Twistronic heterostructures have recently emerged as a new class of quantum electronic materials with properties determined by the twist angle between the adjacent two-dimensional materials. Here, we study moiré superlattice minibands in graphene (G) encapsulated in hexagonal boron nitride (hBN) with an almost perfect alignment with both the top and bottom hBN crystals. We show that, for such an orientation of the unit cells of the hBN layers that locally breaks inversion symmetry of the graphene lattice, the hBN/G/hBN structure features a kagome network of topologically protected states with energies near the miniband edge, propagating along the lines separating the areas with different miniband Chern numbers.

We explore band structures of one-dimensional open systems described by periodic non-Hermitian operators, based on continuum models and tight-binding models. We show that imaginary scalar potentials do not open band gaps but instead lead to the formation of exceptional points as long as the strength of the potential does not exceed a threshold value, which is in contrast to closed systems where real potentials open a gap with infinitesimally small strength. The imaginary vector potentials hinder the separation of low-energy bands because of the lifting of degeneracy in the free system. In addition, we construct tight-binding models through bi-orthogonal Wannier functions based on Bloch wavefunctions of the non-Hermitian operator and its Hermitian conjugate. We show that the bi-orthogonal tight-binding model well reproduces the dispersion relations of the continuum model when the complex scalar potential is sufficiently large.

We report the magnetic properties of the $A$-site spinel compound ${\mathrm{MnSc}}_2{\mathrm{Se}}_4$. The macroscopic magnetic measurements uncover successive magnetic transitions at $T_{\mathrm{N}1}=2.04$ K, followed by two further transitions at $T_{\mathrm{N}2}=1.8$ K and $T_{\mathrm{N}3}=1.6$ K. Neutron powder diffraction reveals that both $T_{\mathrm{N}2}<T<T_{\mathrm{N}1}$ and $T<T_{\mathrm{N}3}$ orders are associated with the propagation vector $k=(3/4,3/4,0)$, while the magnetic structures are collinear amplitude modulated and helical, respectively. Using neutron powder spectroscopy we demonstrated the effect of substitution of S by Se on the magnetic exchange. The energy range of the spin-wave excitations is suppressed due to the chemical pressure of the $X$ ion in ${\mathrm{MnSc}}_2{X}_4$ ($X=\text{S}$, Se) spinels.

Magnetic order is frustrated on the orthorhombic lattice of van der Waals layered FeOCl. Antiferromagnetic (AFM) order is attained at ambient pressure upon cooling through $T_N=81$ K, due to an accompanying monoclinic lattice distortion lifting the magnetic frustration. Within the paramagnetic state at 293 K, an incommensurate structural modulation appears above a critical pressure of $p_c\approx 15$ GPa, while orthorhombic symmetry is retained. This modulation is related to an optimization of the packing of chlorine atoms within the van der Waals gap. Here, we report four new phases in the pressure-temperature $(p,T)$ phase diagram of FeOCl below room temperature. High-pressure–low-temperature single-crystal x-ray diffraction (SXRD) up to 37.8 GPa reveals that, at 100 K, the AFM transition occurs at $p=7.3\pm 1.3$ GPa. The pressure coefficient of $\mathrm{\Delta }{T}_N/\mathrm{\Delta }p=2.13$ K/GPa explains that FeOCl remains paramagnetic up to the highest measured pressure of 33.3 GPa at 293 K. At 6 and 100 K, the structural modulation appears around $p_c\approx 15$ GPa within the AFM ordered phase with monoclinic symmetry. The monoclinic and triclinic lattice distortions increase with pressure up to $\gamma =90.64{\left(1\right)}^{\circ }$, much larger than the maximum value of $90.1^{\circ }$, that can be reached upon cooling at ambient pressure. The structural evolution provides the geometrical basis for the increase with the pressure of direct $3d-3d$ exchange and superexchange interactions. It is proposed, that a strong monoclinic lattice distortion may be of importance for understanding the properties of single-layer FeOCl materials.

This work extends the analysis of the generalized multifractality of critical eigenstates at the spin quantum Hall transition in two-dimensional disordered superconductors [J. F. Karcher et al., Ann. Phys. 435, 168584 (2021)]. A mapping to classical percolation is developed for a certain set of generalized-multifractality observables. In this way, exact analytical results for the corresponding exponents are obtained. Furthermore, a general construction of positive pure-scaling eigenfunction observables is presented, which permits a very efficient numerical determination of scaling exponents. In particular, all exponents corresponding to polynomial pure-scaling observables up to the order $q=5$ are found numerically. For the observables for which the percolation mapping is derived, analytical and numerical results are in perfect agreement with each other. The analytical and numerical results unambiguously demonstrate that the generalized parabolicity (i.e., proportionality to eigenvalues of the quadratic Casimir operator) does not hold for the spectrum of generalized-multifractality exponents. This excludes Wess-Zumino-Novikov-Witten models, and, more generally, any theories with local conformal invariance, as candidates for the fixed-point theory of the spin quantum Hall transition. The observable construction developed in this work paves a way to the investigation of generalized multifractality at Anderson localization critical points of various symmetry classes.

We investigate second-order magnetic responses of quantum magnets against ac magnetic fields. We focus on the case where the $z$ component of the spin is conserved in the unperturbed Hamiltonian and the driving field is applied in the $xy$ plane. We find that linearly polarized driving fields induce a second-harmonic response, while circularly polarized fields generate only a zero-frequency response, leading to a magnetization with a direction determined by the helicity. Employing an unbiased numerical method, we demonstrate the nonlinear magnetic effect driven by the circularly polarized field in the XXZ model and show that the magnitude of the magnetization can be predicted by the dynamical spin structure factor in the linear response regime.

We study theoretically the formation of the nuclear-spin polaron state in semiconductor nanosystems within the Lindblad equation approach. To this end, we derive a general Lindblad equation for the density operator that complies with the symmetry of the system Hamiltonian and address the nuclear-spin polaron formation for localized charge carriers subject to an arbitrarily anisotropic hyperfine interaction when optically cooling the nuclei. The steady-state solution of the density matrix for an anisotropic central spin model is presented as a function of the electron and nuclear spin bath temperature. Results for the electron-nuclear spin correlator as well as data for the nuclear spin distribution function serve as a measure of spin-entanglement. The features in both of them clearly indicate the formation of the nuclear polaron state at low temperatures where the crossover regime coincides with an enhancement of quantum fluctuations and agrees with the mean-field prediction of the critical temperature line. We can identify two distinct polaron states dependent upon the hyperfine anisotropy, which are separated by a quantum phase transition at the isotropic point. These states are reflected in the temporal spin autocorrelation functions accessible in experiment via spin-noise measurements.

In this article, we report on a critical assessment of dielectric function calculations in electron gas through the comparison of different modeling methods. This work is motivated by the fact that the dielectric function is a key quantity in the multiple scattering description of plasmon features in various electron-based spectroscopies. Starting from the standard random phase approximation (RPA) expression, we move on to correlation-augmented RPA, then damped RPA models. Finally, we study the reconstruction of the dielectric function from its moments, using the Nevanlinna and memory function approaches. We find the memory function method to be the most effective, being highly flexible and customizable.

Focusing on the implications of recent experiments on Majorana zero modes in semiconductor-superconductor (SM-SC) heterostructures, we critically examine the quantization of the zero-bias differential conductance as a possible unambiguous signature of Majorana physics in the presence of disorder. By numerically calculating the zero-bias conductance (ZBC) maps as function of Zeeman splitting and chemical potential for different disorder realizations, we find that the large topological region associated with the clean system, which is characterized by a quantized ZBC height $2e^2/h$, breaks up into progressively smaller “islands” as the disorder strength increases. For strong disorder we show that the presence of small islands with ZBC value (approximately) equal to $2e^2/h$, which we refer to as “quantized islands,” represents a unique signature of Majorana physics supporting partially separated Majorana modes (ps-MMs). Because of the small area/volume of these quantized islands in the parameter space, observing them in experiments may require sample selection and the systematic scanning of a large volume in the control parameter space. Upon decreasing disorder, the quantized islands increase in size and eventually coalesce into large topological regions. We conclude that the observation of quantized islands with ZBC value approximately equal to $2e^2/h$ demonstrates unambiguously the presence of the key ingredients necessary for Majorana physics, provides an excellent diagnostic tool for evaluating the disorder strength, and, consequently, represents the next natural milestone in Majorana search.

Focusing on the recently-discovered candidate topological insulator $\alpha \text{−}{\text{(BEDT-TSeF)}}_2{\mathrm{I}}_3$—having two-dimensional charge-neutral Dirac cones in a low symmetry lattice—we combine ab initio and extended-Hubbard model calculations to deal with spin-orbit and nonlocal repulsive interactions, and find a realization of an interaction-induced quantum spin Hall (QSH) insulator, similar to the one proposed in the honeycomb lattice under next-nearest-neighbor repulsions. In the absence of repulsive interactions, a topological insulator appears by the spin-orbit coupling and is characterized by a nonzero spin Chern number. By considering up to next-nearest-neighbor repulsions at Hartree-Fock level, the intrinsic spin-orbit gap is found to grow by orders of magnitude and a QSH insulating phase appears that has both a finite spin Chern number and order parameter. Transport coefficients and spin susceptibility are calculated and found to consistently account for most of the experimental findings, including the metal-to-insulator crossover occurring at $\sim 50\mathrm{K}$ as well as the Berry phase change from 0 to $\pi$ under hydrostatic pressure. We argue that such a QSH insulating phase does not necessitate a sizable spin-orbit interaction to generate a large insulating gap, which is highly advantageous for the search of novel topological phases in generic materials having low symmetry lattice and/or small spin-orbit coupling.

We introduce a many-body theory for thermal far-field emission of dipolar dielectric and metallic nanoparticles in the vicinity of a substrate within the framework of fluctuational electrodynamics. Our theoretical model includes the possibility to define the temperatures of each nanoparticle, the substrate temperature, and the temperature of the background thermal radiation, separately. To demonstrate the versatility of our method, we apply it in an exemplary way by discussing the thermal radiation of four particle assemblies of SiC and Ag nanoparticles above a planar SiC and Ag substrate. Furthermore, we use discrete dipole approximation to determine the thermal emission of a spherical nanoparticle in free space and close to a substrate. Finally, the calculation of the thermal far-field radiation of a sharp Si tip close to a SiC substrate using the discrete dipole approximation including the near-field scattering by the tip as well as the thermal emission of the tip and the contribution of the substrate which is partially blocked by the tip serves as another example.

This work reports the dynamics of transient hole doping in epitaxial graphene devices by using nitric acid as an adsorbent. The timescales associated with corresponding desorption processes are extracted from the data. The understanding of reversible hole doping without gating is of crucial importance to those fabricating devices with a particular functionality. Measurements of the electrical and optical properties of several devices postexposure were performed with transport temperatures between 300 and 1.5 K. Ambient conditions are applied to nontransport measurements to replicate the most likely laboratory conditions for handling devices using this doping method. The relevant timescales from transport measurements are compared with results from Raman spectroscopy measurements.

In a previous work [Rokaj et al., Phys. Rev. Lett. 123, 047202 (2019)] a translationally invariant framework called quantum-electrodynamical Bloch (QED-Bloch) theory was introduced for the description of periodic materials in homogeneous magnetic fields and strongly coupled to the quantized photon field in the optical limit. For such systems, we show that QED-Bloch theory predicts the existence of fractal polaritonic spectra as a function of the cavity coupling strength. In addition, for the energy spectrum as a function of the relative magnetic flux we find that a terahertz cavity can modify the standard Hofstadter butterfly. In the limit of no quantized photon field, QED-Bloch theory captures the well-known fractal spectrum of the Hofstadter butterfly and can be used for the description of two-dimensional materials in strong magnetic fields, which are of great experimental interest. As a further application, we consider Landau levels under cavity confinement and show that the cavity alters the quantized Hall conductance and that the Hall plateaus are modified as ${\sigma }_{xy}=e^2\nu /h(1+{\eta }^2)$ by the light-matter coupling $\eta$. Most of the aforementioned phenomena should be experimentally accessible, and corresponding implications are discussed.

Magnetic frustration in insulators often induces cooperative atomic displacements to form a polar structure with long-range ordered electric dipoles, commonly known as improper multiferroics. In metals, such a multiferroic phenomenon is not expected to occur because free carriers tend to screen internal electrostatic forces, eliminating long-range ordered electric dipoles. To date, intrinsic multiferroiclike metals have rarely been reported. Here we present a spin-frustration-induced long-range antiparallel ordering of electric dipoles in metallic CrN with an unusual antiferromagnetic ground state. Two equivalent magnetic structures are also iden- tified and linked to two distinct antiparallel polar orders. Such a scenario is analogous to that of conventional improper multiferroics with a magnetic origin of polar transition. The order parameter of structural distortion is linearly coupled to the magnetic order due to a strong magnetoelectric effect. The multiferroiclike metal is in striking contrast to insulating multiferroics, offering an extraordinary archetype for probing new underlying physics and exploring unusual properties for practical applications.

The depinning of nanoscale magnetic textures, such as domain walls, vortices, and skyrmions, is of paramount importance for magnetic storage and information processing. We measure the time-resolved magnetic switching statistics of an individual, non-single-domain ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticle using a micrometer-scale superconducting quantum interference device. A strong narrowing of the waiting-time distributions before reaching the final state is observed as compared to the exponential distribution expected for a single barrier. A model consisting of multiple barriers in series is proposed and used to understand the narrow waiting-time distributions and their evolution with magnetic field. The number of barriers is found to reduce as the thermodynamic switching field is approached and eventually, very close to it, an exponential distribution, resulting from a single barrier, is observed.

Low-energy moiré flat bands in magic-angle twisted bilayer graphene (tBG) have demonstrated incredible potential to exhibit rich exotic quantum phenomena. Theoretically, the moiré flat bands of tBG are based on the extended structures, i.e., the moiré patterns with periodic boundary conditions. However, a fundamental question of whether the flat bands can exist in the graphene moiré patterns with a reduced structure symmetry, such as sample edges, remains unanswered. Here, via scanning tunneling microscopy and spectroscopy, we study the local electronic properties of magic-angle tBG near the sample terminated edge and report a direct observation of breakdown of the moiré flat bands. We show that the moiré electronic structures, including the low-energy flat bands, can sufficiently exist in a complete moiré spot, i.e., a moiré supercell, right at the edge even if the translational symmetry of the moiré patterns is broken in one direction. However, the flat-band characteristic is obviously absent in the incomplete moiré spots that are partly terminated by the edge. Our results indicate that a whole moiré spot is sufficient and indispensable for the generation of the effective moiré flat bands in tBG.

The effective spin Hamiltonian method is very useful for simulating and understanding the behavior of magnetism. However, it is not easy to construct an appropriate spin Hamiltonian for a magnetic system, especially for complex magnets such as itinerant topological magnets. Here, we put forward a machine learning (ML) approach, applying an artificial neural network (ANN) and a local spin descriptor to construct an effective spin Hamiltonian for any magnetic system. The obtained Hamiltonians include an explicit Heisenberg part and an implicit nonlinear ANN part. Such a method successfully reproduces artificially constructed models and also accurately describes the itinerant magnetism of bulk ${\mathrm{Fe}}_3\mathrm{Ge}{\mathrm{Te}}_2$. Our work paves a new way for investigating complex magnetic phenomena (e.g., skyrmions) using ML techniques.