Magnetic skyrmions have potential applications in future high-density information storage devices due to their small size, topologically protected structure, and ultralow energy consumption. Here, we use a variational method to study rich topological spin textures in magnetic nanodisks, including skyrmions and skyrmionlike objects. The existence of skyrmions and $k\pi$-skyrmions in a single-phase nanodisk and a disk with a core-shell structure is investigated theoretically, where the phase diagrams of possible spin states are obtained for various Dzyaloshinskii-Moriya interaction (DMI) constants and disk dimeters. Our calculations suggest that skyrmions can be generated spontaneously in a core-shell nanodisk. We also study the transitions of spin textures in the nanodisk induced by spin currents. This paper may provide guidelines for experimental realization of different topological spin textures in nanodisks with certain DMIs. The simple two-dimensional analytical method used in this paper can be used to study the magnetic phase diagrams of thin films with interface-induced DMIs.

In a two-dimensional superconducting vortex lattice, the melting from the solid to the isotropic liquid can occur via an intermediate phase that retains orientational correlations. The effect of such correlations on transport and their interplay with the quenched disorder remain open questions. We perform magnetotransport measurements in a wide range of temperatures and magnetic fields on a weakly pinned two-dimensional vortex system in amorphous MoGe films. While at high fields, where quenched disorder dominates, we recover the typical strong-glass behavior of a vortex liquid, at low fields the resistivity shows a clear crossover to a fragile vortex glass. Our findings, supported by numerical simulations, suggest that this is a signature of heterogeneous dynamics that arises from the presence of orientational correlations.

The stability, trapping, and mobility of electron holes are investigated in lanthanum ferrite ${\mathrm{LaFeO}}_3$, and in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{FeO}}_{3-\delta }$ ($x\approx 0.1$, 0.4, and 0.6) by hybrid-density-functional and density-functional-$\mathrm{theory}+U$ calculations. In pure ${\mathrm{LaFeO}}_3$, the electron hole is more stable under a localized (polaronic) form than under a delocalized form, the energy difference (self-trapping energy) lies between $\approx -0.3$ and $-0.4$ eV. This self-trapped hole polaron is not strictly localized on a single Fe atom: instead, it occupies a quantum state made of a $3d$ orbital of a Fe atom, strongly hybridized with $2p$ orbitals of four neighboring oxygens. The hole polaron is thus localized on five atoms (among which one single Fe), which can be described as the ${\mathrm{Fe}}^{3+}$ oxidation into ${\mathrm{Fe}}^{4+}$. Electron hole transport results from the combination of onsite reorientations and hoppings, with energy barriers estimated at $\approx 0.01–0.20$ eV and $0.3–0.4$ eV, respectively. The aliovalent substitution of lanthanum by strontium in ${\mathrm{LaFeO}}_3$ induces the presence of localized electron holes, preserving the insulating character of ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{FeO}}_3$, regardless of the studied Sr concentration. The formation energy of the oxygen vacancy in ${\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{FeO}}_3$ ($x\approx 0.1$ and 0.4) is estimated at $\approx +0.8$ eV. This value is here successfully used to quantify the evolution of defect concentration as a function of the oxygen partial pressure.

In this paper, we used symmetry analyses and first-principles calculations to discover seven Cs-Te binary systems that have different crystal structures and can host symmetry-enforced topologically nontrivial phonons: $P{2}_1{2}_1{2}_1\text{−}{\mathrm{Cs}}_2\mathrm{Te}, Pbam$-CsTe, $Pnma\text{−}{\mathrm{Cs}}_2\mathrm{Te}, Pm\overline{3}m$-CsTe, $Cmcm\text{−}{\mathrm{Cs}}_2{\mathrm{Te}}_5, Cmc{2}_1\text{−}{\mathrm{Cs}}_2{\mathrm{Te}}_3$, and $P{2}_1/c\text{−}{\mathrm{CsTe}}_4$. These phonons include charge-two Dirac point phonons, charge-one Weyl point phonons, quadratic contact triple point phonons, triple point phonons, butterflylike Weyl nodal line phonons, Dirac nodal line phonons, Weyl nodal loop phonons, multiple straight Weyl nodal line phonons, nodal cage phonons, one-nodal surface phonons, two-nodal surface phonons, and three-nodal surface phonons. Furthermore, the relationship between the crystal structure and the topological properties was thoroughly investigated in this study. More importantly, the Cs-Te binary systems clearly exhibit phononic arc-shaped, phononic nodal-line-shaped, phononic drumheadlike, and phononic torus surface states, which benefit experimental detections. Our theoretical results not only propose various experimentally prepared Cs-Te binary systems with exotic topological phonons and phononic surface states but also explain the structure-property relationship for Cs-Te binary systems based on symmetry analyses.

The sampling of the configuration space in diffusion Monte Carlo is done using walkers moving randomly. In a previous work on the Hubbard model [R. Assaraf, P. Azaria, M. Caffarel, and P. Lecheminant, Phys. Rev. B 60, 2299 (1999).], it was shown that the probability for a walker to stay a certain amount of time in the same state obeys a Poisson law and that the on-state dynamics can be integrated out exactly, leading to an effective dynamics connecting only different states. Here, we extend this idea to the general case of a walker trapped within domains of arbitrary shape and size. The equations of the resulting effective stochastic dynamics are derived. The larger the average (trapping) time spent by the walker within the domains, the greater the reduction in statistical fluctuations. A numerical application to the Hubbard model is presented. Although this work presents the method for (discrete) finite linear spaces, it can be generalized without fundamental difficulties to continuous configuration spaces.

We discuss the modified quantum electrodynamics from a time-reversal-breaking Weyl semimetal coupled with a $U\left(1\right)$ gauge (electromagnetic) field. A key role is played by the soft dispersion of the photons in a particular direction, say $\widehat{z}$, due to the Hall conductivity of the Weyl semimetal. Due to the soft photon, the fermion velocity in $\widehat{z}$ is logarithmically reduced under renormalization group flow, together with the fine-structure constant. Meanwhile, fermions acquire a finite lifetime from spontaneous emission of the soft photon, namely, the Cherenkov radiation. At low-energy $E$, the inverse of the fermion lifetime scales as ${\tau }^{-1}\sim E/\mathrm{PolyLog}\left(E\right)$. Therefore, even though fermion quasiparticles are eventually well-defined at very low energy, over a wide intermediate energy window the Weyl semimetal behaves like a marginal Fermi liquid. Phenomenologically, our results are more relevant for emergent Weyl semimetals, where the fermions and photons all emerge from strongly correlated lattice systems. Possible experimental implications are discussed.

The pursuit of the non-Hermitian skin effect (NHSE) in various physical systems is of great research interest. Compared with recent progress in nonelectronic systems, the implementation of the NHSE in condensed matter physics remains elusive. Here, we show that the NHSE can be engineered in the mesoscopic heterojunctions (system plus reservoir) in which electrons in two channels of the system moving towards each other have asymmetric coupling to those of the reservoir. This makes electrons in the system move forward and in the opposite direction have unequal lifetimes, and so gives rise to a point-gap spectral topology. Accordingly, the electron eigenstates exhibit NHSE under the open boundary condition, consistent with the description of the generalized Brillouin zone. Such a reservoir-engineered NHSE visibly manifests itself as the nonreciprocal charge current that can be probed by the standard transport measurements. Further, we generalize the scenario to the spin-resolved NHSE, which can be probed by the nonreciprocal spin transport. Our work opens a research avenue for implementing and detecting the NHSE in electronic mesoscopic systems, which will lead to interesting device applications.

Recently, the valley degree of freedom of electrons in two-dimensional materials has been attracting growing attention as an information carrier. Here, on the basis of first-principles calculations, we propose a series of unique 2D valleytronic materials in single-layer $h\text{−}M\mathrm{N}X$ ($M=\mathrm{Ti}$, Zr; Hf, $X=\mathrm{Cl}$, Br) and systematically investigate their valleytronic properties. The underlying valley-contrasting physics including valley spin splitting and valley-dependent optical selection rules in single-layer $h\text{−}M\mathrm{N}X$ is unveiled. Moreover, we find that the intriguing valley polarization in single-layer $h\text{−}M\mathrm{N}X$ can be achieved through both circularly polarized light and a ferromagnetic substrate. The substrate-induced valley polarization in SL $h\text{−}M\mathrm{N}X$ is stacking dependent and can be enhanced by decreasing the interlayer distance. Our findings thus provide a tantalizing platform for operating the valley index in two-dimensional materials.

We investigate lateral recoil forces exerted on nanoparticles located near plasmonic platforms with in-plane nonreciprocal response. To this purpose, we first develop a comprehensive theoretical framework based on the Lorentz force within the Rayleigh approximation combined with nonreciprocal Green's functions and then derive approximate analytical expressions to model lateral recoil forces, demonstrating their explicit dependence on the dispersion relation of the system and unveiling the mechanisms that govern them. In particular, a dominant lateral recoil force component appears due to the momentum imbalance of nonreciprocal surface plasmons supported by the platform. This force can be orders of magnitude larger than other recoil force components, acts only along or against the direction of the external bias, and is quasi-independent of the direction, polarization, and wavelength of the incident plane wave. Lateral recoil forces are explored using drift-biased graphene metasurfaces, a platform that is also proposed to sort nanoparticles as a function of their size. Nonreciprocal plasmonic systems may enable new venues to trap, bind, and manipulate nanoparticles and to alleviate some of the challenges of conventional optical tweezers.

In this paper we promote the idea of quantum critical lines (inter alia surfaces) as opposed to points. A quantum critical line is obtained when criticality at zero temperature is extended over a continuum in a one-dimensional line. We base our ideas on a simple but exactly solved model introduced by one of the authors involving a one-dimensional quantum transverse field Ising model with added three-spin interaction. While many of the ideas are quite general, there are other aspects that are not. In particular, a line of criticality with continuously varying exponents is not captured. However, the exact solvability of the model gives us considerable confidence in our results. Although the pure system is analytically exactly solved, the disorder case requires numerical analyses based on exact computation of the correlation function in the Pfaffian representation. The disorder case leads to a dynamic structure factor as a function of frequency and wave vector. We expect that the model is experimentally realizable and perhaps many other similar models will be found to explore quantum critical lines.

The interplay between ferroelectricity and band topology can give rise to a wide range of both fundamental and applied research. Here, we map out the emergence of nontrivial corner states in two-dimensional ferroelectrics and, remarkably, demonstrate that ferroelectricity and corner states are coupled together by crystallographic symmetry to realize the electric control of higher-order topology. Implementing density functional theory, we identify a series of experimentally synthesized two-dimensional ferroelectrics, such as ${\mathrm{In}}_2{\mathrm{Se}}_3$, BN, and SnS, as realistic material candidates for the proposed ferroelectric higher-order topological insulators. Our work not only sheds light on traditional ferroelectric materials but also opens an avenue to bridge the higher-order topology and ferroelectricity that provides a nonvolatile handle to manipulate the topology in next-generation electronic devices.

The role of lattice dynamics in unconventional high-temperature superconductivity is still vigorously debated. Theoretical insights into this problem have long been prevented by the absence of an accurate first-principles description of the combined electronic, magnetic, and lattice degrees of freedom. Utilizing the recently constructed ${\mathrm{r}}^2$SCAN density functional that stabilizes the antiferromagnetic (AFM) state of the pristine oxide ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_6$, we faithfully reproduce the experimental dispersion of key phonon modes. We further find significant magnetoelastic coupling in numerous high-energy Cu-O bond stretching optical branches, where the AFM results improve over the soft nonmagnetic phonon bands.

Recent experiments on vanadium-based nonmagnetic kagomé metals $A{\mathrm{V}}_3{\mathrm{Sb}}_5(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ revealed evidence for possible spontaneous time-reversal symmetry (TRS) breaking in the charge density wave (CDW) ordered state. The long-sought-after quantum order of loop currents has been suggested as a candidate for the TRS breaking state. However, a microscopic model for the emergence of the loop-current CDW due to electronic correlations is still lacking. Here, we calculate the susceptibility of the real and imaginary bond orders on the kagomé lattice near van Hove filling, and reveal the importance of next-nearest-neighbor Coulomb repulsion $V_2$ in triggering the instability toward imaginary bond ordered CDW. The concrete effective single-orbital $t\text{−}{V}_1\text{−}{V}_2$ model on the kagomé lattice is then studied, where $t$ and $V_1$ are the hopping and Coulomb repulsion on the nearest-neighbor bonds. We obtain the mean-field ground states, analyze their properties, and determine the phase diagram in the plane spanned by $V_1$ and $V_2$ at van Hove filling. The region dominated by $V_1$ is occupied by a $2a_0\times 2a_0$ real CDW insulator with the inverse of Star-of-David (ISD) bond configuration. Increasing $V_2$ indeed drives a first-order transition from ISD to stabilized loop-current insulators that exhibit four possible current patterns of different topological properties, leading to orbital Chern insulators. We then extend these results away from van Hove filling and show that electron doping helps the stabilization of loop currents, and gives rise to doped orbital Chern insulators with emergent Chern Fermi pockets carrying large Berry curvature and orbital magnetic moment. Our findings provide a concrete model realization of the loop-current Chern metal at the mean-field level for the TRS breaking normal state of the kagomé superconductors.

Vanadium atoms in layered $\mathrm{Li}\mathrm{V}{\mathrm{O}}_2$ form in-plane periodic vanadium trimers at low temperatures, but the trimers appear randomly in the stacking direction because there are many trimer configurations with comparable lattice energy. We detailed an original modeling scheme to represent glassy states with a completely disordered trimer configuration in the stacking structure. Through pair distribution function analysis using this model, we show that the synthesis method can yield two types of low-temperature stacking structures: a completely disordered stacking structure and a short-range order in the stacking structure. The phase-transition temperature of the former sample is about 15 K lower than that of the latter. We discuss that this is due to the strong trimer frustration that appears in the sample without short-range order, which suppresses the phase-transition temperature, similar to the frustration effect in conventional spin systems.

High-intensity THz lasers allow for the coherent excitation of individual phonon modes. The ultrafast control of emergent magnetism by means of phonons opens up new tuning mechanisms for functional materials. While theoretically predicted phonon magnetic moments are tiny, recent experiments hint towards a significant magnetization in various materials. To explain these phenomena, we derive a coupling mechanism between the phonon angular momentum and the electron spin. This coupling introduces the transient level splitting of spin-up and spin-down channels and a resulting magnetization. We estimate this magnetization on the example of the lowest infrared active mode in the perovskite $\mathrm{K}\mathrm{Ta}{\mathrm{O}}_3$. Our results show an electronic magnetic moment of $\approx {10}^{-1}{\mu }_B$ per unit cell, depending on the doping level and electron temperature.

The heat capacity of a small number of ${}^3\mathrm{He}$ atoms dissolved in a submonolayer ${}^4\mathrm{He}$ film has been measured. The measured heat capacity is finite and suggests that ${}^3\mathrm{He}$ atoms are mobile in an areal density regime higher than that of the $\sqrt{3}\times \sqrt{3}$ phase, where ${}^4\mathrm{He}$ films are believed to be solid. At higher areal densities, the measured heat capacity is proportional to $T^2$ and depends on the number of ${}^3\mathrm{He}$ atoms. These behaviors are anomalous to that of a two-dimensional Fermi fluid and cannot be explained by uniform melting of ${}^4\mathrm{He}$ films. One possible explanation for these anomalous behaviors is that helium atoms exhibit fluidity only inside the domain walls of the adsorption structure, and the dissolved ${}^3\mathrm{He}$ atoms gather in them and behave as a one-dimensional Fermi fluid or as Dirac fermions, depending on the structure of the domain walls. The behaviors of the measured heat capacity strongly suggest this possibility.

The local electronic structure of muons (Mu) as dilute pseudohydrogen in single-crystalline $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ has been studied by the muon spin rotation/relaxation $\left(\mu \mathrm{SR}\right)$. High-precision measurements over a long time range of $\sim 25\mu \mathrm{s}$ have clearly identified two distinct Mu states: a quasistatic Mu $\left({\mathrm{Mu}}_1\right)$ and fast-moving Mu $\left({\mathrm{Mu}}_2\right)$. By comparing this result with predictions from the recently established ambipolarity model, these two states are respectively attributed to the relaxed-excited states associated with the donor $\left(E^{+/0}\right)$ and acceptor $\left(E^{-/0}\right)$ levels predicted by density functional theory (DFT) calculations for the interstitial H. Furthermore, the local electronic structure of ${\mathrm{Mu}}_1$ is found to be an OMu-bonded state with three-coordinated oxygen. The structure is almost identical with the thermal equilibrium state of H, and it is found to function as an electron donor. The other ${\mathrm{Mu}}_2$ is considered to be in the hydride state $\left({\mathrm{Mu}}^{-}\right)$ from the ambipolarity model, suggesting that it is in fast diffusion motion through the short-lived neutral state due to the charge exchange reaction with conduction electrons $({\mathrm{Mu}}^{-}\rightleftarrows {\mathrm{Mu}}^0+e^{-})$.

Coupling a one-dimensional quasiperiodic interacting system to a Markovian bath, we study the avalanche instability of the many-body localized phase numerically, finding that many-body localization (MBL) is more stable in pseudorandom quasiperiodic systems than the corresponding randomly disordered systems for a disorder strength $W>8$, potentially up to arbitrarily large system sizes. We support our conclusion by additionally developing real-space RG arguments, and we provide a detailed comparison between quasiperiodic and random MBL from the avalanche instability perspective, concluding that the two belong to different universality classes.

We study the effect of quasiperiodic perturbations on one-dimensional all-bands-flat lattice models. Such networks can be diagonalized by a finite sequence of local unitary transformations parameterized by angles ${\theta }_i$. Without loss of generality, we focus on the case of two bands with bandgap $\mathrm{\Delta }$. Weak perturbations lead to an effective Hamiltonian with both on- and off-diagonal quasiperiodic terms that depend on ${\theta }_i$. For some angle values, the effective model coincides with the extended Harper model. By varying the parameters of the quasiperiodic potentials, we observe localized insulating states and an entire parameter range hosting critical states with subdiffusive transport. For finite quasiperiodic potential strength, the critical-to-insulating transition becomes energy dependent with what we term fractality edges separating localized from critical states.

The Kitaev interaction in a honeycomb lattice with higher-spin $S$ has been one of the central attractions, as it may offer quantum spin liquids. A microscopic theory showed that when the Hund's coupling at the transition metal generates $S>\frac{1}{2}$, the spin-orbit coupling at the heavy ligands provides a route to the Kitaev interaction. However, there have been debates over its strength compared to other symmetry-allowed interactions. Investigating the symmetry of the Hamiltonian for general $S$, we show the magnon energies at two momentum points related by a broken mirror symmetry reflect the Kitaev interaction when a magnetic field is in the mirror plane. Applying the symmetry analysis to $\mathrm{Cr}{\mathrm{I}}_3$ with $S=\frac{3}{2}$ together with the available angle-dependent ferromagnetic resonance data, we estimate the Kitaev interaction out of the full Hamiltonian and find that it is subdominant. Our theory can be tested by inelastic neutron scattering on candidate materials under the proposed magnetic field direction, which will advance the search for general $S$ Kitaev materials.