed protocol combines high sensitivity, broad dynamic range, and sensitivity to both coherent and stochastic signals, with broad applications in condensed matter physics.

Spins associated to optically active defects in diamond provide a versatile platform for quantum science and technology. In this talk, I will discuss our recent advances in realizing programmable quantum simulators based on individually controllable carbon-13 nuclear spins in diamond. I will present how one can use a single nitrogen-vacancy (NV) centre to detect, characterize and control a large number of nuclear spins in its environment [1,2]. By controlling the interactions between the spins it becomes possible to create a variety of many-body Hamiltonians with tuneable parameters. As an example, I will discuss our investigation of a discrete-time crystal stabilized by many-body localization, a new out-of-equilibrium phase of matter [3].

[1] M. H. Abobeih et al., Nature 576, 411 (2019)
[2] C. E. Bradley et al., Phys. Rev. X. 9, 031045 (2019)
[3] J. Randall et al., arXiv:2107.00736 (2021)

Optically active defects in solids with accessible spin states are promising candidates for quantum applications. While several candidates in 3D crystals including diamond and silicon carbide have been extensively studied, the coherent control of color centers in 2D materials is still an open issue. We recently investigated a bright 850nm fluorescence in irradiated hexagonal boron nitride (hBN) and found it spin-dependent. Using magnetic resonance techniques such as optically detected magnetic resonance (ODMR), we identify this fluorescence associated with a particular defect, a negatively charged boron vacancy V_B^-, possessing a spin triplet (S=1) ground state and a zero-field splitting D/h of 3.48 GHz. Moreover, we realized a coherent control of these defects by applying pulsed ODMR protocols. In particular, we measured spin-lattice relaxation time T₁ of 18 µs and spin coherence time T₂ of 2 µs at room temperature. T₁ increases by three orders of magnitude at cryogenic temperature following a T^-5/2 power law, while T₂ remains almost unaffected. Employing electron spin-echo envelope modulation (ESEEM) we were able to separate the quadrupole and hyperfine interactions with the surrounding nuclei. Finally, by applying a two-frequency "hole-burning" technique, we partially decoupled the electronic spin state from its inhomogeneous nuclear environment, which prolonged the spin coherence time by a factor of three. Our results can be important for employment of spin-decorated hBN layers as local probes in hybrid quantum systems, however, requires careful defect and heterostructure engineering.

Color centers such as the NV, SiV and SnV centers in diamond and vacancy centers in silicon carbide are leading qubits for quantum repeater networks. These systems have a spinful ground state, which is used as a qubit to encode information, optical transitions for spin-photon entanglement and long-range quantum communication, and are coupled to nuclear spin qubits which, given their long coherence times, can be used as an additional memory register. This talk will present high-fidelity designs for entangling gates between the defect and nuclear spin qubits which selectively couple one nuclear spin while decoupling the rest. Optical control protocols tailored to SiV and SnV centers in diamond that overcome cross talk and leakage errors will also be discussed.

Interfacial traps states have been a subject of study due to the detrimental effect they have on conventional electronic devices (e.g. source of leakage currents). The same states also deleteriously impact quantum coherent devices though spin-spin interactions (when the trap state is paramagnetic) or charge fluctuations. Recently, an upside for interfacial trap states has been discovered [1]: interfacial spins may serve as quantum sensors due to how transport or recombination through the sensors intimately depends on the local field environment.

In this work, we present theoretical and experimental results determining the role that spin-dependent processes at deep levels plays in governing recombination through interfacial traps. For paramagnetic trap states, capture of electrons or holes is limited by spin statistics. For instance, a neutral trap with S = 1/2 only captures spins with opposite orientation (assuming a negatively charged ground state of S = 0). These spin-dependent processes are incorporated into a generalized Shockley-Read-Hall model. Magnetic interactions (e.g. hyperfine) at each of the combining spins modify capture efficiency; the probability of capture is determined by solving the stochastic Liouville equation for the spin density matrix. Using this formalism, we calculate magnetoresistance for dangling bond defects at the Si/SiO₂ interface. The results from the model agree well with our experiments measuring recombination current in a Si/SiO₂ MOSFET. Chief among our conclusions is that a large number of nuclei interact with the trap spin which necessitates a semiclassical approximation for the hyperfine interaction. Lastly, we examine how the quantitative theory can be utilized in the service of all-electrical quantum magnetometry using interfacial trap states.

[1] C. J. Cochrane, J. Blacksberg, M. A. Anders, P. M. Lenahan, Phys. Rev. X 5, 041023 (2015)
[2] N. J. Harmon, J. P. Ashton, P. M. Lenahan, M. E. Flatté, arXiv:2008.08121v1

The project or effort depicted was or is sponsored by the Department of the Defense, Defense Threat Reduction Agency under Grant HDTRA 1-18-1-0012 and Grant 1-16-0008. The content of the information does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred.

In this contribution, I present our most recent research on magnon-phonon coupling. A fundamental form of magnon-phonon interaction is an intrinsic property of magnetic materials, the “magnetoelastic coupling.” This form of interaction has been the basis for describing magnetostrictive materials and their applications, where strain induces changes in internal magnetic fields. Unlike magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may cause surface magnons via a rotational motion of the lattice in anisotropic magnets [1]. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. We recently demonstrated a nonreciprocal acoustic wave attenuation with an unprecedented ratio of up to 100% rectification, which is understood as a consequence of the magneto-rotation coupling [2]. Considering the broad application of the general acoustic device in sensing, filtering, and information transportation, the intriguing nonreciprocal features of our magneto-rotation devices suggest an extraordinary versatility of acousto-magnetic applications.
As well-known, the coupling of surface acoustic waves with ferromagnetic layers also induces acoustic ferromagnetic resonance, consequently acting as a method of spin current generation [3, 4]. An intriguing scenario is to explore the spin current generation when the strength of magnon-phonon coupling is improved. We have carefully engineered an acoustic wave device in the presence of an acoustic cavity consisting of two acoustic reflectors separated by a distance equal to a multiple of the acoustic wavelength of our device. In our acoustic cavity device, we observed enhancements of up to two times in acoustic ferromagnetic resonance and up to three times more spin current generation than a non-cavity reference device [5]. The apparent discrepancy between the enhancements of energy absorption in acoustic ferromagnetic resonance (two times) and the enhancements of the spin current generation (three times) is an open question. Beyond the enhancement of spin current generation, improvements of magnon-phonon coupling towards the strong coupling regime can have significant consequences such as early on-sets of nonlinear effects, enhancement of magnon propagation length, condensation, and transfer of coherent phase information.

[1] Maekawa and M. Tachiki, Surface acoustic attenuation due to surface spin-wave in ferro- and antiferromagnets AIP Conference Proceedings 29, 542 (1976)
[2] M. Xu, K. Yamamoto, J. Puebla, K. Baumgaertl, B. Rana, K. Miura, H. Takahashi, D. Grundler, S. Maekawa, Y. Otani, Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling. Sci. Adv. 6, eabb1724 (2020)
[3] M. Xu, J. Puebla, F. Auvray, B. Rana, K. Kondou, and Y. Otani, Phys. Rev. B 97, 180301(R) (2018)
[4] J. Puebla, M. Xu, B. Rana, K. Yamamoto, S. Maekawa, and Y. Otani, J. Phys. D 53, 264002 (2020)
[5] Y. Hwang, J. Puebla, M. Xu, A. Lagarrigue, K. Kondou, and Y. Otani, Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity Appl. Phys. Lett. 116, 252404 (2020)

Controlling static and dynamic behaviors of the bubble-like magnetic skyrmion exhibits great potential in the application of spintronics. Especially, as a natural property, temperature plays an important role in the control of skyrmion behaviors. For example, statically, the temperature can lead to the phase transition of the topological magnetic structures (such as skyrmions) by affecting their stability; dynamically, fluctuated or inhomogeneous temperature fields can drive the skyrmion motion. However, due to the limitation of existing hypothesis-based simulation methods, the study of temperature-related behaviors of skyrmions from the perspective of thermodynamic free energy is rare. Here, we developed a temperature-related phase field simulation to predict and explain the skyrmion behaviors in temperature fields. Statically, we demonstrate the skyrmion lattice of a room-temperature ferromagentic thin film lose the thermal stability near the 340 K, and obtain the temperature-magnetic field-strain phase diagrams of topological structures in MnSi thin film, which consists with the experimental results very well. Dynamically, we study the rectilinear motion of the individual asymmetrical skyrmion driven by temperature gradients, and propose a kinematic equation to describe it. Therefore, these works demonstrate the temperature-related phase field simulation is a reliable method to simulate the skyrmion behaviors in temperature fields, and propose the thermodynamics explanation on both static and dynamic features of skyrmions, which is anticipated to be the theoretical support for further research about temperature-related behaviors of skyrmions and their potential applications in functional devices.

Antiferromagnetic (AFM) materials have great potentials for spintronics applications due to their robustness again disturbance. The physics of mutual interactions between lattice and spin degrees of freedom in AFMs, leading to unusual transport phenomena of spin and heat, has been a subject of continuing interest. In the current work, temperature and field dependent inelastic neutron scattering measurement were performed to investigate the magnon-phonon interactions in AFM nickel (II) oxide. Giant phonon anomalies are observed by inelastic neutron scattering in the first Brillouin zone, showing magnon-like behaviors. Atomistic simulation and scattering simulation attribute such anomalies to strong spin-phonon coupling and the broken symmetry of related phonon modes. Such couplings have important implications on the properties of both phonon and spin dynamics. Additionally, the discover provides a direct approach to quantify such spin-lattice interactions via a revised inelastic neutron scattering formalism.

Antiferromagnets are a type of quantum material where electron exchange interactions result in antiparallel or non-collinear microscopic spin correlations with negligible macroscopic magnetization. Given their low-loss THz magnetic resonance and insensitivity to stray fields, antiferromagnetic spintronics holds great potential in realizing high-speed communications and robust in-memory computing¹. Emerging low-dimensional antiferromagnets such as nanoscale oxide thin films and van der Waals layered materials^2,3, are expected to further pave the way for ultimate antiferromagnetic device miniaturization. Here we investigate the THz magnetic dynamics of such novel material systems. Using cryogenic THz emission spectroscopy as a direct probe of the time-dependent magnetization, we observed nontrivial THz signals generated by ultrafast optical pumping of NiO thin films and NiPS₃ crystals. Their unique THz emission dependence as a function of optical fluence, THz polarization, and temperature, shows direct sensitivity to the magnetic ordering symmetry and picosecond spin dynamics. Our findings will advance the understanding of the underlying physics that governs the dissipation and electrodynamics of low-dimensional antiferromagnetic materials with large tunability for ultrafast and compact spintronic devices.

1. Jungwirth, T., Sinova, J., Manchon, A., Marti, X., Wunderlich, J. & Felser, C. The multiple directions of antiferromagnetic spintronics. Nature
Physics 14, 200-203, doi:10.1038/s41567-018-0063-6 (2018).
2. Lee, J.-U., Lee, S., Ryoo, J. H., Kang, S., Kim, T. Y., Kim, P., Park, C.-H., Park, J.-G. & Cheong, H. Ising-Type Magnetic Ordering in
Atomically Thin FePS3. Nano Letters 16, 7433-7438, doi:10.1021/acs.nanolett.6b03052 (2016).
3. Kang, S., Kim, K., Kim, B. H., Kim, J., Sim, K. I., Lee, J.-U., Lee, S., Park, K., Yun, S., Kim, T., Nag, A., Walters, A., Garcia-Fernandez, M., Li, J., Chapon, L., Zhou, K.-J., Son, Y.-W., Kim, J. H., Cheong, H. & Park, J.-G. Coherent many-body exciton in van der Waals antiferromagnet NiPS3. Nature 583, 785-789, doi:10.1038/s41586-020-2520-5 (2020).

Magneto-electroluminescence (MEL) is a powerful tool to study singlet fission and triplet fusion dynamics in organic light emitters as the functional form of the MEL response can be qualitatively attributed to the dominant processes within the device. In this study, we will present a quantitative model for the MEL response, valid over a wide current range, and apply it to devices based on Rubrene, and three solution processable anthradithiophene emitters. By varying the chemical motif, we are able to systematically vary the film structure between highly textured, poly-crystalline to amorphous. We find significant diversity in the MEL, with the textured films showing feature-rich MEL responses. We will show that features in the MEL response coincide with field direction dependent crossings of the projections of the coherent triplet pair state onto the singlet state of the spin-Hamiltonian. Furthermore, we will discuss the effect of the film structure on the observable MEL response.

The current-driven switching of the magnetization of a ferromagnetic (FM) layer in a multilayer stack continues to be one of the intensively investigated topics of research interest both from the fundamental as well as technological points of view. To achieve the switching with lower (critical) current, it is highly desirable that the FM layer must possess a high value of spin-polarization (such as in Heusler alloys), sufficiently low Gilbert damping α_int and preferably low values of both saturation magnetization M_S and anisotropy energies.
In this work, the effectiveness of two different approaches used in growing sputtered FM films suitable for such current induced switching applications is investigated in detail. In particular, the effectiveness of maintaining high substrate temperature (T_S) during deposition versus in situ annealing of the films at high temperature (T_A) after sputtering at room temperature is compared with an objective to obtain thin films of Heusler alloy Co₂FeAl (CFA) having optimally low α_int and 12-17% smaller M_S by employing ion beam sputtering technique and Si(100) as substrates. In each of the two series of CFA films, appropriately suitable choices of T_S and T_A (lying in 300-773K range) were explored for optimally tailoring their structural, static and dynamic magnetization properties.
Structural studies revealed that although all the CFA films of the two series were polycrystalline but the post-deposition annealed films were structurally optimal with higher density (6.36±0.09 g/cm³) and lower interface roughness (0.48±0.03 nm) compared to the CFA films grown at high T_S (6.23±0.06 g/cm³, 0.61±0.01 nm). While the higher density for the post annealed films is accredited to the removal of the voids at higher T_A, the higher interface roughness of the films grown at higher T_S is attributed to the non-uniform growth of films at higher T_S.
In-plane field-angle dependent longitudinal magneto optical Kerr effect (L-MOKE) study on these CFA films revealed the existence of a coupling between the uniaxial and biaxial magnetic anisotropies, whose origin is attributed to the employed deposition geometry. Contribution of both the anisotropies are separated and the calculations suggest that the uniaxial anisotropy is dominant over biaxial anisotropy in all the CFA films.
Ferromagnetic resonance (FMR) spectroscopy measurements, performed in both the field configuration, viz. in-plane as well as out-of-plane, revealed a lowest value of 1.19 (± 0.08) ×10^-3 of α_int in the film post-annealed at 773K. In addition, the observed non-linear relation between the α_int and the dynamical ‘g’-factor suggests that the contribution of spin-orbit interaction to the damping is far less compared to the damping contribution from the DOS present near the Fermi level. It is remarkable to note that the as-grown CFA films sputtered at room temperature exhibited a record lowest value of 1.73 (±0.09) ×10^-3 of α_int. Attainment of such a small value of Gilbert damping and having moderate magnetization in high spin polarized Co₂FeAl Heusler films sputtered on the Si (100) substrate opens up their great application potential in future spintronics devices.