We investigate the effects of electronic correlations on the Bernevig-Hughes-Zhang model using the real-space density matrix renormalization group (DMRG) algorithm. We introduce a method to probe topological phase transitions in systems with strong correlations using DMRG, substantiated by an unsupervised machine learning methodology that analyzes the orbital structure of the real-space edges. Including the full multi-orbital Hubbard interaction term, we construct a phase diagram as a function of a gap parameter ($m$) and the Hubbard interaction strength ($U$) via exact DMRG simulations on $N\times 4$ cylinders. Our analysis confirms that the topological phase persists in the presence of interactions, consistent with previous studies, but it also reveals an intriguing phase transition from a paramagnetic to a stripey antiferromagnetic topological insulator. The combination of the magnetic structure factor, strength of magnetic moments, and the orbitally resolved density, provides real-space information on both topology and magnetism in a strongly correlated system.

The local density of states (LDOS) of electromagnetic field drives many basic processes associated with light-matter interaction such as the thermal emission of objects, the spontaneous emission of quantum systems, or the fluctuation-induced electromagnetic forces on molecules. Here, we study the LDOS in the close vicinity of magneto-optical metamaterials under the influence of an external magnetic field and demonstrate that it can be efficiently altered in a narrow or a broad spectral range simply by changing the spatial orientation or the magnitude of this field. This result paves the way for an active control of the photonic density of states at deep subwavelength scale.

Size-selected FeRh clusters have been deposited at low energy and under ultra-high vacuum conditions, on a ${\mathrm{BaTiO}}_3$ epitaxial thin film. Using x-ray diffraction in grazing incidence configuration, we have observed the chemical ordering of FeRh nanoparticles into the chemically ordered B2 phase after annealing, while a reciprocal space mapping indicates that particles, despite their random deposition, are finally adopting preferential orientations reflecting an atomic ordering with the ${\mathrm{BaTiO}}_3$ crystal. In addition to the usual epitaxy relationship observed for FeRh thin films, an unexpected orientation is detected (${45}^{\circ }$ in-plane rotation, leading to a new cube-on-cube epitaxy relationship), which must be specific to nanosized FeRh particles.

We present a generalization of the geometric phase to pure and thermal states in $\mathcal{PT}$-symmetric quantum mechanics (PTQM) based on the approach of the interferometric geometric phase (IGP). The formalism first introduces the parallel-transport conditions of quantum states and reveals two geometric phases, ${\theta }^1$ and ${\theta }^2$, for pure states in PTQM according to the states under parallel-transport. Due to the non-Hermitian Hamiltonian in PTQM, ${\theta }^1$ is complex and ${\theta }^2$ is its real part. The imaginary part of ${\theta }^1$ plays an important role when we generalize the IGP to thermal states in PTQM. The generalized IGP modifies the thermal distribution of a thermal state, thereby introducing effective temperatures. At certain critical points, the generalized IGP may exhibit discrete jumps at finite temperatures, signaling a geometric phase transition. We illustrate this phenomenon in PTQM through two examples and compare their differences.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have emerged as a materials paradigm for realizing next-generation on-chip electronic and optoelectronic devices. Strain engineering is actively pursued to tune the electronic properties of 2D TMDCs. However, a generalizable, analytical approach for describing the underlying physics of strain effects on band structure is still lacking. Here, we develop a tight-binding model (TBM) that incorporates strain effect to characterize the band structure tuning of TMDC $({\mathrm{MoS}}_2,$ ${\mathrm{MoSe}}_2,$ ${\mathrm{WS}}_2, \text{and} {\mathrm{WSe}}_2)$ monolayers under biaxial strain fields; strain-dependent Slater-Koster parameters are employed to describe electron hopping and orbital overlap in the strained monolayers. Our approach follows from the Wills-Harrison suggestions of a linear relationship between biaxial strain and Slater-Koster parameters. This leads to a linear dependence of the electronic band gap on applied strain for both direct-indirect $\left({\mathrm{MoX}}_2\right)$ and indirect-direct $\left({\mathrm{WX}}_2\right)$ band gap transitions. We further study the influence of biaxial strain on the energy differences between different high-symmetry points in $k$ space to deduce the physical origin of strain-induced variations in the band gap type and size. In this process, we select different TBMs (6- or 11-band) and compare them with different first-principles calculation results (DFT-PBE or DFT-HSE) to demonstrate the effectiveness and completeness of our method. Building on this model, we also examined the changes in effective mass and optical conductivity of TMDCs under strain, offering insights that can aid in the development of practical device applications utilizing these materials. Our investigation may be extended to general strained monolayer TMDCs, paving the way for exploring the electronic properties of nanotubes, wrinkled 2D materials, and van der Waals heterostructures under inhomogeneous strain.

We analyze, from first-principles calculations, the excitonic properties of monolayer black phosphorus (BP) under strain, as well as of bilayer BP with different stacking registries, as a base platform for the observation and use of hyperbolic polaritons. In the unstrained case, our results confirm the in-plane hyperbolic behavior of polaritons coupled to the ground-state excitons in both mono- and bilayer systems, as observed in recent experiments. With strain, we reveal that the exciton-polariton hyperbolicity in monolayer BP is enhanced (reduced) by compressive (tensile) strain in the zig-zag direction of the crystal. In the bilayer case, different stacking registries are shown to exhibit hyperbolic exciton polaritons with different dispersion, while also peaking at different frequencies. This renders both mechanical stress and stacking registry control as practical tools for tuning physical properties of hyperbolic exciton polaritons in black phosphorus, which facilitates detection and further optoelectronic use of these quasiparticles.

Using first-principles calculations and atomistic spin simulations, we predict stable isolated skyrmions with a diameter below 10 nm in a monolayer of the two-dimensional van der Waals ferromagnet ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$, a material of significant experimental interest. A very large Dzyaloshinskii-Moriya interaction (DMI) is observed due to the intrinsic broken inversion symmetry and strong spin-orbit coupling for monolayer ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$. We show that the nearest-neighbor approximation, often used in the literature, fails to describe the DMI. The strong DMI together with moderate in-plane magnetocrystalline anisotropy energy allows to stabilize nanoscale skyrmions in out-of-plane magnetic fields above $\approx 2$ T. The energy barriers of skyrmions in monolayer ${\mathrm{Fe}}_5{\mathrm{GeTe}}_2$ are comparable to those of state-of-the-art transition-metal ultrathin films. We further predict that these nanoscale skyrmions can be stable for hours at temperatures up to 20 K.

Recently it has been shown experimentally that the superconducting state of ${\mathrm{FeSe}}_{1-x}{\mathrm{S}}_x$ exhibits Bogoliubov Fermi surfaces for $x>0.17$. These Bogoliubov Fermi surfaces appear together with broken time-reversal symmetry and surprisingly demonstrate nematic behavior in a structurally tetragonal phase. Here, through a comprehensive analysis that deduces the structure of Bogoliubov Fermi surfaces from all symmetry-allowed types of translation invariant broken time-reversal symmetry, we find that the origin of nematic Bogoliubov Fermi surfaces is due to magnetic toroidal order that belongs to an $E_u$ irreducible representation of the $D_{4h}$ point group. We further show that this magnetic toroidal order appears as a consequence of either static Néel antiferromagnetic order or due to the formation of a spontaneous pair density wave superconducting order. Finally, we reveal that independent of the presence of Bogoliubov Fermi surfaces, supercurrents will induce Néel magnetic order in many Fe-based superconductors.

Second-order phase transitions appear as a divergence in one of the linear response functions. For a system of correlated electrons, the relevant divergent response can and does involve many-particle observables, most famously the double occupancy. Generally, evaluating the linear response function of many-particle observables requires a many-particle generalization of the Bethe-Salpeter equation. However, here I show that the divergence of linear response functions in dynamical mean-field theory is governed by a two-particle Bethe-Salpeter equation, even for many-particle observables. The reason for this is that the divergence at the second-order phase transition is produced by the self-consistent feedback of the dynamical mean field.

We study plasmons in a rectangular two-dimensional (2D) electron system in the vicinity of a planar metal electrode (gate) and in the presence of a perpendicular uniform magnetic field, using Maxwell's equations and neglecting retardation effects. The conductivity of the 2D system is characterized by the dynamical Drude model without taking collisional relaxation into account, which well describes both high mobility graphene and other field effect transistor structures, including quantum wells like Ga(Al)As, in the terahertz and in some cases subterahertz frequency ranges. Without a magnetic field, we analytically find the current distribution and frequency of plasma eigenmodes when the plasmon wavelength is much larger than the distance to the gate, i.e., in the fully screened limit. To find an approximate solution in a magnetic field, we expand current in the complete set of eigenmodes without magnetic fields. Analytical asymptotic expressions in weak and strong magnetic fields were obtained for the lower modes. Unlike the disk and stripe, the frequencies of these modes tend to zero as the magnetic field tends to infinity. We also discuss a direct analogy to rotational gravity shallow water waves, where size-quantized Poincare waves correspond to size-quantized magnetoplasmons, while Kelvin waves correspond to edge magnetoplasmons.

Models of many-body localization (MBL) can be represented as tight-binding models in the many-body Hilbert space (Fock space). We explore the role of correlations between matrix elements of the effective Fock-space Hamiltonians in the scaling of MBL critical disorder $W_c\left(n\right)$ with the size $n$ of the system. For this purpose, we consider five models, which all have the same distributions of diagonal (energy) and off-diagonal (“hopping”) Fock-space matrix elements but different Fock-space correlations. These include quantum-dot (QD) and one-dimensional (1D) MBL models, their modifications (uQD and u1D models) with removed correlations of off-diagonal matrix elements, as well a quantum random energy model (QREM) with no correlations at all. Our numerical results are in full consistency with analytical arguments predicting $n^{3/4}{(lnn)}^{-1/4}\lesssim W_c\lesssim nlnn$ for the scaling of $W_c\left(n\right)$ in the QD model (we find $W_c\sim n$ numerically), $W_c\left(n\right)\sim \text{const}$ for the 1D model, $W_c\sim nlnn$ for the uQD and u1D models without off-diagonal correlations, and $W_c\sim n^{1/2}lnn$ for QREM. The key difference between the QD and 1D models is in the structure of correlations of many-body energies. Removing off-diagonal Fock-space correlations makes both these models “maximally chaotic”. Our findings demonstrate that the scaling of $W_c\left(n\right)$ for MBL transitions is governed by a combined effect of Fock-space correlations of diagonal and off-diagonal matrix elements.

We consider the problem of local correlations in the kicked, dual-unitary coupled maps on $D$-dimensional lattices. We demonstrate that for $D\ge 2$, fully dual-unitary systems exhibit ultralocal correlations: The correlations between any pair of operators with a local support vanish in a finite number of time steps. In addition, for $D=2$, we consider the partially dual-unitary regime of the model, where the dual-unitarity applies to only one of the two spatial directions. For this case, we show that correlations generically decay exponentially and provide an explicit formula for the correlation function between the operators supported on two and four neighboring sites.

Recently observed skyrmionic, bubble, stripelike spin textures, along with multiple possible magnetic and related electrical phases, have renewed interest in doped rare-earth perovskite material, lanthanum strontium manganite $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ (LSMO). Microscopic mechanisms that lead to exotic spin dynamics, spin structures, and electrical transport of LSMO over the varied temperature range are still not well understood. In the present study, temperature-dependent micro-Raman spectroscopic measurements indicate the intriguing trend of the evolution of spin-phonon and electron-phonon couplings with temperature across multiple magnetic and electrical phases in LSMO ($x=0.125$, 0.175) having orthorhombic and rhombohedral phases. Electrical transport and magnetization studies revealed a metal-insulator phase transition in LSMO-0.125, whereas the system remains in the metallic phase in LSMO-0.175 over the low-temperature ferromagnetically ordered state. The observed anomalous Raman spectral shifts across magnetic phase transition temperatures relate to the complex nature of the coupling between spin and phonon dynamics in these compounds. Especially, the systematic modulation in the change in Raman shift with temperature in the ferromagnetic metallic phase, over which these materials are expected to exhibit nearly zero-field stripe spin structure, is noteworthy. The onset of such a trend is observed just above the paramagnetic-to-ferromagnetic phase transition temperature in LSMO-0.125. The unusual variation of Raman spectral linewidth at the crossover between the insulating and metallic ferromagnetic phase in LSMO-0.125 indicates an intertwined electron density of states near Fermi energy and phonon dynamics at this juncture. To this end, we report the expected signature of the existence of orbital polaron in the ferromagnetic insulating phase of LSMO-0.125 by studying the detailed spectral profiles of high wave-number Raman modes under different excitation wavelengths and varying temperatures. Combined manifold spin-phonon coupling schemes and the existence of orbital polaron in the ferromagnetic insulating phase, as could be qualitatively deliberated from Raman measurements, offer a crucial ingredient for a better understanding of microscopic mechanisms for the formation of exotic phases and various spin textures in LSMO.

An excitation of highly nonlinear, complex magnetization dynamics in a ferromagnet, for example chaos, is a new research target in spintronics. This technology is applied to practical applications such as random number generators and information processing systems. One way to induce complex dynamics is applying feedback effect to the ferromagnet. The role of the feedback electric current on the magnetization dynamics was studied in the past. However, there is another way to apply feedback effect to the ferromagnet, namely feedback magnetic field. In this paper, we developed both numerical and theoretical analyses on the role of the feedback magnetic field causing complex magnetization dynamics. The numerical simulation indicates the change of the dynamical behavior from a simple oscillation with a unique frequency to complex dynamics such as amplitude modulation and chaos. The theoretical analyses on the equation of motion qualitatively explain several features found in the numerical simulations, exemplified as an appearance of multipeak structure in the Fourier spectra. The difference of the role of the feedback electric current and magnetic field is also revealed from the theoretical analyses.

Control of polymorphism in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ poses a significant challenge. In this study, we investigate the phase stability and transitions of α-, β-, and κ-${\mathrm{Ga}}_2{\mathrm{O}}_3$ structures using first-principles calculations. Our analysis of the pressure and energy relationship of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ polymorphs demonstrates that external pressure can induce the transition from the ground-state monoclinic β to the corundum α phase, but not to the orthorhombic κ phases because of the small volumetric changes and relatively large energy differences. However, taking κ-${\mathrm{Ga}}_2{\mathrm{O}}_3$ as an example, we propose that introducing holes (e.g., substituting Ga with a valence II ion) can stabilize the metastable phases in combination with pressure because the valence bands of the κ phase are higher than those in the α and β phases. These findings not only rationalize the absence of direct observation of the β→κ transitions with conventional external pressure, but also offer insights into the underlying physical mechanism behind the experimentally observed ion irradiation-induced transitions to metastable phases in ${\mathrm{Ga}}_2{\mathrm{O}}_3$.

The quantum material MnAs manifests both a magnetic and structural phase transition at 318 K. The origin of this anomaly has been the focus of both experimental and theoretical work for many years. Earlier theoretical work usually involved using many empirical parameters in a stat-mech approach. More recent applications of density functional theory (DFT) have seen some researchers suggesting soft mode phonons coupled to spin may be a promising theoretical direction. Here I introduce a different phonon-magnon coupling in which soft mode phonons resonantly excite magnons localized to distances between in-plane Mn-Mn atoms. I show that these magnons reflect between Mn-Mn atoms setting up antiferromagnetic standing waves. Using a quantum stat-mech approach, I show that this mechanism predicts the disordered Mn-Mn distances, as well as the anomalous temperature behaviors of the lattice parameters and magnetic fields. I further predict hysteresis and magnetic heating/cooling in agreement with experiment, as well as the anomalous thermal and elastic properties.

For quantum many-body systems in one dimension, computational complexity theory reveals that the evaluation of ground-state energy remains elusive on quantum computers, contrasting the existence of a classical algorithm for temperatures higher than the inverse logarithm of the system size. This highlights a qualitative difference between low- and high-temperature states in terms of computational complexity. Here, we describe finite-temperature states using the matrix product state formalism. Within the framework of random samplings, we derive an analytical formula for the required number of samples which provides both quantitative and qualitative measures of computational complexity. At high and low temperatures, its scaling behavior with system size is linear and quadratic, respectively, demonstrating a distinct crossover between these numerically difficult regimes of quantitative difference.

Antiferromagnetic van der Waals family $\mathit{M}\text{P}{\mathit{X}}_3(M=\text{Fe},\text{Mn},\text{Co},\text{and}\text{Ni};X=\text{S}\text{and}\text{Se})$ have attracted significant research attention due to the possibility of realizing long-range magnetic order down to the monolayer limit. Here, we perform inelastic neutron scattering measurements on single-crystal samples of ${\mathrm{MnPSe}}_3$, a member of the $\mathit{M}\mathrm{P}{\mathit{X}}_3$ family, to study the spin dynamics and determine the effective spin model. The excited magnon bands are well characterized by a spin model, which includes a Heisenberg term with three intraplane exchange parameters ($J_1=-0.73$ meV, $J_2=-0.014$ meV, $J_3=-0.43$ meV) and one interplane parameter ($J_c=-0.054$ meV), and an easy-plane single-ion anisotropy term ($D=-0.035$ meV). Additionally, we observe the intersection of the magnon and phonon bands but no anomalous spectral features induced by the formation of magnon-phonon hybrid excitations at the intersecting region. We discuss possible reasons for the absence of such hybrid excitations in ${\mathrm{MnPSe}}_3$.

Ultrafast switching of magnetic materials has been shown to be predominantly thermally driven, but excess heating limits the energy efficiency of this process. By employing atomistic spin-lattice dynamics simulations, we show that efficient coherent magnetization switching of an insulating magnet can be triggered by a THz excitation of phonons. We find that switching is driven by excitation near the $P$ point of the phonon spectrum in conditions where spins typically cannot be excited and when manifold $k$ phonon modes are accessible at the same frequency. Our model determines the necessary ingredients for low-dissipative switching and provides insight into THz-excited spin dynamics with a route to energy efficient ultrafast devices.