We present a comparative investigation of singlet ground state induced magnetism for singlet, doublet, and triplet excited CEF states of non-Kramers $f$ electrons relevant primarily for Pr- and U-based compounds. This type of magnetic order is of the intrinsic quantum nature because it requires the superposition of singlet ground state with excited states due to nondiagonal matrix elements of the effective intersite exchange to generate local moments. In contrast to conventional magnets, the local moments and their ordering appear simultaneously at the transition temperature. It is finite only if the control parameter proportional to the ratio of exchange strength to level splitting exceeds a critical value marking the quantum critical point of the models. We determine the dependence of transition temperature, saturation moment, renormalized level splitting, specific heat jumps, and low-temperature susceptibility as a function of control parameters. Furthermore, the temperature dependence of these quantities is calculated for control parameters above and below the quantum critical point and the distinction to conventional magnetism is discussed. In addition, we investigate the dynamical properties of the three models, deriving the magnetic exciton dispersion and their critical behavior. In particular, the conditions for true and arrested soft-mode behavior at the ordering wave vector are identified.

Motivated by recent observation of the quantum spin Hall effect in monolayer germanene and twisted bilayer transition-metal dichalcogenides (TMDs), we study the topological phases of moiré twisted bilayers with time-reversal symmetry and spin $s_z$ conservation. By using a continuum model description, which can be applied to both germanene and TMD bilayers, we show that at small twist angles the emergent moiré flat bands can be topologically nontrivial due to inversion symmetry breaking. Each of these flat bands admits a lowest-Landau-level description for each spin projection in the chiral limit and at magic twist angle. This allows for the construction of a many-body Laughlin state with time-reversal symmetry, which can be stabilized by a short-range pseudopotential, and therefore serves as an ideal platform for realizing the so-far elusive fractional quantum spin Hall effect with emergent spin-1/2 U(1) symmetry.

The unconventional anomalous Hall effect was observed in the antiferromagnetic state of ${\mathrm{EuZn}}_2{\mathrm{Sb}}_2$. Scaling of unconventional Hall conductivity with the longitudinal conductivity, and the magnitude of the Hall angle indicate spin chirality despite a collinear magnetic structure. Anomalies in magnetoresistance culminate in the same fields, in which the unconventional anomalous Hall resistance has maxima. The monotonous decrease of their magnitude with increasing temperature belittles the role of spin fluctuations here, which are important in isostructural compounds. These observations point to a prominent role of scalar spin chirality within domain walls, when the magnetic field tilts the Eu moments. Simple calculation of such spin chirality shows it is strongest in fields that are characteristic for anomalous magnetotransport.

We report a theoretical study of multiterminal Josephson junctions under the influence of a magnetic field $B$. We consider a ballistic rectangular two-dimensional metal $N_0$ connected by the edges to the left, right, top, and bottom superconductors $S_L$, $S_R$, $S_T$, and $S_B$, respectively. We numerically calculate in the large-gap approximation the critical current $I_c$ versus $B$ between the left and right $S_L$ and $S_R$ for various aspect ratios, with the top and bottom $S_T$ and $S_B$ playing the role of superconducting mirrors. We find the critical current $I_c$ to be enhanced by orders of magnitude, especially at long distance, due to the phase rigidity provided by the mirrors. We obtain magnetic oscillations resembling those of a superconducting quantum interference device. With symmetric couplings, the self-consistent superconducting phase variables of the top and bottom mirrors take the values 0 or $\pi$, as for emerging Ising degrees of freedom. We propose a simple effective Josephson junction circuit model that is compatible with these microscopic numerical calculations. From the $I_c\left(B\right)$ patterns we infer where the supercurrent flows in various device geometries. In particular in the elongated geometry, we show that the supercurrent flows between all pairs of contacts, which allows exploring the full phase space of the relevant phase differences.

Bosonic collective modes are ubiquitous in metals, but over a wide range of energy and momenta suffer from Landau damping, decaying into the continuum of particle-hole excitations. Here we point out that interactions can suppress this decay, protecting a finite fraction of the total spectral weight associated with the collective mode, e.g., a plasmon. The underlying mechanism is level repulsion between a discrete mode and the continuum. We demonstrate the effect using a number of simplified models of strongly correlated Fermi-liquid metals, including a solvable random flavor model in the large-$N$ limit. We discuss in detail the possibility of observing such an avoided decay for plasmons in (moiré) graphenelike systems.

In contrast to the ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs) family exhibiting the ubiquitous $2\times 2\times 2$ charge density wave (CDW) order, only ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$ in the ${\mathrm{RV}}_6{\mathrm{Sn}}_6$ (R = Sc, Y, La) family displays the unusual CDW with a $\sqrt{3}\times \sqrt{3}$ in-plane ordering and a tripling of the unit cell along the $c$ axis. Here, using first-principles density-functional theory calculations, we show that both the $\sqrt{3}\times \sqrt{3}\times 2$ and $\sqrt{3}\times \sqrt{3}\times 3$ CDW orderings can be driven by a Jahn-Teller-like effect where the Sn atoms residing in the kagome bilayers partially undergo delicately different schemes of interlayer dimerization, accompanied by charge redistribution between such Sn atoms and band-gap opening. Counterintuitively, whereas the $\sqrt{3}\times \sqrt{3}\times 2$ phase is energetically more stable than $\sqrt{3}\times \sqrt{3}\times 3$, the latter is thermodynamically stabilized above the CDW transition temperature $T_{\mathrm{CDW}}$ by its higher configurational entropy contributed by degenerate fluctuating phases, and is kinetically selected below $T_{\mathrm{CDW}}$, as corroborated by experimental observation of the accompanying first-order phase transition. Our findings reveal the order-disorder nature of the $\sqrt{3}\times \sqrt{3}\times 3$ CDW phase transition in ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$, with broader implications for understanding charge orderings and CDW fluctuations in other kagome metals.

The dynamics of the diffusion flow of holes photoinjected into a mesoscopic GaAs channel of variable width, where they, together with background electrons, form a hydrodynamic electron-hole fluid, is studied using time-resolved microphotoluminescence. It is found that the rate of recombination of photoinjected holes, which is proportional to the rate of their flow, decreases when holes pass through the expanded sections of the channel. In fact, this is the Venturi effect, which consists in a decrease in the velocity of the fluid in the expanded sections of the pipe. Moreover, a nonuniform diffusion velocity profile is observed, similar to the parabolic Hagen-Poiseuille velocity profile, which indicates a viscous hydrodynamic flow. It is shown that in agreement with a theory, the magnetic field strongly suppresses the viscosity of the electron-hole fluid. Additional evidence of the viscous nature of the studied electron-hole fluid is the observed increase in the recombination rate with increasing temperature, which is similar to the decrease in the electrical resistance of viscous electrons with temperature.

We present a comprehensive study of the magnetic properties of the strongly anisotropic ferrimagnet ${\mathrm{ErFe}}_5{\mathrm{Al}}_7$ in pulsed magnetic fields up to 30 T applied along the hard magnetization axis within the basal plane of the tetragonal lattice around the compensation temperature ($T_{\mathrm{comp}}$). Macroscopic measurements showed two anomalies at about 8 T and 25 T in a small temperature range around $T_{\mathrm{comp}}$. High-field x-ray magnetic circular dichroism (XMCD) data at the Er $M_5$- and the Fe $L_3$-edge resonances provide insight into the element-selective magnetization processes, revealing a coherent rotation of Er $4f$ and Fe $3d$ moments, with stepwise jumps including an unexpected one from an easy to a hard magnetization axis. XMCD at the Er $L_3$-edge resonance elucidates the role of Er $5d$ electrons in coupling the Er $4f$ and the Fe $3d$ moments. Finally, an in-plane anisotropy constant was evaluated from a simulation of the magnetization process at temperatures well below $T_{\mathrm{comp}}$ using a two-sublattice model.

The admixture of spin-singlet and spin-triplet pairing states in superconductors can be typically induced by breaking spatial inversion symmetry. Employing the numerically exact auxiliary-field quantum Monte Carlo method, we study such mixed-parity pairing phenomena of attractive fermions with Rashba spin-orbit coupling (SOC) in a two-dimensional optical lattice at finite temperature. We systematically explore the evolution of the essential pairing structure in both singlet and triplet channels versus the temperature, fermion filling, and SOC and interaction strengths, via computing the finite-size results of the condensate fraction and pair wave function. Our numerical results reveal that the singlet channel dominates in the fermion pairing and the triplet pairing has a relatively small contribution to the superfluidity for physically relevant parameters. In contrast to the singlet channel mainly consisting of the on-site Cooper pairs, the triplet pairing has plentiful patterns in real space with the largest contributions from several nearest neighbors. As the SOC strength increases, the pairing correlation is first enhanced and then suppressed for triplet pairing while it is simply weakened in the singlet channel. We have also obtained the Berezinskii-Kosterlitz-Thouless transition temperatures through the finite-size analysis of the condensate fraction. Our results can serve as a quantitative guide for future optical lattice experiments as well as accurate benchmarks for theories and other numerical methods.

Magnetic skyrmions are topological spin textures with potential applications in future spintronic devices. Spin-polarized current is an effective way to manipulate magnetic skyrmions. Recent experiments show that the critical current density to move the skyrmion decays gradually with the pulse width [Wang et al., Nat. Commun. 13, 1593 (2022)]. In this work, we numerically studied the critical current density of the skyrmion depinning in the presence of currents. In the framework of the Thiele equation, the current density plays a role as the potential that acts on the skyrmion. In the limit of zero damping, the system reduces to a Hamiltonian system even in the presence of current. Thus the induced potential due to the disorders determines the critical current density. The temperature enhances the skyrmion depinning process, which depends on the current density and the pulse width. The mean first passage time for the skyrmion depinning is calculated using the finite element method, which explains the inverse relation between the current density and the pulse width.

In this work, we develop a theory of (meta)stable states of Néel-type skyrmions in weak nonuniform magnetic fields. We claim an Ansatz for modeling the nonsymmetric magnetization that can be implied for both analytics and numerical simulations. Our theory accounts for changes in the size of skyrmion parameters and also includes deformations from the centrally symmetric shape. The Ansatz streamlines the analytic calculation of the skyrmion free energy, enhancing the efficiency of the minimizing process. Performing the minimization in two stages, one can find all the minima, global and local, of the free energy, discovering the stable and metastable states. We apply the developed methodology to investigate the (meta)stable configurations of skyrmions influenced by the stray field of a Pearl vortex. Our study reveals the dependence of skyrmion spatial parameters on the vortex field effective strength and presents a phase diagram identifying regions where metastable configurations are predicted. Corroborated by micromagnetic simulations, our findings offer a detailed perspective on the interaction between magnetic skyrmions and superconducting vortices.

A wide range of two-dimensional (2D) materials have exhibited anomalous electrical properties when surrendered to pressure, along with the unsolved long-standing mystery of the relationship between dimension and superconductivity (SC). Investigation of group IVB transition-metal trichalcogenides (TMTCs) with anisotropic structures is expected to uncover novel physics and shed more light on this relationship in 2D materials. As a typical member in TMTCs, the $\mathrm{ZrS}{\mathrm{e}}_3$ sample with a quasi-one-dimensional structure was discovered to change from a semiconductor into a bulk superconducting phase at 16.0 GPa, evidenced by the electrical transport and alternating-current magnetic susceptibility measurements under pressure. A maximum $T_{\mathrm{c}}-5.5$ K was achieved at 23.1 GPa, concomitantly with an n-p carrier-type switch considered as Lifshitz transition. Besides, an isostructural phase transition was confirmed via in situ x-ray-diffraction measurement at 25.5 GPa, indicating the preservation of the quasi-one-dimensional structure throughout the pressure range. The present results indicate that the atomic spacing and in-plane anisotropy play an important role in the formation of bulk (filamentary) SC in group IVB TMTCs.

Numerical solvers for strongly correlated electronic systems poses a significant challenge in condensed matter theory. In recent years, extensive exploration of strongly correlated effects has been carried out based on exactly solvable Hatsugai-Kohmoto-like models. In this work, we investigate a simple two-site cluster extension of the Hatsugai-Kohmoto model. The model exhibits a rich variety of phases, including the charge-$4e$ phase, charge-$2e$ phase, metallic phase, pseudogap phase, and Luttinger liquid phase. These findings contribute to the understanding of strongly correlated physics and superconducting physics.

Optical resonators can support bound states in the continuum (BICs) with infinite quality ($Q$) factors by eliminating radiation losses. However, practical optical resonators only support quasi-BICs with finite $Q$ factors due to the scattering losses caused by inevitable fabrication defects. Merging multiple BICs in momentum space can improve the $Q$ factors of resonators over a broad wavevector range. The dependence of a resonator on high-precision nanofabrication can also be decreased by improving the robustness of the $Q$ factors of quasi-BICs against asymmetric structural parameters, which is much easier to realize high $Q$ factors. Here, we propose an efficient method to merge multiple BICs in the geometrical parameter space by engineering a folded mode induced by Brillouin zone folding. Along with the topological charge evolution process in momentum space, this approach significantly improves the robustness of the $Q$ factor of resonators against perturbations caused by geometric symmetry breaking and wavevector. Compared with fundamental isolated BICs, the merged BICs are more immune to structural disorders. Our approach provides a path to achieve robust ultrahigh-$Q$ resonances, which holds immense potential in enhancing quantum and nonlinear effects, as well as improving the performance of optical sensors and nanolasers.

The band nonparabolicity of indium tin oxide (ITO) polycrystalline thin films is investigated with the quasilinear Kane model through Seebeck and Hall effect measurements. We report Kane model nonparabolic band parameters of $m_0^{*}=0.21{\mathrm{m}}_0$ and $C=0.52{\mathrm{eV}}^{\text{−}1}$ for ITO, in good agreement with historical photoemission, optical, and transport measurements. To do this, the ITO films were doped with nitrogen by ion implantation, with fluences ranging from $5\times {10}^{14}\mathrm{N}{\mathrm{cm}}^{\text{−}2}$ to $5\times {10}^{15}\mathrm{N}{\mathrm{cm}}^{\text{−}2}$. The presence of the nitrogen in the films was verified with x-ray photoelectron spectroscopy, and their acceptor character studied theoretically by density functional theory. Experimentally, the doped nitrogen formed ${{\mathrm{N}}_{\mathrm{O}}}^{\text{−}}$ defects, deep acceptor states that led to a controlled compensation in carrier concentration from $10.1\times {10}^{20}\pm 0.6\times {10}^{20}{\mathrm{cm}}^{\text{−}3}$ to $2.9\times {10}^{20}\pm 0.2\times {10}^{20}{\mathrm{cm}}^{\text{−}3}$. Understanding the band nonparabolicity of degenerately doped transparent conducting oxides is essential for their commercial application in solar cells, transparent thermoelectric generators, and transparent thin film transistors. In this work, the Seebeck and Hall effect approach with the quasilinear Kane model for band nonparabolicity is presented as a practical method by which to study the variation in carrier effective mass without reliance on optical measurements.

We examine the spectrum for lattices with cylindrical and toroidal topology subject to Abelian gauge potentials. Gauges that are equivalent in planar lattices with trivial topology develop differences due to the periodic boundary conditions. But some residual gauge equivalency, evident in the spectrum, is found to remain under specific conditions. This is associated with the gauge structure being commensurate with the lattice periodicity, and the behavior of the associated field along the symmetry axes. The interplay of the gauge choice and the periodic boundary conditions leads to a class of persistent degeneracies that are found to be robust against vast changes in system parameters and even under change of topology from cylinder to torus.

A three-dimensional non-Hermitian Hamiltonian with parity-time symmetry can exhibit a closed exceptional surface (EP surface) in momentum space, which is a non-Hermitian deformation of the degeneracy line. Since the degeneracy line lacks an internal space, the distributions of Berry curvature inside the EP surface become particularly intriguing. This paper studies the distributions taking a toruslike EP surface as an example. In a meridian cross section, the Berry connection exhibits a vortexlike field with only angular components, while the Berry curvature is perpendicular to this cross section; in a equatorial cross section, the Berry curvature forms a closed curve surrounding the central genus. Both Berry connection and curvature converge along the coplanar axis and diverge at the surface. We find the Berry flux depends on the radius of the integration region and is not quantized inside the EP torus. Approaching the surface, the Berry flux tends to infinity and the dynamical phase oscillates violently. We point out that the streamlines of Berry curvature can be used to estimate the zero or nonzero Berry flux. We generalize the above patterns to the case of EP surfaces with complex shapes and present a proposal of realizing the EP surface in an electrical circuit. Our research outcomes enhance the comprehension of EP surfaces and the topological characteristics of non-Hermitian systems with parity-time ($\mathcal{PT}$) symmetry.

A method to simulate local properties and processes in crystals with impurities via constructing cluster models within the frame of the compound-tunable embedding potential (CTEP) and highly accurate ab initio relativistic molecular-type electronic structure calculations is developed and applied to the Ce- and Th-doped yttrium orthophosphate crystals, ${\mathrm{YPO}}_4$, having xenotime structure. Two embedded cluster models are considered, the “minimal” one, ${\mathrm{YO}}_8$@${\mathrm{CTEP}}_{\min }$, consisting of the central ${\mathrm{Y}}^{3+}$ cation and its first coordination sphere of eight ${\mathrm{O}}^{2-}$ anions (i.e., with broken P–O bonds), and its extended counterpart, $\mathrm{Y}{\left({\mathrm{PO}}_4\right)}_6$@${\mathrm{CTEP}}_{\mathrm{ext}}$, implying the full treatment of all atoms of the ${\mathrm{PO}}_4^{3-}$ anions nearest to the central ${\mathrm{Y}}^{3+}$ cation. ${\mathrm{CTEP}}_{\min ,\mathrm{ext}}$ denote here the corresponding cluster environment described within the CTEP method. The Fock space relativistic coupled cluster (FS RCC) theory is applied to the minimal cluster model to study electronic excitations localized on ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Th}}^{3+}$ impurity ions. Calculated transition energies for the cerium-doped xenotime are in a good agreement with the available experimental data (mean absolute deviation of ca. 0.3 eV for $4f\to 5d$ type transitions). For the thorium-doped crystal the picture of electronic states is predicted to be quite complicated, and the ground state is expected to be of the $6d$ character. The uncertainty for the excitation energies of thorium-doped xenotime is estimated to be within 0.35 eV. Radiative lifetimes of excited states are calculated at the FS RCC level for both doped crystals. The calculated lifetime of the lowest $5d$ state of ${\mathrm{Ce}}^{3+}$ differs from the experimentally measured one by no more than twice.

The high-pressure Pbnm-perovskite polymorph of ${\mathrm{MgSiO}}_3$, i.e., bridgmanite (Bm), plays a crucial role in the Earth's lower mantle. It is likely responsible for ∼75 vol. % of this region and its properties dominate the properties of this region, especially its elastic properties that are challenging to measure at ambient conditions. This study combines deep-learning potential (DP) with density-functional theory (DFT) to investigate the structural and elastic properties of Bm under lower-mantle conditions. To simulate this system, we developed a series of potentials capable of faithfully reproducing DFT calculations using different functionals, i.e., local density approximation (LDA), Perdew-Burke-Ernzerhof parametrization (PBE), revised PBE for solids (PBEsol), and strongly constrained and appropriately normed (SCAN) meta–generalized-gradient approximation functionals. Our predictions with DP-SCAN exhibit a remarkable agreement with experimental measurements of high-temperature equations of states and elastic properties and highlight its superior performance, closely followed by DP-LDA in accurately predicting. This hybrid computational approach offers a solution to the accuracy-efficiency dilemma in obtaining precise elastic properties at high pressure and temperature conditions for minerals like Bm, opening a way to study the Earth material's thermodynamic properties and related phenomena.

We explore the fascinating dynamics of ferroelectric materials under illumination, which holds great potential to advance the field of optoferroelectricity. First, we introduce a general Hamiltonian-based model to explain how light exposure weakens ferroelectric polarity and reduces potential barriers in a ferroelectric system. Second, the properties and the dynamic evolution process of ferroelectric $\mathrm{BaTi}{\mathrm{O}}_3$ and SnTe under illumination are investigated using the first-principles methods, validating our model. The results demonstrate that illumination induces the redistribution of charge density and the reorganization of potential energy surface, thereby driving the transition from ferroelectric to paraelectric phases. Our findings reveal the potential for light-modulated ferroelectric memory and could propel the development of advanced ferroelectric devices. This study, therefore, represents a key step toward a comprehensive understanding of optoferroelectricity and its potential applications.