We study a one-dimensional Kitaev model with uniform phase gradient pairing term. We show that the gradient constant significantly affects the phase diagram, which consists of topologically trivial and nontrivial phases, associated with Majorana edge modes. Based on the exact solution, a Bardeen-Cooper-Schrieffer-pair order parameter is introduced to characterize the phase diagram by its value and nonanalytic behavior at phase boundaries. We find that this order parameter obtains its maxima at the triple critical points, at which the pairing phase gradient suppresses the single-particle scattering process due to the coherent destructive interference. In particular, we show that the ground state at such a point possesses exact off-diagonal long-range order (ODLRO), in the thermodynamic limit. Our result provides an example of a gapless $p$-wave superconducting ground state possessing ODLRO.

Due to the chiral anomaly, Weyl semimetals can exhibit a signature topological magnetoelectric response known as an axion term which is determined by the microscopic band structure. In the presence of strong interactions Weyl fermions may form a chiral condensate, with the intrinsic dynamics and fluctuations of the associated condensate phase producing a dynamical contribution to the axion response. Here we show that an imbalance in the density of right- and left-handed electrons drives an instability of the chiral condensate towards finite momentum and leads to strong fluctuations in the axion response. We derive a long-wavelength theory of Lifschitz type governing the dynamics of the Goldstone mode and use this to characterize its associated spatial fluctuations, which manifest as an inhomogeneous anomalous Hall effect. We show that these fluctuations produce signatures in inelastic light-scattering experiments across a broad spectrum of frequencies, and can be used to determine the structure factor for the axionic collective mode.

We present a general framework for the efficient simulation of realistic fermionic systems with modern machine-learning-inspired representations of quantum many-body states, towards a universal tool for ab initio electronic structure. These machine-learning-inspired Ansätze have recently come to the fore in both a (first-quantized) continuum and discrete Fock space representations, where, however, the inherent scaling of the latter approach for realistic interactions has so far limited practical applications. With application to the “Gaussian Process State,” a recently introduced Ansatz inspired by systematically improvable kernel models in machine learning, we discuss different choices to define the representation of the computational Fock space. We show how local representations are particularly suited for stochastic sampling of expectation values, while also indicating a route to overcome the discrepancy in the scaling compared with continuum-formulated models. We are able to show competitive accuracy for systems with up to 64 electrons, including a simplified (yet fully ab initio) model of the Mott transition in three-dimensional hydrogen, indicating a significant improvement over similar approaches, even for moderate numbers of configurational samples.

Magnetic topological materials and their physical signatures are a focus of current research. By first-principles calculations and symmetry analysis, we reveal exotic topological semimetal states in an existing antiferromagnet $\mathrm{Th}\mathrm{Mn}{}_2{\mathrm{Si}}_2$. Depending on the Néel vector orientation, the topological band crossings near the Fermi level form either a double-nodal loop or two pairs of Dirac points, which are all fourfold degenerate and robust under spin-orbit coupling. These topological features produce large Berry connection polarizability, leading to pronounced nonlinear transport effects. Particularly, we evaluate the third-order current response, which dominates the transverse current in $\mathrm{Th}\mathrm{Mn}{}_2{\mathrm{Si}}_2$. We show that the third-order response can be much more sensitive to topological phase transitions than a linear response, which offers a powerful tool for characterizing topological states of matter.

The capabilities of image probe experiments are rapidly expanding, providing new information about quantum materials on unprecedented length- and timescales. Many such materials feature inhomogeneous electronic properties with intricate pattern formation on the observable surface. This rich spatial structure contains information about interactions, dimensionality, and disorder—a spatial encoding of the Hamiltonian driving the pattern formation. Image recognition techniques from machine learning are an excellent tool for interpreting information encoded in the spatial relationships in such images. Here, we develop a deep learning framework for using the rich information available in these spatial correlations in order to discover the underlying Hamiltonian driving the patterns. We first vet the method on a known case, scanning near-field optical microscopy on a thin film of $\mathrm{V}{\mathrm{O}}_2$. We then apply our trained convolutional neural network architecture to new optical microscope images of a different $\mathrm{V}{\mathrm{O}}_2$ film as it goes through the metal-insulator transition. We find that a two-dimensional Hamiltonian with both interactions and random field disorder is required to explain the intricate, fractal intertwining of metal and insulator domains during the transition. This detailed knowledge about the underlying Hamiltonian paves the way for using the model to control the pattern formation via, e.g., tailored hysteresis protocols. We also introduce a distribution-based confidence measure on the results of a multilabel classifier, which does not rely on adversarial training. In addition, we propose a machine-learning-based criterion for diagnosing a physical system's proximity to criticality.

We predict robust Ising ferromagnetism driven by Coulomb interaction in the metallic phase of twisted transition metal dichalcogenide homobilayers for a range of small twist angles. Due to the presence of spin-valley locking and a Chern band, the completely spin-polarized state—a half metal—has a spin gap and exhibits the anomalous Hall effect. We also find that near a magic angle where the Chern band is predicted to be exceptionally flat, the anomalous Hall metal at $1/3$ filling may become unstable at low temperature and change into a $\sqrt{3}\times \sqrt{3}$ charge density wave or a fractional Chern insulator.

We study the dynamics of a monitored single particle in a one-dimensional Anderson-localized system. The time evolution is governed by Hamiltonian dynamics for fixed time intervals, interrupted by local, projective measurements. The competition between disorder-induced localization and measurement-induced jumps leads to interesting behavior of readout-averaged quantities. We find that measurements at random positions delocalize the average position, similar to a classical random walk. Along each quantum trajectory, the particle remains localized, however, with a modified localization length. In contrast to measurement-induced delocalization, controlled measurements can be used to introduce transport in the system and localize the particle at a chosen site. In this sense, the measurements provide a controlled environment for the particle.

We theoretically consider the problem of doping induced insulator to metal transition in bulk semiconductors by obtaining the transition density as a function of compensation, assuming that the transition is an Anderson localization transition controlled by the Ioffe-Regel-Mott (IRM) criterion. We calculate the mean free path, on the highly doped metallic side, arising from carrier scattering by the ionized dopants, which we model as quenched random charged impurities. The Coulomb disorder of the charged dopants is screened by the carriers themselves, leading to an integral equation for localization, defined by the density-dependent mean free path being equal to the inverse of the Fermi wave number, as dictated by the IRM criterion. Solving this integral equation approximately analytically and exactly numerically, we provide detailed results for the localization critical density for the doping induced metal-insulator transition.

Multiple-$Q$ states manifest themselves in a variety of noncollinear and noncoplanar magnetic structures depending on the magnetic interactions and lattice structures. In particular, cubic-lattice systems can host a plethora of multiple-$Q$ states, such as magnetic skyrmion and hedgehog lattices. We here classify momentum-dependent anisotropic exchange interactions in cubic-lattice systems based on magnetic representation analysis. We construct an effective spin model for centrosymmetric cubic space groups, $Pm\overline{3}m$ and $Pm\overline{3}$, and noncentrosymmetric ones, $P\overline{4}3m, P432$, and $P23$: The former include the symmetric anisotropic exchange interaction, while the latter additionally include the Dzyaloshinskii-Moriya interaction. By performing simulated annealing for the $Pm\overline{3}$ model, we demonstrate that the symmetric anisotropic exchange interaction becomes the origin of the multiple-$Q$ states in the ground state. We also analyze the instability toward the multiple-$Q$ states in the presence of the momentum-dependent anisotropic exchange interaction under the $Pm\overline{3}$ symmetry in a complementary way. Our results will be a reference for not only exploring unknown multiple-$Q$ states but also understanding the origin of the multiple-$Q$ states observed in both noncentrosymmetric and centrosymmetric cubic magnets like EuPtSi and ${\mathrm{SrFeO}}_3$.

Topologically stable spin structures known as magnetic antiskyrmions spontaneously emerge in magnets with broken in-plane rotation symmetry when current is injected. While many studies have focused on their behaviors in ferromagnetic (FM) materials, limited attention has been paid to antiskyrmions in their antiferromagnetic (AFM) counterparts. In spintronic applications, the Magnus force of an AFM antiskyrmion cancels out completely due to its comparable but opposite sign spin pattern on the A and B sublattices in the AFM bipartite system, while an FM antiskyrmion exhibits transverse motion when driven by the currents, causing limitations to antiskyrmion motion velocity and information loss when the antiskyrmion is annihilated at the edges of racetracks. Here, we report the nucleation and dynamics of an isolated AFM antiskyrmion and antiskyrmionium induced by current analytically and numerically. We demonstrate how AFM antiskyrmions and antiskyrmioniums are stabilized and manipulated with the same velocity by spin transfer torques and with different velocities by spin-orbit torques. Under suitable conditions, the AFM antiskyrmion and antiskyrmionium can be driven at velocities as high as 6 km/s without the skyrmion Hall effect, far greater than their ferromagnetic counterparts. Furthermore, the critical driving current density of the AFM antiskyrmion (antiskyrmionium) is much larger than that of the FM antiskyrmion (antiskyrmionium). Our results contribute to the development of antiferromagnetic antiskyrmions and antiskyrmioniums by shedding light on their stabilization and dynamics.

The understanding of antiferromagnetic domain walls, which are the interface between domains with different Néel order orientations, is a crucial aspect to enable the use of antiferromagnetic materials as active elements in future spintronic devices. In this work, we demonstrate that in antiferromagnetic NiO/Pt bilayers arbitrary-shaped structures can be generated by switching driven by electrical current pulses. The generated domains are T domains, separated from each other by a domain wall whose spins are pointing toward the average direction of the two T domains rather than the common axis of the two planes. Interestingly, this direction is the same for the whole domain wall indicating the absence of strong Lifshitz invariants. The domain wall can be micromagnetically modeled by strain distributions in the NiO thin film induced by the MgO substrate, deviating from the bulk anisotropy. From our measurements we determine the domain-wall width to have a full width at half maximum of $\mathrm{\Delta }=98\pm 10$ nm, demonstrating strong confinement.

There has been great interest in identifying a Kitaev quantum spin liquid state in frustrated magnets with bond-dependent interactions. In particular, the experimental report of a half-quantized thermal Hall conductivity in $\alpha \text{−}{\mathrm{RuCl}}_3$ in the presence of a magnetic field has generated excitement as it could be strong evidence for a field-induced chiral spin liquid. More recent experiments, however, provide a conflicting interpretation advocating for topological magnons in the field-polarized state as the origin of the nonquantized thermal Hall conductivity observed in their experiments. An inherent difficulty in distinguishing between the two scenarios is that the phase transition between a putative two-dimensional spin liquid and the field-polarized state exists only at zero temperature, while the behavior at finite temperature is mostly crossover phenomena. In this work, we provide insights into the finite-temperature crossover behavior between the spin excitation continuum in a quantum spin liquid and topological magnons in the field-polarized state in three different theoretical models with large Kitaev interactions. These models allow for a field-induced phase transition from a spin liquid (or an intermediate field-induced spin liquid) to the field-polarized state in the quantum model. We obtain the dynamical spin structure factor as a function of magnetic field using molecular dynamics simulations and compute thermal Hall conductivity in the field-polarized regime. We demonstrate the gradual evolution of the dynamical spin structure factor exhibiting crossover behavior near magnetic fields where zero-temperature phase transitions occur in the quantum model. We also examine nonlinear effects on topological magnons and the validity of thermal Hall conductivity computed using linear spin wave theory. We discuss the implications of our results for existing and future experiments.

In this paper, using first-principles calculations, we find that single-face (Janus-type) hydrogenation induces a magnetic transition from a ${120}^{\circ }$ noncollinear antiferromagnet (AFM) to a robust ferromagnet (FM) with high Curie temperature $\left(T_{\mathrm{c}},\sim 345\mathrm{K}\right)$ and large perpendicular magnetic anisotropy (MA, $\sim 2$ meV) in two-dimensional (2D) $\mathrm{Fe}{\mathrm{Se}}_2$. This hydrogenation results in a shift of spin-down states of in-plane orbitals $\left(d_{xy}/d_{x^2-{\mathrm{y}}^2}\right)$ to the Fermi level, which is crucial in inducing the high $T_{\mathrm{c}}$ and large perpendicular MA. To clarify the strong FM, we proposed a mechanism, i.e., FM direct exchange interaction of spin-down half-filled in-plane orbitals. This interaction is different from the traditional direct exchange interactions in most 2D magnets, which tend to form AFM coupling. As the inversion symmetry is broken, a significant Dzyaloshinskii-Moriya interaction (DMI; $\sim 2.6$ meV) is also induced by the Janus-type hydrogenation. It is found that tuning the exchange integral of orbitals can change the sign of its component of the DMI, providing an approach to manipulate the chirality of magnetic order.

In this paper, we report field-induced ferroelectricity in ${\mathrm{BaHoFeO}}_4$ in a narrow range of applied magnetic fields ($1.5\text{T}\le H\le 5\text{T}$). Magnetization measurements reveal three magnetic anomalies, the first one at $T_N^{\text{Fe}}\sim 52.8$ K, the second one at $T_{N_2}\sim 36.4$ K, and the third one at $T_N^{\text{Ho}}\sim 6.8$ K, corresponding to the antiferromagnetic ordering of ${\mathrm{Fe}}^{3+}$, a possible spin structure change of ${\mathrm{Fe}}^{3+}$, and the antiferromagnetic ordering of ${\mathrm{Ho}}^{3+}$ sublattices, respectively. Surprisingly, the heat capacity data did not show any anomaly at $T_N^{\text{Fe}}$ but exhibited a broad maximum at $T_N^{\text{Ho}}$. Below 40 K, isothermal magnetization measurements indicate a metamagnetic transition at 1.5 T. The temperature-dependent dielectric measurements performed below 1 T reveal two anomalies, around 6 and 36 K, corresponding to ${\mathrm{Ho}}^{3+}$ ordering and a possible change in the magnetic phase of the Fe sublattice. These anomalies disappear at higher magnetic fields, and a different dielectric anomaly appears above the metamagnetic transition with a broad asymmetric peak around 13 K ($T_P$), which shifts to higher temperatures, reaches a maximum under 2 T, and then is suppressed further on increasing the magnetic field up to 9 T. A switchable electric polarization appears below 25 K and above the metamagnetic transition field ($H\ge 1.5$ T). Intriguingly, the polarization disappears above 5 T. The presence of electric polarization in intermediate magnetic fields and its strong magnetic field dependence are interesting and make this material unique.

Electron-phonon coupling (EPC) has been broadly considered to govern the charge transport in solids, whereas its effect on thermal conductivity, especially for semiconductors at ambient temperatures, is a widely debated issue in condensed-matter physics. Employing state-of-the-art first-principles calculations to quantify all the possible scattering factors, we show the dominant role of EPC in phonon transport of $n$-doped GaSb, which yields an unprecedented reduction of lattice thermal conductivity and triggers anomalous temperature-independent behavior. The significant EPC impact hinges on the joint effect of multiple conduction pockets, large EPC strength, and bunched heat-carrying acoustic branches, provoking strong electron-phonon scattering to surpass the intrinsic phonon-phonon scattering for all-scale phonons, in contrast to the case in silicon that only long-wave phonons can be scattered by carriers even at high doping level. This work advances the understating of phonon transport in doped semiconductors and stimulates potential heat management application of anomalous thermal conductivity.

Half metals are ferromagnets hosting spin-polarized conducting carriers and are crucial for spintronics applications. The chromium spinel $\mathrm{Hg}{\mathrm{Cr}}_2{\mathrm{Se}}_4$ represents a unique type of half-metal, which features a half-metal to semiconductor transition (HMST) and exhibits colossal magnetoresistance (CMR) across the ferromagnetic-paramagnetic (FM-PM) transition. Using angle-resolved photoemission spectroscopy, we find that the Fermi surface of $n$-type $\mathrm{Hg}{\mathrm{Cr}}_2{\mathrm{Se}}_4$ ($n\text{−}\mathrm{Hg}{\mathrm{Cr}}_2{\mathrm{Se}}_4$) consists of a single electron pocket which moves above the Fermi level $\left(E_{\mathrm{F}}\right)$ upon the FM-PM transition, leading to the HMST. Such a Lifshitz transition manifests a giant band splitting which originates from the exchange interaction unveiled with a specific chemical nonstoichiometry. The exchange band splitting and the chemical nonstoichiometry are two key ingredients to the HMST and CMR, consistent with our ab initio calculation. Our findings provide spectroscopic evidence of the electronic origin of the anomalous properties of $\mathrm{Hg}{\mathrm{Cr}}_2{\mathrm{Se}}_4$, which address the unique phase transition in half-metals.

Neural quantum states are a new family of variational Ansätze for quantum-many body wave functions with advantageous properties in the notoriously challenging case of two spatial dimensions. Since their introduction, a wide variety of different network architectures have been employed to study paradigmatic models in quantum many-body physics with a particular focus on quantum spin models. Nonetheless, many questions remain about the effect that the choice of architecture has on the performance on a given task. In this work, we present a unified comparison of a selection of popular network architectures and symmetrization schemes employed for ground-state searches of prototypical spin Hamiltonians, namely, the two-dimensional transverse-field Ising model and the $J_1\text{−}{J}_2$ model. In the presence of a nontrivial sign structure of the ground states, we find that the details of symmetrization crucially influence the performance. We describe this effect in detail and discuss its consequences, especially for autoregressive models, as their direct sampling procedure is not compatible with the symmetrization procedure that we found to be optimal.

The band structures of transverse electric (TE) polarized dispersive photonic crystals are generally more complicated than those of transverse magnetic (TM) polarized photonic crystals. Here, by simplifying the governing equations of a TE polarized dispersion system and conducting the band structure analysis, we deduce the Hermitian eigenstates of the vector electric field. The results exhibit the double degeneracy eigenstates in the triangular lattice for lower eigenfrequencies and numerous plasmon-induced flat band modes in the interface of the plasma and air for higher eigenfrequencies. Moreover, we illustrate that the surface plasmon modes tend to present the quantized eigenfrequencies, which is due to the boundary condition imposed by the cylindrical geometry. However, when the eigenfrequency approaches the surface plasma frequency, the localization of the field on the boundary becomes stronger, which weakens the coupling between sites and reduces the dependence of the flat bands on the lattice structure. Unlike the generation mechanism of the lattice-induced flat bands, such plasma frequency-dependent localization at the interface enables a robust flat band characteristic immune to the lattice disorder and provides a novel degree of freedom to control the energy band. Our findings are expected to be useful for electromagnetic wave manipulation and field enhancement.

The relatively low efficiency and costly composition of conventional thermoelectric generators (TEGs) hinder pragmatic usage despite their high Carnot efficiency. A PN-junction-based TEG is one candidate to resolve the noted drawbacks, but electron-hole recombination at the junction severely reduces the efficiency and limits the compactness. Here, we propose the nanoscale homojunction thermoelectric generator by engineering the ${\mathrm{MoS}}_2$ grain boundary. Ab initio scanning Seebeck microscopy revealed that the influence of deep-level wave functions of atomic defects broadens out to a nanoscale, leading to the doping-tunable Seebeck domain with a different magnitude or even inverted sign. This thermopower utilization using vestiges of localized defect states enables exceptional integration density in a single material, while the simple oxygen substitution inflates the efficiency by barricading the recombination at the junction. Our finding suggests that inevitable structural defects could pave the way for actualizing productive thermoelectric energy recycling at the nanoscale.

We study the electronic conductance of dissipative Floquet Majorana zero modes (FMZMs) in a periodically driven nanowire coupled to a quantum dot. We use a numerical method which can accurately take into account the dissipation effects from the superconducting bath, which causes the FMZMs to have a finite lifetime. Our results show that, in the weak nanowire-dot coupling regime, the peak conductance at zero temperature of the resonant dot can be well approximated by a universal function of the FMZM lifetime rescaled with the nanowire-dot coupling strength. For a long FMZM lifetime, the conductance approaches the characteristic quantized value of $G=e^2/2h$, whereas $G\to e^2/h$ (uncoupled dot) as the FMZMs' lifetime goes to zero. Moreover, we show how the conductance-lifetime relation is modified when considering the full Floquet structure of the nonequilibrium Green's function, undesired coupling to non-Majorana states, a nonresonant quantum dot, and finite-temperature effects. Assuming good control over the system parameters, our method can be used to test the presence and lifetime of FMZMs in such devices, which is key for any practical application of these topological states.