We study a problem of interacting fractional charges with the $J_1\text{−}{J}_2\text{−}{J}_3$ Ising model on a checkerboard lattice under magnetic field. As a result of the interplay between repulsive interactions and particle density tuning by a magnetic field, the fractional charges form a classical spin liquid (CSL) phase. The CSL phase is composed of degenerate spin configurations, which can be mapped to the trimer covering of dual square lattice. The CSL state shows macroscopic ground-state entropy, implying the emergence of a novel quantum spin liquid phase when quantum fluctuations are turned on. In addition to the CSL phase, the system exhibits multiple magnetization plateaus, reflecting the fertile screening processes of dimer-monomer mixtures.

The interplay of the magnetic field and interlayer correlations was experimentally investigated in a quasi-two-dimensional quantum magnet $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$. For this purpose, extensive quantum Monte Carlo (QMC) studies of the finite-temperature properties of the spin-1/2 Heisenberg antiferromagnet (HAF) on the rectangular and the spatially anisotropic zig-zag square lattices were performed. The QMC studies revealed the equivalency of both models for any values of magnetic fields and spatial anisotropies $R=J_{\mathrm{interchain}}/J_{\mathrm{intrachain}}$. The analysis based on the decomposition of both lattices into local Hamiltonians confirmed the equivalence. Despite the large influence of the interlayer coupling and rather complicated distribution of the intralayer exchange pathways in $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$, considering several constraints in the data analysis enabled the extraction of the main features of the low-dimensional magnetic subsystem from the specific heat and magnetization. It was found that $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$ can be treated as the realization of the spin-1/2 HAF on the rectangular/zig-zag square lattice with the intralayer spatial anisotropy $R\approx 0.2$, the intrachain exchange coupling 2.20 ± 0.15 K, the saturation field 3.7 ± 0.1 T, the spin-flop field about 0.1 T, and the spin anisotropy of the orthorhombic symmetry. The possibility to investigate magnon instabilities in the strong-field regime of $\mathrm{Cu}\left(en\right){\mathrm{Cl}}_2$ is discussed.

The unconventional superconductivity in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ continues to defy a unified interpretation. In this paper, we focus on some novel aspects of its superconducting pairing by exploiting the orbital degree of freedom in this material. The multiorbital nature, combined with the symmetry of the orbitals involved, leads to a plethora of exotic Cooper pairings not accessible in single-orbital systems. Essential physics is illustrated first using a two-orbital model with $d_{xz}$ and $d_{yz}$ orbitals. We classify the gap functions according to the underlying lattice symmetries, analyze the effective theories of a few representative pairings, and make connections to ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ in the course. In particular, we show how spin-orbit coupling may entangle spin-triplet and spin-singlet pairings. For completeness, the classification is generalized to the three-orbital model involving the $d_{xy}$ orbital as well. The orbital-basis approach distinguishes from the itinerant-band description for ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ and hence offers an alternative perspective to investigate its enigmatic superconducting state.

We present a symmetric double-sided pillared phononic crystal (PPnC) where either quantum spin Hall effect (QSHE) or quantum valley Hall effect (QVHE) is emulated by solely imposing different perturbations to the geometry. When occurring at the deep subwavelength scale, the valley-polarized edge state emulating QVHE becomes evanescent at the zigzag termination and can be enhanced by a locally resonant mode. When occurring at high frequency, the refracted patterns (refracted modes and angles) can be tailored. Furthermore, we demonstrate the occurrence of the pseudospin and pseudospin-valley combined edge states and the valley-dependent propagation at a T junction constructed by PPnCs supporting QSHE and QVHE, respectively.

Anderson localization is fundamentally relevant to all physical systems where coherent waves evolve in the presence of disorder. Physically, any disorder must have a finite spectral bandwidth, hence the disorder is always correlated. Here, we study the regime of localization mediated by virtual transitions in correlated disorder. We find that wave packets centered outside of the spectral extent of the disorder can localize, with localization length almost as short as for localization via first-order transitions. In this regime, virtual transitions lead to phenomena with profound significance, such as mobility edges, and strong localization in regions of momentum space where otherwise localization would be extremely weak. Remarkably, in two-dimensional systems, we show localization can occur in directions that cannot be reached by direct scattering from the disordered potential.

Using spin-dependent specular and off-specular polarized neutron reflectivity (PNR), we report the observation of a twisted helical magnetic structure with planar 2π domain walls (DWs) and highly correlated magnetic domains in a Gd/Co multilayer. Specular PNR with a polarization analysis reveals the formation of planar 2π DWs below a compensation temperature ($T_{\mathrm{comp}}$), resulting in a negative exchange bias in this system. Off-specular PNR with spin polarization showed the development of magnetic inhomogeneities (increase in magnetic roughness) for the central part (thickness $\sim 25–30\AA$) of each Gd layer, where magnetization is aligned perpendicular (in plane) to an applied field. This is contributing towards an antisymmetric magnetoresistance at $T_{\mathrm{comp}}$ in the system. The magnetic roughness is vertically correlated and results in a Bragg sheet observed in the spin-flip channel of off-specular PNR data. The growth and tunability of highly correlated magnetic inhomogeneities (roughness) and the domain structure around $T_{\mathrm{comp}}$ in a combination of a twisted helical magnetic structure with planar 2π DWs will be key for applications in all-spin-based technology.

Laser pulse induced terahertz (THz) emission from Ta/${\left({\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}\right)}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1–0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_x{\mathrm{Fe}}_{1-x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

Here, we predict the effect of the spontaneous deformation of a flexible ferromagnetic ribbon induced by Dzyaloshinskii-Moriya interaction (DMI). The geometrical form of the deformation is determined both by the type of DMI and by the equilibrium magnetization of the stripe. We found three different geometrical phases, namely, (i) the DNA-like deformation with the stripe central line in the form of a helix, (ii) the helicoid deformation with the straight central line, and (iii) cylindrical deformation. In the main approximation the magnitude of the DMI-induced deformation is determined by the ratio of the DMI constant and the Young's modulus. It can be effectively controlled by the external magnetic field, which can be utilized for the nanorobotics applications. All analytical calculations are confirmed by numerical simulations.

The high-temperature structural phase transition of the potential topological material ${\mathrm{NbNiTe}}_2$ has been studied in detail. At high temperatures, the structure is centrosymmetric with orthorhombic space-group $P\frac{2}{m}\frac{2}{n}\frac{2_1}{a}\left(Pmna\right)$, and distorts at 373 K to a monoclinic structure with space-group $P11\frac{2_1}{a}(P{112}_1/a)$. A small signature in specific heat and commensurate small release of entropy are observed at the phase transition. The low-temperature monoclinic structure retains its inversion center and the symmetries associated with the orthorhombic $c$ axis. At high temperatures, ${\mathrm{NbNiTe}}_2$ shows phonon instabilities at $\mathrm{\Gamma }$ and $X$, which are removed by the structural phase transition. The electronic structure of ${\mathrm{NbNiTe}}_2$ is only slightly affected, indicating this ferroic phase transition is primarily due to steric effects.

Spin pumping into proximity-polarized Pt was investigated by magnetic damping studies of Permalloy(Py)/Pt and Py/Pt/Py structures for a Pt thickness range of $0<d_{\mathrm{Pt}}<3$ nm. The observed exponential increase in damping with increasing $d_{\mathrm{Pt}}$ in Py/Pt is consistent with previous studies of single crystal Fe/Pt. However, at present there are no studies of magnetic double layer structures, ferromagnet/Pt/ferromagnet. The main reason is that the presence of an induced magnetic moment in Pt, by proximity effect, leads to a strong interlayer exchange coupling. Proximity effect has so far not been addressed in the framework of spin pumping. Due to the ferromagnetic interlayer exchange coupling mediated by Pt, the magnetic response of the Py/Pt/Py structure results in optical (out-of-phase) and acoustic (in-phase) resonance modes. The magnetic damping of both modes has a different dependence on $d_{\mathrm{Pt}}$ than that of the Py/Pt structure. The study of Py/Pt/Py is sensitive to both the magnitude and phase of spin pumping and thus provides a stringent test of validity for spin-pumping theory for systems with proximity magnetism. We found that the results in Py/Pt and Py/Pt/Py can be self-consistently interpreted by a standard spin pumping theory with ${\tilde{g}}_{\uparrow \downarrow }=4.3\times {10}^{15}\pm 0.4\times {10}^{15}{\mathrm{cm}}^{-2}$ and ${\lambda }_{\mathrm{sd}}=1.1\pm 0.1$ nm. We found no evidence of any additional contributions to the interface damping.

Coupled quantum wells have been proposed as candidates for highly polarizable structures due to their near-degenerate and dipole-coupled electronic states. Hence, many interesting applications in linear and nonlinear optics can be envisioned. We analyze this proposal considering a simple structure with a delta-function barrier separating the wells. While very substantial Stark shifts are certainly predicted for this geometry, perturbative estimates based on polarizabilities (and hyperpolarizabilities) fail beyond a critical field strength that depends inversely on the barrier. Hence, a giant Stark effect due to near-degenerate states is invariably limited by a small critical field. Our analytical (hyper) polarizability expressions are applied to find quantitative Stark shifts for GaAs quantum wells and transition-metal dichalcogenide bilayers. The predicted Stark shifts and critical fields agree with the field dependence observed in a range of available experiments.

Recently, we have proposed an unusual mechanism of superconducting current that is specific for quantum Hall edge channels connected to superconducting electrodes. We have shown that the supercurrent can be mediated by a nonlocal electron-electron interaction that provides an opportunity for a long-distance information transfer in the direction opposite to the electron flow. A convenient model for such interaction is that of an external circuit. The consideration has been performed for the case of a single channel. In order to facilitate the experimental verification and the observation of peculiar features of the effect, in this paper, we provide a more detailed description of the phenomenon and extend the results to more sophisticated setups. We establish that the dynamical phase contributes to superconducting interference; this being the manifestation of the channel chirality. We consider setups that include the scattering between quantum Hall channels of opposite direction and multiple superconducting contacts. For a single quantum Hall constriction, we derive a general and comprehensive relation for the interaction-induced supercurrent in terms of scattering amplitudes and demonstrate the nonlocal nature of the current by considering its sensitivity to scattering. In multiterminal setups, we reveal the characteristic phase dependences of the supercurrents explaining those in terms of interference of Andreev reflection processes. For more complex setups encompassing, at least, two constrictions, we find an interplay between noninteracting and interaction-induced currents and contributions of more complex interference processes.

Weak topological insulators and Dirac semimetals are gapped and nodal phases with distinct topological properties, respectively. Here, we propose a topological phase that exhibits features of both and is dubbed composite Dirac semimetal (CDSM). In its bulk, the CDSM has a pair of Dirac points and a pair of bands inverted along a high-symmetry path. At side surfaces, a pair of Fermi arcs connecting the projected Dirac points coexist with a pair of Fermi loops traversing the surface Brillouin zone. A nonsymmorphic symmetry dictates degeneracies between the Fermi arcs and the Fermi loops. We characterize the CDSM by multiple topological invariants and show that, under a transition without breaking any symmetry, it deforms into a topological crystalline insulator hosting two pairs of surface Fermi loops. We demonstrate the CDSM in two models and predict its realization in the KAuTe-family materials.

We study the Anderson orthogonality catastrophe, and the corresponding x-ray edge problem, in systems that are at a localization transition driven by a deterministic quasiperiodic potential. Specifically, we address how the ground state of the Aubry-André model, at its critical point, responds to an instantaneous local quench. At this critical point, both the single-particle wave functions and the density of states are fractal. We find, numerically, that the overlap between postquench and prequench wave functions, as well as the “core-hole” Green function, evolve in a complex, nonmonotonic way with system size and time, respectively. We interpret our results in terms of the fractal density of states at this critical point. In a given sample, as the postquench time increases, the system resolves increasingly finely spaced minibands, leading to a series of alternating temporal regimes in which the response is flat or algebraically decaying. In addition, the fractal critical wave functions give rise to a quench response that varies strongly from site to site across the sample, which produces broad distributions of many-body observables. Upon averaging this broad distribution over samples, we recover coarse-grained power laws and dynamical exponents characterizing the x-ray edge singularity. We discuss how these features can be probed in ultracold atomic gases using radio-frequency spectroscopy and Ramsey interference.

Axion insulator is a new state of quantum matter with a vanishing Chern number but a quantized topological axion response. We study the critical properties of the magnetic-field-induced quantum phase transition in axion insulators. Take the even septuple layer film ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ as a concrete example, We find the mirror symmetry breaking from in-plane magnetic field could induce an axion insulator to quantum anomalous Hall (QAH) insulator transition, which belongs to the generic integer quantum Hall plateau transition. The microscopic model proposed here explains the out-of-plane magnetic-field-induced QAH plateau transition observed in this system experimentally. The chiral Majorana fermion does not necessarily emerge at the QAH plateau transition in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ due to strong exchange field, but may be quite feasible in its descendent materials ${\mathrm{MnBi}}_4{\mathrm{Te}}_7$ and ${\mathrm{MnBi}}_6{\mathrm{Te}}_{10}$.

A Weyl semimetal is a three-dimensional topological gapless phase. In the presence of strong enough disorder it undergoes a quantum transition towards a diffusive metal phase whose universality class depends on the range of disorder correlations. Similar to other quantum transitions driven by disorder, the critical wave functions at the semimetal–diffusive-metal transition exhibit multifractality. Using renormalization group methods we study the corresponding multifractal spectrum as a function of the range of disorder correlations for generic disorder including random scalar and vector potentials. We also discuss the relation between the geometric fluctuations of critical wave functions and the broad distribution of the local density of states (DOS) at the transition. We derive a new scaling relation for the typical local DOS and argue that it holds for other disorder-driven transitions in which both the average and typical local DOS vanish on one side of the transition. As an illustration we apply it to the recently discussed unconventional quantum transition in disordered semiconductors with power-law dispersion relation near the band edge.

Recent demonstrations of electrical detection and manipulation of antiferromagnets (AFMs) have opened new opportunities toward robust and ultrafast spintronics devices. However, it is difficult to establish the connection between the spin-transport behavior and the microscopic AFM domain states in thin films due to the lack of a real-time imaging technique under the electric field. Here we report a large magneto-optical birefringence effect with polarization rotation up to 60 millidegrees in thin NiO(001) films at room temperature. Such large optical polarization rotation allows us to directly observe AFM domains in thin-film NiO by utilizing a wide-field optical microscope. Complementary x-ray magnetic linear dichroism–photoemission electron microscopy measurement further confirms that the optical contrast is related to the NiO AFM domain. We examine the domain pattern evolution at a wide range of temperatures and with the application of external magnetic field. Comparing to large-scale-facility techniques such as x-ray photoemission electron microscopy, using a wide-field, tabletop optical imaging method in reflection geometry enables straightforward access to domain configurations of single-layer AFMs.

We investigate the phase diagram of an elastic interaction model for spin crossover (SC) materials on a triangular lattice, in which the long-range (LR) interaction of elastic origin, antiferromagnetic-like nearest-neighbor (NN) interaction $J_1$, and ferromagnetic (F)-like next-NN interaction $J_2$ interplay nontrivially. We find that for relatively weak elastic interactions in the model, two critical lines with the three-state Potts universality and the Berezinskii-Kosterlitz-Thouless (BKT) phase line merge at the BKT high temperature end point at zero effective field $H=0$. In contrast, for relatively large elastic interactions, the two critical lines and the coexistence line between the ferrimagnetic (FR)-like phases merge at a critical end point at $H=0$. The phase diagrams are also characterized by tricritical points, at which F-like spinodal, FR-like spinodal, and critical lines merge, and the metastable regions of the F-like and FR-like phases expand for stronger elasticity. We also find that, in the model with only the NN interaction $J_1$, for relatively weak elastic interactions, the critical end point is located at a very low temperature, the disordered phase appears at low temperatures, and the metastable regions of the F-like and FR-like phases are very small. The SR interactions are relevant for the critical lines, while the spinodal lines are caused by the LR interaction of elastic origin, which is independent of the strength of elasticity. Making use of these phase diagrams with the different characteristics, we show a wide variety of SC transitions with one to four steps, in which a second-order (continuous), first-order (discontinuous), or BKT (continuous) transition can occur at each step.

The microscopic mechanism of the interaction of exchange-coupled Fe and V spins in a $\text{Fe}{\text{V}}_2{\text{O}}_4$ ferrimagnet with the electric field has been described. The derived effective energy operator makes it possible to calculate the electric polarization vector using the information about the magnetic structure. The calculated electric polarization is consistent with the available experimental data.

Complex systems with coexisting polarity, chirality, and incommensurate magnetism are of great interest because they open new degrees of freedom in the interaction between crystal and magnetic subsystems and therefore they host a plethora of intriguing physical properties. Here we report the optical properties and lattice and spin dynamics of ${\mathrm{Ni}}_2{\mathrm{InSbO}}_6$ single crystals. This material was studied with the use of polarized optical microscopy and micro-Raman spectroscopy in the temperature range 10–300 K. ${\mathrm{Ni}}_2{\mathrm{InSbO}}_6$ crystallizes in a polar structure described by the noncentrosymmetric space group $R3$ and the two types of structural domains were visualized due to the natural optical activity of opposite chirality. Raman tensor elements of most $A$ and $E$ phonons along with their symmetry were determined. The manifestation of LO-TO splitting was observed for the $A$ modes. By tracking the temperature dependencies of phonon frequencies, the well pronounced spin-phonon interaction was observed for several modes below and above the Néel transition temperature $T_N =76\mathrm{K}$. In the antiferromagnetic phase, a broad excitation centered at $247{\mathrm{cm}}^{-1}$, was detected and assigned to the two-magnon mode and this value was used for estimating exchange parameters through linear spin-wave theory calculations.