Engineering topological boundary states have attracted enormous interest with great insight into both the fundamental understanding and future applications of topological states. Despite the fact that several proposals have been made to achieve the second-order corner states from the first-order helical and/or chiral edge states, manipulation of corner states without topological phase transition remain elusive. Here, taking $\gamma \text{−}{\mathrm{FeO}}_2\mathrm{H}$ monolayer and $1{\mathrm{T}}^{\prime }\text{−}{\mathrm{CrCoS}}_4$ monolayer as the material candidates, we propose a general mechanism for manipulating corner states without topological phase transition in two-dimensional intrinsic triferroics. Under in-plane spontaneous polarization, electrons acquire a transverse velocity, leading to the emergence of nontrivial corner states located at the mirror-symmetric corners perpendicular to the polarization direction. Remarkably, in contrast to previously reported proposals with topological phase transition, we put forward that the spatial distribution of the corner states can be effectively engineered by a ferroelastic switching with the second-order topological insulator remaining intact, thereby driving the advancement of multiferroics in topological spintronics.

Quasi-one-dimensional structures, single chains of Pt and Co atoms embedded into mirror twin boundaries in two-dimensional ${\mathrm{MoS}}_2$, have recently been fabricated [Guo et al., Nat. Synth. 1, 245 (2022)]. Using both collinear and noncollinear first-principles calculations, we predict that other transition metals can form similar structures, and that these systems should possess exciting electronic and magnetic properties and specifically exhibit half-metallic electronic behavior. We further analyze the magnetocrystalline anisotropy of these systems and show that they possess unique easy axes and varying strengths of anisotropy, making thus efficient magnetization switching possible. We finally discuss the potentials of using single-atom chains embedded in ${\mathrm{MoS}}_2$ and other transition metal dichalcogenides in spintronic devices.

While many studies point towards the existence of many-body localization (MBL) in one dimension, the fate of higher-dimensional strongly disordered systems is a topic of current debate. The latest experiments as well as several recent numerical studies indicate that such systems behave many-body localized—at least on practically relevant timescales. However, thus far, theoretical approaches have been unable to quantitatively reproduce experimentally measured MBL features—an important requirement to demonstrate their validity. In this Letter, we use fermionic quantum circuits as a variational method to approximate the full set of eigenstates of two-dimensional MBL systems realized in fermionic optical lattice experiments. Using entanglement-based features, we obtain a phase transition point in excellent agreement with the experimentally measured value. Moreover, we calculate the filling-fraction-dependent MBL phase diagram. We argue that our approach best captures the underlying charge-density-wave experiments and compute the mean localization lengths, which can be compared to future experiments.

The ultrahigh-pressure structural evolution and phase diagram of silica (${\mathrm{SiO}}_2$) from 10 TPa (${10}^{12}\mathrm{Pa}$) to 200 TPa are studied. Using a combination of ab initio simulations and a structure search algorithm, we reveal the phase diagram of ${\mathrm{SiO}}_2$ above 10 TPa and confirm that $I4/mmm{\mathrm{SiO}}_2$ with the coordination number of 10 is the most stable phase of ${\mathrm{SiO}}_2$ at ultrahigh pressures. The phase transition pressures from ${\mathrm{Fe}}_2\mathrm{P}$-type to $I4/mmm$ phase are obtained for different temperatures. To correctly model the ultrahigh-pressure structures, the inner-shell electron interactions are necessary to be considered. The pseudopotentials with different valence electron configurations are tested and the results show that neglecting the inner-shell electrons can predict incorrect phase transition sequence and thermodynamic stability. Based on simulations with all-electron pseudopotentials, it is found that the $I4/mmm{\mathrm{SiO}}_2$ is the most stable ten-coordinated structure up to 200 TPa. The thermodynamic stability of $I4/mmm{\mathrm{SiO}}_2$ up to a temperature of 8 kK is inferred from the Gibbs free energy calculations and the dynamic stability of $I4/mmm{\mathrm{SiO}}_2$ at 180 TPa is demonstrated by phonon calculations.

In synthetic antiferromagnets (SAFs), antiferromagnetic (AFM) order and synthesis using conventional sputtering techniques is combined to produce systems that are advantageous for spintronics applications. Here we present the preparation and study of SAF multilayers possessing both perpendicular magnetic anisotropy and the Dzyaloshinskii-Moriya interaction. The multilayers have an antiferromagnetically aligned ground state but can be forced into a full ferromagnetic (FM) alignment by applying an out-of-plane field $\sim 100$ mT. We study the spin textures in these multilayers in their ground state as well as around the transition point between the AFM and FM states at fields $\sim$ 40 mT by imaging the spin textures using complementary methods: photoemission electron, magnetic force, and Lorentz transmission electron microscopies. The transformation into a FM state by field proceeds by a nucleation and growth process, where skyrmionic nuclei form and then broaden into regions containing a ferromagnetically aligned labyrinth pattern that eventually occupies the whole film. Remarkably, this process occurs without any significant change in the net magnetic moment of the multilayer. The mix of antiferromagnetically and ferromagnetically aligned regions on the micron scale in the middle of this transition is reminiscent of a first-order phase transition that exhibits phase coexistence. These results are important for guiding the design of spintronic devices whose operation is based on spin textures in perpendicularly magnetized SAFs.

Although ${\mathrm{BiFeO}}_3$ is one of the most studied multiferroic materials, recent magnetization and neutron scattering studies have revealed a new magnetic phase in this compound—the transverse conical phase. To study the collective spin excitations of this phase, we performed terahertz spectroscopy in magnetic fields up to 17 T at and above room temperature. We observed five spin-wave branches in the magnetic phase with long-wavelength conical modulation. Using a numerical spin dynamics model, we found two kinds of excitations with magnetic moments oscillating either along or perpendicular to the static fields. Remarkably, we detected strong directional dichroism, an optical manifestation of the magnetoelectric effect, for one spin-wave mode of the conical phase. According to our experiments, the stability of the conical state is sensitive to the magnetic-field history, and it can become (meta)stable at or close to zero magnetic field, which may allow exploiting its magnetoelectric properties at room temperature.

Many magnetic systems display hard-axis collapse—distinct peaks in the remanence and coercivity angular variations, centered ${90}^{\circ }$ off of the easy-magnetization direction. It has recently been discovered that these may also show recoil branches that lie outside the major hysteresis loop. In the present work we examined the magnetization reversal in such systems using vector magnetometry. For Co and Ni films with thickness lower than 25 nm we identified that when the magnetic field is applied in the vicinity of the hard axis and a recoil branch initiates from a state with zero transversal magnetization, the latter remains virtually zero up to the coercive field of the longitudinal component. We showed that this peculiar feature strongly endorses the presumption that these films consist of effectively noninteracting grains in a bidomain magnetic state. We also carried out phenomenological simulations, essential for understanding the magnetization reversal mechanism and anisotropy. Finally, for thin films with collapsed hard axes we presented a simple procedure for evaluating the easy-axis misalignment angle of the bidomain state as well as the anisotropy and intragrain coupling parameters.

The development of machine-learning interatomic potentials has immensely contributed to the accuracy of simulations of molecules and crystals. However, creating interatomic potentials for magnetic systems that account for both magnetic moments and structural degrees of freedom remains a challenge. This work introduces SpinGNN, a spin-dependent interatomic potential approach that employs the graph neural network (GNN) to describe magnetic systems. SpinGNN consists of two types of edge GNNs: Heisenberg edge GNN (HEGNN) and spin-distance edge GNN (SEGNN). HEGNN is tailored to capture Heisenberg-type spin-lattice interactions, while SEGNN accurately models multibody and high-order spin-lattice coupling. The effectiveness of SpinGNN is demonstrated by its exceptional precision in fitting a high-order spin Hamiltonian and two complex spin-lattice Hamiltonians with great precision. Furthermore, it successfully models the subtle spin-lattice coupling in $\mathrm{BiFe}{\mathrm{O}}_3$ and performs large-scale spin-lattice dynamics simulations, predicting its antiferromagnetic ground state, magnetic phase transition, and domain-wall energy landscape with high accuracy. Finally, we perform spin-lattice simulations over one million atoms across GPUs in parallel. Our study broadens the scope of graph neural network potentials to magnetic systems, serving as a foundation for carrying out large-scale spin-lattice dynamic simulations in first-principle accuracy on such systems.

Feasibility of accurate quantum calculations is often restricted by the dimensionality of the truncated Hilbert space required for the numerical computations. The present work demonstrates Bayesian machine learning (ML) models that use quantum properties in an effectively lower-dimensional Hilbert space to make predictions for the Hamiltonian parameters that require a larger basis set as applied to a classical problem in quantum statistical mechanics, the polaron problem. We consider two polaron models: the Su-Schrieffer-Heeger (SSH) model and the mixed SSH - Holstein model. We demonstrate ML models that can extrapolate polaron properties in the phonon frequency. We consider the sharp transition in the ground-state momentum of the SSH polaron and examine the evolution of this transition from the antiadiabatic regime to the adiabatic regime. We also demonstrate Bayesian models that use the posterior distributions of highly approximate quantum calculations as the prior distribution for models of more accurate quantum results. This drastically reduces the number of fully converged quantum calculations required to map out the polaron dispersion relations for the full range of Hamiltonian parameters of interest.

Quasi-one-dimensional correlated electronic multiorbital systems with either ladder or chain geometries continue attracting considerable interest due to their complex electronic phases arising from the interplay of the hopping matrix, the crystal-field splitting, the electronic correlations (Hubbard repulsion $U$ and Hund coupling $J_{\mathrm{H}}$), and strong quantum fluctuations. Recently, the intriguing cobalt zigzag chain system ${\mathrm{BaCoTe}}_2{\mathrm{O}}_7$, with electronic density $n=7$, was prepared experimentally. Here, we systematically study the electronic and magnetic properties of this quasi-one-dimensional compound from the theoretical perspective. Based on first-principles density functional theory calculations, strongly anisotropic one-dimensional electronic Co $3d$ bands were found near the Fermi level. By evaluating the relevant hopping amplitudes, we provide the magnitude and origin of the nearest-neighbor (NN) and next-nearest-neighbor (NNN) hopping matrices in ${\mathrm{BaCoTe}}_2{\mathrm{O}}_7$. With this information, we constructed a three-orbital electronic Hubbard model for this zigzag chain system, and studied two cases: with only a NN hopping matrix, and with NN plus NNN hopping matrices. Introducing the Hubbard and Hund couplings and studying the model via the density matrix renormalization group method, we constructed the ground-state phase diagram. A robust staggered $\uparrow \text{−}\downarrow \text{−}\uparrow \text{−}\downarrow$ antiferromagnetic (AFM) region was found when only the NN hopping matrix in the chain direction was employed. However, for the realistic case where the NNN hopping matrix is also included, the dominant state becomes instead a block AFM $\uparrow \text{−}\uparrow \text{−}\downarrow \text{−}\downarrow$ order, in agreement with experiments. The system displays Mott insulator characteristics with three half-filled orbitals, when the block AFM order is stable. Our results for ${\mathrm{BaCoTe}}_2{\mathrm{O}}_7$ provide guidance to experimentalists and theorists working on this zigzag one-dimensional chain and related materials.

We solve the problem of an interacting quantum dot embedded in a Josephson junction between two superconductors with finite charging energy described by the transmon (Cooper pair box) Hamiltonian. The approach is based on the flat-band approximation of the Richardson model, which reduces the Hilbert space to the point where exact diagonalization is possible while retaining all states that are necessary to describe the low-energy phenomena. The presented method accounts for the physics of the quantum dot, the Josephson effect, and the Coulomb repulsion (charging energy) at the same level. In particular, it captures the quantum fluctuations of the superconducting phase as well as the coupling between the superconducting phase and the quantum dot (spin) degrees of freedom. The method can be directly applied for modeling Andreev spin qubits embedded in transmon circuits in all parameter regimes, for describing time-dependent processes, and for the calculation of transition matrix elements for microwave-driven transmon, spin-flip, and mixed transitions that involve coupling to charge or current degrees of freedom.

We present a comprehensive investigation of the light-matter interaction dynamics in two-dimensional materials coupled with a spectrally isolated cavity mode in the strong coupling regime. The interaction between light and matter breaks the translational symmetry of excitons in the two-dimensional lattice and results in the emergence of a localized polariton state. Employing an approach involving transformation to exciton reaction coordinates, we derive a Markovian master equation to describe the formation of a macroscopic population in the localized polariton state. Our study shows that the construction of a large-scale polariton population is affected by correction terms addressing the breakdown of translational symmetry. Increasing the spatial width of the cavity mode increases the Coulomb scattering rates while the correction terms saturate and affect the system's dynamics progressively less. Tuning the lattice temperature can induce bistability and hysteresis with different origins than those recognized for quantum wells in larger microcavities. We identify a limit temperature $T_{\mathrm{l}}$ as a key factor for stimulated emissions and forming a macroscopic population, enriching our understanding of strong coupling dynamics in systems with extreme confinement.

In this article, we carry out an in-depth study of the scanning tunneling microscopy-induced luminescence spectra (STML) of individual plasmonic nanoparticles measured in air. When compared to the results of far-field light scattering measured under the same ambient conditions, the STML measurements show spectral shifts and peak broadening of hundreds of meV, even when a nonplasmonic tip is used for STML. We simulate the near-field excitation and the effect of the tip using the finite-element method and show that these effects alone cannot explain the spectral shifts and peak broadening observed for STML experiments in air. However, the experimental results are well reproduced in the numerical simulations if the screening effect of a water meniscus bridge present in the tip-nanoparticle gap is considered. Our results pave the way for finer interpretations of STML experiments in air, where ignoring this additional screening effect can lead to an incorrect mode assignment of the observed resonances.

We show that many-body fermionic non-Hermitian systems require two distinct sets of topological invariants to describe the topology of energy bands and quantum states, respectively, with the latter yet to be explored. We identify 10 symmetry classes—determined by particle-hole, linearized time-reversal, and linearized chiral symmetries. Each class has a topological invariant associated with each dimension, dictating the topology of quantum states. These findings pave the way for a deeper understanding of the topological phases of many-body non-Hermitian systems.

In the present paper, we utilize the coupled-oscillator model describing the hybridization of optical phonons in touching and/or overlapping particles in order to study the Raman spectra of nanoparticles organized into various types of regular and random arrays including nanosolids, porous media, and agglomerates with tightly bonded particles. For the nanocrystal solids, we demonstrate that the ratio of the size variance to the coupling strength allows us to judge the character (localized or propagating) of the optical phonon modes, which left the particles of their origin and spread throughout an array. The relation between the shift and the broadening of the Raman peak and the coupling strength and the disorder is established for nanocrystal solids, agglomerates, and porous media providing us with information about the array structure, the structure of its constituents, and the properties of optical phonons.

If a two-level system coupled to a single-mode cavity is strongly driven by an external laser, instead of a continuous accumulation of photons in the cavity, oscillations in the mean photon number occur. These oscillations correspond to peaks of finite width running up and down in the photon number distribution, reminiscent of wave packets in linear chain models. A single wave packet is found if the cavity is resonant to the external laser. Here, we show that for finite detuning, multiple packet structures can exist simultaneously, oscillating at different frequencies and amplitudes. We further study the influence of dissipative effects resulting in the formation of a stationary state, which, depending on the parameters, can be characterized by a bimodal photon number distribution. While we give analytical limits for the maximally achievable photon number in the absence of any dissipation, surprisingly, dephasing processes can push the photon occupations towards higher photon numbers.

In interacting theories, hydrodynamics describes the universal behavior of states close to local thermal equilibrium at late times and long distances in a gradient expansion. In the hydrodynamic regime of metals, momentum relaxes slowly with a rate $\mathrm{\Gamma }$, which formally appears on the right-hand side of the momentum dynamical equation and causes a Drude-like peak in the frequency dependence of the thermoelectric conductivities. Here we study the structure and determine the physical implications of momentum-relaxing gradient corrections beyond Drude, i.e., arising at subleading order in the gradient expansion. We find that they effectively renormalize the weight of the Drude pole in the thermoelectric conductivities, and contribute to the dc conductivities at the same order as previously known gradient corrections of translation-invariant hydrodynamics. Turning on a magnetic field, extra derivative corrections appear and renormalize the cyclotron frequency and the Hall conductivity. This relaxed hydrodynamics provides a field-theoretic explanation for previous results obtained using gauge-gravity duality. In strongly coupled metals where quasiparticles are short-lived and which may be close to a hydrodynamic regime, the extra contributions we discuss are essential to interpret experimentally measured magnetothermoelectric conductivities. Specializing to metals close to a Fermi liquid phase, the effective mass measured either through the specific heat or the spectral weight of the Drude-like peak are found to differ, as was indeed reported in overdoped cuprate superconductors. More generally, we expect such terms to be present in any hydrodynamic theory with approximate symmetries, which arise in many physical systems.