Noninteracting particles obeying certain fractional statistics have been predicted to exhibit superconductivity. We discuss the issue in an attractively interacting system of spinful semions on a lattice by numerically investigating the presence of off-diagonal long-range order at zero temperature. For this purpose we construct a Hubbard model wherein two semions with opposite spin can virtually coincide while maintaining consistency with the fractional braiding statistics. Clear off-diagonal long-range order is seen in the strong coupling limit, consistent with the expectation that a pair of semions obeys Bose statistics. We find that the semion system behaves similarly to a system of fermions with the same attractive Hubbard $U$ interaction for a wide range of $U$, suggesting that semions also undergo a BCS to BEC crossover as a function of $U$.

Dislocations are topological defects known to be crucial in the onset of plasticity and in many properties of crystals. Classical elasticity fails to fully explain their dynamics under extreme conditions of high gradients of strain and small scales, which can nowadays be scrutinized. Previous works have proposed a type of force acting on dislocations when they are in the presence of strain gradients, the so-called core force. Here we designed a two-dimensional atomistic simulation through which we confirm the existence of this force, probe its properties, and measure its coefficients. The results of our simulations agree with the assumption that the core energy is the origin for the core force, but the coefficients of this force cannot be predicted by the approaches previously proposed. We show that the measured coefficients can be used to obtain the core energy in some systems such as ours. This way of evaluating the core energy can be compared to other existing approaches. Moreover, we show that the core force implies the existence of nonreciprocal interactions between dislocations. We believe that a correct consideration of the core force can be considered in mechanical modeling and in some theories trying to explain strange phenomena in plasticity.

The correlation of the microstructure and the magnetic properties of ${\mathrm{Ni}}_{\text{2}}{\mathrm{MnAl}}_{\text{0.5}}{\mathrm{Ga}}_{\text{0.5}}$ and ${\mathrm{Ni}}_{\text{2}}\mathrm{MnAl}$ Heusler alloys is studied in samples of distinct $\mathrm{L}{2}_{\text{1}}$ order states by neutron powder diffraction and small-angle scattering. The extracted correlation lengths of the structural and magnetic order agree over all annealing states, which implies the structural antiphase domain boundaries to be identical to the magnetic domain walls, and thus to inherently give rise to a magnetization reversal. The findings are discussed in the context of recent small-angle neutron scattering studies on related systems, and their wider implications are pointed out.

Electrocaloric materials of solid state hold great promise for next-generation high-efficiency cooling technology. Besides the normal electrocaloric effect (ECE) in ferroelectrics or relaxors, anomalous ECE with decreasing temperature upon application of an electric field is known to occur in antiferroelectrics (AFEs), and previous understanding refers to the field-induced canting of electric dipoles if there is no phase transition. Here we use a first-principles-based method to study the ECE in Nd-substituted ${\mathrm{BiFeO}}_3$ (BNFO) perovskite solid solutions, which has a Pnma-type AFE ground state. We demonstrate another scenario to achieve and explain anomalous ECE, emphasizing that explicit consideration of octahedral tiltings is indispensable for a correct understanding. This mechanism may be general for AFEs for which the antipolar mode is not the primary order parameter or for other commonly occurring Pnma-type perovskites, even without being AFEs. We also find that the negative ECE can reach a large magnitude in BNFO.

Two-dimensional (2D) materials that exhibit vertical polarization might provide ground-breaking device applications, compatible with electric-field switching even in the presence of a surface-depolarizing field. Here, using first-principles calculations, we focus on vertical cationic off-plane displacements in transition metal trioxides ${M\mathrm{O}}_3$ $(M=\mathrm{W}, \mathrm{Mo})$, with a configuration similar to ${\mathrm{CrI}}_3$-like hexagonal trihalides. We further design a ${\mathrm{WMoO}}_6$ monolayer, with W and Mo arranged in an energetically stable honeycomb-checkerboard pattern, showing a net vertical ferroelectric polarization and a wide band gap. Moreover, the structural, electronic, and ferroelectric properties of ${\mathrm{WMoO}}_6$ monolayers are found to be significantly strain tunable. Our results put forward a transition metal trioxide monolayer with switchable vertical ferroelectricity, thus broadening the range of 2D functional ferroelectrics.

We report on the results of ${}^{23}\mathrm{Na}$ NMR in the honeycomb lattice magnet ${\mathrm{Na}}_2{\mathrm{Co}}_2{\mathrm{TeO}}_6$, which has been nominated as a Kitaev material. Measurements of magnetic shift and width of the NMR line as functions of temperature and magnetic field show that a spin-disordered phase does not appear up to a field of 9 T. In the antiferromagnetic phase just below the Néel temperature $T_N$, we find a temperature region extending down to $\sim T_N/2$, where the nuclear spin-lattice relaxation rate $1/T_1$ remains enhanced and is further increased by a magnetic field. This region crosses over to a low-temperature region characterized by the rapidly decreasing $1/T_1$, which is less field-sensitive. These observations suggest incoherent spin excitations with a large spectral weight at low energies in the intermediate temperature region transforming to more conventional spin-wave excitations at low temperatures. The drastic change of the low-energy spin dynamics is likely caused by strong damping of spin waves activated only in the intermediate temperature region, which may be realized for triple-$\mathbf{q}$ magnetic order possessing partially disordered moments as scattering centers of spin waves. In the paramagnetic phase near $T_N$, dramatic field suppression of $1/T_1$ is observed. From analysis of the temperature dependence of $1/T_1$ based on the renormalized-classical description of a two-dimensional quantum antiferromagnet, we find the field-dependent spin stiffness constant that scales with $T_N$ as a function of magnetic field. This implies field suppression of the energy scale characterizing both two-dimensional spin correlations and three-dimensional long-range order, which may be associated with an increasing effect of frustration in magnetic fields.

The influence of nanoparticle geometrical and percolation effects on the electrical transport and magnetotransport properties of the system are investigated in a number of binary networks composed of superconducting (${\mathrm{MgB}}_2$), ferromagnetic (${\mathrm{CrO}}_2$ or ${\mathrm{La}}_{1/3}{\mathrm{Sr}}_{2/3}{\mathrm{MnO}}_3$), and insulating (${\mathrm{LiCoO}}_2$ or ${\mathrm{Cr}}_2{\mathrm{O}}_3$) nanoparticles. For all-metal ${\mathrm{CrO}}_2/{\mathrm{MgB}}_2$ binary composites an anomalously high resistance state with two distinct percolation thresholds is found within a narrow range of the constituent composition, as a result of the strong suppression of heterointerfacial conductance between superconducting and half-metallic (${\mathrm{CrO}}_2$) nanoparticles and an unexpectedly large value of the percolation threshold of ${\mathrm{MgB}}_2$. The latter can be attributed to the geometric mismatch between the constituent nanoparticles and the related particle packing and excluded volume effects. The filling factor in this binary composite attains a maximum near the same percolation threshold and exhibits a power-law scaling behavior, which would indicate another geometric phase transition. The geometric effect of constituent particles is also verified through the large changes of percolation threshold in different types of combinations of binary components. Interestingly, the percolation transitions in these binary nanocomposites, either in the case of single or double percolation, are found to be governed by an unconventional scaling behavior of resistance just below the percolation threshold, due to mechanisms determined by heterogeneous interfaces of particles that are absent in conventional single-component percolation systems. In addition, magnetotransport is investigated in ${\mathrm{CrO}}_2/{\mathrm{MgB}}_2$ junctions, demonstrating nonhysteretic low-temperature magnetoresistance at the half-metal/Type II superconductor interface with the highest values around $42\%$ at liquid helium temperatures obtained at volume fraction between the two percolation thresholds. The results show large variations of magnetoresistance that are highly sensitive to the constituent composition and the sample temperature near the percolation thresholds, demonstrating the interrelation between the structural or geometrical property of the system and materials functionality.

Ratchet motion of superconducting vortices, which is a directional flow of vortices in superconductors, is highly useful for exploring quantum phenomena and developing superconducting devices, such as superconducting diode and microwave antenna. However, because of the challenges in the quantitative characterization of the dynamic motion of vortices, a phase diagram of the vortex ratchet motion is still missing, especially in the superconductors with low dimensional structures. Here we establish a quantitative phase diagram of the vortex ratchet motion in a highly anisotropic superlattice superconductor, ${\left(\mathrm{SnS}\right)}_{1.17}\mathrm{Nb}{\mathrm{S}}_2$, using nonreciprocal magnetotransport. The ${\left(\mathrm{SnS}\right)}_{1.17}\mathrm{Nb}{\mathrm{S}}_2$, which possesses a layered atomic structure and noncentrosymmetry, exhibits nonreciprocal magnetotransport in a magnetic field perpendicular and parallel to the plane, which is considered a manifest of ratchet motion of superconducting vortices. We demonstrated that the ratchet motion is responsive to current excitation, magnetic field, and thermal perturbation. Furthermore, we extrapolated a giant nonreciprocal coefficient $\left(\gamma \right)$, which quantitatively describes the magnitude of the vortex ratchet motion, and eventually established phase diagrams of the ratchet motion of the vortices with a quantitative description. Last, we propose that the ratchet motion originates from the coexistence of pancake vortices and Josephson vortices. The phase diagrams are desirable for controlling the vortex motion in superlattice superconductors and developing next-generation energy-efficient superconducting devices.

The strongly correlated bosons in flat-band systems are an excellent platform to study a wide range of quantum phenomena. Such systems can be realized in optical lattices filled with ultracold atomic gases. In this paper we study the Bose-Hubbard model in the Lieb lattice by means of the time-dependent Gutzwiller mean-field approach. We find that in the Mott insulator phase the excitation modes are gapped and display purely particle or purely hole character, while in the superfluid phase the excitation spectrum is gapless. The geometry of the Lieb lattice leads to a nonuniform order parameter and nonuniform oscillation energy in the ground state. This results in additional anticrossings between dispersive bands in the excitation spectra, while the flat bands remain insensitive to this effect. We analyze the oscillations of the order parameter on the sublattices as well as the particle-hole character of the excitations. For certain model parameters we find simultaneous pure phase and pure amplitude oscillations within the same mode, separated between the sublattices. Also, we propose a simple method to differentiate between the hole and particle superfluid regions in the Lieb lattice by in situ measurement of the atom population on the sublattices.

Gaussian fermionic matrix product states (GfMPS) form a class of ansatz quantum states for 1d systems of noninteracting fermions. We show, for a simple critical model of free hopping fermions, that (i) any GfMPS approximation to its ground state must have bond dimension scaling superpolynomially with the system size, whereas (ii) there exists a non-Gaussian fermionic MPS approximation to this state with polynomial bond dimension. This proves that, in general, imposing Gaussianity at the level of the tensor network may significantly alter its capability to efficiently approximate critical Gaussian states. We also provide numerical evidence that the required bond dimension is subexponential, and thus can still be simulated with moderate resources.

We investigate the ground-state properties of the nearest-neighbor $S=1$ pyrochlore Heisenberg antiferromagnet using two complementary numerical methods, the density-matrix renormalization group (DMRG) and pseudofermion functional renormalization group (PFFRG). Within DMRG, we are able to reliably study clusters with up to 48 spins by keeping 20 000 SU(2) states. The investigated 32-site and 48-site clusters both show indications of a robust $C_3$ rotation symmetry breaking of the ground-state spin correlations and the 48-site cluster additionally features inversion symmetry breaking. Our PFFRG analysis of various symmetry-breaking perturbations corroborates the findings of either $C_3$ or a combined $C_3$/inversion symmetry breaking. Moreover, in both methods the symmetry-breaking tendencies appear to be more pronounced than in the $S=1/2$ system.

Elemental substitutions are successfully used for the optimization of thermoelectric properties of a specific material; it requires, however, a deep understanding of its impact. For this purpose, based on the full-Heusler material ${\mathrm{Fe}}_2\mathrm{VAl}$, various compounds (${\mathrm{Fe}}_{2-x}{\mathrm{Cr}}_x\mathrm{VAl}$) were synthesized by substituting Cr for Fe in a wide range from $x$ = 0.005 up to $x$ = 0.4. X-ray diffraction analysis revealed full solubility of Cr for all concentrations. Bulk thermoelectric properties, such as electrical resistivity, Seebeck coefficient, thermal conductivity, and Hall resistivity were measured from 2 K up to 780 K, and all results were discussed in the context of the outcome of density functional calculations. Transport anomalies, resembling Kondo scattering, were observed for all samples below 30 K. Finally, an increased effective number of valence electrons of 7 for Cr was phenomenologically determined, which revealed good agreement with other $p$-type doping studies of ${\mathrm{Fe}}_2\mathrm{VAl}$.

In view of electro-optic modulators and switches operating in the terahertz frequency range, we study the absorbance of a topological insulator (TI) thin film subjected to a static electric field. Adopting the Hamiltonian for the three-dimensional ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ class of TIs including terms to second order in the electron wave vector and to first order in the electric field, we present an effective model for a TI thin film. We demonstrate the distinct influences of in-plane and out-of-plane external electric fields, the in-plane electric field-induced dichroism, and the electric field and chemical potential tuning of the absorbance edge. Under the influence of an out-of-plane electric field, a TI film with a thickness of about 2 nm exhibits a considerable absorbance at the absorbance edge. Of prime practical importance, the absorbance edge can be shifted about $0.3\mathrm{eV}$ with the application of moderate electric fields about $0.4\mathrm{V}{\mathrm{nm}}^{-1}$ or with chemical potential variations about $0.3\mathrm{eV}$.

Using the determinant quantum Monte Carlo method, we investigate the metal-insulator transitions induced by the stripe of charge density in an interacting two-dimensional Dirac fermion system. The stripe will introduce the transport anisotropy and insulating intermediate phase into the system, accompanied by the change of band structure and a peak of density of states around Fermi energy. In the case of strong correlation, stripe exhibits competition with Coulomb repulsion through closing the energy gap and disrupting the magnetic order and finally drives the system in the Mott insulating phase back to the metallic state. Our results may provide a feasible way to modify transport properties by setting charge stripes in experiments.

Spin-orbit interaction (SOI) plays a fundamental role in many low-dimensional semiconductor and hybrid quantum devices. In the rapidly evolving field of semiconductor spin qubits, SOI is an essential ingredient that can allow for ultrafast qubit control. The exact manifestation of SOI in a given device is, however, often both hard to predict theoretically and probe experimentally. Here, we develop a detailed theoretical connection between the leakage current through a double quantum dot in Pauli spin blockade and the underlying SOI in the system. We present a general analytic expression for the leakage current, which allows to connect experimentally observable features to both the magnitude and orientation of an effective spin-orbit field acting on the moving carriers. Motivated by the large recent interest in hole-based quantum devices, we further zoom in on the case of Pauli blockade of hole spins, assuming a strong transverse confinement potential. In this limit we also find an analytic expression for the current at low external magnetic field, that includes the effect of hyperfine coupling of the hole spins to randomly fluctuating nuclear spin baths. This result can be used to extract information about both hyperfine and spin-orbit coupling parameters for hole spins in devices with a significant fraction of nonzero nuclear spins.

We develop a theory of anomalous elasticity in disordered two-dimensional flexible materials with orthorhombic crystal symmetry. Similar to the clean case, we predict the existence of infinitely many flat phases with anisotropic bending rigidity and Young's modulus showing power-law scaling with momentum controlled by a single universal exponent the very same as in the clean isotropic case. With an increase of temperature or disorder, these flat phases undergo a crumpling transition. Remarkably, in contrast to the isotropic materials where crumpling occurs in all spatial directions simultaneously, the anisotropic materials crumple into a tubular phase. In distinction to the clean case in which the crumpling transition happens at unphysically high temperatures, a disorder-induced tubular crumpled phase can exist even at room-temperature conditions. Our results are applied to anisotropic atomic single layers doped by adatoms or disordered by heavy ions bombarding.

We consider the driven dynamics of Wigner crystals interacting with random disorder. Using numerical simulations, we find a rich variety of transport phenomena as a function of charge density, drive, and pinning strength. For weak pinning, the system forms a defect-free crystal that depins elastically. When the pinning is stronger, a pinned glass phase appears that depins into a filamentary flow state, transitions at higher drives into a disordered flow phase, and finally forms a moving smectic. Within the filamentary flow phase, the conduction curves can show switching dynamics as well as negative differential conductivity in which switching events cause some flow channels to be blocked. The velocity noise in the filamentary flow regime exhibits narrow band characteristics due to the one-dimensional nature of the motion, while the moving smectic has narrow band velocity noise with a washboard frequency. In the disordered flow state, the noise power reaches a peak value and the noise has a $1/f$ character. Our transport results are consistent with recent experimental transport studies in systems where Wigner crystal states are believed to be occurring. Below the conduction threshold, we find that avalanches with a power-law size distribution appear when there are sudden local rearrangements of charges from one pinned configuration to another.

Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as an ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two silver nanostripes with different dimensions placed on both sides of an ultrathin active dielectric spacing layer. The introduced gain leads to an exceptional point formation in the linear operation regime, where unidirectional perfect transmission due to loss compensation is achieved from the presented non-Hermitian parity-time symmetric nanoscale system. The demonstrated plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when its nonlinear response is considered, mainly due to its spatially asymmetric geometry combined with enhanced nonlinearity. Strong and broadband nonreciprocity with unity transmission contrast from opposite directions is realized under relatively low-input laser intensity values. Furthermore, asymmetric nonlinear third-harmonic generation with a pronounced contrast is also illustrated by the same nonreciprocal metasurface design. The findings of this work can lead to the realization of various self-induced nonreciprocal nanophotonic configurations, such as integrated ultrafast switches, ultrathin protective coatings, and asymmetric directional imaging devices.

Current-induced spin-orbit torque (SOT) in normal metal/ferromagnet (NM/FM) bilayers bears great promise for the development of low-power spin-based devices, but the microscopic origin of purely interfacial SOTs in ultrathin systems is not yet fully understood. Here, we show that a linear response theory with a nonperturbative treatment of spin-dependent interactions and impurity scattering potential predicts dampinglike (DL) SOTs that are strictly absent in perturbative approaches. The technique is applied to a two-dimensional Rashba-coupled FM (the paradigmatic model of a NM/FM interface), where higher-order scattering processes encoding skew scattering from nonmagnetic impurities allow for current-induced spin polarization with nonzero components along all spatial directions. This is in stark contrast to previous results of perturbative methods (neglecting skew scattering), which predict a coplanar spin-polarization locked perpendicular to the charge current as a result of the conventional Rashba-Edelstein effect. Furthermore, the angular dependence of ensuing SOTs and their dependence upon the scattering potential strength is analyzed numerically. Simple analytic expressions for the spin-density–charge-current response function and related SOT efficiencies are obtained in the weak scattering limit. We find that the extrinsic DL torques driven by impurity scattering reaches efficiencies of up to 7% of the fieldlike (Rashba-Edelstein) torque. Our microscopic theory shows that bulk phenomena, such as the spin Hall effect, are not a necessity in the generation of the DL SOTs of the type observed in experiments on ultrathin systems.

We construct a heavy fermion representation for twisted bilayer graphene (TBG) systems. Two local orbitals (per spin/valley) are analytically found, which are exactly the maximally localized zero modes of the continuum Hamiltonian near the AA-stacking center. They have similar properties to the Wannier functions found in a recent study, but also have a clear interpretation as the zeroth pseudo Landau levels (ZLL) of Dirac fermions under the uniform strain field created by twisting. The electronic states of TBG can be viewed as the hybridization between these ZLL orbitals and other itinerant states, which can be obtained following the standard procedure of orthogonalized plane wave method. The “heavy fermion” model for TBG separates the strongly correlated components from the itinerant components and provides a solid base for the comprehensive understanding of the exotic physics in TBG.