Ultrafast manipulation of magnetic states is one of the necessities in modern data storage technology. Quantum antiferromagnets are promising candidates in this respect. The orientation of the order parameter, the sublattice magnetization, can be used to encode “0” and “1” of a bit. Then, the switching of magnetization and the full control of its reorientation are crucial for writing data. We show that the magnetization can be switched efficiently by an external short THz pulse with relatively low amplitude. The coupling to the spin degrees can be direct via the magnetic field or indirect via the electric field inducing spin-polarized charge currents. Our description is based on time-dependent Schwinger boson mean-field theory, which includes the intrinsic dephasing mechanisms beyond a macrospin description. The findings help to introduce the use of antiferromagnets in data storage technology.

Magnetic skyrmions became critical elements in spintronics. Our study, through micromagnetic simulations, reveals that skyrmions experience chaotic changes in topological charge when exposed to sinusoidal variations in perpendicular magnetic anisotropy. This work introduces the following concept: a logic device based on a single skyrmion, which we envision as a stepping stone in chaotic computing. This conceptual device is designed to perform various logic operations in a reconfigurable format. We demonstrate the concept's feasibility by implementing sixteen 2-input logic gates, whose functionality depends on the skyrmion's initial conditions. These initial conditions can be manipulated by a current, accompanied by spin-orbit torque and the subsequent skyrmion-Hall effect. Our study presents an optimized logic operation map, highlighting specific initial conditions for corresponding logic operations. This work may provide the concept of a magnetic-field-free, low-power logic device, potentially contributing significantly to the development of advanced spintronic computing paradigms.

Andreev reflection is a fundamental transport process occurring at the junction between a normal metal and a superconductor (an N-S junction), when an incident electron from the normal side can only be transmitted in the superconductor as a Cooper pair, with the reflection of a hole in the normal metal. As a consequence of the spin singlet nature of the BCS Cooper pairs, the current due to Andreev reflection at an N-S junction is always symmetric in spin. Using a Keldysh Nambu Floquet approach, combining analytical and numerical calculations, we study in details the AC transport at an N-S junction, when the two spin components in the normal metal are driven by different periodic drives. We show that, in the Andreev regime, i.e., when the superconducting gap is much larger than the frequency of the drives, the spin-resolved photoassisted currents are always equal even if the two drives are different. In addition, we show that in this regime the excess noise depends only on the sum of the periodic drives, and we consider in particular the case of Lorentzian pulses (Levitons). We also show how these properties get modified when going beyond the Andreev regime. Finally we give a simple analytical proof of the special properties of the Andreev regime using an exact mapping to a particular N-N junction.

Investigating finite-temperature effects on quantum phases is key to their experimental realization. Finite temperature and the interplay between quantum and thermal fluctuations can undermine properties and/or key features of quantum systems, but they can also bring about interesting phenomena. In this paper we present a comprehensive investigation of the finite-temperature phase diagram of two-dimensional lattice dipolar bosons coupled to a high-finesse optical cavity. Interestingly, we observe that checkerboard density-density correlations are enhanced at finite temperature. Indeed, we found that finite temperature drives a superfluid ground state into a normal state, which will then develop checkerboard order at higher temperatures. We show that this effect is solely due to the cavity-mediated interactions. We also confirm that the supersolid checkerboard phase survives for a wide range of filling factors up to a temperature scale of the order of half hopping amplitude, while the checkerboard diagonal order can survive up to temperatures of a few hopping amplitudes.

Polymer nanocomposites generally exhibit unexpected dielectric/electrical performance far beyond the sum of every component, which is mainly due to the interface effect induced by the differences in structures and properties between the nanofillers and the polymer matrix. However, understanding the capricious interface effect in different polymer nanocomposites remains a major challenge. Here, we perform density functional theory calculations to investigate the atomic/molecular configurations and local charge behaviors of heterogeneous interfaces between the fillers of perovskites, oxides, two-dimensional materials, and polar/nonpolar polymers. Our findings demonstrate that atomic reconfiguration takes place during the formation of the inorganic/organic interface in order to minimize the overall energy of the system. Significant charge accumulation occurs at heterogeneous interfaces due to electron redistribution, especially for the examples of $\mathrm{Hf}{\mathrm{O}}_2$ and negatively charged $\mathrm{C}{\mathrm{a}}_2\mathrm{N}{\mathrm{b}}_3{\mathrm{O}}_{10}$. When applying an electric field, local polarization, especially around the interface, will be distorted and enhanced as a result of interfacial interaction. Even for the nonpolar polymer with linear dielectric oxides such as $\mathrm{Ti}{\mathrm{O}}_2$, induced dipole moments also appear near the interface, leading to the improvement of overall polarizability. The outcomes of our study verify that the variable electrical behaviors at the interfaces are highly dependent on the feature of every component constituting the inorganic/organic interface, which offers valuable insights for optimizing the experimental design of heterogeneous interfaces in polymer nanocomposites.

The quasi-two-dimensional magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$, stabilized via nonequilibrium molecular beam epitaxy, is investigated by resonant soft x-ray scattering. Kiessig fringes are observed, confirming a high sample quality and a thin film thickness of 10 septuple layers $(\sim 13 \mathrm{nm})$. An antiferromagnetic Bragg peak is observed at the structurally forbidden reflection, whose magnetic nature is validated by studying its temperature, energy, and polarization dependence. Through a detailed analysis, an A-type antiferromagetic order with in-plane moments is implied. This alternative spin structure in ${\mathrm{MnBi}}_2{\mathrm{Se}}_4$, in contrast to the Ising antiferromagnetic states in other magnetic topological insulators, might be relevant for hosting new topological states.

Magnetic skyrmions with topologically nontrivial spin textures form a variety of periodic structures depending on microscopic interactions and lattice symmetry. We theoretically investigate a transformation between triangular and square skyrmion crystals against an external magnetic field in a polar tetragonal magnet. By performing the simulated annealing for a classical spin model, we show that the competition of the Dzyaloshinskii-Moriya interaction at multiple wave vectors is a key ingredient in inducing the structural transition in terms of the skyrmion crystals. The present results indicate the importance of magnetic frustration in momentum space as the origin of exotic topological phase transitions.

We report high-quality single-crystal growth, x-ray diffraction, magnetic susceptibility [χ($T,H$)], magnetization [$M$($H$)], heat capacity $\left[C_{\mathrm{P}}(T,H)\right]$, electrical resistivity [ρ($T,H$)], and electron spin resonance (ESR) measurements of GdAsSe as functions of temperature and magnetic field. We identify an antiferromagnetic phase transition at $T_{\mathrm{N}}\sim 11.9\pm 0.2$ K and construct magnetic phase diagrams for $H||ab$ and $H||c$ axes based on the χ($T,H$) and $C_{\mathrm{P}}$($T,H$) data. Isothermal $M$($H$) curves along the $H||ab$ direction at 3 K exhibit a field-induced spin orientation at $H_{\mathrm{C}}\sim 3.78\mathrm{T}$. Both $M$($H$) and χ($T,H$) indicate an easy-plane-type anisotropy. The Curie-Weiss analysis of the high-temperature paramagnetic χ($T$) yields a negative Weiss temperature, suggesting dominant antiferromagnetic interactions between the Gd ions. Magnetic entropy reaches 83% of R ln8 at $T_{\mathrm{N}}$. The presence of residual entropy above $T_{\mathrm{N}}$ and the persistence of ESR critical broadening up to $\sim 3T_{\mathrm{N}}$ alludes to a degree of magnetic frustration in the studied material. The ρ($T$) data above $T_{\mathrm{N}}$ is well fitted to the Bloch-Grüneisen theory for metals. Further, density functional theory calculations reveal an antiferromagnetic ground state where the Gd atoms are coupled ferromagnetically in the $ab$ plane and antiferromagnetically along the $c$ axis.

In this study, we explore the Josephson current-phase relation within a planar diffuse tunneling superconducting multilayer junction subjected to a parallel magnetic field. Our investigation involves computing the supercurrent associated with a fixed jump in the phase of the order parameter at each of the two insulating interfaces, allowing us to derive the current-phase relation for the junction. Employing perturbation theory in junction conductance, we determine both the first and second harmonics of the current-phase relation under specific magnetic field conditions. Notably, the presence of a strong spin-orbit interaction in the middle region of the junction introduces an anomalous Josephson effect. The interplay between spin-orbit and Zeeman interactions results in the emergence of an effective vector potential. This specific characteristic induces a phase shift in each harmonic of the current-phase relation without altering the overall shape of the relation. The mechanism for the Josephson diode effect is discussed for disordered junctions of multiband superconductors.

Superconducting diode effects (SDEs) occur in systems with asymmetric critical supercurrents $|I_{+}^c|\ne |I_{-}^c|$ yielding dissipationless flow in one direction (e.g., $+)$, while dissipative transport in the opposite direction $(-)$. Here we investigate the SDE in a phase-biased $\varphi$ Josephson junction with a double-barrier resonant-tunneling InAs nanowire nested between proximitized InAs/Al leads with finite momentum $\hslash q$ Cooper pairing. Within the Bogoliubov-de Gennes (BdG) approach, we obtain the exact BCS ground state energy ${\mathcal{E}}_G(q,\varphi )$ and $I_{+}^c\ne \left|I_{-}^c\right|$ from the current-phase relation $I_G(q,\varphi )\sim {\partial }_{\varphi }{\mathcal{E}}_G(q,\varphi )$. The SDE arises from the accrued Andreev phase shifts $\delta {\varphi }_{L,R}(q,\varphi )$ leading to asymmetric BdG spectra for $q\ne 0$. Remarkably, the diode efficiency $\gamma =(I_{+}^c-|I_{-}^c\left|\right)/(I_{+}^c+|I_{-}^c\left|\right)$ shows multiple Fabry-Perot resonances $\gamma \simeq 26\%$ at the double-barrier Andreev bound states as the well depth $V_g$ is varied. Our $\gamma$ also features sign reversals for increasing $q$ and high sensitiveness to fermion-parity transitions. The latter enables $I_{+}^c\left({\varphi }_{+}\right)\rightleftarrows I_{-}^c\left({\varphi }_{-}\right)$ switchings over narrow phase windows, i.e., ${\varphi }_{+},{\varphi }_{-}\in \mathrm{\Delta }\varphi \ll \pi$, possibly relevant for future superconducting electronics.

Weyl semimetals with magnetic ordering provide a promising platform for the investigation of rare topological effects such as the anomalous Hall effect, resulting from the interplay of nontrivial bands with various spin configurations. The materials $R\mathrm{AlSi}$, where $R$ represents a rare-earth element, are prominent representatives of Weyl semimetals, where the Weyl states are induced by space inversion symmetry breaking, and in addition, for several rare-earth elements $R$, enhanced by time-reversal symmetry breaking through the formation of a magnetic order at low temperature. We report optical signatures of Weyl fermions in the magnetic compounds CeAlSi, PrAlSi, NdAlSi, and SmAlSi as well as the nonmagnetic family member LaAlSi by broad-frequency infrared spectroscopy at room temperature, i.e., in the paramagnetic phase. A similar profile of the optical conductivity spectrum and a metallic character are observed for all compounds, with LaAlSi showing the strongest free charge-carrier contribution. Furthermore, the linear-in-frequency behavior of the optical conductivity of all investigated compounds indicates the presence of Weyl nodes in close proximity to the Fermi energy, resulting from inversion symmetry breaking in noncentrosymmetric structures. According to the characteristics of these linear slopes, the $R\mathrm{AlSi}$ compounds are expected to host mainly type-II Weyl states with overtilted Weyl cones. The results are compared to the optical response of the closely related $R\mathrm{AlGe}$ materials, which are considered as potential hybridization-driven Weyl-Kondo systems.

We study a generalization of the well-known classical two-dimensional square lattice compass model of XY spins (sometimes referred to as the ${90}^{\circ }$ compass model), which interpolates between the XY model and the compass model. Our model possesses the combined $C_4$ lattice and spin rotation symmetry of the compass model but is free of its fine-tuned subsystem symmetries. Using both field theoretic arguments and Monte Carlo simulations, we find that our model possesses a line of critical points with continuously varying exponents of the Ashkin-Teller type terminating at the four-state Potts point. Further, our Monte Carlo study uncovers that beyond the four-state Potts point, the line of phase transition is connected to the lattice-nematic Ising phase transition in the square lattice compass model through a region of first-order transitions.

We describe how the spin Hall effect (SHE) can be studied from ab initio by combining density functional theory with the nonequilibrium Green's functions technique for quantum transport into the so-called $\mathrm{DFT}+\mathrm{NEGF}$ method. After laying down our theoretical approach, in particular discussing how to compute charge and spin bond currents, $\mathrm{DFT}+\mathrm{NEGF}$ calculations are carried out for ideal clean systems. In these, the transport is ballistic and the linear response limit is met. The SHE emerges in a central region attached to two leads when we apply a bias voltage so that electrons are accelerated by a uniform electric field. As a result, we obtain a finite spin Hall current and, by performing a scaling analysis with respect to the system size, we estimate the “ballistic” spin Hall conductivity (SHC). We consider $5d$ metals with fcc and bcc crystal structures, finding that the SHC exhibits a rough qualitative dependence on the $d$-band filling, and comment on these results in relation to the existing literature. Finally, within the same $\mathrm{DFT}+\mathrm{NEGF}$ approach, we also predict the appearance of a current-induced spin dipole moment inside the materials' unit cell and estimate its magnitude.

We study postquench dynamics of charge density wave (CDW) order in the square-lattice $t\text{−}V$ model. The ground state of this system at half-filling is characterized by a checkerboard modulation of particle density. A generalized self-consistent mean-field method, based on the time-dependent variational principle, is employed to describe the dynamical evolution of the CDW states. Assuming a homogeneous CDW order throughout the quench process, the time-dependent mean-field approach is reduced to the Anderson pseudospin method. Quench simulations based on the Bloch equation for pseudospins produce three canonical behaviors of order-parameter dynamics: phase-locked persistent oscillation, Landau-damped oscillation, and dynamical vanishing of the CDW order. We further develop an efficient real-space von Neumann equation method to incorporate dynamical inhomogeneity into simulations of quantum quenches. Our large-scale simulations uncover complex pattern formations in the postquench CDW states, especially in the strong quench regime. The emergent spatial textures are characterized by super density modulations on top of the short-period checkerboard CDW order. Our demonstration of pattern formation in quenched CDW states, described by a simple broken $Z_2$ symmetry, underscores the importance of dynamical inhomogeneity in quantum quenches of many-body systems with more complex orders.

In a two-dimensional system, Rashba spin-orbit coupling can lift spin degeneracy and give the opposite spin chirality to two split Fermi circles from two Rashba bands. Here, we propose a generic model that can produce unconventional Rashba bands. In such cases, the two Fermi circles from the two bands exhibit the same spin chirality. When various interactions are taken into account, many unique physics can emerge in systems with unconventional Rashba bands compared to those with conventional Rashba bands. For instance, we study the spin galvanic effect by considering two cases: one with potential impurity scattering and the other with magnetic impurity scattering. In both cases, we find that the efficiency of the spin galvanic effect is strongly enhanced in unconventional Rashba bands compared to conventional Rashba bands. More intriguingly, we find that the efficiency of conventional Rashba bands is insensitive to either potential or magnetic impurity scattering. However, the efficiency of unconventional Rashba bands can be further enhanced by magnetic impurity scattering compared to the potential impurity scattering. Thus, unconventional Rashba bands can produce giant spin galvanic effect. These results demonstrate that this model is useful for exploring abnormal physics in systems with unconventional Rashba bands.

Electron doping induces metal-to-insulator transition (MIT) in ${\mathrm{SmNiO}}_3$ as realized by experiments. While earlier density functional theory (DFT) studies with static correlations fell short of explaining the recent MIT observations at lower hydrogen concentrations, we present a comprehensive computational investigation employing an advanced approach. We combine DFT with dynamical mean field theory $(\mathrm{DFT}+\mathrm{DMFT})$ to efficiently analyze the insulating behavior of hydrogen-doped ${\mathrm{SmNiO}}_3$. In contrast to previous theoretical works, our calculations predict an insulator transition occurring at a reduced doping level of H:Ni = 0.5:1. Specifically, while the $\mathrm{DFT}+U$ method reveals a gap opening between $p\text{−}\mathrm{to}\text{−}d$ orbitals, the DMFT approach highlights a gap opening between $d\text{−}\mathrm{to}\text{−}d$ orbitals. Our findings uncover a selective Mott transition in site and orbital characteristics, with the Ni ions proximate to the doped hydrogen exhibiting Mott-like traits. Notably, DMFT calculations highlight a pronounced dependence on Hund's parameter $J$, implying the presence of Hundness in the Mott insulator. This study underscores the necessity of accounting for dynamical correlations to accurately describe the electronic structure of strongly correlated electron-doped rare-earth nickelates.

Haah's cubic code is the prototypical type-II fracton topological order. It instantiates the no stringlike operator property that underlies the favorable scaling of its code distance and logical energy barrier. Previously, the cubic code was only explored in translation-invariant systems on infinite and periodic lattices. In these settings, the code distance scales superlinearly with the linear system size, while the number of logical qubits within the degenerate ground space exhibits a complicated functional dependence that undergoes large fluctuations within a linear envelope. Here, we extend the cubic code to systems with open boundary conditions and crystal lattice defects. We characterize the condensation of topological excitations in the vicinity of these boundaries and defects, finding that their inclusion can introduce local stringlike operators and enhance the mobility of otherwise fractonic excitations. Despite this, we use these boundaries and defects to define new encodings where the number of logical qubits scales linearly without fluctuations, and the code distance scales superlinearly, with the linear system size. These include a subsystem encoding with open boundary conditions and a subspace encoding using lattice defects.

The spin-boson (SB) model plays a central role in studies of dissipative quantum dynamics, due to bothits conceptual importance and relevance to a number of physical systems. Here, we provide rigorous bounds of the computational complexity of the SB model for the physically relevant case of a zero temperature ohmic bath. We start with the description of the bosonic bath via its Feynman-Vernon influence functional (IF), which is a tensor on the space of the trajectory of an impurity spin. By expanding the kernel of the IF via a sum of decaying exponentials, we obtain an analytical approximation of the continuous bath by a finite number of damped bosonic modes. We bound the error induced by restricting bosonic Hilbert spaces to a finite-dimensional subspace with small boson numbers, which yields an analytical form of a matrix-product state (MPS) representation of the IF. We show that the MPS bond dimension D scales polynomially in the error on physical observables as well as in the evolution time T, $D\propto T^4/{\epsilon }^2$. This bound indicates that the SB model can be efficiently simulated using a polynomial in time-computational resources.

${\mathrm{Ce}}_5{\mathrm{Si}}_3$ exhibits geometrical frustration in its crystal structure and possesses a Schottky-type anomaly, implying dimer formation based on specific heat measurements. We performed inelastic neutron scattering experiments on polycrystalline samples to observe microscopical evidence of dimer formation. Crystalline electric field excitations were observed at 17, 27, 39, and 63 meV, and magnetic excitation caused by spin-dimer formation with dispersion was observed at approximately 0.6 meV. The magnetic excitation observed at low energies may originate from spin-dimer formation, which exhibits large dispersion that cannot be explained using the conventional Shastry–Sutherland lattice model, implying that the excited-state pseudotriplet may split.

Materials with tilted Dirac cones, such as $8\text{−}Pmmn$ borophene, are being explored for valleytronic applications as the tilting direction is different for nonequivalent valleys. In this paper, a valley-filtering device based on electrostatic waveguides is proposed. First, these waveguides are examined from a theoretical point of view. An inner product is defined starting from the probability current density along the waveguide axis. It is shown that the bound modes with real eigenvalues are mutually orthogonal and orthogonal with respect to all radiating modes. In a next step, by exploiting these orthogonality properties, a simulation procedure is introduced based on an explicit, symplectic partitioned Runge-Kutta time-stepping method specifically adapted for this problem. Finally, this approach is applied to the situation of a waveguide nanoconstriction and it is demonstrated that this structure can function as a valley filter. Within a certain window in the energy domain, transmission is practically zero for one valley, while being almost perfect for the other one. The effect of several design variables, such as length and width of the constriction, is carefully investigated. Moreover, the effect of misalignment between the tilting direction and the waveguide axis is assessed, showing that the proposed valley-filtering design is robust against deviations up to several tens of degrees. In addition, the simulation results reveal that the dispersion relation of the waveguide modes is not necessarily monotonic, which can give rise to oscillations in the transmission function due to interference effects.