We present a molecular orbital candidate ${\mathrm{Ba}}_4{\mathrm{TaMn}}_3{\mathrm{O}}_{12}$ with a face-sharing octahedra trimer, by comparing it with a related compound ${\mathrm{Ba}}_4{\mathrm{NbMn}}_3{\mathrm{O}}_{12}$. The synthesis of the polycrystalline powder is optimized by suppressing the secondary impurity phase via x-ray diffraction. Magnetic susceptibility measurements on the optimized samples reveal a weak magnetic hysteresis with magnetic transitions consistent with heat capacity results. The effective magnetic moments from susceptibility indicate a strongly coupled $S=2$ antiferromagnetic trimer at around room temperature, whereas the estimated magnetic entropy from heat capacity suggests the localized $S=3/2$ timer. These results can be explainable by a partial molecular orbital state, in which three $t_{2g}$ electrons are localized in each Mn ion and one $e_g$ electron is delocalized over two-end Mn ions of the trimer based on density functional theory calculations. This unconventional 3d orbital state is comprehended as a consequence of competition between the hybrid interatomic orbitals within the Mn trimer and the local moment formation by on-site Coulomb correlations.

We calculate the screened electron-electron interaction for the charge-disproportionated insulator ${\mathrm{CaFeO}}_3$ using the constrained random-phase approximation (cRPA). While in many correlated materials the formation of a Mott-insulating state is driven by a large local Coulomb repulsion, represented by the Hubbard $U$, several cases have been identified more recently in which $U$ is strongly screened and instead the Hund's interaction $J$ dominates the physics. Our results confirm a strong screening of the local Coulomb repulsion $U$ in ${\mathrm{CaFeO}}_3$, whereas $J$ is much less screened and can thus stabilize a charge-disproportionated insulating state. This is consistent with the case of the rare-earth nickelates, where similar behavior has been demonstrated. In addition, we validate some common assumptions used for parametrizing the local electron-electron interaction in first-principles calculations based on density-functional theory (DFT), we assess the dependence of the interaction on the choice of correlated orbitals, and we discuss the use of the calculated interaction parameters in $\mathrm{DFT}+U$ calculations of ${\mathrm{CaFeO}}_3$. Our work also highlights certain limitations for the direct use of cRPA results in DFT-based first-principles calculations, in particular for systems with strong entanglement between the correlated and uncorrelated bands.

Skyrmioniums are doughnutlike topological spin textures in chiral magnets, which are able to move with zero skyrmion Hall effect. The annihilation of a skyrmionium involves the topological transition through singularity formation at the continuum level. Here we analytically and numerically study a type of thermal annihilation of a skyrmionium under the framework of micromagnetism. The skyrmionium initially exists in an ultrathin ferromagnetic film in the exchange-dominated regime and is stabilized under a uniform perpendicular magnetic field. The thermal annihilation of the “skyrmion” inside the skyrmionium results in the transformation of the skyrmionium to a skyrmion. We derive an analytical formula in the Arrhenius form describing the annihilation rate of this process. We find that the analytical solutions agree well with the micromagnetic simulation results. Besides, from the large deviation point of view, we derive the dynamical path for the collapse of a skyrmionium. Our results are useful for understanding the thermal stability of skyrmioniums and may lead to spintronic applications based on the thermal manipulation of skyrmioniums.

Magnetic anisotropies often originate from the spin-orbit coupling and determine magnetic ordering patterns. We develop a microscopic theory for dc electric-field controls of magnetic anisotropies in magnetic Mott insulators and discuss its applications to Kitaev materials and topological spin textures. Throughout this paper, we take a microscopic approach based on Hubbard-type lattice models, tight-binding models with onsite interactions. We derive a low-energy spin Hamiltonian from a fourth-order perturbation expansion of the Hubbard-type model. We show in the presence of a strong intra-atomic spin-orbit coupling that dc electric fields add non-Kitaev interactions such as a Dzyaloshinskii-Moriya interaction and an off-diagonal ${\mathrm{\Gamma }}^{\prime }$ interaction to the Kitaev-Heisenberg model and can induce a topological quantum phase transition between Majorana Chern insulating phases. We also investigate the interatomic Rashba spin-orbit coupling and its effects on topological spin textures. dc electric fields turn out to create and annihilate magnetic skyrmions, hedgehogs, and chiral solitons. We propose several methods of creating topological spin textures with external electromagnetic fields. Our theory clarifies that the strong but feasible electric field can control Kitaev spin liquids and topological spin textures.

Moiré structures, along with line-graph-based $d$-electron systems, represent a setting to realize flat bands. One form of the associated strong correlation physics is the Kondo effect. Here, we address the recently observed Kondo-driven heavy fermion state and its destruction in AB-stacked hetero-bilayer transition metal dichalcogenides, which can be controlled by the gate voltages. By studying an effective interacting Hamiltonian using the slave spin approach, we obtained a phase diagram with the total filling factor and the displacement field strength as the tunable parameters. In an extended range of the tunable displacement field, our numerical results show that the relative filling of the $d$ orbital, which is associated with the highest moiré band from the ${\mathrm{MoTe}}_2$ layer, is enforced to be ${\nu }_d\approx 1$ by the interaction. This agrees with the experimental observation. We also argue that the observed high coherence temperature scale could be explained by the non-negligible bandwidth of the $d$ orbital. Our results set the stage to address the amplified quantum fluctuations that the Kondo effect may produce in these structures and new regimes that the systems open up for Kondo-destruction quantum criticality.

Dynamical decoupling is effective in reducing gate errors in most quantum computation platforms and is therefore projected to play an essential role in future fault-tolerant constructions. In superconducting circuits, however, it has proven difficult to utilize the benefits of dynamical decoupling. In this work, we present a theoretical proposal that incorporates a continuous version of dynamical decoupling, namely, spin locking, with a coupler-based CZ gate for transmons and provide analytical and numerical results that demonstrate its effectiveness.

We investigate the magnetoresistance of a dual-gated bilayer $\mathrm{Mo}{\mathrm{S}}_2$ encapsulated by hexagonal boron nitride. At low magnetic fields ($\left|B\right|<0.5\mathrm{T}$), we observe a negative magnetoresistance, which we identify as the weak localization effect. We determine both the phase coherence length and mean free path as a function of electron density and displacement field. Both characteristic lengths show a similar monotonic increase with electron density, while they are not affected by the displacement field. We further investigate the dephasing mechanism by measuring the temperature dependence of the phase coherence length. Our results suggest that when only the lower spin-orbit split bands ($K\uparrow , K^{\prime }\downarrow$) contribute to transport is Coulomb scattering the dominant source of decoherence, while intervalley scattering seems not to play a relevant role in this regime. This observation is consistent with the picture of spin-polarized valleys (spin-valley locking), where the intrinsic spin-orbit coupling protects the spin states, rather than introducing an additional dephasing mechanism as in other materials.

A heat modulator is proposed based on a voltage-biased Aharonov-Bohm interferometer. Once an electrical bias is applied, Peltier effects give rise to a flow of heat that can be modulated by a magnetic flux. We determine the corresponding temperature changes using a simple thermal model. Our calculations demonstrate that the modulated temperature difference can be as large as 80 mK at base temperature about 600 mK with relative temperature variations reaching 10%. Our model also predicts, quite generally, the emergence of spin-polarized heat flows without any ferromagnetic contacts, if Rashba spin-orbit interaction is combined with the applied magnetic flux, which potentially paves the way towards caloritronic information processing.

${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and related compounds are prototype three-dimensional topological insulators with a single Dirac cone in the surface band structure. While the topological surface states can be characterized with surface-sensitive methods, the underlying bulk energy band inversion has not been investigated in detail. Here, a study is presented that combines density-functional theory and nuclear magnetic resonance to explore the nontrivial band edge of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. It is found that the topological band inversion is not a discrete reversal of the order of the valence and conduction band at the $\mathrm{\Gamma }$ point. Rather, the bands closest to the Fermi level become well mixed and spread evenly below and above the band gap, such that the characters of the valence- and conduction-band edges become indistinguishable. Beside those bands relevant for the band inversion, i.e., Bi and Se $p_{\mathrm{z}}$, also Bi $p_{\mathrm{x}}$ and $p_{\mathrm{y}}$ states are involved. As a part of this mixture of states, the band inversion shows no edges in $k$ space.

The interplay between non-Hermitian effects and topological insulators has become a frontier of research in non-Hermitian physics. However, the existence of a non-Hermitian skin effect for topological-protected edge states remains controversial. In this paper, we discover a new type of the non-Hermitian skin effect called the non-Hermitian chiral skin effect. Non-Hermitian chiral skin effect is induced by inhomogeneous edge dissipation under closed boundary conditions. In particular, the chiral modes of the non-Hermitian chiral skin effect localize around “topological defects” characterized by global dissipation rather than being confined to the system boundaries. We show its detailed physical properties by taking the non-Hermitian Haldane model as an example. As a result, the intrinsic mechanism of the hybrid skin-topological effect in Chern insulators is fully understood via the non-Hermitian chiral skin effect. Therefore this work will be helpful for the research on both non-Hermitian physics and topological quantum states.

We systematically investigate the strain-engineered magnon states in two-dimensional (2D) ferromagnetic monolayers. By suitable engineering of an inhomogeneous strain, we demonstrate the emergence of magnon Landau levels and magnon snake states in 2D ferromagnetic monolayers. We show a magnon valley Hall effect and valley filter without relying on any external fields. Our proposal offers us another way to manipulate magnon valley transport and construct different types of flexible spintronic devices, and is experimentally feasible for 2D ferromagnetic materials by using state-of-art techniques.

The orbital Hall effect can generate currents of angular momentum more efficiently than the spin Hall effect in most metals. However, so far, it has only been understood as a steady-state phenomenon. In this theoretical study, the orbital Hall effect is extended into the time domain. We investigate the orbital angular momenta and their currents induced by a femtosecond laser pulse in a Cu nanoribbon. Our numerical simulations provide detailed insights into the laser-driven electron dynamics on ultrashort timescales with atomic resolution. The ultrafast orbital Hall effect described in this paper is consistent with the familiar pictorial representation of the static orbital Hall effect, but we also find pronounced differences between physical quantities that carry orbital angular momentum and those that carry charge. For example, there are deviations in the time series of the respective currents. This paper lays the foundations for investigating ultrafast Hall effects in confined metallic systems.

Numerous measurements of the dielectric constant $\varepsilon$ of the recently discovered ferroelectric nematic (${\mathrm{N}}_{\mathrm{F}}$) liquid crystal (LC) phase report extraordinarily large values of $\varepsilon$′ (up to ∼30 000) in the ${\mathrm{N}}_{\mathrm{F}}$ phase. We show that what is in fact being measured in such experiments is the high capacitance of the nonferroelectric, interfacial, insulating layers of nanoscale thickness that bound the ${\mathrm{N}}_{\mathrm{F}}$ material in typical cells. Polarization reorientation as a linear response to AC electric field drive renders the ${\mathrm{N}}_{\mathrm{F}}$ layer effectively electrically conductive, exhibiting low resistance that enables the charging of the interfacial capacitors. We analyze the commonly employed parallel-plate cell filled with ${\mathrm{N}}_{\mathrm{F}}$ material of high polarization P, oriented parallel to the plates at zero applied voltage. Minimization of the dominant electrostatic energy renders P spatially uniform and orients the polarization to make the electric field in the ${\mathrm{N}}_{\mathrm{F}}$ as small as possible, a condition under which the voltage applied to the cell appears almost entirely across the high-capacity interfacial layers. This coupling of orientation and charge creates a combined polarization-external capacitance (PCG) Goldstone reorientation mode requiring applied voltages orders of magnitude smaller than that of the ${\mathrm{N}}_{\mathrm{F}}$ layer alone to effectively transport charge across the ${\mathrm{N}}_{\mathrm{F}}$ layer. The ${\mathrm{N}}_{\mathrm{F}}$ layer acts as a low-value resistor, and the interfacial capacitors act as reversible energy storage reservoirs, lowering the restoring force (mass) of the PCG mode and producing strong reactive dielectric behavior. Analysis of data from a variety of experiments on ferroelectric LCs (chiral smectics C, bent-core smectics, and the ${\mathrm{N}}_{\mathrm{F}}$ phase) supports the PCG model, showing that deriving dielectric constants from electrical impedance measurements of high-polarization ferroelectric LCs, without properly accounting for the self-screening effects of polarization charge and the capacitive contributions of interfacial layers, can result in overestimation of the $\varepsilon$′ values of the LC by many orders of magnitude.

We consider a smooth interface between a topological nodal-line semimetal and a topologically trivial insulator (e.g., the vacuum) or another semimetal with a nodal ring of different radius. Using a low-energy effective Hamiltonian including only the two crossing bands, we show that these junctions accommodate a two-dimensional zero-energy level and a set of two-dimensional dispersive bands, corresponding to states localized at the interface. We characterize the spectrum, identifying the parameter ranges in which these states are present, and highlight the role of the nodal radius and the smoothness of the interface. We also suggest material-independent ways to detect and identify these states, using optical conductivity and infrared absorption spectroscopy in magnetic field.

The quantum phase transition between the low-field fracton phase with type-II fracton excitations and the high-field polarized phase is investigated in the two-dimensional self-dual quantum Newman-Moore model. We apply perturbative and numerical linked-cluster expansions to calculate the ground-state energy per site in the thermodynamic limit, revealing a level crossing at the self-dual point. In addition, high-order series expansions of the relevant low-energy gaps are determined using perturbative continuous unitary transformations indicating no gap closing. Our results therefore predict a first-order phase transition between the low-field fracton and the high-field polarized phase at the self-dual point.

We investigate the $c$-direction conduction in CrSBr in the linear regime, which is not accessible in other van der Waals (vdW) magnetic semiconductors, because of the unmeasurably low current. The resistivity, which is ${10}^8-{10}^{11}$ times larger than in the $a$ and $b$ directions, exhibits magnetic state dependent thermally activated and variable range hopping transport. In the spin-flip phase at 2 T, the activation energy is 20 meV lower than in the antiferromagnetic state due to a downshift of the conduction band edge, in agreement with ab initio calculations. In the variable range hopping regime, the average hopping length decreases from twice the interlayer distance to the interlayer distance at 2 T because in the antiferromagnetic state the large exchange energy impedes electrons hopping between adjacent layers. Our work demonstrates that the linear transport regime provides new information about electronic processes in vdW magnetic semiconductors and shows how magnetism influences these processes both in real and reciprocal space.