We report ${}^{125}\mathrm{Te}$ NMR measurements of the topological quantum material ${\mathrm{ZrTe}}_5$. Spin-lattice relaxation results, well explained by a theoretical model of Dirac electron systems, reveal that the topological characteristic of ${\mathrm{ZrTe}}_5$ is $T$ dependent, changing from weak topological insulator to strong topological insulator as temperature increases. Electronic structure calculations confirm this ordering, the reverse of what has been proposed. NMR results demonstrate a gapless Dirac semimetal state occurring at a Lifshitz transition temperature, $T_c=85$ K, in our crystals. We demonstrate that the changes in NMR shift at $T_c$ also provide direct evidence of band inversion when the topological phase transition occurs.

We study the transport properties of an electron-hole double layer consisting of barrier-separated InAs/InGaSb quantum wells. We focus on measurements of four-terminal resistivity of a Hall-bar sample as a function of electron $\left(n\right)$ and hole $\left(p\right)$ density, that are being tuned by a pair of top and bottom gates. In zero magnetic field, we clearly observe an insulating phase which occurs at a charge neutral point, below a critical carrier density $n=p<1\times {10}^{11}\mathrm{c}{\mathrm{m}}^{-2}$. This phase is characterized by a narrow and thermally activated resistance peak and an anomalous Hall resistance. This observation reinforces our previous finding of an excitonic insulator. Remarkably, when the layer densities are being tuned into imbalance, here $p\gg n$, a broader resistance peak emerges. We discuss this phase with respect to a possible (theoretically predicted) charge density wave ground state. Both phases can persist above ∼25 K, indicating robust correlations in the electron-hole double layers.

Spectroscopic techniques are vital to determine the energy distribution of trapped states in semiconducting materials to assess the quality and efficiency of electronic devices. However, there is a need for a sensitive spectroscopic technique that can be used with atomically thin materials which are thinner than the Debye screening length as the current methods, such as deep level trap spectroscopy and admittance spectroscopy, are incompatible with the long emission times and temperature sensitivities of these materials. In this work we expand the threshold voltage transient phenomenon into an energy dispersive spectroscopic technique, dubbed the threshold voltage transient spectroscopy technique. This is applied to few-layer ${\mathrm{MoTe}}_2$ with the trap concentration and subsequently the density of trap states found in a region between the valence band edge and the midgap, which clearly shows the density of states in the tail end of the valence band.

Controlled extraction of spin-polarized currents from the surface of topological insulators (TIs) would be an important step to use TIs as spin-electronic device materials. One way is to utilize the surface photovoltage (SPV) effect, by which the surface current may flow upon irradiation of light. To date, unipolar SPV has been observed on TIs, while the realization of ambipolar SPV is crucial for taking control over the direction of the flow. By using time-resolved photoemission, we demonstrate the ambipolar SPV realized on the TI ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. The topological surface states showed downward and upward photovoltaic shifts for the $n$- and $p$-type samples, respectively. We also discerned the photogenerated carriers accumulated in the surface states for $>4\mu \mathrm{s}$. We provide the keys besides the in-gap Fermi level to engineer the SPV on TIs.

In this paper, we prepare heavily electron doped $\mathrm{L}{\mathrm{a}}_{0.04}\mathrm{B}{\mathrm{a}}_{0.96}\mathrm{Sn}{\mathrm{O}}_3$ (LBSO) thin films on a $\mathrm{SrTi}{\mathrm{O}}_3$ (STO) substrate by the pulsed laser deposition method under different oxygen pressures, in which different levels of oxygen vacancy concentrations are present. Normally, oxygen vacancies are implicitly assumed as isolated point defects that dope the materials with electron carriers, whereas oxygen-deficient LBSO films exhibit an apparent metal-to-insulator transition combined with lower carrier concentrations and mobility with increased oxygen vacancies, which are concluded from the transport measurement data. Considering the almost constant density of threading dislocations and the higher formation energies for other intrinsic crystal defects, the observed transport trends are ascribed to the extra oxygen vacancies introduced. Therefore, strong electron localization originating from the interaction between the oxygen vacancy and La impurity is proposed for interpreting the behavior in oxygen-deficient LBSO films. Oxygen-deficient crystal structure models for LBSO have been optimized and the electronic structures are revealed by first-principles calculations based on the density functional theory. The partial density of states results indicate that strong electron localization comes from the deep strongly localized states in the forbidden gap, which are mainly composed of hybridized orbitals of $\mathrm{O}2p$ with $\mathrm{Sn}5s5p$ and some amount of $\mathrm{Sn}4d$. Thus, the control of the oxygen defects and the related electronic states in barium stannate is essential for achieving optimal electrical performance and as potential applications in perovskite heterostructure devices.

A modified periodic boundary condition adequate for non-Hermitian topological systems is proposed. Under this boundary condition, a topological number characterizing the system is defined in the same way as in the corresponding Hermitian system, and hence, at the cost of introducing an additional parameter that characterizes the non-Hermitian skin effect, the idea of bulk-edge correspondence in the Hermitian limit can be applied almost as it is. We develop this framework through the analysis of a non-Hermitian Su-Schrieffer-Heeger model with chiral symmetry and prove the bulk-edge correspondence in a generalized parameter space. A finite region in this parameter space with a nontrivial pair of chiral winding numbers is identified as topologically nontrivial, indicating the existence of a topologically protected edge state under an open boundary.

We study the pseudospin of an electron pair in superconducting transition-metal dichalcogenide monolayers. We show that the pseudospin affects the electric transport property of the lateral heterojunction of the superconducting and metallic monolayers by using both analytic and numerical methods. The pseudospins of two electrons forming a Cooper pair are parallel to each other, unlike the real spin, which is antiparallel, and the direction of parallel pseudospins varies with the Fermi energy. The pseudospin is aligned in the same direction for the Fermi energy close to the valence band top, but it has a chiral configuration in the wave number space for the Fermi energy far from the band edge. Moreover, we investigate the electronic transport with the formation of a Cooper pair, the Andreev reflection, in a lateral heterojunction of semiconducting and superconducting transition-metal dichalcogenides, ${\mathrm{MoSe}}_2$ and ${\mathrm{NbSe}}_2$ monolayers with charge doping, respectively. The conductance decreases with the superconducting transition under the electron-doped and slightly hole doped junctions for the slight source-drain bias. However, in the highly hole doped junction, the junction shows an increase of conductance with the transition. We reveal that the qualitative change in the electronic transport property is attributed to the variation of the pseudospin configuration in the Fermi surface with the charge density.

Among numerous multiferroic phenomena observed in spin frustrated lattice, giant magnetoelectricity in honeycomb lattice ${(\mathrm{Fe},\mathrm{Mn})}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ has stimulated great interest and substantial effort in searching for novel members in this 238 family. In this work, we synthesize successfully compound ${\mathrm{Co}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, a structural analogue of ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$, and present a series of characterizations on its structural, magnetic, and electric properties. An antiferromagnetic transition takes place at the Neel temperature $T_N=39\mathrm{K}$ with appearance of electric polarization and dielectric anomaly, which provides clear evidence of simultaneous magnetic and ferroelectric transitions. The neutron powder diffraction (NPD) and magnetic susceptibility data confirm the $c$-axis collinear antiferromagnetic orders and emergent ferroelectric polarization. In particular, such antiferromagnetic order is relatively robust against magnetic field up to 9 T, different from ${\mathrm{Fe}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ with ferrimagnetic transition or ${\mathrm{Mn}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ with spin flop in the low-field region. Our data on single crystals demonstrate the second-order magnetoelectric effect in terms of magnetic field dependence of ferroelectric polarization response, while no linear magnetoelectric response is allowed. It is suggested that ${\mathrm{Co}}_2{\mathrm{Mo}}_3{\mathrm{O}}_8$ provides a unique platform on which rich multiferroic physics in the presence of collinear magnetic order can be explored.

We investigated experimentally and computationally the concentration dependence of the electronic specific heat coefficient $\gamma$ in the $\mathrm{Y}{\left({\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x\right)}_2$ pseudobinary Laves phase system. The experimentally observed maximum in $\gamma \left(x\right)$ around the magnetic phase transition was interpreted within the local density approximation combined with the virtual crystal approximation. To explain the formation of the observed maximum, we analyzed theoretically the dependence of the magnetic energy, magnetic moments, densities of states, and Fermi surfaces on the Co concentration. Furthermore, we carried out the calculations of the density of states (DOS) at the Fermi level as a function of fixed spin moment. The calculated Co concentration at which $\gamma$ takes the maximum value ($x_{\max -\mathrm{LDA}-\mathrm{VCA}}=0.91$) stays in good agreement with the measured value ($x_{\max -\mathrm{expt}}=0.925$). We conclude that the observed maximum in $\gamma \left(x\right)$ results from the presence of the sharp DOS peak in the vicinity of the Fermi level.

We study the magnetization dynamics of a bulk single crystal ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ (CGT), by means of broadband ferromagnetic resonance (FMR), for temperatures from 60 K down to 2 K. We determine the Kittel relations of the fundamental FMR mode as a function of frequency and static magnetic field for the magnetocrystalline easy—and hard—axis. The uniaxial magnetocrystalline anisotropy constant is extracted and compared with the saturation magnetization, when normalized with their low temperature values. The ratios show a clear temperature dependence when plotted in the logarithmic scale, which departs from the predicted Callen-Callen power law fit of a straight line, where the scaling exponent $n$ , $K_u\left(T\right)\propto {[M_s\left(T\right)/M_s\left(2\mathrm{K}\right)]}^n$, contradicts the expected value of 3 for uniaxial anisotropy. Additionally, the spectroscopic $g$ factor for both the magnetic easy—and hard—axis exhibits a temperature dependence, with an inversion between 20 K and 30 K, suggesting an influence by orbital angular momentum. Finally, we qualitatively discuss the observation of multidomain resonance phenomena in the FMR spectras, at magnetic fields below the saturation magnetization.

Building on the growing evidence based on NMR, magnetization, neutron scattering, electron spin resonance, and specific heat that, under pressure, ${\mathrm{SrCu}}_2{\left({\mathrm{BO}}_3\right)}_2$ has an intermediate phase between the dimer and the Néel phase, we study the competition between two candidate phases in the context of a minimal model that includes two types of intra- and interdimer interactions without enlarging the unit cell. We show that the empty plaquette phase of the Shastry-Sutherland model is quickly replaced by a quasi-one-dimensional full plaquette phase when intra- and/or interdimer couplings take different values, and that this full plaquette phase is in much better agreement with available experimental data than the empty plaquette one.

We report spin-entropy-induced thermopower and the occurrence of a spin-blockade effect in stoichiometric disordered CoO. Cation defect-driven distortion in the octahedral ligand field of CoO leads to a charge transfer process and favors the stabilization of ${\mathrm{Co}}^{+3}$ charge states at defect adjacent atomic sites. Moreover, a higher extent of local stoichiometric disruption triggers the spin crossover and magnetic collapse into a ${\mathrm{Co}}^{+3}$ state. Degenerated spin-orbital states on vacancy neighbored atomic sites render the spin-orbital degeneracy to enhance the thermopower in CoO. Furthermore, we unravel an operating spin-blockade effect in CoO. The localized combination of active magnetic states—high-spin ${\mathrm{Co}}^{+2}$ and neutral magnetic states—low-spin ${\mathrm{Co}}^{+3}$ on alternate atomic sites suppress the charge carrier hopping due to a spin blockade. In the pursuit of efficient thermoelectric material, the present investigation explores the potential of the recipe of spin entropy and defect-engineered CoO.

We use large-scale Monte Carlo simulations to test the Weinrib-Halperin criterion that predicts new universality classes in the presence of sufficiently slowly decaying power-law correlated quenched disorder. While new universality classes are reasonably well established, the predicted exponents are controversial. We propose a method of growing such correlated disorder using the three-dimensional Ising model as a benchmark system for both generating disorder and studying the resulting phase transition. Critical equilibrium configurations of a disorder-free system are used to define the two-value distributed random bonds with a small power-law exponent given by the pure Ising exponent. Finite-size scaling analysis shows a new universality class with a single phase transition, but the critical exponents ${\nu }_d=1.13\left(5\right),{\eta }_d=0.48\left(3\right)$ differ significantly from theoretical predictions. We find that depending on the details of the disorder generation, disorder-averaged quantities can develop peaks at two temperatures for finite sizes. Finally, a layer model with the two values of bonds spatially separated in halves of the system genuinely has multiple phase transitions, and thermodynamic properties can be flexibly tuned by adjusting the model parameters.

We propose another approach to overcome the difficulties of previous ab initio methods used to study lattice dynamics in disordered systems, such as alloys. Group III nitrides and arsenides are used as prototypical systems to validate the developed methodology. The phonon behaviors of ternary alloys, for specific concentrations, are calculated with a methodology based on the ab initio calculations of dynamics of the respective nitrides' and arsenides' bulks. The generalized phonons' behavior of long wavelengths for ternary alloys were simulated employing the generalized quasichemical approximation method to account for the inherent statistical disorder of the system. The model describes the evolution of optical phonons to the $\mathrm{\Gamma }$ point as a function of molar fraction for zinc blende ternary alloys for III arsenides and III nitrides for any arbitrary compositions. We have found that the obtained results are in good agreement with experimental data taken from Raman and IR measurements available in the literature.

The magnetic moment of titanium in $\mathrm{Sm}{\left({\mathrm{Fe}}_{0.8}{\mathrm{Co}}_{0.2}\right)}_{11}\mathrm{Ti}$ was evaluated directly with x-ray magnetic circular dichroism, and magnitude of titanium's magnetic moment was identified with powder neutron Rietveld analysis. It was demonstrated experimentally that titanium in $\mathrm{Sm}{\left({\mathrm{Fe}}_{0.8}{\mathrm{Co}}_{0.2}\right)}_{11}\mathrm{Ti}$ has a magnetic moment of about $1{\mu }_{\mathrm{B}}$, and that magnetic moment of titanium couples antiferromagnetically to those of the host elements (iron and cobalt) at room temperature. From first-principles calculation, the magnitude of titanium's magnetic moment could be explained by Friedel's concept of the virtual bound state and reverse spin extended to the neighboring iron sites.

We report the structural, magnetic, and magnetocaloric properties of ${\mathrm{Co}}_2{\mathrm{Cr}}_{1-x}{\mathrm{Ti}}_x\mathrm{Al}$ ($x=$ 0–0.5) Heusler alloys for spintronic and magnetic refrigerator applications. Room-temperature x-ray diffraction and neutron diffraction patterns along with Rietveld refinements confirm that the samples are of single phase and possess a cubic structure. Interestingly, magnetic susceptibility measurements indicate a second-order phase transition from paramagnetic to ferromagnetic where the Curie temperature ($T_C$) of ${\mathrm{Co}}_2\mathrm{CrAl}$ increases from 330 K to 445 K with Ti substitution. Neutron powder diffraction data of the $x=$ 0 sample across the magnetic phase transition taken in a large temperature range confirm the structural stability and exclude the possibility of antiferromagnetic ordering. The saturation magnetization of the $x=$ 0 sample is found to be 8000 emu/mol (1.45 ${\mu }_{\mathrm{B}}$/f.u.) at 5 K, which is in good agreement with the value $(1.35\pm 0.05{\mu }_{\mathrm{B}}$/f.u.) obtained from the Rietveld analysis of the neutron powder diffraction pattern measured at a temperature of 4 K. By analyzing the temperature dependence of the neutron data of the $x=$ 0 sample, we find that the change in the intensity of the most intense Bragg peak (220) is consistent with the magnetization behavior with temperature. Furthermore, an enhancement of change in the magnetic entropy and relative cooling power values has been observed for the $x=$ 0.25 sample. Interestingly, the critical behavior analysis across the second-order magnetic phase transition and extracted exponents ($\beta \approx$ 0.496, $\gamma \approx$ 1.348, and $\delta \approx$ 3.71 for the $x=$ 0.25 sample) suggest the presence of long-range ordering, which deviates toward 3D Heisenberg-type interactions above $T_C$, consistent with the interaction range value $\sigma$.

Motivated by recent reports of a field-induced intermediate phase (IP) in the antiferromagnetic honeycomb Kitaev model that may be a spin liquid whose nature is distinct from the Kitaev ${\mathbb{Z}}_2$ phase, we present a detailed numerical study on the nature and dynamical response (such as dynamical spin-structure factors and resonant inelastic x-ray scattering intensities) of this field-induced IP and neighboring phases in a family of Kitaev-based models related by hidden symmetries and duality transformations. We further show that the same field-induced IP can appear in models relevant for $\alpha -{\mathrm{RuCl}}_3$, which exhibit a ferromagnetic Kitaev coupling and additional interactions. In $\alpha -{\mathrm{RuCl}}_3$, the IP represents a new phase, that is likely independent from the putative field-induced (spin-liquid) phase recently reported from thermal Hall conductivity measurements.

We generate many-body corrected tight-binding Hamiltonians for topological insulators of the ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ family. To this end, we use ab initio calculated parameters extracted from $GW$ calculations, thus capturing many-body exchange and correlation effects, in contrast to previous tight-binding models. We investigate the effect of many-body renormalizations on the electronic structure of bulk and surface states of semi-infinite systems as well as thin films of these materials. It is shown that the $GW$ self-energy correction brings about profound changes not only in the band-gap values but also in the band dispersion around the inverted gaps with respect to standard density-functional theory (DFT). These changes substantially improve the agreement with experiment. We discuss the strong renormalization effect as being a result of the characteristic overestimation of inverted gaps by standard approximations of DFT (opposite to the underestimation of gaps in topologically trivial materials). In particular, we analyze the consequences that these renormalizations have on the dispersions of the topological surface states and on the surface resonances. For reference, the tight-binding Hamiltonians are provided in the Supplemental Material [30].