The ${A\mathrm{V}}_3{\mathrm{Sb}}_5$ $(A=\mathrm{K}, \mathrm{Rb}, \mathrm{Cs})$ kagome materials host an interplay of emergent phenomena including superconductivity, charge density wave states, and nontrivial electronic structure topology. The band structures of these materials exhibit a rich variety of features like Dirac crossings, saddle points associated with van Hove singularities, and flat bands prompting significant investigations into the in-plane electronic behavior. However, recent findings including the charge density wave ordering and effects due to pressure or chemical doping point to the importance of understanding interactions between kagome layers. Probing this $c$-axis electronic structure via experimental methods remains challenging due to limitations of the crystals and, therefore, rigorous computational approaches are necessary to study the interlayer interactions. Here we use first-principles approaches to study the electronic structure of ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ with emphasis on the $k_z$ dispersion. We find that the inclusion of nonlocal and dynamical many-body correlation has a substantial impact on the interlayer band structure. We present band behavior that additionally supports the integration of symmetry in accurately plotting electronic structures and influences further analysis like the calculation of topological invariants.

Strong disorder has a crucial effect on the electronic structure in quantum materials by increasing localization, interactions, and modifying the density of states. ${\mathrm{Bi}}_x\mathrm{TeI}$ films grown at room temperature and $230\mathrm{K}$ exhibit dramatic magnetotransport effects due to disorder, localization, and electron correlation effects, including a metal-insulator transition at a composition that depends on growth temperature. The increased disorder caused by growth at 230 K causes the conductivity to decrease by several orders of magnitude for several compositions of ${\mathrm{Bi}}_x\mathrm{TeI}$. The transition from metal to insulator with decreasing composition $x$ is accompanied by a decrease in the dephasing length, which leads to the disappearance of the weak-antilocalization effect. Electron-electron interactions cause low temperature conductivity corrections on the metallic side and Efros-Shklovskii variable range hopping on the insulating side, effects which are absent in single crystalline ${\mathrm{Bi}}_x\mathrm{TeI}$. The observation of a tunable metal-insulator transition and the associated strong localization and quantum effects in ${\mathrm{Bi}}_x\mathrm{TeI}$ shows the possibility of tuning spin transport in quantum materials via disorder.

Ge/SiGe heterostructure quantum well structures based on Ge two-dimensional hole gas have become one of the most promising research directions for preparing spin quantum bits due to their low disorder and high mobility. In this study, high-quality virtual substrates were epitaxially grown on 8-in silicon substrates using reduced pressure chemical vapor deposition. The surface roughness of the samples was optimized by adjusting key parameters such as the thickness of the reverse gradient buffer layer and ${\mathrm{Si}}_{1-x}{\mathrm{Ge}}_x$ buffer layer. Based on the strain modulation of dislocation dynamics, high-quality strained quantum wells with a density of stress accumulation points (DSAP) of $0.301/\mu {\mathrm{m}}^2$ and surface RMS roughness less than 3 nm were achieved. Ultimately, an ultrashallow heavily constrained and undoped quantum well with a well depth of 15 nm, in-plane compressive strain of ${\varepsilon }_{\parallel }=-1.19\%$, and a mobility of $3.382\times {10}^5{\mathrm{cm}}^2/\mathrm{Vs}$ was obtained. A novel characterization method for quantum wells was proposed based on the defined DSAP. The ultrashallow quantum well depth and higher compressive strain enable the quantum well to maintain a high effective $g$ factor (up to 8.3), becoming the cradle for fully electrically controlled and microwave coupled quantum bits.

Despite the fact that first-principles methods are critical tools in the study and design of materials today, the accuracy of density functional theory (DFT) prediction is fundamentally reliant on the exchange-correlation functional chosen to approximate the interactions between electrons. Although the general improvement in accurately calculating the bandgap with the Heyd-Scuseria-Ernzerhof (HSE) hybrid-functional method over the conventional semilocal DFT is well accepted, other properties such as formation energy have not been systematically studied and have yet to be evaluated thoroughly for different classes of materials. A high-throughput hybrid-functional DFT investigation on materials bandgaps and formation energies is therefore performed in this work. By evaluating over a thousand materials, including metals, semiconductors, and insulators, we have quantitatively verified that the materials bandgaps obtained through HSE [mean absolute error (MAE) = 0.687 eV] are more accurate than those from the Perdew-Burke-Ernzerhof (PBE) functional (MAE = 1.184 eV) when compared to the experimental values. For formation energies, PBE systematically underestimates the magnitude of the formation enthalpies (MAE = 0.175 eV/atom), whereas formation enthalpies obtained from the HSE method are generally more accurate (MAE = 0.147 eV/atom). We have also found that HSE significantly increases the accuracy of formation energy prediction for insulators and strongly bound compounds. A primary application of this new dataset is achieved by building a cokriging multifidelity machine learning (ML) model to quickly predict the bandgaps with HSE-level accuracy when its PBE bandgap is available from DFT calculations. The preliminary goal of our ML model, benchmarked in this work, is to select the semiconductors and insulators which may have been mislabeled as metals from the DFT-PBE calculations in the existing Open Quantum Materials Database. The performance of the cokriging model in reliably predicting HSE bandgaps with quantified model uncertainty is analyzed by comparing the results against published experimental data from the literature.

In a comparative and systematic study on finite-size particles the isostructural van der Waals materials ScOCl, ScOBr, and FeOCl, are investigated. The origin of unconventional magnetic properties of FeOCl ($S$ = 5/2) is elucidated. Experimental evidence for fluctuations are presented and discussed in relation to soliton condensation into topological edge states. The anisotropy within ${}_{\infty }^2\left[{\mathrm{FeO}}_{4/4}{\mathrm{Cl}}_{2/2}\right]$ corrugated layers gives rise to the formation of solitons (1D) with parameters given here and to the development of 2D-Ising magnetic order around 20 K. The latter is identified by a $\lambda$ anomaly in the specific heat and a kink in the magnetic susceptibility. Concomitantly, the canting angle of moments along the spin-chain direction changes. We attribute this to a magnetoelastic effect. The finite size of the chains ($\approx 143$ Fe-sites $\approx {\xi }_s$ at 20 K) results in the confinement of solitons, which condense into breathers. In small applied magnetic fields the dynamics unfold and a spin-glass-like phase below 5 K is established. Phonon anharmonicities revealed by Raman scattering are explained by diffusional dynamics and are in agreement with the derived soliton energy from Mössbauer spectroscopy.

$\alpha$-Sn thin films can exhibit a variety of topologically nontrivial phases. Both studying the transitions between these phases and making use of these phases in eventual applications requires good control over the electronic and structural quality of $\alpha$-Sn thin films. $\alpha$-Sn growth on InSb often results in out-diffusion of indium, a $p$-type dopant. By growing $\alpha -\mathrm{Sn}$ via molecular beam epitaxy on the Sb-rich $\mathrm{c}(4\times 4)$ surface reconstruction of InSb(001) rather than the In-rich $\mathrm{c}(8\times 2)$, we demonstrate a route to substantially decrease and minimize this indium incorporation. The reduction in indium concentration allows for the study of the surface and bulk Dirac nodes in $\alpha$-Sn via angle-resolved photoelectron spectroscopy without the common approaches of bulk doping or surface dosing, simplifying topological phase identification. The lack of indium incorporation is verified in angle-resolved and angle-integrated ultraviolet photoelectron spectroscopy as well as in clear changes in the Hall response.

Raman spectroscopy is a powerful and nondestructive method that is widely used to study the vibrational properties of solids or molecules. Simulations of finite-temperature Raman spectra rely on obtaining polarizabilities along molecular-dynamics trajectories, which is computationally highly demanding if calculated from first principles. Machine learning force fields (MLFF) are becoming widely used for accelerating molecular-dynamics simulations, but machine-learning models for polarizability are still rare. In this work, we present and compare three polarizability models for obtaining Raman spectra in conjunction with MLFF molecular-dynamics trajectories: (i) a model based on projection to primitive cell eigenmodes, (ii) a bond polarizability model, and (iii) symmetry-adapted Gaussian process regression (SA-GPR) using a smooth overlap of atomic positions. In particular, we investigate the accuracy of these models for different systems and how much training data are required. Models are first applied to boron arsenide, where the first- and second-order Raman spectra are studied as well as the effect of boron isotopes. With ${\mathrm{MoS}}_2$ we study the applicability of the models for highly anisotropic systems and for simulating resonant Raman spectra. Finally, inorganic halide perovskites are studied with a particular interest in simulating the spectra across phase transitions. All models can be used to efficiently predict polarizabilities and are applicable to large systems and long simulation times, and while all three models were found to perform similarly for BAs and ${\mathrm{MoS}}_2$, only SA-GPR offers sufficient flexibility to accurately describe complex anharmonic materials like the perovskites.

Engineering heterostructures with various types of quantum materials can provide an intriguing playground for studying exotic physics induced by the proximity effect. Here, we report on the successful synthesis of iron-based superconductor $\mathrm{FeS}{\mathrm{e}}_x\mathrm{T}{\mathrm{e}}_{1-x}$ (FST) thin films across the entire composition range of $0\le x\le 1$ and its heterostructure with a magnetic topological insulator (TI) by using molecular beam epitaxy. Superconductivity is observed in the FST films with an optimal superconducting transition temperature $T_c\sim 12$ K at around $x=0.1$. We found that superconductivity survives in the very Te-rich films ($x\le 0.05$), showing stark contrast with bulk crystals with suppression of superconductivity due to an appearance of bicollinear antiferromagnetism accompanied by a monoclinic structural transition. By examining thickness $t$ dependence of magnetic susceptibility and electrical transport properties, we observed a trend where anomalies associated with the first-order structural transition broaden in films with below $t\sim 100$ nm. We infer this observation suggests a suppression of the structural instability near substrates. Furthermore, we fabricated an all chalcogenide-based heterointerface between FST and a magnetic TI ${\left(\mathrm{Cr},\mathrm{Bi},\mathrm{Sb}\right)}_2\mathrm{T}{\mathrm{e}}_3$, observing both superconductivity and a large anomalous Hall conductivity. The anomalous Hall conductivity increases with decreasing temperature, approaching the quantized value of $e^2/h$ down to the measurable minimum temperature at $T_c$. The result suggests coexistence of magnetic and superconducting gaps at low temperatures opening at the top and bottom surfaces, respectively. Our magnetic TI/superconductor heterostructure could be an ideal platform to explore chiral Majorana edge mode.

We examined the piezomagnetic effect in an antiferromagnet MnTe, which is a candidate material for altermagnetism with a high critical temperature. We observed that the magnetization develops with the application of stress and revealed that the piezomagnetic coefficient $Q$ is $1.38\times {10}^{-8}$ ${\mu }_{\mathrm{B}}$/Mn/MPa at 300 K. The poling-field dependence of magnetization indicates that the antiferromagnetic domain can be controlled using the piezomagnetic effect. We demonstrate that piezomagnetic effect is one of the suitable methods for the detection of broken time-reversal symmetry and the domain control in antiferromagnets with broken time-reversal symmetry, including altermagnets.

The structure of $\mathrm{LuF}{\mathrm{e}}_2{\mathrm{O}}_4$ below the charge ordering temperature of $T_{\mathrm{CO}}=320$ K has been studied immensely due to the question of whether charge ordering on the mixed valence Fe sublattice leads to a macroscopic electric polarization. In contrast, the local structure associated with polaron fluctuations above $T_{\mathrm{CO}}$, from which the charge ordered structure emerges, is largely unknown. In this work, we characterize this local structure using the x-ray three-dimensional difference pair distribution function ($3\mathrm{D}\text{−}\mathrm{\Delta }\mathrm{PDF}$). Atomic correlations extracted directly from the $3\mathrm{D}\text{−}\mathrm{\Delta }\mathrm{PDF}$ are used to propose models for the local structure, which are simulated using Monte Carlo methods. It is found that the primary Fourier components of the diffuse scattering can be described by the large, correlated Lu displacements. The local structure of Lu displacements draws conceptual parallels to the classical models of triangular Ising antiferromagnets with the added complexity of interplane interactions. The weaker features of the $3\mathrm{D}\text{−}\mathrm{\Delta }\mathrm{PDF}$ are modeled by displacement correlations involving Fe and O, the latter resulting in an increased coordination number of Lu compared with the average structure. Analysis of the Fe-coordination environments in the atomic configurations obtained from Monte Carlo simulations reveals a strong correlation between distortion of the coordination environment and the +II oxidation state of Fe, which suggests a Jahn-Teller polaronic nature of the charge carriers.

We combine (scanning) transmission electron microscopy, electron energy loss spectroscopy (EELS), and x-ray photoelectron spectroscopy (XPS) with density functional theory (DFT) to investigate the complex interplay of surface energetics and polar discontinuity compensation in the interface formation of (001) ${\mathrm{LaInO}}_3/{\mathrm{BaSnO}}_3$-based heterostructures. We present evidence from both experiment and theory that the $\mathrm{BaSn}{\mathrm{O}}_3$ surface with BaO termination is energetically favored over a wide range of chemical potentials. However, overgrowth of the nonpolar BaO-terminated surface with $\mathrm{LaIn}{\mathrm{O}}_3$ results in a $\mathrm{Sn}{\mathrm{O}}_2$-terminated interface. EELS and XPS show that this interfacial termination exchange is mediated by Ba segregation to the growth surface. Our DFT calculations show that the efficient compensation of the polar discontinuity, attributed to polar distortions within $\mathrm{BaSn}{\mathrm{O}}_3$ at the $\mathrm{Sn}{\mathrm{O}}_2$-terminated interface, serves as the driving force behind the observed Ba segregation. This intricate interplay underscores the importance of polar discontinuity compensation as a pivotal factor influencing interface formation in perovskite systems.

The fracture of polymer networks and gels has a significant impact on the performance of these versatile and widely used materials, and a molecular-level understanding of the fracture process is crucial for the design of new materials. Combining molecular dynamics simulations and network analysis techniques, we demonstrate that in the initial undeformed state of model end-linked polymer networks, polymer strands with fewer topological defects in their local surroundings, higher geodesic edge betweenness centrality values compared to the system average, and greater alignment to the loading direction are more prone to breaking under uniaxial tensile deformation.

The quasi-one-dimensional chiral compound ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ has been extensively studied as a prime example of a topological Weyl semimetal. Upon crossing its phase transition temperature $T_{\mathrm{CDW}}\approx 263\mathrm{K}, {\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ exhibits incommensurate charge density wave (CDW) modulations described by the well-defined propagation vector $\sim (0.05,0.05,0.11)$, oblique to the ${\mathrm{TaSe}}_4$ chains. Although optical and transport properties greatly depend on chirality, there is no systematic report about chiral domain size for ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$. In this study, our single-crystal scattering refinements reveal a bulk iodine deficiency, and Flack parameter measurements on multiple crystals demonstrate that separate ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ crystals have uniform handedness, supported by direct imaging and helicity-dependent terahertz emission spectroscopy. Our single-crystal x-ray scattering and calculated diffraction patterns identify multiple diffuse features and create a real-space picture of the temperature-dependent ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ crystal structure. The short-range diffuse features are present at room temperature and decrease in intensity as the CDW modulation develops. These transverse displacements, along with electron pinning from the iodine deficiency, help explain why ${\left({\mathrm{TaSe}}_4\right)}_2\mathrm{I}$ behaves as an electronic semiconductor at temperatures above and below $T_{\mathrm{CDW}}$, despite a metallic band structure calculated from density functional theory of the ideal structure.

Quantum technologies based on quantum point defects in crystals require control over the defect charge state. Here we tune the charge state of shallow nitrogen-vacancy and silicon-vacancy centers by locally oxidizing a hydrogenated surface with moderate optical excitation and simultaneous spectral monitoring. The loss of conductivity and change in work function due to oxidation are measured in atmosphere using conductive atomic force microscopy and Kelvin probe force microscopy (KPFM). We correlate these scanning probe measurements with optical spectroscopy of the nitrogen-vacancy and silicon-vacancy centers created via implantation 15–25 nm beneath the diamond surface and annealing. The observed charge state of the defects as a function of optical exposure demonstrates that laser oxidation provides a way to precisely tune the Fermi level over a range of at least 2.00 eV. We also observe a significantly larger oxidation rate for implanted surfaces compared to unimplanted surfaces under ambient conditions. Combined with knowledge of the electron affinity of a surface, these results suggest KPFM is a powerful, high-spatial-resolution technique to advance surface Fermi level engineering for charge stabilization of quantum defects.

We investigated changes in the structural and electronic properties of single-crystalline $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ films across temperature-induced phase transitions. Based on resistivity and synchrotron x-ray diffraction measurements, we determined the phase diagram of $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$. The characteristic properties of $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ were classified into three temperature regions: the insulating λ phase at $T$ ≤ 350 K, the conductive λ phase at 350 K ≤ $T$ ≤ 460 K via the metal-insulator transition, and the metallic α phase at $T$ ≥ 460 K. Detailed crystallographic analysis revealed that the monoclinic λ phase exhibited a second-order structural phase transition to the orthorhombic α phase, accompanied by a gradual deformation of the monoclinic angle. These results suggest that unusual temperature-induced changes in $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ originate from similar atomic conformations of the λ and α phases. Our findings provide guidelines for controlling multiple phase transitions in $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$ polymorphs using external stimuli, which is crucial for optoelectronic and heat-storage applications of $\mathrm{T}{\mathrm{i}}_3{\mathrm{O}}_5$.

We describe a first-principles statistical mechanics method to calculate the free energies of crystalline alloys that depend on temperature, composition, and strain. The approach relies on an extension of the alloy cluster expansion to include an explicit dependence on homogeneous strain in addition to site occupation variables that track the degree of chemical ordering. The method is applied to the Si-Ge binary alloy and is used to calculate free energies that describe phase stability under arbitrary epitaxial constraints. We find that while the incoherent phase diagram (in which coexisting phases are not affected by coherency constraints) hosts a miscibility gap, coherent phase equilibrium predicts ordering and negative enthalpies of mixing. Instead of chemical instability, the chemomechanical free energy exhibits instabilities along directions that couple the composition of the alloy with a volumetric strain order parameter. This has fundamental implications for phase field models of spinodal decomposition as it indicates the importance of gradient energy coefficients that couple gradients in composition with gradients in strain.

We report a set of scanning tunneling microscopy (STM) and spectroscopy (STS) experiments studying native defects in CVT grown $1T\text{−}{\mathrm{TaS}}_2$. Six different sample surfaces from four bulk crystals were investigated. Wide area imaging reveals a prevalence of nanometer-scale electronic inhomogeneities due to native defects, with pristine regions interspersed. These inhomogeneities appear in typical as-grown crystals and coexist with a well-formed commensurate charge density wave of $1T\text{−}{\mathrm{TaS}}_2$ at low temperatures. Electronic inhomogeneities show up both as variations in the apparent height in STM and in the local density of states in STS; the bands can shift by 60 meV and the gap varies by more than 100 meV across inhomogeneities. These inhomogeneities are present in similar concentration across large-scale areas of all samples studied, but do not influence the charge density wave formation on local or global scales. The commensurate charge density wave exhibits long-range order and remains locally intact in the presence of these inhomogeneities.

Solid phase epitaxy is a crystallization technique used to produce high-quality thin films. Lateral solid phase epitaxy furthermore enables the realization of nonplanar structures, which are interesting, e.g., in the field of spintronics. Here, we demonstrate lateral solid phase epitaxy of yttrium iron garnet over an artificial edge, such that the crystallization direction is perpendicular to the initial seed. We use single-crystalline garnet seed substrates partially covered by a ${\mathrm{SiO}}_x$ film to study the lateral crystallization over the ${\mathrm{SiO}}_x$ mesa. The yttrium iron garnet layer retains the crystal orientation of the substrate not only when in direct contact with the substrate but also across the edge on top of the ${\mathrm{SiO}}_x$ mesa. By controlling the crystallization dynamics it is possible to almost completely suppress the formation of polycrystals and to enable epitaxial growth of single-crystalline yttrium iron garnet on top of mesas made from ceramic materials. From a series of annealing experiments, we extract an activation energy of $3.0\mathrm{eV}$ and a velocity prefactor of $6.5\times {10}^{14}\mathrm{nm}/\mathrm{s}$ for the lateral epitaxial crystallization along the $\langle 100\rangle$ direction. Our results pave the way to engineer single-crystalline nonplanar yttrium iron garnet structures with controlled crystal orientation.

The nitrogen-vacancy (NV) center in diamond, a well-studied, optically active spin defect, is the prototypical system in many state-of-the-art quantum sensing and communication applications. In addition to the enticing properties intrinsic to the NV center, its diamond host's nuclear and electronic spin baths can be leveraged as resources for quantum information rather than considered solely as sources of decoherence. However, current synthesis approaches result in stochastic defect spin positions, reducing the technology's potential for deterministic control and yield of NV spin bath systems, as well as scalability and integration with other technologies. Here, we demonstrate the use of theoretical calculations of electronic central spin decoherence as an integral part of an NV spin bath synthesis workflow, providing a path forward for the quantitative design of NV center-based quantum sensing systems. We use computationally generated coherence data to characterize the properties of single NV center qubits across relevant growth parameters to find general trends in coherence time distributions dependent on spin bath dimensionality and density. We then build a maximum likelihood estimator with our theoretical model, enabling the characterization of a test sample through NV $T_2^{*}$ measurements. Finally, we explore the impact of dimensionality on the yield of strongly coupled electron spin systems. The methods presented herein are general and applicable to other qubit platforms that can be appropriately simulated.

Rydberg states of excitons can reach microns in size and require extremely pure crystals. We introduce an experimental method for the rapid and spatially resolved characterization of Rydberg excitons in copper oxide (${\mathrm{Cu}}_2\mathrm{O}$) with submicron resolution over large zones. Our approach involves illuminating and imaging the entire sample on a camera to realize a spatially resolved version of resonant absorption spectroscopy, without any mobile part. This yields spatial maps of Rydberg exciton properties, including their energy, linewidth, and peak absorption, providing a comprehensive quality assessment of the entire sample in a single shot. Furthermore, by imaging the sample photoluminescence over the same zone, we establish a strong relationship between the spectral quality map and the photoluminescence map of charged oxygen vacancies. This results in an independent, luminescence-based quality map that closely matches the results obtained through resonant spectroscopy. Our findings reveal that Rydberg excitons in natural ${\mathrm{Cu}}_2\mathrm{O}$ crystals are predominantly influenced by optically active charged oxygen vacancies, which can be easily mapped. Together, these two complementary methods provide valuable insights into ${\mathrm{Cu}}_2\mathrm{O}$ crystal properties.