Spin-crossover and Prussian blue analog materials are investigated in two dimensions with the three-state Blume-Emery-Griffiths model where each spin interacts with its nearest neighbors and next-nearest neighbors and may be either in high-spin or low-spin state. The interactions through the system are strongly dependent on the instantaneous distance between atoms and are magnetic and elastic in nature. Finite-size effects have been detected at finite temperature on the model. The thermal distortion of the lattice configuration due to lattice unit displacements strengthened the thermal spin transition that occurred. The generated numerical results are obtained by two-step Monte Carlo simulations where used thermodynamic parameters allowed to establish a rich phase diagram. Gradual and first-order transitions with thermally induced hysteresis phenomena have been observed. Near the thermal hysteresis loops, the model exhibits throughout relaxation curves lattice configurations evolving through two-dimensional nucleation and growth processes that are enhanced with suitable values of the model parameters.

A large unidirectional magnetoresistance (UMR) ratio of $\mathrm{UMR}/R_{xx}\approx 0.36\%$ is found in W/CoFeB metallic bilayer heterostructures at room temperature. Three different regimes in terms of the current dependence of the UMR ratio are identified: a spin-dependent-scattering mechanism regime at small current densities $J\approx {10}^9\mathrm{A}/{\mathrm{m}}^2$ (UMR ratio $\propto J$), a spin-magnon-interaction mechanism regime at intermediate $J\approx {10}^{10}\mathrm{A}/{\mathrm{m}}^2$ (UMR ratio $\propto J^3$), and a spin transfer torque (STT) regime at $J\approx {10}^{11}\mathrm{A}/{\mathrm{m}}^2$ (UMR ratio independent of J). We verify the direct correlation between this large UMR and the transfer of spin angular momentum from the W layer to the CoFeB layer by both field-dependent and current-dependent UMR characterizations. Numerical simulations further confirm that the large STT-UMR stems from the tilting of the magnetization affected by the spin Hall effect-induced STTs. An alternative approach to estimate dampinglike spin torque efficiencies from magnetic heterostructures is also proposed.

Spin-crossover (SCO) molecules have two accessible states: one high spin (HS) and one low spin (LS). We show that on the pyrochlore lattice elastic interactions between SCO molecules can give rise to three different spin-state ice phases (denoted ${\text{H}}_n{\text{L}}_{4-n}$ with $n=1, 2, \mathrm{or} 3)$—characterized by the “ice rules” that every tetrahedron must contain $n$ HS and $4-n$ LS metals. Each is a “Coulomb phase” where a local ice rule can be mapped to a divergence-free gauge field and the low-energy excitations carry a spin fractionalized midway between the LS and HS states. To date, no phase with the $n=1$ and $n=3$ ice rules has been observed. In SCO materials, the subtle competition between the entropies and enthalpies of the HS and LS states allows temperature or pressure to change the ice rules, allowing straightforward access to the $n=1$ and $n=3$ phases.

We show that in type-II superconductors a magnetic field applied transversely to correlated columnar disorder drives a phase transition to a distinct “smectic” vortex glass (SmVG) state. SmVG is characterized by an infinitely anisotropic electrical transport, resistive (dissipationless) for current perpendicular to (along) columnar defects. Its positional order is also quite unusual, long-ranged with true Bragg peaks along columnar defects and logarithmically rough vortex lattice distortions with quasi-Bragg peaks transverse to columnar defects. For low temperatures and sufficiently weak columnar-only disorder, SmVG is a true topologically ordered “Bragg glass,” characterized by a vanishing dislocation density. At sufficiently long scales the residual ever-present point disorder converts this state to a more standard but highly anisotropic vortex glass.

It is possible to simulate the dynamics of a single spin-$1/2$ ($\mathcal{PT}$-symmetric) system by conveniently embedding it into a subspace of a larger Hilbert space, undergoing unitary dynamics governed by a Hermitian Hamiltonian. Our goal is to analyze a many-body generalization of this idea, i.e., embedding many-body non-Hermitian dynamics. As a first step in this direction, we investigate embedding of “$N$” noninteracting spin-$1/2$ ($\mathcal{PT}$-symmetric) degrees of freedom, thereby unfolding the complex nature of the embedding Hamiltonian. It turns out that the resulting Hermitian Hamiltonian of $N+1$ spin halves comprises “all to all”, $q$-body interaction terms ($q=1,...,N+1$) where the additional spin-$1/2$ is a part of the larger embedding space. We show that the presence of finite entanglement in the eigenstates of the resulting cluster of $N+1$ spin halves ensures the nonvanishing probability of post-selection of the additional spin-1/2, which is essential for the embedding to be practicable. Finally, we also note that our study can be identified with a central spin model where orthogonality catastrophe owing to the finite entanglement plays a central role in protecting the additional spin-1/2 degree of freedom from decoherence.

Overlap between neighboring atomic wave functions is a central feature of conducting solids. In heavy-fermion materials, $f$-electron orbitals in the lattice lie on the boundary between fully localized and hybridized in an energy band. This dichotomy gives rise to a range of behaviors, including antiferromagnetism, unconventional superconductivity, and the ability to tune from one ground state to the other continuously. Measuring the degree of this hybridization by traditional methods is challenging and indirect. We utilize an approach using NMR to determine the magnetic couplings between the $f$ electrons and neighboring nuclear spins in a series of ${\mathrm{CeRh}}_{1-x}{\mathrm{Ir}}_x{\mathrm{In}}_5$ crystals and find that the hybridization is strongly direction dependent in this important class of superconducting heavy-fermion materials. Our results demonstrate that hyperfine coupling measurements provide a quantitative measure of orbital anisotropy.

Flat-band models have been of particular interest from both fundamental aspects and realization in materials. Beyond the canonical examples such as Lieb lattices and line graphs, a variety of tight-binding models are found to possess flat bands. However, the analytical treatment of dispersion relations is limited, especially when there are multiple flat bands with different energies. In this paper, we present how to determine flat-band energies and wave functions in tight-binding models on decorated diamond and pyrochlore lattices in generic dimensions $D\ge 2$. For two and three dimensions, such lattice structures are relevant to various organic and inorganic materials, and thus our method will be useful to analyze the band structures of these materials.

Monolayer transition metal dichalcogenides $M{X}_2$ $(M=\mathrm{Mo}, \mathrm{W} \mathrm{and} X=\mathrm{Te}, \mathrm{Se}, \mathrm{S})$ in $1T^{\prime }$ structure were predicted to be quantum spin Hall insulators based on first-principles calculations, which were quickly confirmed by multiple experimental groups. For a better understanding of their properties, in particular, their responses to external fields, we construct a realistic four-band tight-binding (TB) model by combining the symmetry analysis and first-principles calculations. Our TB model respects all symmetries and can accurately reproduce the band structure in a large energy window from $-0.3$ eV to 0.8 eV. With the inclusion of spin-orbital coupling (SOC), our TB model can characterize the nontrivial topology and the corresponding edge states. Our TB model can also capture the anisotropic strain effects on the band structure and the strain-induced metal-insulator transition. Moreover, we found that although $M{X}_2$ share the same crystal structures and have the same crystal symmetries, the orbital composition of states around the Fermi level are qualitatively different and their lower-energy properties cannot be fully described by a single $\mathit{k}·\mathit{p}$ model. Thus, we construct two different types of $\mathit{k}·\mathit{p}$ models, one for ${M\mathrm{S}}_2$ and $M{\mathrm{Se}}_2$, the other for $M{\mathrm{Te}}_2$, respectively. Benefiting from high accuracy and simplicity, our TB and $\mathit{k}·\mathit{p}$ models can serve as a solid and concrete starting point for future studies of transport, superconductivity, strong correlation effects, and twistronics in $1T^{\prime }$ transition metal dichalcogenides.

Current-induced spin and orbital polarizations are theoretically studied in a helical atomic chain with $p_x, p_y$, and $p_z$ orbitals in each atom in the linear response to the chemical-potential difference between source and drain electrodes. It is shown that these polarizations are parallel to the helix axis and proportional to the curvature of the cylinder surface defined by the helix in the small curvature region. This proportionality suggests that the curvature generates the coupling between the current and the atomic orbital angular momentum and therefore influences the magnitude of induced spin polarization through the $LS$ coupling. It is also found that current-induced polarizations are nonzero even when the pitch is enforced to vanish. This suggests the possibility that nonchiral structures with curvature can produce current-induced polarizations. With this possibility in mind, it is proposed that the chirality in the current, rather than that in the structure, determines the existence of such current-induced polarizations.

We study fractional boundary charges (FBCs) for two classes of strongly interacting systems. First, we study strongly interacting nanowires subjected to a periodic potential with a period that is a rational fraction of the Fermi wavelength. For sufficiently strong interactions, the periodic potential leads to the opening of a charge density wave gap at the Fermi level. The FBC then depends linearly on the phase offset of the potential with a quantized slope determined by the period. Furthermore, different possible values for the FBC at a fixed phase offset label different degenerate ground states of the system that cannot be connected adiabatically. Next, we turn to the fractional quantum Hall effect (FQHE) at odd filling factors $\nu =1/(2l+1)$, where $l$ is an integer. For a Corbino disk threaded by an external flux, we find that the FBC depends linearly on the flux with a quantized slope that is determined by the filling factor. Again, the FBC has $2l+1$ different branches that cannot be connected adiabatically, reflecting the $(2l+1)$-fold degeneracy of the ground state. These results allow for several promising and strikingly simple ways to probe strongly interacting phases via boundary charge measurements.

We report the results of muon spin relaxation ($\mu \mathrm{SR}$) and ${}^{29}\mathrm{Si}$ NMR measurements carried out on the cubic chiral magnet EuPtSi. This compound exhibits a helimagnetic transition at $T_{\mathrm{N}}=4$ K, and hosts a unique skyrmion phase characterized by a short modulation period and strong anisotropy under magnetic fields. Our zero-field $\mu \mathrm{SR}$ experiments revealed the development of critical slowing down of Eu spin fluctuations over a relatively wide critical region $(T-T_{\mathrm{N}})/T_{\mathrm{N}}<5$ above $T_{\mathrm{N}}$. We also found from ${}^{29}\mathrm{Si}$ NMR that the spin fluctuations are strongly suppressed by magnetic field in the paramagnetic state above 20 K. These characteristic spin dynamics observed over a wide region of temperature and magnetic field suggest the presence of magnetic frustration in the spin system. Such frustration would underlie the mechanism stabilizing the short-period skyrmion lattice observed in this compound.

The two-dimensional (2D) transition metal dichalcogenides (TMDs) have been proposed as candidates for the channel material in future field effect transistor designs. The heterophase design, which utilizes the metallic T or ${\mathrm{T}}^{\prime }$ phase of the TMD as contacts to the semiconducting H-phase channel, has shown promising results in terms of bringing down the contact resistance of the device. In this work, we use ab-initio calculations to demonstrate how atomic-scale and quantum effects influence the ballistic transport properties in such heterophase transistors with channel lengths up to 20 nm. We investigate how the charge transfer depends on the carrier density both in ${\mathrm{T}}^{\prime }\text{−}\mathrm{H} {\mathrm{MoTe}}_2$ Schottky contacts and in planar ${\mathrm{T}}^{\prime }\text{−}\mathrm{H}\text{−}{\mathrm{T}}^{\prime } {\mathrm{MoTe}}_2$ transistors. We find that the size of the Schottky barrier and the charge transfer is dominated by the local atomic arrangements at the interface and the doping level. Furthermore, two types of quantum states have a large influence on the charge transport: Interface states and standing waves in the semiconductor due to quantum confinement. We find that the latter can be associated with rises in the current by more than an order of magnitude due to resonant tunneling. Our results demonstrate the quantum mechanical nature of these 2D transistors and highlight several challenges and possible solutions for achieving a competitive performance of such devices.

We present both time-averaged and time-resolved transport measurements of a two-dimensional electron (Wigner) crystal on the surface of superfluid helium confined in a narrow microchannel. We find that the field-current characteristics of the driven crystal obtained by the time-averaged measurements exhibit oscillations and negative differential conductivity. This unusual transport behavior was observed previously by Glasson et al. [Phys. Rev. Lett. 87, 176802 (2001)] and was attributed to a nonequilibrium transition of the electron system to a novel dynamically ordered phase of current filaments aligned along the channels. Contrarily to this explanation, our time-resolved transport measurements reveal that oscillating field-current characteristics appear due to dynamical decoupling (slipping) and recoupling (sticking) of the uniform electron crystal to the liquid helium substrate. Our result demonstrates that this unusual nonlinear transport effect is intrinsic, does not depend on the device geometry, and is associated with the dynamical interaction of Wigner crystal with the surface excitations of the liquid substrate.

Current-induced spin polarization (CISP) in the topological insulator (TI) ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ has been observed via circularly polarized photoconductive differential current (CPDC). It is found that the CPDC induced by the CISP is proportional to the square of the longitudinal electric field when the electric field is weak. The intensity of the CPDC decreases and finally reverses the sign when the thickness of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films is increased from 3 to 20 QL, owing to the switching of the dominant contribution to the CPDC from the top surface states to the bottom surface states. The spin polarization degree (SPD) of the photoconduction and the critical electric field $E_c$ at which the SPD becomes saturated are determined, and it is revealed that the SPD decreases while $E_c$ increases with the increasing of the thickness of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films. In addition, it is demonstrated that the CPDC excited by 1064 nm is more than one order larger than that excited by 1342 nm light, which is due to the larger optical absorption coefficient when excited by 1064 nm light. The temperature dependence of the CPDC and SPD of ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ films is also investigated, and it is found that the SPD induced by CISP first increases and then decreases with decreasing temperature. This work proposes a method using CPDC to investigate the CISP of TIs.

We develop a first-principles theory for Schottky barrier physics. The Poisson equation is solved self-consistently with the electrostatic charge density over the entire barrier using the density functional theory (DFT) electronic structure converged locally, allowing computation of a Schottky barrier entirely from DFT involving thousands of atomic layers in the semiconductor (SC). The induced charge in the bulk consists of conduction and valence band charges from doping and band bending, as well as charge from the evanescent states in the gap of the SC. The Schottky barrier height (SBH) is determined when the induced charge density and the induced electrostatic potential reach self-consistency. Tests on the GaAs-graphene and Si/Al heterostructures yield SBH, width, along with depletion and inversion layers obtained self-consistently as functions of temperature and bulk doping.

Efficient control of a magnetization without an application of the external magnetic fields is the ultimate goal of spintronics. We demonstrate that in monolayers of ${\text{CrI}}_3$, magnetization can be switched all optically by application of the resonant pulses of circularly polarized light. This happens because of the efficient coupling of the lattice magnetization with a bright excitonic transition. ${\text{CrI}}_3$ is thus a perspective functional material with high potential for applications in the domains of spintronics and ultrafast magnetic memory.

Highly efficient carrier doping into the spin-orbit coupled Mott insulator ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ is achieved by low-energy hydrogen ion beam irradiation at low temperature. We demonstrate that heavy doping of hydrogen into a ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$ epitaxial thin film induces a large increase in conductivity by band-filling control via electron doping, which is confirmed by Hall effect measurements. The introduction of a large amount of hydrogen and its distribution along the depth direction are clarified by nuclear reaction analysis. The doped interstitial and substitutional hydrogens act as electron donors with minimum perturbation to the lattice, as evidenced by crystal structural analysis and first-principles calculations of the defect formation energy for doped hydrogen. The hydrogen-doping method offers a strategy toward realization of novel quantum phases in strongly correlated spin-orbit entangled systems.

We study a generalized Blume-Capel model on the simple cubic lattice. In addition to the nearest-neighbor coupling there is a next-to-next-to-nearest-neighbor coupling. In order to quantify spatial anisotropy, we determine the correlation length in the high-temperature phase of the model for three different spatial directions. It turns out that the spatial anisotropy depends very little on the dilution or crystal-field parameter $D$ of the model and is essentially determined by the ratio of the nearest-neighbor and the next-to-next-to-nearest-neighbor coupling. This ratio is tuned such that the leading contribution to the spatial anisotropy is eliminated. Next we perform a finite-size scaling (FSS) study to tune $D$ such that also the leading correction to scaling is eliminated. Based on this FSS study, we determine the critical exponents $\nu =0.62998\left(5\right)$ and $\eta =0.036284\left(40\right)$, which are in nice agreement with the more accurate results obtained by using the conformal bootstrap method. Furthermore, we provide accurate results for fixed-point values of dimensionless quantities such as the Binder cumulant and for the critical couplings. These results provide the groundwork for broader studies of universal properties of the three-dimensional Ising universality class.