Symposium S.NM07 : Two-Dimensional Quantum Materials Out of Equilibrium

High pressure is a powerful, reversible and “clean” way to drive materials away from equilibrium, exposing new phases, new states, new effects and new physics that would not exist in ambient condition. For layered materials where neighboring layers are held together by weak van der Waals forces, high pressure are particularly effective in creating these new effects. In this talk, we will discuss some of our findings in using high hydrostatic pressure to modulate the structure and electronic/optical properties of a variety of 2D materials, ranging from single or few layers of transition metal dichalcogenides, and their heterostructures, to new 2D materials created by intercalating non-2D materials.

The last decade has seen an exponential growth in the science and technology of two-dimensional materials. Beyond graphene, there is a huge variety of layered materials that range in properties from insulating to superconducting that can be grown over large scales for a variety of electronic devices and quantum technologies, such as topological quantum computing, quantum sensing, and neuromorphic computing. In this talk, I will discuss the synthesis of a range of 2D layers over large areas for sensing and electronic devices, as well as recent breakthroughs in novel 2D heterostructures and realization of unique 2D allotropes of 3D materials. I will introduce a novel synthesis method, dubbed confinement heteroepitaxy (CHet), that enables the creation of atomically thin metals, enabling a new platform for creating artificial quantum lattices (AQLs) with atomically sharp interfaces and designed properties.

Ultrafast optical spectroscopy has attained prominence due to its ability to resolve dynamics in conventional metals and semiconductors at the fundamental time scales of electron and lattice motion. In recent years, ultrafast optical techniques have become more sophisticated, making it possible to directly access fundamental material parameters in a non-contact manner. In this talk, I will discuss the use of ultrafast optical spectroscopy to track and potentially control carrier dynamics through both space and time in two-dimensional (2D) quantum materials. This will include unraveling charge transfer and diffusion across the interface between lateral transition metal dichalcogenide (TMD) monolayers as well as driving structural dynamics in a topological insulator with intense terahertz (THz) pulses, along with other studies of these fascinating nanosystems.

Electronic devices in the form of high mobility conducting channel with charge carriers can be regulated through the internal charge transfer or chemical doping. Using interlayer interaction through the internal charge transfer to control functional heterostructures with atomic-scale design has become one of the most effective interface-engineering strategies nowadays. Here, we demonstrate the effect of a crystalline LaFeO₃ buffer layer on amorphous and crystalline LaAlO₃/SrTiO₃ heterostructures with different characteristic length of interlayer charge transfer. The LaFeO₃ buffer layer acts as an energetically favored electron acceptor in both LaAlO₃/SrTiO₃ systems, resulting in modulation of interfacial carrier density and hence metal-to-insulator transition. For the amorphous and crystalline LaAlO₃/SrTiO₃ heterostructures, the corresponding metal-to-insulator transition is found when the LaFeO₃ buffer layer thickness crosses 3 and 6 unit cells, respectively. Such different critical LaFeO₃ thicknesses are explained in terms of distinct characteristic lengths of the redox-reaction-mediated and polar-catastrophe-dominated charge transfer, controlled by the interfacial atomic contact and Thomas-Fermi screening effect, respectively. Our results not only shed light on the complex interlayer charge transfer across oxide heterostructures, but also establish a new route to precisely tailor the charge-transfer process at a functional interface by atomically engineered buffer layers.

Control over materials thickness down to the single-atom scale has emerged as a powerful tuning parameter for manipulating not only the single-particle band structures of solids, but increasingly also their interacting electronic states and phases. A particularly attractive materials system in which to explore this is the transition-metal dichalcogenides, both because of their naturally-layered van der Waals structures as well as the wide variety of materials properties which they are known to host. Yet, how their interacting electronic states and phases evolve when thinned to the single-layer limit remains a key open question in many such systems. Here, we use angle-resolved photoemission to investigate the electronic structure and charge density wave (CDW) phases of monolayer TiSe_2,TiTe₂, and VSe₂ epitaxial thin films grown by molecular-beam epitaxy.¹ Three-dimensionality is a core feature of the electronic structure of all of these parent compounds, but we show how their CDW phases not only persist, but are strengthened, in the monolayer limit. In TiSe₂, we observe a strong-coupling and orbital-selective CDW, necessarily without a k_z-selectivity in band hybridisation that is of key importance for the bulk instability.² TiTe₂ is driven into a charge-ordered phase in the monolayer which is not stable in the bulk at all, but our measurements indicate that it is a much weaker-coupling instability than for the Se-based sister compound. In VSe₂, we show how the monolayer hosts a much stronger-coupling CDW instability than for the bulk compound, which in turn drives a metal-insulator transition, removing a competing instability to ferromagnetism.³ We show how ferromagnetism can, however, be re-established via proximity coupling.⁴ Together, these studies point to the delicate balance that can be realized between competing interacting states and phases in monolayer transition-metal dichalcogenides, and suggest new strategies for controlling these.

This work was performed in close collaboration with M.D. Watson, A. Rajan, K. Underwood, J. Feng, D. Biswas, W. Rahim, D.O. Scanlon, G. Vinai, G. Panaccione and colleagues from the Universities of St Andrews, Oxford, Keil, UCL, Diamond, Elettra, and SOLEIL.

¹ Rajan et al., arXiv:1910.03307
² Watson et al., Phys. Rev. Lett. 122 (2019) 076404.
³ Feng et al., Nano Lett. 18 (2018) 4493
⁴ Vinai et al., arXiv:1909.01713 (2019)

One of the most pressing questions in condensed matter physics is how electron correlations play out on two-dimension. For this question, the newly emerged magnetic van der Waals material can be helpful. In particular, TMPS3 with TM=transition metal elements have attracted significant attention as it exhibits all three fundamental magnetic models of magnetism; Ising (FePS3), XY (NiPS3) and Heisenberg (MnPS3) Hamiltonian. Using these materials, we have studied some of the fundamental theorems of modern magnetism: Onsager solution for the Ising model, The Berezinskii–Kosterlitz–Thouless transition for the XY model, and Mermin-Wagner theorem for the Heisenberg model. Besides, we have also investigated how correlation physics plays a role in optical spectroscopy data. In this talk, I will demonstrate how we can use this unique magnetic property of these materials to learn of the old physics.

Owing to their large oscillator strength and strong exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as an attractive material platform for studying strong light-matter coupling and associated phenomena. Following up on our previous work on strong light-matter coupling in 2D TMDs [1, 2], here we will present results on enhancing the nonlinear interaction between the quasiparticles (exciton-polaritons). This is achieved using excited states of excitons (Rydberg states) which have larger Bohr radii. We will also present our recent results on controlling the valley pseudospin via the pseudomagnetic fields in optical cavities and our work on realizing an electrically pumped polariton LED [3]. Finally, we will first present our work on realizing single photon emitters (SPEs) in hexagonal boron nitride (hBN), a van der Waals material, via strain engineering [4] and coupling of these SPEs to high Q silicon nitride microresonators [5].
[1] X. Liu, et al., Nature Photonics 9, 30 (2015)
[2] Z. Sun et al., Nature Photonics 11, 491 (2017)
[3] J. Gu et al. Nature Nanotech. (2019) DOI: 10.1038/s41565-019-0543-6
[4] N. Proscia et al. Optica 5, 1128 (2018)
[5] N. Proscia et al. ArXiv 1906.06546

The study of non-equilibrium properties of quantum phases of matter is important in the context of future quantum technologies, since all devices operate in the non-equilibrium limit. Theoretically, the transverse field Ising model (TFIM) and its disordered variants provide a tractable platform to study effects of quantum quenches (a rapid variation of a non-thermal parameter, leading to interesting transient effects), or quantum annealing (a slow variation of a non-thermal parameter, leading to efficient optimization). While there is enormous theoretical interest in these ideas, they have not yet been thoroughly tested on real Ising magnetic systems. There are well-known magnetic material realizations of the TFIM, CoNb2O6 and LiHoYF4, which display quantum critical points in accessible transverse magnetic field ranges. We have studied these materials following various protocols of time variation of the magnetic field, using ac susceptibility and neutron scattering as probes. I will discuss the non-equilibrium effects we have observed in these TFIM materials.

The recent experimental realization of magnetic two-dimensional van der Waals (m2DvdW) materials has generated significant interest in exploiting such systems for novel 2D magnetism and for developing applications such as energy-efficient ultra-compact spin-based electronics. A better understanding of the magnetic interactions in these systems may help to accelerate the discovery of materials with higher magnetic ordering temperatures, moving these novel systems closer to practical applications. Using linear-response ab initio methods, we investigate the magnetic interactions and spin excitations in various m2DvdW systems, such as CrI₃, CrGeTe₃, VI₃, and Fe₃GeTe₂. Dynamical transverse spin susceptibility is calculated to directly compare with available Inelastic Neutron Scattering (INS) measurements. Accurate descriptions of electronic structure can provide reliable predictions of electronic and magnetic properties. Due to confinement, electrons in these 2D systems are likely to be strongly correlated, and electron screening anisotropic, so that their description may require advanced ab initio methods beyond density functional theory (DFT). Therefore, we also use the quasiparticle self-consistent GW (QSGW) method, a self-consistent many-body perturbation method, to describe the electronic structure and calculate magnetic properties. In contrast to DFT, the electron-electron interaction is evaluated explicitly by calculating the dynamically-screened Coulomb interaction W. We found that the nonlocal electron correlations, which are not included in methods such as DFT+U, have profound effects on the electronic and magnetic properties in these m2dvdW materials. A more elaborate description of electron interactions helps better describe the magnetic interactions in these 2D quantum systems. By comparing the electronic structures obtained in QSGW and DFT, we discuss the applicability and limitations of the widely-used DFT+U methods for these systems. We will also discuss the effects of spin-orbit coupling, the Dzyaloshinskii-Moriya interaction, impurities, pressure, and electric fields on the magnetic interactions and excitations in relevant systems.

Hydrostatic pressure is an effective way in tuning the physical properties of materials. This study examines the impact of hydrostatic pressure on the magnetic properties of 3-d transition metal oxide; CoFe₂O₄. The high cubic magnetocrystalline anisotropy, coercivity, moderate saturation magnetization, chemical stability, wear resistance, electrical insulation, and photo magnetic properties of CoFe₂O₄ have resulted in its study for engineering applications. The ease with which cobalt ferrite can be synthesized makes it suitable for scientific investigations and also increases its potential for cost-effective engineering applications. Our results show that the coercive field is decreased from 1600 Oe to 1000 Oe upon the application of 0.2 GPa, and the saturation magnetization is decreased from 91 emu/g to 88 emu/g, measured at cryogenic temperatures (4 K). The modification in the magnetic exchange interaction upon the application of pressure is believed to alter the magnetic properties. We will present our results based on magnetic experiments collected as a function of pressure (0-1 GPa), temperature (4-400 K), and magnetic field (3T) .

Octahedral layers of metal atom separated and charge-compensated by alkali-metal ions have the ideal architecture to be used as electrodes for Li-ion batteries, ionic conductors, and visible light photocatalysts. Li₈M₂(Te/Sb)₂O₁₂ (M = transition metal) are relatively new members of this class. Forming honeycomb frameworks, they present interesting magnetic phenomena related to frustrated two-dimensional lattices of spins. In the present work, polycrystalline Li₈Cr₂(Te/Sb)₂O₁₂ were synthesized by standard solid-state route. Powder X ray diffraction patterns were recorded to check the phase formation and purity. C_2/m space group was confirmed using Rietveld analysis and lattice parameters are determined to be a = 5.141 Å, b = 8.884 Å, c = 5.143 Å, β = 109.5 Å for Li₈Cr₂Sb₂O₁₂, and a = 5.128 Å, b = 8.850 Å, c = 5.151 Å, β = 109.8 Å for Li₈Cr₂Te₂O₁₂, presenting diminished unit cell sizes compared to that of Li8Co2Te2O12 are a = 5.226 Å, b = 8.892 Å, c = 5.160 Å, β = 110.9 Å. In the case of Li₈Cr₂Sb₂O₁₂ a magnetic phase transition is present at 7.4 K as determined from the derivative, dCp/dT. A similar transition is found in Li₈Co₂Te₂O₁₂ at 9.5 K. Neutron diffraction is underway for comprehending cationic ordering, crystal, and magnetic structure of Li₈Cr₂(Te/Sb)₂O₁₂ as well as its mixed occupancy of the 4g Wyckoff position in the C_2/m space grpup. Our experimental results will highlight the magnetism of the honeycomb layers of Cr and the ionic diffusion of inter-layer Lithium and will be of interest to magnetism as well as battery research.

Proton irradiation as a means of modifying the magnetic properties of layered van der Waals (vdW) materials has considerable, yet unexplored potential. Although vdW crystals display strong correlation and stability, their susceptibility to various kinds of stimuli promotes research into the flexible control of magnetism via proton irradiation for various applications. In this study, layered ferromagnetic vdW CrSiTe₃ (CST) crystals were subjected to proton irradiation at fluences ranging from 10¹⁶ to 10¹⁸ protons/cm². Low-temperature and isothermal magnetometry was conducted with a Quantum Design MPMS 3 on both pristine and irradiated samples. Direction dependent isothermal magnetization confirms the presence of magnetocrystalline anisotropy, with a stable easy axis along the in-plane direction. Furthermore, at the fluence of 5 x 10¹⁶ protons/cm², we have observed a 35% increase in saturation magnetization from 25 emu/g to 34 emu/g at T = 2 K. We will present and discuss our experimental findings on saturation magnetization, coercive field, magnetocrystalline anisotropy, and Curie temperature on proton irradiated CST.

The van der Waals (vdWs) class of materials offer a new approach for two-dimensional magnetism allowing spin fluctuations to be tuned upon exfoliation of layers. The magnetic properties with pressure are investigated for the highly anisotropic MX₃ (M = Cr, V; X = Cl, Br, I) family. A clear understanding of the bulk magnetic structure and its tunability can be used as a means towards predicting material properties upon exfoliation. As research on pressure induced magnetic properties of exfoliated CrX₃ is at its nascent stage, hydrostatic pressure as a probe to induce a change in magnetic properties is employed in this study of bulk CrBr₃. Furthermore, pressure studies on CrI₃, VI₃, and Cr₂Ge₂Te₆ have appeared in literature reporting that pressure can modify the spin-lattice coupling and long-range magnetic order, however, the origin of magnetic behavior and whether pressure increases or decreases the long-range magnetic ordering is under debate. This study as a whole will take a deeper look into characterizing the magnetic properties of these crystals to understand the underlying magnetic behavior. Results for CrBr₃ upon the application of 0.4 and 0.8 GPa show a decrease in magnetization with no significant change in Curie temperature (T_C) with pressures of 0, 0.4, 0.8 GPa with T_C ≈ 33, 32, 31 K, respectively. In the future, further insight will be collected by looking at the local electronic and magnetic structure of these materials by employing x-ray magnetic circular dichroism (XMCD) (Cr L2,3 edge) measurements upon the application of pressure (0-10 GPa). In this work we will present all comprehensive experimental findings.

Low dimensional magnetic lattices offer the possibility of realizing flatbands in the magnon spectrum which can then lead to dissipation-less spin transport and associated magnon Hall effect. One could expect to find a magnon insulator, similar to a topological insulator. In the present work we present a rather less-studied honeycomb material Na₂Ni₂TeO₆ where our preliminary density functional theory calculations of magnetic structure shows departures from reported structures. Our samples of Na₂Ni₂TeO₆ confirmed hexagonal P6₃/mcm space group with refined lattice parameters, a=5.2023(1)Å and c=11.1552(8)Å. The bulk magnetism for the present sample is characterized using magnetic susceptibility and specific heat, both of which confirm a phase transition at 28 K. Application of 8 T magnetic field only slightly polarizes the transition. We obtain a Curie-Weiss temperature of -9.7(2)K and effective paramagnetic moment of 2.24(4)m_B/Ni. This matches well with the spin-only moment of Ni²⁺. Elastic and inelastic neutron scattering experiments are currently underway and reveal a rather flat spin wave excitation at 5 meV. Combining neutron diffraction with the DFT results, we would arrive at an accurate estimation of the exchange constants for Na₂Ni₂TeO₆.

The discovery of atomically thin carbon sheets known as graphene in 2004 has prompted intense scientific research on synthesizing and characterizing two-dimensional (2D) nanomaterials. It has been demonstrated both experimentally and theoretically that the 2D confinement of electrons offer distinct optoelectronic properties unlike their corresponding bulk materials including band-gap tunability and improved charge transport properties. Over the last 15 years several types of 2D nanomaterials such as metal oxides, transition metal dichalcogenides, layered metal hydroxides, and MXenes have been extensively studied. More recently organic-inorganic hybrid perovskite 2D sheets have been synthesized and characterized for their use in solution-processable photovoltaic devices. Perovskites possess defect tolerant electronic properties, wide band gap control, and large absorption coefficients necessary for many applications; however, lead free compositions along with combining 2D syntheses and assemblies need further research to reduce environmental toxicity. Cs₃Bi₂X₉ (X=Cl, Br, and I) is a perovskite family material with a hexagonal crystal structure. Herein, we report the synthesis of Cs₃Bi₂X₉ nanosheets with specific halide control by a low temperature colloidal synthetic method, for the first time. UV-visible absorption spectroscopy and powdered X-ray diffraction (XRD) characterizations, along with electron microscopy (EM) analyses, shows formation of atomically-thin perovskite nanosheets. EM analyses also observed formation of stacked nanosheets as superlattice structures. Furthermore, we conducted small angle X-ray scattering techniques to provide an in-depth study of the factors such as the van der Waals forces induced from core stoichiometry, crystal structure, and ligand-ligand interactions between sheets impacting the self-assembly of 2D nanosheets into larger stacked superstructures. We hypothesize that these superstructures are self-assembled mainly through ligand-ligand interactions. The unique properties of the perovskite family, in combination with the well-studied benefits of 2D nanomaterials, suggest that our 2D Cs₃Bi₂X₉ will display well-controlled optoelectronic and charge transport properties leading to advanced solid-state device applications.

Tracing the flow of particulate matter through harsh conditions (such as a chemical explosions) requires a rugged tag that can be measured by a unique, identifiable signature. Small semiconductor core-shell particles of ZnS@CdSe provide a unique, tunable photoluminescent signature that can be adjusted by the material’s composition and core/shell thickness. The particles have been ruggedized by the growth of a silica layer around the quantum dots (QDs) that acts as a sacrificial layer during finite periods of elevated temperatures and pressures. Incorporating the QDs into a matrix allows for the identification of the debris by its unique photoluminescence. The silicated QD-bound powders were suspended in a hydrated silica gel pending incorporation into temperature-resistant paints and synthetic stone. Initial explosive testing of the tags embedded in enclosures and concrete showed the deposition and movement of particles based on size. The incorporation of temperature-resistant QD-bound powders has enabled unique identifiers, which allows for the tracking of mass through explosive events and other inaccessible environments.

Ideal two-dimensional (2D) Heisenberg magnets lack long range order [1]. However, the XY model with spins confined to a plane shows a topological phase transition at a finite temperature corresponding to binding and unbinding of vortices [2,3]. Experimental evidence for such Berezinskii-Kosterlitz-Thouless (BKT) transitions has been difficult to obtain in condensed matter systems, where, even a weak interlayer coupling that is invariably present leads to long-range order, pre-empting the BKT transition. The BKT signatures are still discernible above the long-range ordering temperature, however, in the characteristic exponential temperature dependence of the coherence length of the fluctuations. In this work we report that an applied magnetic field can induce such BKT correlations not only in quasi 2-dimensional systems but also in nominally 3-dimensional manganites undergoing antiferromagnetic transitions. We arrive at this unexpected conclusion based on our studies of temperature dependence of electron spin resonance (ESR) linewidth ΔH(T) of Cr³⁺ doped bismuth strontium manganite Bi_0.5Sr_0.5Mn_1-xCr_xO₃ (x= 0.04, 0.1) (BSMCO).
BSMCO [4] belongs to the family of mixed valent manganites of the type ReAMnO₃ where Re is a trivalent rare earth ion or Bi³⁺ and A is a divalent alkaline earth ion, which are intensely studied in the last few years [5]. ΔH(T) provides the most important probe to study the spin interactions in these strongly correlated spin systems. We find that ΔH(T) observed in BSMCO (current work), as well as in La_.05Ca_.95MnO₃ and CaMnO₃ [6] is better (than the usually adopted critical state model) described by the BKT scenario which predicts [7]
ΔH_BKT(T) = ΔH_∞ exp [ 3b/(T/T_BKT — 1)^0.5 ] + mT + ΔH_ind , T > T_BKT
where T_BKT is the BKT transition temperature and b = π/2 for a square lattice and the last two terms are included to account for the high temperature and temperature independent behaviour. This is unexpected as these manganites have a 3D structure and the BKT model addresses systems with spin and spatial dimensions of two. We understand this result in terms of an effective symmetry reduction induced by i) the magnetic field applied in the ESR experiment and ii) the intrinsic anisotropy arising from microscopic details such as doping and phase separation - contributing to an effectively 2-dimensional XY easy plane anisotropy. Nanometric scale spin clusters similar to the ones observed in La doped CaMnO₃ [8] could conceivably play the role of vortices in these 3D materials. The sensitivity of the BKT behaviour to applied field is also supported by a re-analysis of the field dependence of the ΔH(T) in the quasi-2D antiferromagnetic compound BaNi₂V₂O₈ reported by Heinrich at al., [9]. For undoped manganites we find T_BKT is of the order of the magnetic interaction energy, suggesting that the applied field could be the sole origin of the BKT behaviour [10]. We shall also address the interesting observation that in BSMCO (x=0.04, 0.1), T_BKT is composition-independent while the magnetic properties are quite sensitive to composition. <span style="font-size:10.8333px">.</span>
SVB and AA thank the Indian National Science Academy, the National Academy of Sciences, India and the Indian Academy of Sciences for support.

References:
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[2] V. L. Berezinskii, Sov. Phys. JETP 32, 493 (1971)
[3] J. M. Kosterlitz and D. J. Thouless, Journal of Physics C: Solid State Physics 6, 1181 (1973)
[4] K. S. Bhagyashree, L. R. Goveas, and S. V. Bhat, Applied Magnetic Resonance 50,1049 (2019)
[5] Y. Tokura, Rep. Progr. Phys., 69, 797 (2006)
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[7] M. Hemmida et al., Phys. Rev. B95, 224101 (2017)
[8] E. Granado et al., Phys. Rev. B68, 134440 (2003)
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[10] A. Ashoka, K. S. Bhagyashree and S. V. Bhat, arXiv:1809.07635v3

Strain describes the deformation of a material as a result of applied stress. It has been widely employed to probe transport properties of materials, ranging from semiconductors to correlated materials. In order to understand, and eventually control, transport behavior under strain, it is important to quantify the effects of strain on the electronic band structure, carrier density and mobility. Here, we demonstrate that much information can be obtained by exploring a novel experimental observable: magneto-elastoresistance (MER), which refers to magnetic field-driven changes of the elastoresistance. We use this powerful approach to study the combined effect of strain and magnetic fields on the semi-metallic transition metal dichalcogenide WTe₂. We discover that WTe₂ shows a large and temperature non-monotonic elastoresistance, driven by uniaxial stress, that can be tuned by magnetic field. Using first-principle and analytical low-energy model calculations, we provide a semi-quantitative understanding of our experimental observations. We show that in WTe₂ the strain induced change of the carrier density dominates the observed elastoresistance. In addition, the change of the mobilities can be directly accessed using MER. Our analysis also reveals the importance of a heavy hole band near the Fermi level on the elastoresistance at intermediate temperatures.

At the time of its discovery, antiferromagnetic (AFM) order was considered as an interesting but essentially useless phenomenon in terms of application. Now, however, the AFM state is being regarded as highly promising for spintronic applications. The fully compensated magnetic order makes it robust against external magnetic fields and at the same time leads to vanishing magnetic stray fields, thus reducing crosstalk between neighbouring AFM domains. Furthermore, the AFM order permits spin dynamics in the THz-regime with AFM switching processes on the picosecond timescale. Because of the absence of a macroscopic magnetization, ultrafast detection of the AFM state is challenging, however. In my talk, I present various laser-optical approaches to overcome this. On the one hand, linear and nonlinear laser-optical processes allow us to track the ultrafast three-dimensional motion of the AFM order parameter through space. By doing that, we show that, in stark contrast to ferromagnets, damping effects play an important role during the optical excitation of the AFM state and offer an efficient handle for all-optical AFM order-parameter switching ("writing an AFM bit") [1]. Furthermore, THz time-domain spectroscopy offers a novel approach for studying the competition of AFM order and a magnetically screened spin-liquid state with the formation of heavy fermions and their disintegration near the quantum-critical points in materials like CeCu_6-xAu_x and YbRh₂Si₂ [2, 3].

[1] C. Tzschaschel et al., Nature Comms. 10, 3995 (2019)
[2] C. Wetli et al., Nature Phys. 14, 1103 (2018)
[3] S. Pal et al., Phys. Rev. Lett. 122, 096401 (2019)

Quantum materials offer not only a vast array of potential for technological advancement, but also provide a testbed for the laws of nature in condensed matter. One area to advance the frontier in this area is through the measurement of fast, spontaneous fluctuations of quantum order. This is at the heart of the essential physics in these types of solids, but has remained largely unexplored. The key component we use is the application of a newly developed coherent scattering method which can access these fluctuations on the relevant energy scales. This new tool has been demonstrated on a topological magnetic material, and early results point to the tremendous potential for this approach to provide important new insight into fundamental open questions in condensed matter. It allows for the direct, element-specific and momentum-resolved measurement of the fluctuations of a complex material and to connect them to the requisite response functions calculated from first principles. By analyzing subtle X-ray variations known as "speckle", statistics is used to extract the stochastic fluctuation information of the system. This offers the ability to address a range of important current problems in materials. In this talk, we will first discuss the newly developed tools which are used to measure the material properties, based on short pulses from an x-ray free electron laser. We will then present how this has been applied to skyrmions and the measurement of nanosecond fluctuations. This will be followed by an examination of new results on skyrmion phyiscs near the critical point of the skyrmion ordered lattice phase. We will end with an outlook of how this progress can help to tackle the question of fluctuations in unconventional superconductors.

This work at SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through the Materials Sciences and Engineering Division, contract DEAC02-76SF00515. J. J. Turner acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences through the Early Career Research Program.

Controlling the interlayer twist angle in artificial two-dimensional (2D) van der Waals (vdW) heterostructures offers an experimental route to create moire superlattice. One can create exotic electronic states by minimizing electronic band width with tunable moire length scale. However, in the small twist angle regime, vdW interlayer interaction can cause significant structural reconfiguration at the interface, creating the arrays of domain structures. In this presentation, we will discuss the atomic reconstruction at twisted vdW interfaces and its effect on electronic structure and electrical transport behavior. Furthermore, we note that multiple domain boundaries at the reconstructed interface join together to create energetically unfavorable nodes forming vortex-like structures. We will discuss our recent efforts to understand the topological nature of those defects.

Proton irradiation as a form of defect engineering is used to inhibit the crystal structure of van der Waals crystals in order to provoke a change in the magnetic properties. In this study, the magnetocaloric effect, heat capacity, and critical behavior of transition metal trichalcogenides as a function of proton irradiation were analyzed for Mn₃Si₂Te₆, Fe_2.7GeTe₃, and CrSiTe₃. The crystals were irradiated at fluences of 1x10¹⁵, 5x10¹⁵, 1x10¹⁶ and 1x10¹⁸ H⁺/cm² with a 2 MeV beam. Following proton irradiation, the Curie temperatures (T_C) were obtained experimentally using heat capacity and magnetization as a function of temperature, and by critical exponent analysis. Critical exponents were calculated using the Kouvel-Fisher model and modified Arrott plot. In this work, the correlation between proton irradiation and the resulting critical behavior, magnetocaloric effect, and Curie temperatures will be discussed.

Control over the interactions between light and matter underlies many classical and quantum applications. In recent years, 2D layered semiconductors have gained prominence for optoelectronics because of their strong excitonic features and capacity for van der Waals assembly in layered heterostructures. Through integration with various photonics devices, the interactions between these materials and light can be tailored and new physical regimes can be achieved, highlighting the importance of understanding photonic dressed states of 2D materials. One of the unique features of monolayer materials such as transition metal dichalcogenides is the valley pseudospin addressable by circularly-polarized light. Understanding how this valley polarization can be manipulated is an exciting goal with implications for devices and quantum information science. Here, I discuss hybrid light-matter dressed states, or exciton-poIaritons, in transition metal dichalcogenides embedded in dielectric microcavities. These strongly-coupled polaritons preserve the valley pseudospin known from bare monolayers [1]. Distinct behavior of valley-polarized exciton-polaritons can be accessed with microcavity engineering by tuning system parameters such as cavity decay rate and exciton-photon coupling strength. In the opposite dressed state regime of weak light-matter coupling, intense off-resonant light can be used to shift energy levels though the Stark effect [2]. Applied to valley-sensitive states, this shift can be used for coherent manipulation of valley pseudospin. I will show how this approach can be translated to valley-selective exciton-polaritons. Cavity reflectance spectra exhibit a simultaneous shift of both polariton branches when ultrafast coincident pump and probe pulses are co-circularly-polarized, and no appreciable shift when they are cross-polarized. This valley-selective polariton Stark shift combines both dressed state regimes of strong, near resonant interactions with weak, off-resonant optical interactions, providing a new tool for state control in coherent valleytronics. Exploiting photonic dressed states can be viewed as another approach in the toolbox for manipulating the optoelectronic properties of 2D materials and their heterostructures.

This work was supported by DOE (DE-SC0012130) and ONR (N00014-16-1-3055).

[1] Y.-J. Chen, et al. Nature Photonics 11, 431 (2017).
[2] T. LaMountain, et al. Phys. Rev. B 97, 045307 (2018).

Van der Waals (vdW) magnetism is the topic of intense research interest in the community of 2D magnetic materials. Proton irradiation is an effective tool in controlling the magnetic properties of vdW magnets by modifying the spin-lattice coupling. In the present work, the magnetic properties of vdW magnet, such as Mn₃Si₂Te₆ (MST) have been studied at varying proton irradiation fluences of 1x10¹⁵, 5x10¹⁵, 1x10¹⁶, and 1x10¹⁸ H⁺/cm². The magnetization changes more prominently in the ferrimagnetic region (temperature < 74 K, where the saturation magnetization (M_S) reaches a maximum value of 25.1 emu/g (53% increase) at the fluence of 5x10¹⁵ H⁺/cm² and decreases with further increase in the fluence. Electron Paramagnetic Resonance (EPR) spectroscopy was used to probe whether proton irradiation caused defects that may alter the magnetic properties of MST. From the EPR measurements taken at 80 K and 50 K, no signals corresponding to defects arose from the pristine and all the fluence rate spectra except for two intrinsic Mn signals. Raman spectroscopy suggests a change in the in-plane Tellurium vibrational modes, as reflected from a similar trend between the M_S and the Raman E_g peak. Additionally, XPS spectroscopy shows a decrease in the Te 3d5 peaks’ intensity as the fluence changes. The changes observed in the M_S are suggested to come from the modified Mn-Te interactions through spin-lattice coupling.

Understanding the interactions leading to magnetic quantum phenomena in a wide range of quantum materials is extremely important for development of new quantum materials and future technologies. Although neutron scattering is a powerful tool for this purpose, it also faces a range of challenges, such as the lack of sizable high-quality crystals, limited high pressure capabilities, and the difficulty in disentangling the intrinsic quantum phenomena versus effects from defects and site-disorder. Recently we have introduced the local magnetic susceptibility methods for studying quantum materials, which can be used for small crystals and powder samples. A single crystal neutron diffractometer DEMAND at the High Flux Isotope Reactor at the Oak Ridge National Laboratory has been upgraded for studying magnetic materials under various extreme sample environment conditions. In this talk, I will introduce the local site magnetic susceptibility methods, the current capabilities at the DEMAND, and our recent studies on two-dimensional quantum materials.

Particulate flow measurements require environmentally inert and size-selected particles, with unique identifiers. Agglomerated microscale silica covered quantum dots (QDs) were synthesized and used for explosive mass-deposition measurements within fabricated enclosures. Experimental determination of luminescent intensity, as well as chemical-environmental tolerance, were conducted to ensure the inert nature and unique identifier for each size-selected band. Reagent concentration control was used to produce silicated/QD luminescent particles with bands centered from 100 nm to 4 microns. The ability to uniformly size particles and link the size to an easily identifiable characteristic, such as luminescence, allows for the creation of flow-based particle distribution description. This is a valuable tool for the analysis of turbulent flow in highly variable cavities. Future modeling with these particles can be used to predict the deposition and settling location of particulates and the environmental flow of debris in turbulent fluid environments.

Non-equilibrium phases have developed as an important topic in the realm of layered materials, especially in the two-dimensional limit. Of particular interest is the case of magnetism in the single layer limit[1]. Using external inputs ranging from static to dynamic, the states in these systems can be strongly manipulated to drive transitions between different spin configurations[2]. However, the challenge is to follow the emergent or collapses of specific phases in systems that are at the two-dimensional limit. Here I will focus on using X-rays to explore the magnetic order parameter directly for the cases involving antiferromagnetic and ferromagnetic states. The use of polarized X-rays in either scattering or absorption experiments, has a long history of probing magnetic states. With synchrotron and free-electron laser sources, we can extend this to the ultrafast domain to follow the dynamic response of the magnetic configuration. Here I will present examples involving dynamic collapse of antiferromagnetic order in nickelates to the optical conversion of antiferromagnetic to ferromagnetic order in manganites[3]. I will use these ultrathin oxide films as examples of exploration of magnetism in the two-dimensional limit using X-rays.

The work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. The development of the materials and ultrafast experiments is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC-0012375.

References:
[1] M. Gibertini, M. Koperski, A. F. Morpurgo, and K. S. Novoselov, Nature Nanotechnology 14, 408 (2019).
[2] H. Li, S. Ruan, and Y. J. Zeng, Adv Mater 31, 1900065 (2019).
[3] J. Zhang, X. Tan, M. Liu, S. W. Teitelbaum, K. W. Post, F. Jin, K. A. Nelson, D. N. Basov, W. Wu, and R. D. Averitt, Nat Mater 15, 956 (2016).

Intrinsically two-dimensional materials have a variety of properties that make them attractive for potential topological devices, but the electronic and topological properties of this class of materials remains under-explored. Using density functional theory-based spin-orbit spillage, Wannier-interpolations, and related techniques, we identify topologically non-trivial intrinsic 2D materials in several classes, include magnetic and non-magnetic materials, as well as insulators and semimetals. Using JARVIS-DFT 2D material dataset we first identify materials with high spin-orbit spillage among 683 materials resulting in 108 materials with high-spillage values. Then we use Wannier-interpolation to carry-out Z2, Chern-number, anomalous hall conductivity, Curie temperature surface and edge state calculations to identify topological insulators and semimetals such as quantum spin-hall and anomalous-hall insulators (QSHI, QAHI) , magnetic and non-magnetic semimetals. For a subset of predicted QAHI materials, we run GW+SOC and GGA+U calculations. We find that as we introduce many-body effects only few materials retain non-trivial band-topology suggesting the importance of high-level DFT methods in predicating 2D topological materials. However, as an initial step, the automated spillage screening and Wannier-approach provide useful predictions for finding new topological materials.

van der Waals layered magnets such as CrX₃ (X = Cl, Br, I) have received a great deal of research excitement in the recent past due to their exfoliable nature and layer dependent magnetic properties, as well as their potential applications in future data storage devices. Interestingly, these compounds reveal strong halogen size dependent magnetic properties due to the introduction of strong spin-orbit coupling as one moves from Cl to I. This variation can be perfectly captured by employing electron spin resonance (ESR) spectroscopy, which is highly sensitive to underlying magnetic interactions. In this work, we report the ESR properties of these compounds such as g-value, line broadening, and signal intensity as a function of microwave frequency (9-240 GHz), temperature (4-500 K), and angle of rotation (0-360^o). We compare and contrast the ESR properties of these three compounds and discuss their two-dimensional correlations across the magnetic ordering temperature, magnetic exchange interactions, and spin dynamics. The experimental findings collected from this work will have greater implications in understanding the few-layer magnetism in these materials, which is being intensely researched in the 2D magnetism community.

Transition metal dichalcogenides (TMDC) are an emerging class of quantum materials because they are 2D topological insulators in the 1T' phase and Weyl semimetals in the Td phase. To optimize the properties of these materials, synthetic methods to tune their Fermi level and bulk gap opening are necessary. This can be realized by synthesizing alloys with a wide range of adjustable compositions. Mismatched alloys, such as MoS_2-xTe_x, are alloys formed by elements far away from each other in the periodic table. Due to the large mismatch in atomic radius and electronegativity, a small addition of the alloying element can induce dramatic modifications in the electronic structure of the material. However, such immiscible alloys are hard to synthesize as bulk crystals by conventional crystal growth methods, especially in metastable phases such as the 1T' phase. Here, we report the bottom-up synthesis of 2D MoS_2-xTe_x mismatched alloys in the 1T' phase by chemical vapor deposition through non-equilibrium methods. We introduce K⁺ during the synthesis to grow the 1T' phase, followed by fast cooling to stabilize this phase to room temperature. With this approach, MoS_2-xTe_x alloys can be grown on various substrates including mica, sapphire and gold. The 1T' structure of the flakes are confirmed by Raman spectroscopy and scanning transmission electron microscopy. Our results open new opportunities in tailoring the structure and properties of quantum materials by alloying mismatched elements through non-equilibrium synthesis.

Research is sponsored by the U.S. Dept. of Energy, Office of Science, Basic Energy Sciences, Materials Science and Engineering Div. (synthesis science) and Scientific User Facilities Div. (characterization science).

In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here we report a notable exception in an insulator — ytterbium dodecaboride (YbB₁₂). The resistivity of YbB₁₂ exhibits distinct quantum oscillations despite having a much larger magnitude than in metals. This unconventional oscillation is shown to arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conven- tional Fermi liquid theory of metals. The large effective masses indicate the presence of a Fermi surface consisting of strongly correlated electrons. Quantum oscillations are also observed in the magnetization of YbB₁₂. Our result reveals a mysterious dual nature of the ground state in YbB12: it is both a charge insulator and a strongly correlated metal.

The spin-orbit coupled Mott insulator Sr₂IrO₄ has attracted considerable interest because of its exotic J_eff = 1/2 Mott state arising from the interplay of on-site Coulomb repulsion and strong spin-orbit coupling. We have investigated magnetization dynamics of this enigmatic compound. Specifically, we measured coherent magnons of the J_eff = 1/2 Mott state using Kerr rotation following excitation with either mid-infrared 9 μm (below the charge gap), or near-infrared 1.3 μm (above the charge gap) circularly polarized pulses. For both pump wavelengths, the 2D in-plane B_2g coherent magnon oscillation of frequency ~0.5 THz is observed. In this talk, we show that the excitation pathways are different for these two excitation wavelengths. In particular, coherent magnon excitation with 9 μm pulses arises from the inverse Faraday effect that, microscopically, is related to two-magnon processes. Notably, coherent magnon generation with 9 μm pulses is nearly an order of magnitude more efficient in comparison to 1.3 μm pulses, without excitation of carriers across the Mott gap.


Acknowledgement: This research is supported by the Army Research Office MURI grant ARO W911NF-16-1-0361, “Floquet engineering and metastable states.”

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. This talk will describe our implementation of magnetic resonant inelastic X-ray scattering at free-electron lasers to directly determine the magnetic dynamics after photo-doping the Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇ [1-3]. We find that the non-equilibrium states host strong 2D magnetic correlations even in cases where 3D long-range magnetic order is completely suppressed [1]. The marked difference in these 2D and 3D timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics. We also uncover distinct differences in the magnetic dynamics between insulators Sr₂IrO₄ and Sr₃Ir₂O₇, which we attribute to a “spin-bottle-neck” effect [3].

Excitons in two-dimensional semiconductors exhibit rich dynamics. Their fluorescence is sensitive to the nanoscopic environment. They are confined in the vertical direction while extending and diffusing along the atomically thin plane. Excitons can also interact with each other, notably to reduce light emission at high densities through exciton-exciton annihilation.
In this presentation, first, we will demonstrate that an atomically thin semiconductor can display substantial temporal fluctuations in fluorescence intensity influenced by its environment. We show blinking and flickering synchronized over monolayer domains tens of micrometers in size. We analyze the fluctuation statistics to draw analogies with blinking in conventional quantum dots.
Second, we will discuss strategies to increase the amount of photons emitted by a 2D semiconductor based on nanophotonic enhancement. We study how exciton diffusion impacts fluorescence enhancement using nanophotonic structures. As mobile excitons diffuse through optical hotspots, the careful balance of diffusion constants and nanophotonic geometry can lead to enhanced or suppressed fluorescence. Finally, we investigate the effect of nanophotonic enhancement on exciton-exciton annihilation.
Our results open new vistas for nanoscale photonics and optoelectronics with atomically thin semiconductors. Controlling exciton dynamics in the form of fluctuations, diffusion, or annihilation has direct implications for stable single-photon sources, molecular sensors based on excitonic fluorescence, or high-power light-emitting devices.

The confinement of electrons in semiconductor quantum wells, and the resulting quantization of motion and energy, allows the engineering of electronic band structure and resulting device properties. However, this possibility has remained largely unexplored for the case of ultra-thin two-dimensional (2D) van der Waals crystals. Here, we apply the quantum well concept to 2D materials, using the thickness of exfoliated 2D crystals to control the quantum well dimensions in few-layer indium selenide. This approach allows high-precision control of the subband energies, with their uniformity giving high quality electronic transport properties to the system.

Here, for the first time, we present a comprehensive study of the full subband structure of atomically thin 2D semiconductors, applied to InSe[1]. The subbands of the conduction band are fully mapped using resonance tunneling spectroscopy, in agreement with theoretical density of states calculations. On the valence band side, we map the subbands using photolumiscence excitation (PLE), interpreting the results using theoretical calculations of the optical absorption and comparing them with ARPES data. Further evidence of quantum confinement is explored, including quantum-confined Stark shifts of the subbands on application of a bias across the crystal, and an increase in the binding energy of the excitonic resonance in absorption in the thinnest films is seen.

The analogy between ultra-thin van der Waals crystals and conventional semiconductor quantum wells can be clearly traced, with this work paving the way to the application of 2D materials in infrared and terahertz light sources employing intersubband optical transitions.

[1] J. Zultak et. al., arXiv:1910.04215 (2019)

Owing to strong electronic and dielectric confinement effects, strongly bound two-dimensional excitons are observed in layered hybrid organic-inorganic perovskites[1,2]. Given the relevance of polaronic effects in their 3D counterparts, here we ask if such effects are consequential for excitons in 2D HOIPs. We argue that the peculiar lattice interactions are manifested intrinsically in the exciton spectral structure, which is comprised of multiple non-degenerate resonances with distinct photo-physical characteristics[2]. We highlight their population[3] and dephasing dynamics[4] that point to the apparently deterministic role of polaronic effects. We contend that an interplay of long-range and short-range exciton-lattice couplings give rise to exciton polarons, which fundamentally establishes their effective mass and radius, and consequently, their quantum dynamics[2]. Finally, we highlight opportunities for developing rigorous description of exciton polarons in 2D-HOIPs to advance their fundamental understanding as model systems for materials with strong lattice-mediated correlations.

[1] F. Thouin, D. Valverde-Chavez, C. Quarti, D. Cortecchia, I. Bargigia, D. Beljonne, A. Petrozza, C. Silva and A. R. Srimath Kandada, Phonon coherences reveal the polaronic character of the excitons in two-dimensional lead-halide perovskites, Nature Materials, 18, 349-356 (2019).
[2] A. R. Srimath Kandada and C. Silva, Perspective: Exciton polarons in two-dimensional hybrid metal-halide perovskites, arXiv:1908.03909[cond-mat.mtrl-sci].
[3] F. Thouin, A. R. Srimath Kandada, D. Valverde-Chavez, D. Cortecchia, I. Bargigia, A. Petrozza, X. Yang, E. R. Bittner and C. Silva, Electron-phonon couplings inherent in polarons drive exciton dynamics in two-dimensional metal-halide perovskites, Chem. Mater., 31, 7085-7091 (2019).
[4] F. Thouin, D. Cortecchia, A. Petrozza, A. R. Srimath Kandada and C. Silva, Enhanced screening and spectral diversity in many-body elastic scattering of excitons in two-dimensional hybrid metal-halide perovskites, Phys. Rev. Res, in press.

Two dimensional electron gases (2DEG) in graphene^[1] and black Phosphorus (bP) ^[2] have been explored in quantum transport measurements, yet unlike graphene, the optical properties 2DEGs in bP are relatively unknown. The crystal structure of bP is anisotropic^[3], so it is a candidate material to exhibit naturally-occurring tunable hyperbolic dispersion. According to calculations^[4], plasmons (low energy collective mode excitations) in bP exist in the mid to far-infrared regime and exhibit hyperbolic dispesion and anisotropy in different frequency windows.
In order to probe plasmonic resonances we have performed far-field infrared spectroscopy measurements (with both thermal source and synchrotron beam at ALS, Berkeley) to observe gate-tunable polarized absorption in patterned bP nano-resonators encapsulated with hBN (to prevent oxidation). Ribbons of bP 100-150nm wide were fabricated via electron beam-lithography and reactive ion etching, aligned to its two principal crystal axes identified with polarized Raman spectroscopy and characterized with SEM. A vertical Fabry-Perot resonator was also used to enhance the weak plasmonic absorption. Polarized infrared extinction shows clear anisotropy in the 35-50µm regime, along with characteristic Fabry-Perot (F-P) peaks. Finite difference time domain (FDTD) simulations show good agreement with experimental results. However, these measurements are limited by two factors – patterning bP induces scattering centers leading to higher damping; dry etching of bP limits the accessible wave vectors, leading to resonances in the far IR with poor signal to noise ratio. To circumvent these limitations we designed a structure comprised of unpatterned bP (giving much higher quality 2DEGs) placed close to metallic resonators, which can be made at the 30-50nm scale, cladded by hBN spacer layers, enabling excitation of acoustic plasmons. FDTD simulations predict gate-tunable anisotropic/hyperbolic plasmonic resonances in 10-20µm range. Higher order optical modes from the metallic resonators and bP plasmon-hBN phonon coupling are observed. Fabrication techniques for such devices will be discussed, along with characterization techniques like polarized Raman spectroscopy, SEM and two-terminal field effect transport measurements. Also, polarized infra-red extinction and its dependence on charge density, duty cycle and hBN spacer thickness will be shown if time permits. Taken together, these optical studies offer a thorough understanding of low energy excitations in bP and suggest the potential for infrared fingerprinting in the THz range.

1. “Experimental observation of the quantum Hall effect and Berry's phase in graphene”, Y. Zhang et al., Nature 438, 201-204 (2005)
2. “Quantum oscillations in a two-dimensional electron gas in black phosphorus thin films”, L. Li et al., Nature Nanotechnology 10, 608–613 (2015)
3. “The renaissance of black phosphorus”, X. Ling et al., PNAS 112 (15) 4523-4530, (2015)
4. “Plasmons and Screening in Monolayer and Multilayer Black Phosphorus”, T. Low et al., PRL 13, 106802 (2014)

In this talk I will discuss how ultrafast optical techniques can be used to probe magnetic correlations and modify magnetic interactions in van der Waals materials. As a testbed I will discuss CrSiTe₃, which is composed of van der Waals bonded sheets of ferromagnetic interacting Heisenberg spins that, in isolation, would be impeded from long-range order by the Mermin-Wagner theorem. Using an optical second harmonic generation based probe of spin correlations, I will show how CrSiTe₃ evades this law via a two-step crossover from two- to three-dimensional magnetic short-range order above its Curie temperature. Having understood the interplay between short-range correlations and magneto-elastic distortions, I will then demonstrate, using coherent phonon spectroscopy, how optically induced ligand-to-metal charge transfer excitations can be used to transiently enhance magnetic super-exchange in CrSiTe₃.

We report a novel 0D-2D hybrid heterostructure displaying light-driven capacitive charging dynamics through solid state photodoping. We exploit the coupling between indium tin oxide (ITO) nanocrystals and two-dimensional molybdenum disulfide (MoS₂) to absorb and directly store the energy of the incoming light as extra charges within the ultra-thin heterostructure. By illuminating this hybrid system with light beyond the bandgap of ITO photo-electrons and holes are generated inside the nanocrystal and rapidly separated from each other. Photo-generated holes are quenched at the surface of the semiconductor and injected into the MoS₂ monolayer, which acts as a hole collector. The photo-excited electrons accumulate in the ITO nanocrystal leading to the photodoping of the hybrid nanostructure. Multiple electrons can be excited and permanently stored displaying the behavior of a light-driven miniaturized capacitor. During the photodoping process we detect variations in the relative contributions of excitons and trions to the emission of 2D-MoS₂: while the overall photoluminescence peak shifts over time to higher energies the exciton population is enhanced and the trion population suppressed. We extract the temporal evolution of the free carriers in the MoS₂, which mimics the charging kinetics of a capacitive system. Further studies unveiled that the solid state photodoping process occurs over two main timescales, with most of the variations happening in the first 30 seconds and remaining established after the photoexcitation is switched off. Moreover, by probing the system with light at different wavelengths we identify two distinct processes competing with each other: one reversible due to the interaction between the MoS₂ and air molecules and one irreversible due to the injection of extra holes transferred from the ITO nanocapacitors. Quantitative analysis suggests that each nanocrystal can store, in average, 40 optically generated electrons and that the p-type photodoping of 2D MoS₂ reaches values compatible with electrostatic gating. These studies present an innovative step towards a light-driven hybrid 0D-2D nanocapacitor, opening prospects for the development of new solutions to directly store the solar energy.

The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipativeless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such goal. Yet a challenge remains on how to selectively control surface helical spin transport and surface-bulk coupling. Here we use the mid-infrared and terahertz (THz) photoexcitation of intraband transitions and/or coherence optical phonons to enable ultrafast manipulation of surface THz conductivity in a topological insulator. We will also discuss how to extend the ultra-broadband, wavelength-selective pumping to emerging topological semimetals. Imposing electronic and vibrational coherence into topological matter may become a universal light control principle for reinforcing the protected quantum transport.

Valley pseudo-spin and broken inversion symmetry leads to valley-specific optical and transport properties in monolayer transition metal dichalcogenides (TMDC). Selective population of the degenerate K and K’ valleys via right- and left-hand circularly polarized light has fueled interest in the potential of TMDCs for Valleytronic applications, where the momentum state is manipulated to perform logical operations.

Intense circularly polarized light fields can break the valley degeneracy, allowing the energy levels of each valley to be independently tuned via a valley-selective form of the optical Stark effect.^1,2 These strong light-matter interactions have historically been described by a two-level dressed-atom picture, which works well for semiconductors far from resonance where the Rabi frequency is larger than the exciton formation rate. Recently, the need to extend this model to include the additional energy levels associated with intervalley biexcitons has been demonstrated,³ however the basic assumption of non-interacting particles remained. Here, we report that this assumption is not applicable for excitation near resonance in monolayer WS₂,⁴ where an excitonic model⁵ of the optical Stark effect that includes many body effects is required.

We use ultrafast transient absorption spectroscopy to show that that the valley-selective optical Stark effect produces a blue shift for both below- and above-resonant excitation.⁴ This coherent light-matter interaction is inconsistent with the two-level dressed-atom picture, which predicts a change in the direction of the optical Stark shift when tuning through resonance. Instead, our observations are well described by an excitonic model⁵ of the optical Stark effect that includes exciton-exciton interactions in addition to the exciton photon interaction. Here, we confirm the prediction from this theory that repulsion between virtual excitons will be the dominant contribution to the optical Stark effect when the excitation wavelength is detuned from resonance by less than the exciton binding energy. This observation demonstrates the need to include many-body Coulomb effects when describing the optical Stark effect in low-dimensional semiconductors that support bound excitons.


References
1. Kim, et al., Science 346,1205 (2014)
2. Sie, et al., Nat. Mater. 14290 (2015)
3. Yong, et al., Nat. Phys. 141092 (2018)
4. Cunningham, et al., Nat. Commun. (in press)
5. Schmitt-Rink, et al., Phys. Rev. Lett. 572752 (1986)

Floquet states are photon-dressed electronic states that emerge when driven by a time-periodic optical excitation. The presence of these states changes the equilibrium picture of electronic transitions into one where transitions between Floquet states are allowed. We have recently studied the indirect exchange (RKKY) interaction between ferromagnetic chains mediated by itinerant electrons that are driven strongly out of equilibrium by circularly polarized light. Emergence of non-equilibrium Floquet states leads to new and qualitatively different regimes of RKKY coupling, in which the sign (ferromagnetic or antiferromagnetic) and the period of the RKKY oscillation become tunable through the frequency and amplitude of the laser. Another phenomenon where Floquet driving plays a distinctive role is the tunneling dynamics in a tunneling junction. We have investigated the tunneling current between graphene layers under strong polarized radiation, predicting periodic suppression of the photon-assisted tunneling current when the bias voltage is equal to integer multiples of the photon frequency. Our findings on these Floquet systems demonstrate the feasibility of optically controlling the indirect exchange interaction and tunneling in van der Waals materials with coherent laser fields.

Janus two-dimensional (2D) materials present a new form of quantum materials predicted to exhibit topological phases and the Rashba-type spin-orbit-coupling. Despite the fact that Janus monolayers have now been experimentally realized, the reliable synthesis of these metastable materials requires precise synthetic control. Pulsed laser deposition (PLD) plasma plumes offer hyperthermal kinetic energies that have been used for decades to synthesize thin films of many materials and are currently being explored for the growth of 2D transition metal dichalcogenides (TMDC) in metastable phases, alloys, and heterostructures. Here, we demonstrate PLD for the selective replacement of sulfur by selenium in monolayer (ML) WS₂ crystals at low (250-600 ^oC) temperatures to form 2D Janus TMDCs. Using in situ plasma diagnostics, the kinetic energy of Se species generated by pulsed laser ablation of Se targets was moderated in the < 20 eV/atom range by collisions with inert Ar atoms in order to determine the thresholds for top- and bottom-layer selenization of monolayer WS₂ crystals on both TEM grids and SiO₂/Si substrates. Subsequent analysis by Z-contrast STEM images, XPS, PL, and Raman spectroscopy were used to measure the degree of alloying and confirm the Janus monolayer formation as a function of kinetic energy, revealing discrete kinetic energy thresholds: (1) for irreparable damage to the monolayers, (2) for replacement of S atoms on both the top and bottom of the monolayer to enable full or partial conversion into WSe₂; and (3) for the selective replacement of only the top S atoms (facing the Se beam), enabling the formation of high-quality Janus WSeS and MoSeS monolayers. The experimental thresholds are consistent with both density functional theory calculations of energy barriers for Se migration through WS₂, and with molecular dynamic simulations the selenization process. The generalization of this synthetic approach to many other Janus monolayer systems will be described. Synthesis science was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES), Materials Sciences and Engineering Division. Characterization science at CNMS was supported by the Scientific User Facilities Division, BES.

Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in the two-dimensional (2D) materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, we report a room-temperature colossal MR of up to 5,000% at 9 T in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO₂ terminated SrTiO₃, we demonstrate a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices. Strikingly, a colossal MR of >1,000% was also achieved in the terraced graphene even at a high carrier density of ~10¹² cm^-2. Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. Our results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

Strain describes the deformation of a material as a result of applied stress. It has been widely employed to probe transport properties of materials, ranging from semiconductors to correlated materials. In order to understand, and eventually control, transport behavior under strain, it is important to quantify the effects of strain on the electronic band structure, carrier density and mobility. Here, we demonstrate that much information can be obtained by exploring a novel experimental observable: magneto-elastoresistance (MER), which refers to magnetic field-driven changes of the elastoresistance. We use this powerful approach to study the combined effect of strain and magnetic fields on the semi-metallic transition metal dichalcogenide WTe₂. We discover that WTe₂ shows a large and temperature non-monotonic elastoresistance, driven by uniaxial stress, that can be tuned by magnetic field. Using first-principle and analytical low-energy model calculations, we provide a semi-quantitative understanding of our experimental observations. We show that in WTe₂ the strain induced change of the carrier density dominates the observed elastoresistance. In addition, the change of the mobilities can be directly accessed using MER. Our analysis also reveals the importance of a heavy hole band near the Fermi level on the elastoresistance at intermediate temperatures.

The demand for disorder-tolerant quantum logic and spin electronics can be met by generating and controlling dissipativeless spin currents protected by topology. Dirac fermions with helical spin-locking surface transport offer a way of achieving such goal. Yet a challenge remains on how to selectively control surface helical spin transport and surface-bulk coupling. Here we use the mid-infrared and terahertz (THz) photoexcitation of intraband transitions and/or coherence optical phonons to enable ultrafast manipulation of surface THz conductivity in a topological insulator. We will also discuss how to extend the ultra-broadband, wavelength-selective pumping to emerging topological semimetals. Imposing electronic and vibrational coherence into topological matter may become a universal light control principle for reinforcing the protected quantum transport.

Control over the interactions between light and matter underlies many classical and quantum applications. In recent years, 2D layered semiconductors have gained prominence for optoelectronics because of their strong excitonic features and capacity for van der Waals assembly in layered heterostructures. Through integration with various photonics devices, the interactions between these materials and light can be tailored and new physical regimes can be achieved, highlighting the importance of understanding photonic dressed states of 2D materials. One of the unique features of monolayer materials such as transition metal dichalcogenides is the valley pseudospin addressable by circularly-polarized light. Understanding how this valley polarization can be manipulated is an exciting goal with implications for devices and quantum information science. Here, I discuss hybrid light-matter dressed states, or exciton-poIaritons, in transition metal dichalcogenides embedded in dielectric microcavities. These strongly-coupled polaritons preserve the valley pseudospin known from bare monolayers [1]. Distinct behavior of valley-polarized exciton-polaritons can be accessed with microcavity engineering by tuning system parameters such as cavity decay rate and exciton-photon coupling strength. In the opposite dressed state regime of weak light-matter coupling, intense off-resonant light can be used to shift energy levels though the Stark effect [2]. Applied to valley-sensitive states, this shift can be used for coherent manipulation of valley pseudospin. I will show how this approach can be translated to valley-selective exciton-polaritons. Cavity reflectance spectra exhibit a simultaneous shift of both polariton branches when ultrafast coincident pump and probe pulses are co-circularly-polarized, and no appreciable shift when they are cross-polarized. This valley-selective polariton Stark shift combines both dressed state regimes of strong, near resonant interactions with weak, off-resonant optical interactions, providing a new tool for state control in coherent valleytronics. Exploiting photonic dressed states can be viewed as another approach in the toolbox for manipulating the optoelectronic properties of 2D materials and their heterostructures.

This work was supported by DOE (DE-SC0012130) and ONR (N00014-16-1-3055).

[1] Y.-J. Chen, et al. Nature Photonics 11, 431 (2017).
[2] T. LaMountain, et al. Phys. Rev. B 97, 045307 (2018).

Understanding the interactions leading to magnetic quantum phenomena in a wide range of quantum materials is extremely important for development of new quantum materials and future technologies. Although neutron scattering is a powerful tool for this purpose, it also faces a range of challenges, such as the lack of sizable high-quality crystals, limited high pressure capabilities, and the difficulty in disentangling the intrinsic quantum phenomena versus effects from defects and site-disorder. Recently we have introduced the local magnetic susceptibility methods for studying quantum materials, which can be used for small crystals and powder samples. A single crystal neutron diffractometer DEMAND at the High Flux Isotope Reactor at the Oak Ridge National Laboratory has been upgraded for studying magnetic materials under various extreme sample environment conditions. In this talk, I will introduce the local site magnetic susceptibility methods, the current capabilities at the DEMAND, and our recent studies on two-dimensional quantum materials.

In metals, orbital motions of conduction electrons on the Fermi surface are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators. Here we report a notable exception in an insulator — ytterbium dodecaboride (YbB₁₂). The resistivity of YbB₁₂ exhibits distinct quantum oscillations despite having a much larger magnitude than in metals. This unconventional oscillation is shown to arise from the insulating bulk, even though the temperature dependence of the oscillation amplitude follows the conven- tional Fermi liquid theory of metals. The large effective masses indicate the presence of a Fermi surface consisting of strongly correlated electrons. Quantum oscillations are also observed in the magnetization of YbB₁₂. Our result reveals a mysterious dual nature of the ground state in YbB12: it is both a charge insulator and a strongly correlated metal.

The recent experimental realization of magnetic two-dimensional van der Waals (m2DvdW) materials has generated significant interest in exploiting such systems for novel 2D magnetism and for developing applications such as energy-efficient ultra-compact spin-based electronics. A better understanding of the magnetic interactions in these systems may help to accelerate the discovery of materials with higher magnetic ordering temperatures, moving these novel systems closer to practical applications. Using linear-response ab initio methods, we investigate the magnetic interactions and spin excitations in various m2DvdW systems, such as CrI₃, CrGeTe₃, VI₃, and Fe₃GeTe₂. Dynamical transverse spin susceptibility is calculated to directly compare with available Inelastic Neutron Scattering (INS) measurements. Accurate descriptions of electronic structure can provide reliable predictions of electronic and magnetic properties. Due to confinement, electrons in these 2D systems are likely to be strongly correlated, and electron screening anisotropic, so that their description may require advanced ab initio methods beyond density functional theory (DFT). Therefore, we also use the quasiparticle self-consistent GW (QSGW) method, a self-consistent many-body perturbation method, to describe the electronic structure and calculate magnetic properties. In contrast to DFT, the electron-electron interaction is evaluated explicitly by calculating the dynamically-screened Coulomb interaction W. We found that the nonlocal electron correlations, which are not included in methods such as DFT+U, have profound effects on the electronic and magnetic properties in these m2dvdW materials. A more elaborate description of electron interactions helps better describe the magnetic interactions in these 2D quantum systems. By comparing the electronic structures obtained in QSGW and DFT, we discuss the applicability and limitations of the widely-used DFT+U methods for these systems. We will also discuss the effects of spin-orbit coupling, the Dzyaloshinskii-Moriya interaction, impurities, pressure, and electric fields on the magnetic interactions and excitations in relevant systems.

High pressure is a powerful, reversible and “clean” way to drive materials away from equilibrium, exposing new phases, new states, new effects and new physics that would not exist in ambient condition. For layered materials where neighboring layers are held together by weak van der Waals forces, high pressure are particularly effective in creating these new effects. In this talk, we will discuss some of our findings in using high hydrostatic pressure to modulate the structure and electronic/optical properties of a variety of 2D materials, ranging from single or few layers of transition metal dichalcogenides, and their heterostructures, to new 2D materials created by intercalating non-2D materials.

Quantum materials offer not only a vast array of potential for technological advancement, but also provide a testbed for the laws of nature in condensed matter. One area to advance the frontier in this area is through the measurement of fast, spontaneous fluctuations of quantum order. This is at the heart of the essential physics in these types of solids, but has remained largely unexplored. The key component we use is the application of a newly developed coherent scattering method which can access these fluctuations on the relevant energy scales. This new tool has been demonstrated on a topological magnetic material, and early results point to the tremendous potential for this approach to provide important new insight into fundamental open questions in condensed matter. It allows for the direct, element-specific and momentum-resolved measurement of the fluctuations of a complex material and to connect them to the requisite response functions calculated from first principles. By analyzing subtle X-ray variations known as "speckle", statistics is used to extract the stochastic fluctuation information of the system. This offers the ability to address a range of important current problems in materials. In this talk, we will first discuss the newly developed tools which are used to measure the material properties, based on short pulses from an x-ray free electron laser. We will then present how this has been applied to skyrmions and the measurement of nanosecond fluctuations. This will be followed by an examination of new results on skyrmion phyiscs near the critical point of the skyrmion ordered lattice phase. We will end with an outlook of how this progress can help to tackle the question of fluctuations in unconventional superconductors.

This work at SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through the Materials Sciences and Engineering Division, contract DEAC02-76SF00515. J. J. Turner acknowledges support from the U.S. Department of Energy, Office of Science, Basic Energy Sciences through the Early Career Research Program.

The spin-orbit coupled Mott insulator Sr₂IrO₄ has attracted considerable interest because of its exotic J_eff = 1/2 Mott state arising from the interplay of on-site Coulomb repulsion and strong spin-orbit coupling. We have investigated magnetization dynamics of this enigmatic compound. Specifically, we measured coherent magnons of the J_eff = 1/2 Mott state using Kerr rotation following excitation with either mid-infrared 9 μm (below the charge gap), or near-infrared 1.3 μm (above the charge gap) circularly polarized pulses. For both pump wavelengths, the 2D in-plane B_2g coherent magnon oscillation of frequency ~0.5 THz is observed. In this talk, we show that the excitation pathways are different for these two excitation wavelengths. In particular, coherent magnon excitation with 9 μm pulses arises from the inverse Faraday effect that, microscopically, is related to two-magnon processes. Notably, coherent magnon generation with 9 μm pulses is nearly an order of magnitude more efficient in comparison to 1.3 μm pulses, without excitation of carriers across the Mott gap.


Acknowledgement: This research is supported by the Army Research Office MURI grant ARO W911NF-16-1-0361, “Floquet engineering and metastable states.”

The study of non-equilibrium properties of quantum phases of matter is important in the context of future quantum technologies, since all devices operate in the non-equilibrium limit. Theoretically, the transverse field Ising model (TFIM) and its disordered variants provide a tractable platform to study effects of quantum quenches (a rapid variation of a non-thermal parameter, leading to interesting transient effects), or quantum annealing (a slow variation of a non-thermal parameter, leading to efficient optimization). While there is enormous theoretical interest in these ideas, they have not yet been thoroughly tested on real Ising magnetic systems. There are well-known magnetic material realizations of the TFIM, CoNb2O6 and LiHoYF4, which display quantum critical points in accessible transverse magnetic field ranges. We have studied these materials following various protocols of time variation of the magnetic field, using ac susceptibility and neutron scattering as probes. I will discuss the non-equilibrium effects we have observed in these TFIM materials.

Ideal two-dimensional (2D) Heisenberg magnets lack long range order [1]. However, the XY model with spins confined to a plane shows a topological phase transition at a finite temperature corresponding to binding and unbinding of vortices [2,3]. Experimental evidence for such Berezinskii-Kosterlitz-Thouless (BKT) transitions has been difficult to obtain in condensed matter systems, where, even a weak interlayer coupling that is invariably present leads to long-range order, pre-empting the BKT transition. The BKT signatures are still discernible above the long-range ordering temperature, however, in the characteristic exponential temperature dependence of the coherence length of the fluctuations. In this work we report that an applied magnetic field can induce such BKT correlations not only in quasi 2-dimensional systems but also in nominally 3-dimensional manganites undergoing antiferromagnetic transitions. We arrive at this unexpected conclusion based on our studies of temperature dependence of electron spin resonance (ESR) linewidth ΔH(T) of Cr³⁺ doped bismuth strontium manganite Bi_0.5Sr_0.5Mn_1-xCr_xO₃ (x= 0.04, 0.1) (BSMCO).
BSMCO [4] belongs to the family of mixed valent manganites of the type ReAMnO₃ where Re is a trivalent rare earth ion or Bi³⁺ and A is a divalent alkaline earth ion, which are intensely studied in the last few years [5]. ΔH(T) provides the most important probe to study the spin interactions in these strongly correlated spin systems. We find that ΔH(T) observed in BSMCO (current work), as well as in La_.05Ca_.95MnO₃ and CaMnO₃ [6] is better (than the usually adopted critical state model) described by the BKT scenario which predicts [7]
ΔH_BKT(T) = ΔH_∞ exp [ 3b/(T/T_BKT — 1)^0.5 ] + mT + ΔH_ind , T > T_BKT
where T_BKT is the BKT transition temperature and b = π/2 for a square lattice and the last two terms are included to account for the high temperature and temperature independent behaviour. This is unexpected as these manganites have a 3D structure and the BKT model addresses systems with spin and spatial dimensions of two. We understand this result in terms of an effective symmetry reduction induced by i) the magnetic field applied in the ESR experiment and ii) the intrinsic anisotropy arising from microscopic details such as doping and phase separation - contributing to an effectively 2-dimensional XY easy plane anisotropy. Nanometric scale spin clusters similar to the ones observed in La doped CaMnO₃ [8] could conceivably play the role of vortices in these 3D materials. The sensitivity of the BKT behaviour to applied field is also supported by a re-analysis of the field dependence of the ΔH(T) in the quasi-2D antiferromagnetic compound BaNi₂V₂O₈ reported by Heinrich at al., [9]. For undoped manganites we find T_BKT is of the order of the magnetic interaction energy, suggesting that the applied field could be the sole origin of the BKT behaviour [10]. We shall also address the interesting observation that in BSMCO (x=0.04, 0.1), T_BKT is composition-independent while the magnetic properties are quite sensitive to composition. <span style="font-size:10.8333px">.</span>
SVB and AA thank the Indian National Science Academy, the National Academy of Sciences, India and the Indian Academy of Sciences for support.

References:
[1] N. D. Mermin and H. Wagner, Phys. Rev. Lett. 17, 1133 (1966)
[2] V. L. Berezinskii, Sov. Phys. JETP 32, 493 (1971)
[3] J. M. Kosterlitz and D. J. Thouless, Journal of Physics C: Solid State Physics 6, 1181 (1973)
[4] K. S. Bhagyashree, L. R. Goveas, and S. V. Bhat, Applied Magnetic Resonance 50,1049 (2019)
[5] Y. Tokura, Rep. Progr. Phys., 69, 797 (2006)
[6] E. Granado et al., Phys. Rev. Lett. 86, 5385 (2001)
[7] M. Hemmida et al., Phys. Rev. B95, 224101 (2017)
[8] E. Granado et al., Phys. Rev. B68, 134440 (2003)
[9] M. Heinrich et al., Phys. Rev. Lett. 91, 137601 (2003)
[10] A. Ashoka, K. S. Bhagyashree and S. V. Bhat, arXiv:1809.07635v3

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. This talk will describe our implementation of magnetic resonant inelastic X-ray scattering at free-electron lasers to directly determine the magnetic dynamics after photo-doping the Mott insulators Sr₂IrO₄ and Sr₃Ir₂O₇ [1-3]. We find that the non-equilibrium states host strong 2D magnetic correlations even in cases where 3D long-range magnetic order is completely suppressed [1]. The marked difference in these 2D and 3D timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics. We also uncover distinct differences in the magnetic dynamics between insulators Sr₂IrO₄ and Sr₃Ir₂O₇, which we attribute to a “spin-bottle-neck” effect [3].

Ultrafast optical spectroscopy has attained prominence due to its ability to resolve dynamics in conventional metals and semiconductors at the fundamental time scales of electron and lattice motion. In recent years, ultrafast optical techniques have become more sophisticated, making it possible to directly access fundamental material parameters in a non-contact manner. In this talk, I will discuss the use of ultrafast optical spectroscopy to track and potentially control carrier dynamics through both space and time in two-dimensional (2D) quantum materials. This will include unraveling charge transfer and diffusion across the interface between lateral transition metal dichalcogenide (TMD) monolayers as well as driving structural dynamics in a topological insulator with intense terahertz (THz) pulses, along with other studies of these fascinating nanosystems.

Controlling the interlayer twist angle in artificial two-dimensional (2D) van der Waals (vdW) heterostructures offers an experimental route to create moire superlattice. One can create exotic electronic states by minimizing electronic band width with tunable moire length scale. However, in the small twist angle regime, vdW interlayer interaction can cause significant structural reconfiguration at the interface, creating the arrays of domain structures. In this presentation, we will discuss the atomic reconstruction at twisted vdW interfaces and its effect on electronic structure and electrical transport behavior. Furthermore, we note that multiple domain boundaries at the reconstructed interface join together to create energetically unfavorable nodes forming vortex-like structures. We will discuss our recent efforts to understand the topological nature of those defects.

Non-equilibrium phases have developed as an important topic in the realm of layered materials, especially in the two-dimensional limit. Of particular interest is the case of magnetism in the single layer limit[1]. Using external inputs ranging from static to dynamic, the states in these systems can be strongly manipulated to drive transitions between different spin configurations[2]. However, the challenge is to follow the emergent or collapses of specific phases in systems that are at the two-dimensional limit. Here I will focus on using X-rays to explore the magnetic order parameter directly for the cases involving antiferromagnetic and ferromagnetic states. The use of polarized X-rays in either scattering or absorption experiments, has a long history of probing magnetic states. With synchrotron and free-electron laser sources, we can extend this to the ultrafast domain to follow the dynamic response of the magnetic configuration. Here I will present examples involving dynamic collapse of antiferromagnetic order in nickelates to the optical conversion of antiferromagnetic to ferromagnetic order in manganites[3]. I will use these ultrathin oxide films as examples of exploration of magnetism in the two-dimensional limit using X-rays.

The work at Argonne is supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. The development of the materials and ultrafast experiments is supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC-0012375.

References:
[1] M. Gibertini, M. Koperski, A. F. Morpurgo, and K. S. Novoselov, Nature Nanotechnology 14, 408 (2019).
[2] H. Li, S. Ruan, and Y. J. Zeng, Adv Mater 31, 1900065 (2019).
[3] J. Zhang, X. Tan, M. Liu, S. W. Teitelbaum, K. W. Post, F. Jin, K. A. Nelson, D. N. Basov, W. Wu, and R. D. Averitt, Nat Mater 15, 956 (2016).

The last decade has seen an exponential growth in the science and technology of two-dimensional materials. Beyond graphene, there is a huge variety of layered materials that range in properties from insulating to superconducting that can be grown over large scales for a variety of electronic devices and quantum technologies, such as topological quantum computing, quantum sensing, and neuromorphic computing. In this talk, I will discuss the synthesis of a range of 2D layers over large areas for sensing and electronic devices, as well as recent breakthroughs in novel 2D heterostructures and realization of unique 2D allotropes of 3D materials. I will introduce a novel synthesis method, dubbed confinement heteroepitaxy (CHet), that enables the creation of atomically thin metals, enabling a new platform for creating artificial quantum lattices (AQLs) with atomically sharp interfaces and designed properties.

Owing to their large oscillator strength and strong exciton binding energy, two-dimensional (2D) transition metal dichalcogenides (TMDs) have emerged as an attractive material platform for studying strong light-matter coupling and associated phenomena. Following up on our previous work on strong light-matter coupling in 2D TMDs [1, 2], here we will present results on enhancing the nonlinear interaction between the quasiparticles (exciton-polaritons). This is achieved using excited states of excitons (Rydberg states) which have larger Bohr radii. We will also present our recent results on controlling the valley pseudospin via the pseudomagnetic fields in optical cavities and our work on realizing an electrically pumped polariton LED [3]. Finally, we will first present our work on realizing single photon emitters (SPEs) in hexagonal boron nitride (hBN), a van der Waals material, via strain engineering [4] and coupling of these SPEs to high Q silicon nitride microresonators [5].
[1] X. Liu, et al., Nature Photonics 9, 30 (2015)
[2] Z. Sun et al., Nature Photonics 11, 491 (2017)
[3] J. Gu et al. Nature Nanotech. (2019) DOI: 10.1038/s41565-019-0543-6
[4] N. Proscia et al. Optica 5, 1128 (2018)
[5] N. Proscia et al. ArXiv 1906.06546