We investigate the crystal structures, phase transitions, and phase stability of undoped and Ca-modified $\mathrm{NaNb}{\mathrm{O}}_3$ through a combined analysis of high-resolution synchrotron radiation x-ray and neutron diffraction, convergent-beam electron diffraction, and density functional theory calculations. It is demonstrated that the antiferroelectric (AFE)-R phase is stabilized over a wide temperature range of 200–800 K by Ca modification, which is associated with the formation of Na vacancies at the A site. Our results reveal that the high-temperature AFE-R phase of undoped $\mathrm{NaNb}{\mathrm{O}}_3$ exhibits Pmmn symmetry whereas the room-temperature AFE-R phase of the Ca-modified $\mathrm{NaNb}{\mathrm{O}}_3$ adopts Pnma symmetry. We found that the Pmmn phase of $\mathrm{NaNb}{\mathrm{O}}_3$ is stabilized by temperature-driven isostatic pressure accompanied by lattice expansion, whereas the Pnma one of the Ca-modified $\mathrm{NaNb}{\mathrm{O}}_3$ is induced by composition-induced chemical pressure along with lattice shrinkage.

Solving for the time evolution of a many particle system whose dynamics is governed by Lindblad equation is hard. We extend the use of the transfer matrix approach to a class of Lindblad equations that admit a closed hierarchy of two point correlators. An example that we treat is the $XX$ spin chain, i.e., free fermions, subject to the local onsite dephasing, but can be extended to other Hermitian dissipators, e.g., nonlocal dephasing. We find a simple expression of the Green's function in the Laplace domain. The method can be used to get analytical results in the thermodynamic limit, for instance, to get the evolution of the magnetization density and to explicitly see the crossover between ballistic and diffusive behavior, or to show that the correlations between operators at distance $l$ decay with time as $1/t^{\lceil l/2\rceil +1/2}$. It also provides a fast numerical method to determine the evolution of the density with a complexity scaling with the system size more favorably than in previous methods, easily allowing one to study systems with $\sim {10}^6$ spins.

Skyrmions are nanosized magnetic whirls attractive for spintronic applications due to their innate stability. They can emulate the characteristic behavior of various spintronic and electronic devices such as spin-torque nano-oscillators, artificial neurons and synapses, logic devices, diodes, and ratchets. Here, we show that skyrmions can emulate the physics of an $\mathit{RC}$ circuit—the fundamental electric circuit composed of a resistor and a capacitor—on the nanosecond time scale. The equation of motion of a current-driven skyrmion in a quadratic energy landscape is mathematically equivalent to the differential equation characterizing an $\mathit{RC}$ circuit: the applied current resembles the applied input voltage and the skyrmion position resembles the output voltage at the capacitor. These predictions are confirmed via micromagnetic simulations. We show that such a skyrmion system reproduces the characteristic exponential voltage decay upon charging and discharging the capacitor under constant input. Furthermore, it mimics the low-pass filter behavior of $\mathit{RC}$ circuits by filtering high frequencies in periodic input signals. Since $\mathit{RC}$ circuits are mathematically equivalent to the leaky-integrate-fire (LIF) model widely used to describe biological neurons, our device concept can also be regarded as a perfect artificial LIF neuron.

When a charge current is injected into an altermagnet along a suitable crystallographic direction, a transverse spin current can be generated. This so-called spin-splitter effect does not rely on spin-orbit coupling and is thus distinct from the spin Hall effect. The spin-splitter effect was predicted by ab initio calculations and has been experimentally confirmed. To utilize the spin-splitter effect for practical purposes in spintronic devices, it is important to understand (i) how the system parameters affect the transverse spin current, such as filling fraction, altermagnetic strength, interface parameters, spin-orbit interactions, and impurities, and (ii) determine the properties of any associated spin accumulation, which is the measurable quantity. Here, we determine the answer to these questions and provide a real-space visualization of the spin flow and spin accumulation due to the spin-splitter effect. We utilize the nonequilibrium Keldysh Green's-function method on a two-dimensional square lattice to this end. We find that the presence of edges induces oscillations in the spin accumulation and strongly modifies the signal for small samples. At half filling, the spin accumulation acquires an anomalous pattern and the spin-splitter effect vanishes. We prove analytically that this follows from a combined particle-hole and spin-reversal symmetry of the model used for the altermagnetic state. Increasing the altermagnetic strength leads to a larger spin accumulation, as expected. However, when adding Rashba spin-orbit interaction, providing an additional spin Hall signal, we find that the spin accumulation is not simply the sum of the spin Hall and spin-splitter contribution. Finally, we show that the spin-splitter effect is robust towards moderate impurity scattering with a potential of the same order as the hopping parameter, which facilitates its observation in real materials.

We propose a superconducting circuit that supports the propagation of Korteweg–de Vries (KdV)-type voltage solitons. By employing Cooper-pair boxes as nonlinear capacitors, we realize a dual system to the previously reported current solitons based on nonlinear inductors. The proposed voltage solitons manifest as propagating capacitors, carrying a velocity-dependent induced charge. This unique characteristic facilitates local electrostatic interactions, which are dual to the magnetic interactions mediated by fluxons.

We study the intravalley spin-polarized superconductivity in rhombohedral tetralayer graphene, which has been discovered experimentally in Han et al. [arXiv:2408.15233]. We construct a minimal model for the intravalley spin-polarized superconductivity, assuming a simplified anisotropic interaction that depends only on the angle between the incoming and outgoing momenta. Despite the absence of Fermi surface nesting, we show that superconductivity can emerge near the Van Hove singularity with the maximal $T_c$ near a bifurcation point of the peaks in the density of states. We identify the $p+ip$, $h+ih$, and the nodal $f$-wave pairings as the possible states, which are all pair density wave orders due to the intravalley nature. Furthermore, these pair density wave orders require a finite attractive threshold for superconductivity, resulting in a narrow stripe shape of superconducting region, consistent with experimental findings. We point out that the Kohn-Luttinger mechanism is a plausible explanation with a dominant $p+ip$ pairing. The possibility of realizing intravalley spin-polarized superconductivity in other rhombohedral graphene systems is also discussed.

The discoveries that the unconventional stoichiometric hydrides ${\mathrm{SH}}_3$ and ${\mathrm{P}}_4{\mathrm{H}}_6$ become superconductors with critical temperatures ($T_c$) of 203 and 103 K at high pressures have motivated extensive investigations of hydrogen-rich compounds. The As-H system, the hydride of the same main group, has also been predicted to have a variety of stoichiometric ratios with high $T_c$ under high pressure. However, its high-pressure stoichiometry evolution and superconductivity remain unknown. Here, we explored the stoichiometric evolution and thermodynamic process of the As-H system under high pressure. We observed that ${\mathrm{AsH}}_3$ underwent liquefaction and solidification at approximately 0.01 and 1.7 GPa, respectively. Upon further compression, ${\mathrm{AsH}}_3$ dimerizes into ${\mathrm{As}}_2{\mathrm{H}}_4$ at 7.0 GPa. ${\mathrm{As}}_2{\mathrm{H}}_4$ is unstable, decomposing into gray arsenic at pressures above 20.0 GPa or amorphous arsenic upon decompression. This comprehensive exploration of the high-pressure stoichiometric evolution of As-H is crucial for exploring superconductivity in the As-H system and understanding the internal arsenic cycling in planets like Jupiter and Saturn.

Weyl semimetals are promising quantum materials with gapless electronic excitations and distinctive topological properties. In this paper, we explore the high harmonic generation in three-dimensional Weyl semimetal ${\mathrm{WP}}_2$ based on semiconductor Bloch equations in the Wannier gauge. Selection rules and mechanisms for harmonic generation driven by a single pulse in different polarization state are discussed. We also find that the harmonic yield exhibits an oscillating increase with the laser intensity due to the quantum interference between long and short trajectories. Moreover, the results also reveal an anomalous ellipticity dependence, with the total harmonic yield first decreasing and then increasing. Our simulations capture the complex band structure and dipole moments, providing valuable insights for strong-field-driven electron dynamics in Weyl semimetals.

Quantum anomalous Hall effect (QAHE) is significant for future low-power electronics devices, where a main challenge is realizing QAHE at high temperatures. In this work, based on experimentally reported two-dimensional (2D) germanene and magnetic semiconductors ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ and ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$, and the first-principles calculations, germanene/magnetic semiconductor heterostructures are investigated. Topologically nontrivial edge states and quantized anomalous Hall conductance are demonstrated. It is shown that the QAHE temperature can be enhanced to approximately 62 K in germanene/monolayer (ML) ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ with 2.1% tensile strain, 64 K in germanene/bilayer (BL) ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ with 1.4% tensile strain, and 50 K in germanene/ML ${\mathrm{Cr}}_2{\mathrm{Si}}_2{\mathrm{Te}}_6$ with 1.3% tensile strain. With increasing tensile strain of these heterostructures, the band gap decreases and the Curie temperature rises, and the highest temperature of QAHE is obtained. Since these 2D materials were discovered in recent experiments, our results provide promising materials for achieving high-temperature QAHE.

We derive asymptotically exact formulas for the equilibrium magnetic stripe period in ultrathin films with out-of-plane anisotropy that include the full domain wall magnetic dipolar energy. Starting with the reduced two-dimensional micromagnetic model valid for thin films, we obtain the leading-order approximation for the energy per unit volume in the vanishing film thickness limit in the case of Bloch and Néel wall rotations. Its minimization in the stripe period leads to an analytical expression for the equilibrium period with a prefactor proportional to the Bloch wall width. The constant in the prefactor, which is related to the long-range dipolar interactions, is carefully evaluated. This results in a remarkable agreement of the stripe domain energy density and stripe period predicted by our analytical formulas with micromagnetic simulations. Our formula can be used to accurately deduce magnetic parameters from the experimental measurements of the stripe period and to systematically predict the equilibrium stripe periods in ultrathin films.

The emergent inductor, which is a concept of an inductor employing quantum mechanics on helical magnets, has been studied actively from both theoretical and experimental aspects. Interestingly, finite inductance has been observed not only in spiral magnetic phases but also across various other magnetic phases, although the underlying mechanism behind emergent inductance in nonhelical magnets remains unresolved. In this paper, we broaden the concept of emergent inductance to encompass nonhelical magnets and establish a comprehensive formalism of the emergent inductance generated by magnetization dynamics. Using a diagrammatic approach, we derive the linear response of current density mediated by magnetization dynamics and formulate impedance and inductance for general magnetic materials. We reveal that the emergent inductance is composed of two distinct components, each contributing positively or negatively. Notably, the positive inductance arises from Ohmic dissipation including the effect of the emergent electric field (Berry phase in real space) while the negative inductance arises from the polarization of itinerant electrons (Berry curvature in $k-t$ space). We also apply the present method to quasi-1D models and show the numerical results of the inductance, the spin dynamics, the impedance, and the $Q$ value.

Doping nonmagnetic semiconductors with magnetic impurities is a feasible way to obtain diluted magnetic semiconductors (DMSs). It is generally accepted that for the most extensively studied DMS, (Ga, Mn)As, its highest Curie temperature ${\mathrm{T}}_{\text{C}}$ was achieved at 200 K with a Mn concentration of approximately 16% in experiments. A recent experiment reported record-breaking high electron and hole mobilities in the semiconductor BAs [Science 377, 437 (2022)]. Since BAs shares the same zinc-blende structure with GaAs, here we predict four DMSs (B, Mn)$X$ ($X=\mathrm{N}$, P, As, Sb) by density functional theory calculations. Using a rescaling method to diminish the overestimation of Curie temperature, our results indicate that a significantly higher ${\mathrm{T}}_{\text{C}}$ in the range of 467 to 485 K for (B, Mn)As with a Mn concentration of around 15.6% and even higher ${\mathrm{T}}_{\text{C}}$ values above the room temperature for (B, Mn)P with a Mn concentration exceeding 9.4%. Furthermore, using the method of ab initio scattering and transport (amset) with first-principles material parameters, we have predicted a hole mobility of 48.9 ${\mathrm{cm}}^{\text{2}}{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 300 K for (B, Mn)As with the hole concentration of about $n_{\text{h}}=$ 4.2 $\times {10}^{\text{19}}{\mathrm{cm}}^{-3}$, which is about two times larger than the hole mobility at 300 K in the calculations for (Ga, Mn)As. The hole mobility of (B, Mn)As can be enhanced faster than that of (Ga, Mn)As when the hole concentration is decreased. Our findings predict the emergence of a new family of DMS, (B, Mn)$X$, and are expected to stimulate both experimental and theoretical studies of the DMS with possible high $T_{\text{C}}$.

The kagome metal $\mathrm{Sc}{\mathrm{V}}_6\mathrm{S}{\mathrm{n}}_6$ displays a variety of exotic electronic states, including charge density wave (CDW) order, nontrivial topological states, Van Hove singularity (VHS), and electronic nematic order, which strongly correlate with the V kagome lattice. We unveil herein that the partial substitution of V by Mn in the kagome lattice $\mathrm{Sc}{\mathrm{V}}_6\mathrm{S}{\mathrm{n}}_6$ results in multiple magnetic transitions including the long-range magnetic order above room temperature with the transition temperature of $\sim 320$ K. In addition, the Mn substitution suppresses the CDW order and the VHSs, while leading to unconventional anomalous and other anomalous Hall effects caused by both intrinsic Berry curvature and extrinsic skew scattering. Our work provides valuable insights into the correlation between magnetism and the electronic band structure of the kagome lattice, which is instructive for future research on this system.

We investigate the robustness of virtual topological states—topological phases away from the Fermi energy—against the electron-electron interaction and band filling. As a case study, we employ a realistic model to investigate the properties of vacancy-driven topological phases in transition metal dichalcogenides (TMDs) and establish a connection between the degree of localization of topological wave functions, the vacancy density, and the electron-electron interaction strength with the topological phase robustness. We demonstrate that electron-electron interactions play a crucial role in degrading topological phases thereby determining the validity of single-particle approximations for topological insulator phases. Our findings can be naturally extended to virtual topological phases of a wide range of materials.

Two-dimensional topological insulators (2D TIs) exhibit a characteristic Landau level spectrum in which a pair of zeroth Landau levels disperse oppositely as a result of band inversion. The two Landau levels cross at a critical magnetic field, marking the transition to a trivial phase. Experiments sometimes show an avoided crossing, signifying hybridization (mixing) of the zeroth Landau level wave functions. Here, we theoretically identify the essential symmetries that protect against zeroth Landau level mixing, using thin films of cadmium arsenide (${\mathrm{Cd}}_3{\mathrm{As}}_2$), a prototype 2D TI, as an example. We show that avoided crossings that result from zeroth Landau level mixing indicate the breaking of the four-fold rotational symmetry, inherent to bulk ${\mathrm{Cd}}_3{\mathrm{As}}_2$. We discuss potential reasons for the symmetry breaking, including the formation of novel correlated states, and the relevance of our results to other materials.

Rare-earth trihydride ($R{\mathrm{H}}_3$) compounds exhibit intriguing coupled electronic and structural properties as a function of doping, hydrogen vacancies, and thermodynamic conditions. Theoretical studies of these materials typically rely on density functional theory (DFT), including the use of small supercells that may underestimate strong correlation effects and structural distortions which in turn may influence their metallicity. Here, we elucidate the roles of lattice distortions and correlation effects on the electronic properties of pristine and doped $R{\mathrm{H}}_3$ compounds by adopting $\mathrm{DFT}+\mathrm{U}$ and quantum Monte Carlo (QMC) methods. Linear-response constrained DFT (LR-cDFT) methods find Hubbard U $\approx 2$ eV for $R_d$ orbitals and $\mathrm{U}\approx 6$ eV for ${\mathrm{H}}_s/{\mathrm{N}}_p/{\mathrm{O}}_p$ orbitals. The small U on ${\mathrm{Lu}}_d$ orbitals is consistent with QMC calculations on ${\mathrm{LuH}}_3$ and ${\mathrm{LuH}}_{2.875}{\mathrm{N}}_{0.125}$. In pure face-centered-cubic (FCC) $R{\mathrm{H}}_3$ ($R=\mathrm{Lu},\mathrm{Y}$) compounds, neither DFT nor $\mathrm{DFT}+\mathrm{U}$ with the self-consistently determined U is enough to create a band gap, however a supercell with hydrogen distortions creates a small gap whose magnitude increases when performing $\mathrm{DFT}+\mathrm{U}$ with self-consistently determined U values. Correlation effects, in turn, have a moderate influence on the coupled structural and electronic properties of doped $R{\mathrm{H}}_3$ compounds and may be important when considering the competition between structural distortions and superconductivity.

Symmetry protected topology has been studied extensively since the mid-2000s, but the topology protected by spin symmetry has just begun to be studied. In this work, based on spin-symmetry analysis, we propose that a class of Weyl nodal line metals is protected by the spin symmetry. Then, by the first-principles electronic structure calculations, we predict that both altermagnetic ${\mathrm{Nb}}_2{\mathrm{FeB}}_2$ and ${\mathrm{Ta}}_2{\mathrm{FeB}}_2$ are node-network metals protected by the spin symmetry. Moreover, both altermagnetic ${\mathrm{Nb}}_2{\mathrm{FeB}}_2$ and ${\mathrm{Ta}}_2{\mathrm{FeB}}_2$ have nodal rings protected by the mirror symmetry and Dirac points protected by nonsymmorphic spin symmetry. Furthermore, both altermagnetic ${\mathrm{Nb}}_2{\mathrm{FeB}}_2$ and ${\mathrm{Ta}}_2{\mathrm{FeB}}_2$ transform node-network metal phase into Weyl metal phase when considering spin-orbit coupling. Therefore, our work not only enriches the topological phases protected by spin symmetry but also provides an excellent material platform to investigate the exotic physical properties of multiple altermagnetic topological metal phases in experiment.

The Kohn-Sham density functional method for the fractional quantum Hall (FQH) effect has recently been developed by mapping the strongly interacting electrons into an auxiliary system of weakly interacting composite fermions (CFs) that experience a density-dependent effective magnetic field. This approach has been successfully applied to explore the edge reconstruction, fractional charge, and fractional braiding statistics of quasiparticle excitations. In this work, we investigate composite fermion excitons in the bulk of the disk geometry. By varying the separation of the quasiparticle-quasihole pairs and calculating their energy, we compare the dispersion of the exciton with results from other numerical methods, such as exact diagonalization (ED) and Monte Carlo (MC) simulation. Furthermore, from the spectral function calculation, we identify chiral “graviton” excitations: a spin $-2$ mode for the particle-like Laughlin state and a spin 2 mode for the hole-like Laughlin state. This method can be extended to construct neutral collective excitations for other fractional quantum Hall states in disk geometry.