Utilization of van der Waals magnetic materials as the host of topologically protected skyrmionic and other complex spin textures has been drawing increasing attention. Here, using first-principles-based calculations, we predict a new stable magnetic ${\mathrm{CrGe}(\mathrm{Se},\mathrm{Te})}_3$ Janus monolayer with a strong Dzyaloshinskii-Moriya interaction (DMI), due to the breaking of inversion symmetry. Consequently, nanometric skyrmions (that carry a topological charge of $\pm 1$) and skyrmionium states (with a zero topological charge) can spontaneously form in the absence of magnetic field. We further unveil a subtle competition between DMI and frustration as key for stabilizing skyrmioniums.

Physical properties of a topological origin are known to be robust against small perturbations. This robustness is both a source of theoretical interest and a driver for technological applications, but presents a challenge when looking for new topological systems: Small perturbations cannot be used to identify the global direction of change in the topological indices. Here, we overcome this limitation by breaking the symmetries protecting the topology. The introduction of symmetry-breaking terms causes the topological indices to become smooth, nonquantized functions of the system parameters, which are amenable to efficient design algorithms based on gradient methods. We demonstrate this capability by designing discrete and continuous phononic systems realizing conventional and higher-order topological insulators.

This paper reports a unique behavior expected for the optical excitations in one-dimensional Mott insulators. When the intersite Coulombic attraction does not work between two elementary charge excitations, i.e., a doublon and a holon, the lowest optical excitations are described as an unbound pair of them, still having finite mutual correlation due to the forbidden double occupancy on the same site. We find that, in such a situation, the pairs obey a particle statistics that exceeds that of ordinary bosons. This feature appears most directly in the enhancement factor of the matrix element for the pair photocreation. We discuss the possible mechanism for this ultrabosonic behavior and also demonstrate that this behavior is maintained even if we include a small amount of interiste Coulomibic interaction. Lastly, this enhancement factor almost coincides with that for ordinary bosons when the degree of the intersite interaction exceeds a certain value that switches on the formation of a bound doublon-holon pair.

In organic molecular semiconductors (OSCs), fluctuation of transfer integrals originating from thermally induced molecular vibrations is suggested to cause large scatterings of carriers, and to be a most important factor for the suppression of their carrier mobility. The intrinsic carrier mobility under such a fluctuation of transfer integrals is calculated using the transient localization theory, in which the estimation of transfer-integral fluctuation depending on each OSC is indispensable. In the present study, we provide a methodology to evaluate nonlocal electron-phonon couplings in OSCs using the density functional theory, which enables us to evaluate precisely the fluctuation magnitude of transfer integrals. Our method is based upon the combination of the frequency correction to reduce numerical inaccuracies in normal-mode frequencies, the extraction of tight binding parameters using maximally localized Wannier functions, and the explicit consideration of anharmonicity of phonons. We apply this method to classical OSCs, tetracene and rubrene, and a recently developed high-mobility OSC, $3,11\text{−}\text{didecyl-dinaphtho}[2,3\text{−}d:2\prime ,3\prime \text{−}d\prime ]\mathrm{benzo}[1,2\text{−}b:4,5\text{−}b\prime ]\mathrm{dithiophene}$ $\left({\mathrm{C}}_{10}\text{−}\mathrm{DNBDT}\text{−}\mathrm{NW}\right)$. We succeeded in identifying the low-frequency vibrations dominating the fluctuation of transfer integrals at room temperature, which we consider to be the main factors to limit the intrinsic mobility.

We theoretically investigate the thermoelectric transport in the nanoribbon of the magnetic topological insulator with a domain wall in the linear and nonlinear regimes. The Lorenz number $L$, the Seebeck coefficients $S_c$, and the thermoelectrical figure of merit $ZT$ in the linear response regime are obtained by the nonequilibrium Green's function method. These thermoelectric coefficients strongly depend on the configuration of the domain wall, that is, the configuration of the domain wall can regulate the thermoelectric transport performance. We also discuss the effect of the width of nanoribbon and the thickness of the domain wall on the thermoelectric coefficient. The results show that the Néel-type wall is more dependent on the size effect of the domain wall than the Bloch-type wall. For a device with a given width, we can always allow the setup to be in high performance thermoelectric transport by adjusting the configuration of the domain wall. Moreover, considering the disorder, the transmission coefficient is robust against disorder even if the disorder strength is very strong, and $S_c$ and $ZT$ are robust against moderate disorder. Finally, for a nonlinear situation, we calculate the maximum power-generation efficiency $\eta$ and the equivalent thermoelectric figure of merit $Z{T}_M$, indicating the potential application as an effective thermoelectric device.

Using THz spectroscopy, we show that the spin-wave spectrum of multiferroic ${\mathrm{BiFeO}}_3$ in its high-field canted antiferromagnetic state is well described by a spin model that violates rhombohedral symmetry. We demonstrate that the monoclinic distortion of the canted antiferromagnetic state is induced by the single-ion magnetoelastic coupling between the lattice and the two nearly antiparallel spins. The revised spin model for ${\mathrm{BiFeO}}_3$ contains two new single-ion anisotropy terms that violate rhombohedral symmetry and depend on the direction of the magnetic field.

${\mathrm{Ag}}_2{\mathrm{CrO}}_2$ is an $S=\frac{3}{2}$ triangular lattice antiferromagnet in which long-range magnetic order with a fivefold magnetic unit cell appears below the transition temperature ($T_{\mathrm{N}}$) of 24 K. The long-range magnetic order is accompanied by a structural transition. Inelastic neutron scattering experiments were performed to investigate the magnetic interactions, anisotropy, and correlations in this material. Above $T_{\mathrm{N}}$, diffuse scattering originating from the short-ranged magnetic correlations consistent with a ${120}^{\circ }$ structure was observed. The magnetic excitations below $T_{\mathrm{N}}$ were reproduced reasonably well using a linear spin-wave model with further-neighbor interactions and easy-axis anisotropy, which are predicted to be due to a previously proposed spin-lattice coupling [Wang and Vishwanath, Phys. Rev. Lett. 100, 077201 (2008)].

In frustrated magnetic systems with competing interactions fluctuations can lift the residual accidental degeneracy. We argue that the state selection may have different outcomes for quantum and thermal order by disorder. As an example, we consider the semiclassical Heisenberg fcc antiferromagnet with only the nearest-neighbor interactions. Zero-point oscillations select the type 3 collinear antiferromagnetic state at $T=0$. Thermal fluctuations favor instead the type 1 antiferromagnetic structure. The opposite tendencies result in a finite-temperature transition between the two collinear states. Competition between effects of quantum and thermal order by disorder is a general phenomenon and is also realized in the $J_1\text{−}{J}_2$ square-lattice antiferromagnet at the critical point $J_2=\frac{1}{2}{J}_1$.

The entropic signatures of magnetic phase transitions in the skyrmion lattice host compounds $\mathrm{Mn}{\mathrm{Si}}_{0.962}{\mathrm{Al}}_{0.038}$ and ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}\mathrm{Si}$ were investigated through low field magnetization and ac susceptibility measurements. These data indicate that the conical to skyrmion transition that occurs with the application of magnetic field in $\mathrm{Mn}{\mathrm{Si}}_{0.962}{\mathrm{Al}}_{0.038}$ is characterized by clear discontinuity in the magnetic entropy as expected for first order topological phase transition. These same magnetoentropic features are negligibly small in isostructural ${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}\mathrm{Si}$ due to the level of chemical substitution related disorder and differences in the spin dynamics (range and timescales). Despite the obvious similarities in the magnetic structures of these two compounds, the transitions between these phases is substantially different indicating a surprising nonuniversality to the magnetic phase transitions in this class of materials.

We theoretically explore two-dimensional (2D) moiré heterostructures in lattice-commensurate magnetic fields as platforms for quantum simulation of a paradigmatic model of non-Fermi-liquid physics: a Fermi surface coupled to a fluctuating gauge field. In these moiré Hofstadter (MH) systems, long-wavelength acoustic phonons exhibit singular interactions with electrons analogous to those of electrons with 2D gauge fields. This leads to a breakdown of Fermi-liquid theory at low temperatures. We show that a combination of large moiré unit-cell size, tunable Fermi-surface topology, and enhanced coupling to interlayer sliding modes enhances these effects by over many orders of magnitude compared with bulk crystals, placing them within experimental reach. Though we find that the asymptotic low-temperature non-Fermi-liquid regime remains at prohibitively low temperatures, striking precursor non-Fermi-liquid signatures can be observed, and we propose surface acoustic wave attenuation and quantum oscillation transport experiments. We also study the motion of MH acoustic polarons, which we predict exhibit logarithmically diverging effective mass and unconventional magnetic field scaling for scaling of cyclotron resonance frequency and quantum oscillation amplitude.

Traditional band theory of perfect crystalline solids often uses as input the structure deduced from diffraction experiments; when modeled by the minimal unit cell this often produces a spatially averaged model. The present study illustrates that this is not always a safe practice unless one examines if the intrinsic bonding mechanism is capable of benefiting from the formation of a distribution of lower symmetry local environments that differ from the macroscopically averaged structure. This can happen either due to positional or to magnetic symmetry breaking. By removing the constraint of a small crystallographic cell, the energy minimization in the density functional theory finds atomic and spin symmetry breaking, not evident in conventional diffraction experiments but being found by local probes such as atomic pair distribution function analysis. Here we report that large atomic and electronic anomalies in bulk tetragonal FeSe emerge from the existence of distributions of local positional and magnetic moment motifs. The found symmetry-broken motifs obtained by minimization of the internal energy represent what chemical bonding in the tetragonal phase prefers as intrinsic energy lowering (stabilizing) static distortions. This explains observations of band renormalization, predicts orbital order and enhanced nematicity, and provides unprecedented close agreement with spectral function measured by photoemission and local atomic environment revealed by the pair distribution function. While the symmetry-restricted strong correlation approach has been argued previously to be the exclusive theory needed for describing the main peculiarities of FeSe, we show here that the symmetry-broken mean-field approach addresses numerous aspects of the problem, provides intuitive insight into the electronic structure, and opens the door for large-scale mean-field calculations for similar $d$-electron quantum materials.

The passage of an electric current in a material can cause a biased mass transport at its surface. This migration phenomenon is intimately related to the microscopic details of atomic processes of diffusion and attachment/detachment at step edges. Using low-energy electron microscopy, we have examined in operando under an electric current the migration of $\mathrm{Si}\left(111\right)\text{−}1\times 1$ advacancy islands confined on $\mathrm{Si}\left(111\right)\text{−}7\times 7$ terraces. The islands move opposite to the current direction, with velocity increasing with the radius. The effective valence of Si adatoms is $2.8\pm 0.5$ and the kinetic length of attachment-detachment is about 500 nm. The analysis of the island's shape reveals that the electric current significantly biases the kinetic rate of mass transfers at step edges modifying the overall island's shape.

We report on theoretical and experimental study of the spin polarization recovery and Hanle effect for the charge carriers interacting with the fluctuating nuclear spins in the semiconductor structures. We start the theoretical description from the simplest model of static and isotropic nuclear spin fluctuations. Then we describe the modification of the polarization recovery and Hanle curves due to the anisotropy of the hyperfine interaction, finite nuclear spin correlation time, and the strong pulsed spin excitation. For the latter case, we predict the appearance of the resonant spin amplification in the Faraday geometry and of the quantum Zeno effect. The set of the experimental results for various structures and experimental conditions is chosen to highlight the specific effects predicted theoretically. We show that the joint analysis of the spin polarization recovery and the Hanle effect is a very valuable tool for addressing carrier spin dynamics in semiconductors and their nanostructures.

The nondispersive flat band in the $\alpha \text{−}{T}_3$ model leads to a conventional integer quantum Hall effect (IQHE) for Dirac particles as well as the existence of the zero Hall-conductivity plateau ($\nu =0$) at the band center. In this work, we study the broken-flat-band effect on the IQHE by introducing the disorder or staggered lattice potential in the system. It is found that the zero-Hall plateau is quite vulnerable and readily destroyed by weak disorders and even the neighboring plateaux $\nu =\pm 2$ are also smeared. When a uniform staggered lattice potential is turned on without disorder, a new serial of integer Hall plateaux with ${\sigma }_{xy}=\nu e^2/h$ ($\nu =\pm 2,\pm 4,\cdots$) grow out in the zero-Hall conductivity regime and show no relativistic properties. This emergent IQHE is independent of the strong magnetic field and even fully valley polarized. The findings indicate that the broken flat band in the $\alpha \text{−}{T}_3$ model shall make a great difference on the transport properties of Dirac particles.

Understanding strongly interacting electrons enables the design of materials, nanostructures, and devices. Developing this understanding relies on the ability to tune and control electron-electron interactions by, e.g., confining electrons to atomically thin layers of two-dimensional crystals with reduced screening. The interplay of strong interactions on a hexagonal lattice with two nonequivalent valleys, topological moments, and the Ising-like spin-orbit interaction gives rise to a variety of phases of matter corresponding to valley and spin-polarized broken-symmetry states. In this paper we describe a highly tunable strongly interacting system of electrons laterally confined to monolayer transition-metal dichalcogenide $\mathrm{Mo}{\mathrm{S}}_2$ by metallic gates. We predict the existence of valley- and spin-polarized broken-symmetry states tunable by the parabolic confining potential using exact diagonalization techniques for up to $N=6$ electrons. We find that the ground state is formed by one of two phases, both spin and valley polarized or valley unpolarized but spin intervalley antiferromagnetic, which compete as a function of electronic shell spacing. This finding can be traced back to the combined effect of Ising-like spin-orbit coupling and weak intervalley exchange interaction. These results provide an explanation for interaction-driven symmetry-breaking effects in valley systems and highlight the important role of electron-electron interactions for designing valleytronic devices.

Both applied electric currents and magnetization dynamics modify the Dzyaloshinskii-Moriya interaction (DMI), which we call current-induced DMI (CIDMI) and dynamical DMI (DDMI), respectively. We report a theory of CIDMI and DDMI. The inverse of CIDMI consists in charge pumping by a time-dependent gradient of magnetization ${\partial }^2\mathit{M}(\mathit{r},t)/\partial \mathit{r}\partial t$, while the inverse of DDMI describes the torque generated by ${\partial }^2\mathit{M}(\mathit{r},t)/\partial \mathit{r}\partial t$. In noncollinear magnets, CIDMI and DDMI depend on the local magnetization direction. The resulting spatial gradients correspond to torques that need to be included into the theories of Gilbert damping, gyromagnetism, and current-induced torques (CITs) in order to satisfy the Onsager reciprocity relations. CIDMI is related to the modification of orbital magnetism induced by magnetization dynamics, which we call dynamical orbital magnetism (DOM), and spatial gradients of DOM contribute to charge pumping. We present applications of this formalism to the CITs and to the torque-torque correlation in textured Rashba ferromagnets.

The insulating ferrimagnet ${\mathrm{Cu}}_2\mathrm{O}{\mathrm{SeO}}_3$ shows a rich variety of phases such as skyrmion lattice and helical magnetism controlled by interplay of different exchange interactions which can be tuned by external pressure. In this work we have investigated pressure-induced phase transitions at room temperature using synchrotron-based x-ray diffraction and Raman-scattering measurements. With first-principles theoretical analysis, we show that spin-spin exchange couplings in the ambient cubic phase are affected notably by hydrostatic pressure. The ambient cubic phase transforms to a monoclinic phase above 7 GPa and then to the triclinic phase above 11 GPa. Emergence of new phonon modes in the Raman spectra confirms these structural phase transitions. Notably, upon decompression, the crystal undergoes a transition to a new monoclinic structure. Atomic coordinates have been refined in the low-pressure cubic phase to capture the Cu-tetrahedra evolution responsible for the earlier reported magnetic behavior under pressure. Our experiments will motivate further studies of its emergent magnetic behavior under pressure.

We present an effective medium description of time-modulated dielectric media. By taking the averaged fields over one modulation period, the relationship between them is derived, therefore defining the different constitutive parameters. In the most general situation, it is found that the effective material is described by means of a spatially and temporally dispersive transverse dielectric function and a constant longitudinal dielectric function. It has been also found that the frequency dependence in the former is weak, in comparison with its wavenumber dependence (spatial dispersion). Different physical consequences of this spatial dispersion are discussed, with special emphasis on the weak dispersion approximation and the limit in which it is found that the effective material behaves as a resonant and isotropic magnetodielectric medium with no additional longitudinal mode, as it is commonly found in spatially dispersive materials. Time-dependent media therefore opens an alternative way of designing dynamically tunable metamaterials.