This paper focuses on the dynamics of a domain wall (DW) displacing along an elliptically bent nanowire (NW) under the action of spin-polarized electric currents and external magnetic fields. Our results evidence that a curvature gradient induces an exchange-driven effective tangential field responsible for pinning a DW at the maximum curvature point in the NW. However, the competition between the torques produced by the external stimuli and the curvature-induced effective fields changes the DW equilibrium position and phase. Therefore, when the external stimuli are below a certain threshold, the DW follows a damped harmonic oscillation around the new equilibrium position. Above this threshold, DW displaces along the NW under an oscillatory translational motion.

We report the thermodynamic properties studied by thermal expansion, magnetostriction, magnetization, and specific heat measurements as well as the low-energy magnetic excitations of ${\mathrm{MnTiO}}_3$ and investigate how magnetoelastic coupling and magnetic anisotropy affect the evolution of long-range order and the magnetic phase diagram. Specifically, we utilize high-resolution capacitance dilatometry and antiferromagnetic resonance (AFMR) studies by means of high-frequency electron spin resonance (HF-ESR) spectroscopy. The role of anisotropy is reflected by spin reorientation at $B_{\mathrm{SF}}\simeq 6\mathrm{T}$ and a corresponding sign change in $\partial T_{\mathrm{N}}/\partial B$. Analysis of the AFMR modes enables us to establish the zero-field excitation gap $\mathrm{\Delta }$ as well as its temperature dependence. We derive the effective anisotropy field $B_{\mathrm{A}}=0.16\left(1\right)\mathrm{T}$ which predominately originates from out-of-plane nearest-neighbor magnetic dipole interactions. Despite the nearly fully quenched orbital moment, our data show pronounced thermal expansion and magnetostriction anomalies at $T_{\mathrm{N}}$ and $B_{\mathrm{SF}}$, respectively, which allows the experimental determination of sizable uniaxial pressure dependencies, i.e., $\partial B_{\mathrm{SF}}/\partial p_{\mathrm{c}}=-0.20\left(2\right)\mathrm{T}/\mathrm{GPa}, \partial T_{\mathrm{N}}/\partial p_b=0.69\left(12\right)\mathrm{K}/\mathrm{GPa}$, and $\partial T_{\mathrm{N}}/\partial p_c=-2.0\left(4\right)\mathrm{K}/\mathrm{GPa}$. While in the thermodynamic properties the presence of short-range magnetic order up to $\simeq 170\mathrm{K}$ is detected by anisotropic lattice distortion and by the violation of a constant Grüneisen behavior, the presence of local magnetic fields is seen in ESR at even higher temperatures of at least $3T_{\mathrm{N}}$.

Easy axis antiferromagnets are robust against external magnetic fields of moderate strength. Spin reorientations in strong fields can provide insight into more subtle properties of antiferromagnetic materials, which are often hidden by their high ground-state symmetry. Here, we investigate theoretically effects of curvature in ring-shaped antiferromagnetic achiral anisotropic spin chains in strong magnetic fields. We identify the geometry-governed helimagnetic phase transition above the spin-flop field between vortex and onion states. The curvature-induced Dzyaloshinskii–Moriya interaction results in the spin-flop transition being of first or second order, depending on the ring curvature. Spatial inhomogeneity of the Néel vector in the spin-flop phase generates weak ferromagnetic response in the plane perpendicular to the applied magnetic field. Our paper contributes to the understanding of the physics of curvilinear antiferromagnets in magnetic fields and guides prospective experimental studies of geometrical effects relying on spin-chain-based nanomagnets.

The second-order magneto-optical effects, represented by Voigt and Schäfer-Hubert effects, are effective methods to detect the spin textures in antiferromagnets, whereas the previous studies are usually limited to collinear antiferromagnets. In noncollinear antiferromagnets, the spin textures characterized by spin chirality have been revealed to play a critical role in many exciting physics. In particular, the first-order topological magneto-optical effects originated from scalar spin chirality have been discovered recently. In this work, using the first-principles calculations and group-theory analysis, we generalize the first-order topological magneto-optical effects to the second-order cases, that is, topological Voigt and Schäfer-Hubert effects, by taking the noncoplanar $3Q$ spin state of $\gamma \text{−}{\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ alloy as an example. The conventional Voigt and Schäfer-Hubert effects are comparatively studied in the collinear $1Q$ and $2Q$ spin states of $\gamma \text{−}{\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ alloy. In addition, the natural linear birefringence due to crystal anisotropy is discussed in the strained $1Q, 2Q$, and $3Q$ states, and a unique fingerprint for experimentally distinguishing the second-order topological magneto-optical effects and natural linear birefringence is identified. Our work brings a topological insight into the second-order magneto-optical effects in noncoplanar antiferromagnets and also provides $\gamma \text{−}{\mathrm{Fe}}_x{\mathrm{Mn}}_{1-x}$ as an attractive material platform for future experimental exploration.

The honeycomb magnet $\alpha \text{−}{\mathrm{RuCl}}_3$ is a prime candidate material for realizing the Kitaev quantum spin liquid (QSL), but it shows long-range magnetic order at low temperature. Nevertheless, its broad inelastic neutron scattering (INS) response at finite frequency has been interpreted as that of a “proximate QSL.” A moderate in-plane magnetic field indeed melts the residual zigzag order, giving rise to peculiar intermediate-field phases before the high-field polarized state. In INS measurements the low-frequency spin waves disappear, leading to a broad scattering continuum in the field-induced intermediate regime, whose nature is currently under debate. Here, we study the magnetic-field-dependent spin dynamics of the $K\text{−}\mathrm{\Gamma }\text{−}{\mathrm{\Gamma }}^{\prime }$ model within a stochastic semiclassical treatment, which incorporates the effect of finite-temperature fluctuations. At temperatures relevant for INS experiments, we show how the excitations of the zigzag phase broaden and that the different intermediate phases all show a similar continuum response. We discuss the implications of our results for experiments and highlight the importance of distinguishing finite-temperature fluctuations from genuine quantum fractionalization signatures in frustrated magnets.

The complete thermodynamic potential of (111) ferroelectric thin films including anisotropic in-plane misfit strain, in-plane shear strain, and external stress is developed based on the Landau-Ginzburg-Devonshire thermodynamic theory. Anisotropic strain-tuned low-symmetry equilibrium phases in (111) single-domain ferroelectric $\mathrm{PbTi}{\mathrm{O}}_3$ films were investigated via the developed nonlinear thermodynamic potential and the misfit strain–misfit strain phase diagram was constructed. It was found that the anisotropic epitaxial strain can effectively control the orientations of polarization vectors, which determine the symmetry of equilibrium phases, and their functionalities. In particular, two types of low-symmetry triclinic phases, disappearing under isotropic strain states, can be stabilized by anisotropic strains. Low-symmetry monoclinic and triclinic ferroelectric phases and field-induced phase transitions were also analyzed in depth. It was indicated that the monoclinic ($M_A^1$) symmetry rhombohedral-like–tetragonal-like transition and triclinic ($T_{\mathrm{r}}^1$)-monoclinic ($M_B^1$) phase transition would give rise to the superior piezoelectric properties as compared to other structure phase transitions. Electric field-enhanced piezoelectric response in triclinic structure originates from the elastic softening that is related to the shear $u_5$. These results provide guidance for interpreting and understanding more experimental observations as well as the designing of high-performance piezoelectric films.

Floquet engineering offers a powerful approach to the generation of nonequilibrium topological phases with large topological invariants and hence a proliferation of topological edge modes. It is thus of importance to develop feasible experimental approaches to detect such topological phases. Using the nitrogen-vacancy center in diamond and its synthetic dimensions, here we experimentally demonstrate how Floquet Chern insulator phases can be clearly detected through imaging the static and dynamic spin textures in momentum space. In particular, we simulate a periodically quenched generalized Haldane model and observe different topological phases with Chern numbers $C=1,2,4$. In addition to confirming the versatility of Floquet driving in generating phases with large Chern numbers, this work clearly establishes an experimental method to detect Floquet topological phases in two and higher spatial dimensions.

We show that localization in quasiperiodically modulated, two-dimensional systems is stable to the presence of a finite density of ergodic grains. This contrasts with the case of randomly modulated systems, where such grains seed thermalizing avalanches. These results are obtained within a quantitatively accurate, self-consistent entanglement mean-field theory, which analytically describes two level systems connected to a central ergodic grain. The theory predicts the distribution of entanglement entropies of each two level system across eigenstates, and the late time values of dynamical observables. In addition to recovering the known phenomenology of avalanches, the theory reproduces exact diagonalization data, and predicts the spatial profile of the thermalized region when the avalanche fails.

Whether disordered and quasiperiodic many-body quantum systems host a long-lived localized phase in the thermodynamic limit has been the subject of intense recent debate. While in one dimension, substantial evidence for such a many-body localized (MBL) phase exists, the behavior in higher dimensions remains an open puzzle. In disordered systems, for instance, it has been argued that rare regions may lead to thermalization of the whole system through a mechanism dubbed avalanche instability. In quasiperiodic systems, however, rare regions are altogether absent and the fate of a putative many-body localized phase has hitherto remained largely unexplored. In this work, we investigate the localization properties of two many-body quasiperiodic models, which are two-dimensional generalizations of the Aubry-André model. By studying numerically the out-of-equilibrium dynamics of large systems, we find very long-lived localization on experimentally relevant time scales. Surprisingly, we also observe large-scale transport along deterministic lines of weak potential, which appear in the investigated quasiperiodic models. Our results demonstrate that quasiperiodic many-body systems have the remarkable and counter-intuitive capability of exhibiting coexisting localization and transport properties—a phenomenon reminiscent of the behavior of supersolids. Our findings are of direct experimental relevance and can be tested, for instance, using state-of-the-art cold atomic systems.

Anderson localization is a universal interference phenomenon occurring when a wave evolves through a random medium and it has been observed in a great variety of physical systems, either quantum or classical. The recently developed localization landscape theory offers a computationally affordable way to obtain useful information on localized modes, such as their location or size. Here we examine this theory in the context of classical waves exhibiting high-frequency localization and for which the original localization landscape approach is no longer informative. Using the so-called Webster's transformation, to convert a classical wave equation into a Schrödinger equation with the same localization properties, and combining a set of frequency-shifted operators, we introduce an optimized localization landscape. This optimized localization landscape offers an affordable way to reveal key information on mode localization across the frequency spectrum.

The magnon Hedin's equations are derived via the Schwinger functional derivative technique, and the resulting self-consistent Green's function (GF) method is used to calculate ground state spin patterns and magnetic structure factors for two-dimensional magnetic systems with frustrated spin-$\frac{1}{2}$ Heisenberg exchange coupling. Compared with random phase approximation treatments, the inclusion of a self-energy correction improves the accuracy in the case of scalar product interactions, as shown by comparisons between our method and exact benchmarks in homogeneous and inhomogeneous finite systems. We also find that, for cross-product interactions (e.g., antisymmetric exchange), the method does not perform equally well, and an inclusion of higher corrections is in order. Aside from indications for future work, our results clearly indicate that the GF method in the form proposed here already shows potential advantages in the description of systems with a large number of atoms as well as long-range interactions.

Multiferroics with coupling between ferroelectricity and magnetism have been pursued for decades. However, their magnetoelectric performances remain limited due to the common trade-off between ferroelectricity and magnetism. Here, a family of nitride perovskites is proposed as multiferroics with prominent physical properties and nontrivial mechanisms. Taking ${\mathrm{GdWN}}_3$ as a prototype, our first-principles calculations found that its perovskite phases own large polarizations (e.g., $111.3\mu \mathrm{C}/{\mathrm{cm}}^2$ for the $R3c$ phase) and a magnetic moment $7{\mu }_{\mathrm{B}}/{\mathrm{Gd}}^{3+}$. More interestingly, its ferroelectric origin is multiple, with significant contributions from both ${\mathrm{Gd}}^{3+}$ and ${\mathrm{W}}^{6+}$ ions, different from its sister member ${\mathrm{LaWN}}_3$ in which the ferroelectricity almost arises from ${\mathrm{W}}^{6+}$ ions only. With decreasing size of rare-earth ions, the $A$ site ions would contribute more and more to the ferroelectric instability. Considering that small rare-earth ions can be primary origins of both proper ferroelectricity and magnetism in nitride perovskites, our work provides a route to pursuit more multiferroics with unconventional mechanisms and optimal performances.

Ferrimagnets, such as ${\mathrm{Gd}}_x\mathrm{Fe}{\mathrm{Co}}_{(1–x)}$, can produce ultrafast magnetic switching and oscillation due to the strong exchange field. The two-sublattice macrospin model has been widely used to explain the experimental results. However, it fails in describing the spatially nonuniform magnetic dynamics which gives rises to many important phenomena such as domain walls and skyrmions. Here we develop a two-dimensional atomistic model and provide a torque analysis method to study the ferrimagnetic oscillation. Under spin-transfer torque, the magnetization oscillates in the exchange mode or the flipped exchange mode. When the Gd composition is increased, the exchange mode first disappears, and then appears again as the magnetization compensation point is reached. We show that these results can only be explained by analyzing the spatial distribution of magnetization and effective fields. In particular, when the sample is small, a spatially nonuniform oscillation is also observed in the square film. Our work reveals the importance of spatial magnetic distributions in understanding the ferrimagnetic dynamics. The method developed in this paper provides an important tool to gain a deeper understanding of ferrimagnets and antiferromagnets. The observed ultrafast dynamics can also stimulate the development of THz oscillators.

Any matrix product state $|\mathrm{\Psi }\rangle$ has a set of associated kept and discarded spaces, needed for the description of $|\mathrm{\Psi }\rangle$, and changes thereof, respectively. These induce a partition of the full Hilbert space of the system into mutually orthogonal spaces of irreducible $n$-site variations of $|\mathrm{\Psi }\rangle$. Here, we introduce a convenient projector formalism and diagrammatic notation to characterize these $n$-site spaces explicitly. This greatly facilitates the formulation of MPS algorithms that explicitly or implicitly employ discarded spaces. As an illustration, we derive an explicit expression for the $n$-site energy variance and evaluate it numerically for a model with long-range hopping. We also describe an efficient algorithm for computing low-lying $n$-site excitations above a finite MPS ground state.

We introduce two exotic lattice models on a general spatial graph. The first one is a matter theory of a compact Lifshitz scalar field, while the second one is a certain rank-2 $\text{U}\left(1\right)$ gauge theory of fractons. Both lattice models are defined via the discrete Laplacian operator on a general graph. We unveil an intriguing correspondence between the physical observables of these lattice models and graph theory quantities. For instance, the ground state degeneracy of the matter theory equals the number of spanning trees of the spatial graph, which is a common measure of complexity in graph theory (“GSD = complexity”). The discrete global symmetry is identified as the Jacobian group of the graph. In the gauge theory, superselection sectors of fractons are in one-to-one correspondence with the divisor classes in graph theory. In particular, under mild assumptions on the spatial graph, the fracton immobility is proven using a graph-theoretic Abel-Jacobi map.

We study a model for photovoltaic energy collection consisting of a Mott-insulating layer in presence of acoustic phonons, coupled to two wide-band fermion leads at different chemical potentials and driven into a nonequilibrium steady state by a periodic electric field. We treat electron correlations with nonequilibrium dynamical mean-field theory using the so-called auxiliary master-equation approach as impurity solver and include dissipation by acoustic phonons via the Migdal approximation. For a small hybridization to the leads, we obtain a peak in the photocurrent as a function of the driving frequency which can be associated with impact ionization processes. For larger hybridizations the shallow peak suggests a suppression of impact ionization with respect to direct photovoltaic excitations. Acoustic phonons slightly enhance the photocurrent for small driving frequencies and suppress it at frequencies around the main peak at all considered hybridization strengths.

We theoretically investigate the optical energy absorption of crystalline silicon subject to dual-color femtosecond laser pulses, using the time-dependent density functional theory. We employ the modified Becke-Johnson exchange-correlation potential, which reproduces the experimental direct band gap energy $E_g$. We consider situations where one color is in the ultraviolet (UV) range above $E_g$ and the other is in the infrared (IR) range below it. The energy deposition is examined as a function of mixing ratio $\eta$ of the two colors with the total pulse energy conserved. Energy transfer from the laser pulse to the electronic system in silicon is dramatically enhanced by simultaneous dual-color irradiation and maximized at $\eta \sim 0.5$. The number of generated carriers, not the absorbed energy per carrier, is increased. The effect is more efficient for lower IR photon energy, or, equivalently, larger vector-potential amplitude. The interplay between intraband electron motion in the valence band (before excitation) driven by the IR component and resonant valence-to-conduction interband excitation (carrier injection) induced by the UV component is identified as the underlying mechanism. The former increases excitable electrons which pass through the $k$ points of resonant transitions. The effect of different multiphoton absorption paths or intraband motion of carriers generated in the conduction band plays a minor role.

Recently, an extremely low lattice thermal conductivity value has been reported for the alkali-based telluride material ${\mathrm{BaIn}}_2{\mathrm{Te}}_4$. The value is comparable with low-thermal conductivity metal chalcogenides, and the glass limit is highly intriguing. Therefore, to shed light on this issue, we performed first-principles phonon thermal transport calculations. We predicted highly anisotropic lattice thermal conductivity along different directions via the solution of the linearized phonon Boltzmann transport equation. More importantly, we determined several different factors as the main sources of the predicted ultralow lattice thermal conductivity of this crystal, such as the strong interactions between low-frequency optical phonons and acoustic phonons, small phonon group velocities, and lattice anharmonicity indicated by large negative mode Grüneisen parameters. Along with thermal transport calculations, we also investigated the electronic transport properties by accurately calculating the scattering mechanisms, namely the acoustic deformation potential, ionized impurity, and polar optical scatterings. The inclusion of spin-orbit coupling (SOC) for electronic structure is found to strongly affect the $p$-type Seebeck coefficients. Finally, we calculated the thermoelectric properties accurately, and the optimal $ZT$ value of $p$-type doping, which originated from high Seebeck coefficients, was predicted to exceed unity after 700 K and have a direction averaged value of 1.63 (1.76 in the $y$-direction) at 1000 K around $2\times {10}^{20}{\mathrm{cm}}^{-3}$ hole concentration. For $n$-type doping, a $ZT$ around $3.2\times {10}^{19}{\mathrm{cm}}^{-3}$ concentration was predicted to be a direction-averaged value of 1.40 (1.76 in the $z$-direction) at 1000 K, mostly originating from its high electron mobility. With the experimental evidence of high thermal stability, we showed that the ${\mathrm{BaIn}}_2{\mathrm{Te}}_4$ compound has the potential to be a promising mid- to high-temperature thermoelectric material for both $p$-type and $n$-type systems with appropriate doping.