The spin-driven component of electric polarization in a single crystal of multiferroic ${\mathrm{BiFeO}}_3$ was experimentally investigated in pulsed high magnetic fields up to 41 T. Sequential measurements of electric polarization for various magnetic field directions provide clear evidence of electric polarization normal to the hexagonal $c$ axis $\left({\mathit{P}}_{\mathrm{t}}\right)$ in not only the cycloidal phase, but also the field-induced canted antiferromagnetic phase. The direction of ${\mathit{P}}_{\mathrm{t}}$ is directly coupled with the ferromagnetic moment in the canted antiferromagnetic phase, and thus controlled by changing the direction of the applied magnetic field. This magnetoelectric coupling is reasonably reproduced by the metal-ligand hybridization model.

Lattice dynamical properties of antiferromagnetic rocksalt oxides are often interpreted using the cubic space group $Fm\overline{3}m$, although below Néel temperature their magnetic substructure possesses a lower symmetry. For example, in the case of NiO, a rhombohedral structural distortion lowers the symmetry to trigonal space group $R\overline{3}m$ below 525 K. We performed hybrid density functional theory calculations on the phonon dispersion relations of MnO, CoO, and NiO, and the lattice thermal conductivity of NiO using both $Fm\overline{3}m$ and $R\overline{3}m$ space groups. The calculated acoustic phonon frequencies of all oxides agree well with the available experimental data, while the optical modes of MnO and CoO show somewhat larger discrepancies. Our calculations show the phonon density of states to be very similar with both studied space groups. The experimental thermal conductivity of antiferromagnetic NiO is reproduced well below the Néel temperature by solving the linearized phonon Boltzmann transport equation.

To study non-Heisenberg effects in the vicinity of spin crossover in strongly correlated electron systems we derive an effective low-energy Hamiltonian for the two-band Kanamori model. It contains a Heisenberg high-spin term proportional to exchange constant as well as a low-spin term proportional to spin gap parameter ${\varepsilon }_s$. Using cluster mean field theory we obtain several non-Heisenberg effects. Near the critical value of spin gap ${\varepsilon }_s^c$ there is a magnetic phase transition of first order. In the vicinity of ${\varepsilon }_s^c$ in the paramagnetic phase we observe nontrivial behavior of the Curie constant in the paramagnetic susceptibility in the wide range of temperature. Reentrant temperature behavior of nearest-neighbor spin-spin correlations is observed at ${\varepsilon }_s>{\varepsilon }_s^c$. Finally, the pressure-temperature magnetic phase diagram for ferroperriclase is obtained using the effective Hamiltonian.

We report the observation of spin transport through antiferromagnetic insulators using spin pumping driven by short-wavelength magnons. The short-wavelength magnons are excited by parametric pumping in a $\mathrm{Pt}/\mathrm{NiO}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ trilayer. The parametrically excited magnons in the ${\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ layer inject a spin current into the NiO layer, which is detected electrically using the inverse spin Hall effect in the Pt layer. We found that the spin-pumping efficiency of the parametrically excited magnons in the $\mathrm{Pt}/\mathrm{NiO}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ film is nearly an order of magnitude smaller than that in the $\mathrm{Pt}/{\mathrm{Y}}_3{\mathrm{Fe}}_5{\mathrm{O}}_{12}$ film. The suppression of the parametric spin-pumping efficiency induced by inserting the NiO layer is comparable to that of the FMR spin-pumping efficiency. We further found that the suppression of the spin-pumping efficiency due to the NiO insertion for the dipole-exchange magnons with the wave number of $k\sim {10}^3-{10}^4{\mathrm{cm}}^{-1}$ is more significant than that for the exchange magnons with $k\sim {10}^5{\mathrm{cm}}^{-1}$. This difference is attributed to two-magnon scattering and spin pinning due to antiferromagnetic grains in the NiO layer. These results provide insight into the understanding of the spin pumping into antiferromagnetic insulators.

Recent progress of quantum simulators provides insight into the fundamental problems of strongly correlated systems. To adequately assess the accuracy of these simulators, the precise modeling of the many-body physics, with accurate model parameters, is crucially important. In this paper, we employed an ab initio exact diagonalization framework to compute the correlated physics of a few electrons in artificial potentials. We apply this approach to a quantum-dot system and study the magnetism of the correlated electrons, obtaining good agreement with recent experimental measurements in a plaquette. Through control of dot potentials and separation, including geometric manipulation of tunneling, we examine the Nagaoka transition and determine the robustness of the ferromagnetic state. While the Nagaoka theorem considers only a single-band Hubbard model, in this work we perform extensive ab initio calculations that include realistic multiorbital conditions in which the level splitting is smaller than the interactions. This simulation complements the experiments and provides insight into the formation of ferromagnetism in correlated systems. More generally, our calculation sets the stage for further theoretical analysis of analog quantum simulators at a quantitative level.

Superconductors with topological surface or edge states have been intensively explored for the prospect of realizing Majorana bound states, which obey non-Abelian statistics and are crucial for topological quantum computation. The traditional routes for making topological insulator/superconductor and semiconductor/superconductor heterostructures suffer fabrication difficulties and can only work at low temperature. Here, we use angle-resolved photoemission spectroscopy to directly observe the evolution of a topological transition of band structure nearby the Fermi level in two-dimensional high-$T_c$ superconductor $\mathrm{FeT}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{e}}_x/\mathrm{SrTi}{\mathrm{O}}_3\left(001\right)$ monolayers, fully consistent with our theoretical calculations. Furthermore, evidence of edge states is revealed by scanning tunneling spectroscopy with assistance of theoretical calculations. Our study provides a simple and tunable platform for realizing and manipulating Majorana states at high temperature.

We investigate the fate of topological states on fractal lattices. Focusing on a spinless chiral $p$-wave paired superconductor, we find that this model supports two qualitatively distinct phases when defined on a Sierpinski gasket. While the trivial phase is characterized by a self-similar spectrum with infinitely many gaps and extended eigenstates, the “topological” phase has a gapless spectrum and hosts chiral states propagating along edges of the graph. Besides employing theoretical probes such as the real-space Chern number, inverse participation ratio, and energy-level statistics in the presence of disorder, we develop a simple physical picture capturing the essential features of the model on the gasket. Extending this picture to other fractal lattices and topological states, we show that the $p+ip$ state admits a gapped topological phase on the Sierpinski carpet and that a higher-order topological insulator placed on this lattice hosts gapless modes localized on corners.

The energy conversion efficiency of solar cells based on multicrystalline silicon is greatly deteriorated by dislocations. However, an in-depth understanding on the dislocation motion dynamics down to atomic scale is still lacking. In this paper, we propose a novel atomistic approach to simulate the kink migration and kink-pair formation which govern dislocation motion in silicon, namely the kinetic activation-relax technique (k-ART). With this method, long timescale events can be simulated and complex energy landscapes can be explored. Four mechanisms for kink migration are observed, with total activation energy of 0.16, 0.25, 0.32, and 0.25 eV. New nontrivial kink structures that participate in kink migration are identified due to the open-ended search algorithm for saddle points in k-ART. In addition, a new pathway for kink-pair formation, with a minimum activation energy of 1.11 eV is discovered. The effect of shear stress on kink migration is also investigated. It shows that shear stress shifts the energy barriers of available events to lower energies, resulting in a change of the preferred kink-migration mechanism and a reduction of kink-pair formation energy.

When a local and attractive potential is quenched in a nanowire, the spectrum changes its topology from a purely continuum to a continuum and discrete portion. We show that, under appropriate conditions, this quench leads to stable coherent oscillations in the observables' time evolution. In particular, we demonstrate that ballistic nanowires with spin-orbit coupling (SOC) exposed to a uniform magnetic field are especially suitable to observe this effect. Indeed, while in ordinary nanowires the effect occurs only if the strength $U_0$ of the attractive potential is sufficiently strong, even a weak value of $U_0$ is sufficient in SOC nanowires. Furthermore, in these systems coherent oscillations in the spin sector can be generated and controlled electrically by quenching the gate voltage acting on the charge sector. We interpret the origin of this phenomenon, analyze the effect of variation of the chemical potential and the switching time of the quenched attractive potential, and address possible implementation schemes.

We have studied how a generic bilayer kagome lattice behaves upon layer rotation. We employed a tight-binding model with one orbital per site and found: (i) for low rotational angles and at low energies, the same flat bands structure, such as in twisted bilayer graphene; although, for a larger value of the magic angle. Moreover, (ii) at high energies due to the superstructure symmetry regions, we found the characteristic three-band dispersion of the kagome lattice. In the latter, its bandwidth decreases for lower angles confining them within a few meV. Therefore, we found, in the twisted kagome lattice, the coexistence of two sets of flat bands in different energies and lying in different spatial regions of the bilayer system.

We study transport between quantum Hall edge states at filling factor $\nu =1$ in the presence of electron-acoustic-phonon coupling. After a Bogoliubov-Valatin transformation, the low-energy spectrum of interacting electron-phonon system is presented. The electron-phonon interaction splits the spectrum into charged and neutral “downstream” and neutral “upstream” modes with different velocities. In the regimes of dc and periodic ac biases the tunneling current and nonequilibrium finite frequency nonsymmetrized noise are calculated perturbatively in tunneling coupling of the quantum point contact. We show that the presence of electron-phonon interaction strongly modifies noise and current relations compared to the free-fermion case.

In addition to the giant peak of the nonlocal resistance $R_{\text{NL}}$, an anomalous negative value of $R_{\text{NL}}$ has been observed in graphene systems, although its formation mechanism is not quite understood yet. In this work, utilizing the nonequilibrium Green's function method, we calculate the local-current flow in an H-shaped noninteracting graphene system located in a ballistic regime. Similar to the previous conclusions obtained from viscous hydrodynamics, the numerical results show that a local-current vortex appears between the nonlocal measuring terminals, and it induces a backflow current and a remarkable negative voltage drop at the probe. Specifically, the stronger the vortex is, the more negative $R_{\text{NL}}$ becomes. In addition, spin-orbital coupling is added as an additional tool to study this exotic vortex, although this coupling is not a driving force for the arising vortex at all. Moreover, a breakdown of the nonlocal Wiedemann-Franz law is obtained in this ballistic system, and two experimental criteria are provided to confirm the existence of this exotic vortex. Notably, it is shown that the vortex actually originates from a collision between the flowing current and the boundaries, due to the long electron mean free path and the resulting ballistic transport caused by the specific linear spectrum of graphene.

Peculiar features exist in the stress-temperature phase stability diagram of ${\mathrm{V}}_2{\mathrm{O}}_3$, such as a first-order phase transition between the paramagnetic insulating and metallic phases that ends with a critical point, quantum phase transition, and a triple point. These features remain largely unexplored, and the exact nature of the phase transitions is not clear due to very limited control over the stress in bulk or film samples. Here, we show the synthesis of single-crystal ${\mathrm{V}}_2{\mathrm{O}}_3$ nanoplates using chemical vapor deposition via van der Waals epitaxy. Thickness of the ${\mathrm{V}}_2{\mathrm{O}}_3$ nanoplates range from a few to hundreds of nanometers, and they can be mechanically exfoliated from the growth substrate. Using Raman spectroscopy on the nanoplates, we reveal that, upon heating, ${\mathrm{V}}_2{\mathrm{O}}_3$ enters a supercritical state for both tensile strained and relaxed crystals with a similar out-of-plane response. Transmission electron microscopy on ${\mathrm{V}}_2{\mathrm{O}}_3$ nanoplates hints at the existence of a structural change when the crystals are heated. Our results show that ${\mathrm{V}}_2{\mathrm{O}}_3$ nanoplates should be useful for studying the physics of the supercritical state and the phase stability of ${\mathrm{V}}_2{\mathrm{O}}_3$ to enable new horizons in applications.

The valley relaxation mechanism of the bright state of positively charged excitons (bright positive trions) in monolayer (1L-) ${\mathrm{WSe}}_2$ is studied through polarization- and time-resolved photoluminescence and transient reflection measurements. A long valley relaxation time, exceeding 100 ps, is observed for positive trions at low temperature, which is notably prolonged when compared with the characteristic excitonic valley relaxation time of 10 ps. With increasing temperature, the relaxation time decreases to a few ps. This finding implies that phonon-mediated intervalley scattering is important for the relaxation process of the valley-polarized bright state of positive trions.

In the study of the anomalous Hall effect, the scaling relations between the anomalous Hall and longitudinal resistivities play the central role. The scaling parameters by definition are fixed as the scaling variable (longitudinal resistivity) changes. Contrary to this paradigm, we unveil that the electron-phonon scattering can result in apparent temperature dependence of scaling parameters when the longitudinal resistivity is tuned through temperature. An experimental approach is proposed to observe this hitherto unexpected temperature dependence. We further show that this phenomenon also exists in the nonlinear Hall effect in nonmagnetic inversion-breaking materials and may help identify experimentally the presence of the side-jump contribution besides the Berry-curvature dipole.

The pressure evolution of the magnetic properties of the ${\mathrm{Ce}}_2{\mathrm{RhIn}}_{7.79}{\mathrm{Cd}}_{0.21}$ heavy fermion compound was investigated by single crystal neutron magnetic diffraction and electrical resistivity experiments under applied pressure. From the neutron magnetic diffraction data, up to $P=0.6\mathrm{GPa}$, we found no changes in the magnetic structure or in the ordering temperature $T_N=4.8\mathrm{K}$. However, the increase of pressure induces an interesting spin rotation of the ordered antiferromagnetic moment of ${\mathrm{Ce}}_2{\mathrm{RhIn}}_{7.79}{\mathrm{Cd}}_{0.21}$ into the $ab$ tetragonal plane. From the electrical resistivity measurements under pressure, we have mapped the evolution of $T_N$ and the maximum of the temperature dependent electrical resistivity ($T_{\mathrm{MAX}}$) as a function of the pressure ($P\lesssim 3.6\mathrm{GPa}$). To gain some insight into the microscopic origin of the observed spin rotation as a function of pressure, we have also analyzed some macroscopic magnetic susceptibility data at ambient pressure for pure and Cd-doped ${\mathrm{Ce}}_2{\mathrm{RhIn}}_8$ using a mean-field model including tetragonal crystalline electric field (CEF). The analysis indicates that these compounds have a Kramers doublet ${\mathrm{\Gamma }}_7^{-}$-type ground state, followed by a ${\mathrm{\Gamma }}_7^{+}$ first excited state at ${\mathrm{\Delta }}_1\sim 80\mathrm{K}$ and a ${\mathrm{\Gamma }}_6$ second excited state at ${\mathrm{\Delta }}_2\sim 270\mathrm{K}$ for ${\mathrm{Ce}}_2{\mathrm{RhIn}}_8$ and ${\mathrm{\Delta }}_2\sim 250\mathrm{K}$ for ${\mathrm{Ce}}_2{\mathrm{RhIn}}_{7.79}{\mathrm{Cd}}_{0.21}$. The evolution of the magnetic properties of ${\mathrm{Ce}}_2{\mathrm{RhIn}}_8$ as a function of Cd doping and the rotation of the direction of the ordered moment for the ${\mathrm{Ce}}_2{\mathrm{RhIn}}_{7.79}{\mathrm{Cd}}_{0.21}$ compound under pressure suggest important changes of the single ion anisotropy of ${\mathrm{Ce}}^{3+}$ induced by applying pressure and Cd doping in these systems. These changes are reflected in modifications in the CEF scheme that will ultimately affect the actual ground state of these compounds.

We establish the bilayer non-Hermitian theory with interlayer coupling between two graphenelike lattices described by non-Hermitian Hamiltonians. Two exceptional rings (ERs) and an exceptional concentric ring (ECR) in this system are discovered. A topological classification has been identified for such non-Hermitian Hamiltonians via their global topological invariants. We find that two ERs and the ECR correspond to the global ${\mathbb{Z}}_3$ topological invariant $V=+1$ and $V=-1$, respectively, whose topological properties are determined by the four-level bands and go beyond the topological properties of a single ER. It is shown that in contrast to the Hermitian case, the topological surface arcs with gain and loss are both present and redistributed in the two ERs and the ECR systems. We further propose an experimental design with bilayer photonic crystals (PCs) to realize the ECR. Our work can potentially pave the way for unveiling the unusual topological properties in such stacked two-dimensional materials and trigger further theoretical and experimental investigations.

We study general entanglement properties of the excited states of the one-dimensional translational invariant free fermions and coupled harmonic oscillators. In particular, using the integrals of motion, we prove that these Hamiltonians independent of the gap (mass) have infinite excited states that can be described by conformal field theories with integer or half-integer central charges. In the case of free fermions, we also show that because of the huge degeneracy in the spectrum, even a gapless Hamiltonian can have excited states with an area law. Finally, we study the universal average entanglement entropy over eigenstates of the Hamiltonian of the free fermions introduced recently in Vidmar et al. [Phys. Rev. Lett. 119, 020601 (2017)]. In particular, using a duality relation, we map the average bipartite entanglement entropy over all the exponential numbers of eigenstates of a generic free fermion Hamiltonian to the problem of the calculation of the average multibipartite entanglement entropy over a single eigenstate of the $\mathit{XX}$ chain. This relation can be useful for experimental measurement of the universal average entanglement entropy. Part of our conclusions can be extended to the quantum spin chains that are associated with the free fermions via Jordan-Wigner transformation.

We explore the correlated electron states in harmonically confined few-electron quantum dots in an external magnetic field by the path-integral Monte Carlo method for a wide range of the field and the Coulomb interaction strength. Using the phase structure of a preceding unrestricted Hartree-Fock calculation for phase fixing, we find a rich variety of correlated states, often completely different from the prediction of mean-field theory. These are finite temperature results, but sometimes the correlations saturate with decreasing temperature, providing insight into the ground-state properties.

Elastic-mechanical deformations are found to dramatically alter the electronic properties of monolayer (ML) $\mathrm{Mo}{\mathrm{S}}_2$; particularly, the low-energy Bloch bands are responsive to a directional strain. In this study, in-plane uniaxial deformation is found to drift the low-energy electron/hole valleys of strained $\mathrm{ML}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ far away from $K/K’$ points in the Brillouin zone (BZ). The amount of drift differs notably from hole to electron bands, where the conduction band minimum (CBM) drifts nearly 2 times more than the valence band maximum (VBM) in response to a progressively increasing strain field (0–10%). The resulting strain-induced valley asymmetry/decoherence can lift the momentum degeneracy of valley carriers at the $K$ point, thereby affecting the low-energy valley excitations ($K$-valley polarization) in a strained $\mathrm{ML}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ lattice. The quantum origin of this decoherent valley arises from the differences in the Bloch orbital wave functions of electron and hole states at the exciton band edges and their deformation under strain. A higher drift (>1.5 times) is noticed when strain is along the zigzag (ZZ) axis relative to the armchair (AC) axis, which is attributed to a faster decline in Young's modulus and Poisson's ratio (PR) along the ZZ direction. A similar valley drift only in the VBM of uniaxially strained $\mathrm{ML}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ was reported in an earlier local density approximation (LDA) based density functional theory (DFT) study [Q. Zhang et al., Phys. Rev. B 88, 245447 (2013)], where a massive valley drift occurring at the CBM was fully overlooked. Moreover, the giant VBM drift reported therein is 6 times the drift observed in our DFT studies based on spin-orbit coupling (SOC) and Perdew-Burke-Ernzerhof generalized gradient approximation (PBE-GGA) functionals. The physical origin of valley drift has been ascertained in our thorough investigations. The robustness of our approach is substantiated as follows. With progressive increase in strain magnitude (0–10%), the band gap remains direct up to 2% uniaxial tensile strain, under SOC, which accurately reproduces the experimental strain-induced direct-to-indirect band gap transitions occurring at ∼2% strain. Based on LDA-DFT [Q. Zhang et al., Phys. Rev. B 88, 245447 (2013)], this crossover in band gap has been incorrectly reported to occur at a higher value of uniaxial strain of 4%. Moreover, the direct SOC band gap shows a linear redshift at a rate of 51–53 meV/(% of strain), under uniaxial tensile strain, which is in excellent quantitative agreement with experimentally observed rates in the redshift of direct excitonic transitions measured in several optical absorption and photoluminescence (PL) spectroscopy experiments. In addition, the Berry curvature Ω($k$) of electron/hole bands gets significantly modulated in strained $\mathrm{ML}\text{−}\mathrm{Mo}{\mathrm{S}}_2$, where the intensity of the flux profile increases as a function of the magnitude of strain with an opposite drift around $K/K’$, when strained along the ZZ/AC direction. A strong strain-valley coupling leads to an enhancement in the strength of spin-orbit induced spin splitting of bands at VBM/CBM, which is sizably enhanced (∼7 meV) simply by the strain-controlled orbital motions. Our findings are of prime importance in the valley physics of $\mathrm{Mo}{\mathrm{S}}_2$. Besides, the important theoretical insights emerging from this work will trigger further experimental investigations on $\mathrm{ML}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ to realize its novel technological potential in nanoelectronics, spintronics, and valleytronics.