Chiral magnets give rise to the antisymmetric Dzyaloshinskii-Moriya interaction, which induces topological nontrivial textures such as magnetic skyrmions. The topology is characterized by integer values of the topological charge. In this work, we performed the Monte Carlo calculation of a two-dimensional model of the chiral magnet. A surprising upturn of the topological charge is identified at high fields and high temperatures. This upturn is closely related to thermal fluctuations at the atomic scale, and is explained by a simple physical picture based on triangulation of the lattice. This emergent topology is also explained by a field-theoretic analysis using $C{P}^1$ formalism.

We show for a simple noncollinear configuration of the atomistic spins (in particular, where one spin is rotated by a finite angle in a ferromagnetic background) that the pairwise energy variation computed in terms of multiple-scattering formalism cannot be fully mapped onto a bilinear Heisenberg spin model even in the absence of spin-orbit coupling. The non-Heisenberg terms induced by the spin-polarized host appear in leading orders in the expansion of the infinitesimal angle variations. However, an $E_g-T_{2g}$ symmetry analysis based on the orbital decomposition of the exchange parameters in bcc Fe leads to the conclusion that the nearest-neighbor exchange parameters related to the $T_{2g}$ orbitals are essentially Heisenberg-like: they do not depend on the spin configuration, and can, in this case, be mapped onto a Heisenberg spin model even in extreme noncollinear cases.

Recent pump-probe experiments reported an enhancement of superconducting transport along the $c$ axis of underdoped ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{6+\delta }$ (YBCO), induced by a midinfrared optical pump pulse tuned to a specific lattice vibration. To understand this transient nonequilibrium state, we develop a pump-probe formalism for a stack of Josephson junctions, and we consider the tunneling strengths in the presence of modulation with an ultrashort optical pulse. We demonstrate that a transient enhancement of the Josephson coupling can be obtained for pulsed excitation and that this can be even larger than in a continuously driven steady state. Especially interesting is the conclusion that the effect is largest when the material is parametrically driven at a frequency immediately above the plasma frequency, in agreement with what is found experimentally. For bilayer Josephson junctions, an enhancement similar to that experimentally is predicted below the critical temperature $T_c$. This model reproduces the essential features of the enhancement measured below $T_c$. To reproduce the experimental results above $T_c$, we will explore extensions of this model, such as in-plane and amplitude fluctuations, elsewhere.

Vortex attraction which can cause a bundling of vortices has been observed in a multitude of type-II superconductors. While its underlying mechanisms have been extensively studied, the morphology of the emerging vortex superstructure has only been rarely considered. Here, we present a comprehensive experimental study on the type-II/1 superconductor niobium which focuses on the transformation of its homogeneous vortex lattice into an inhomogeneous domain structure at the onset of vortex attraction. By means of small-angle neutron scattering, ultra-small-angle neutron scattering, and neutron grating interferometry, the vortex lattice and the micrometer-scale vortex domain structure as well as its distribution could be investigated. In particular, we focus on the transformation of the vortex lattice at the transition to the intermediate mixed state, which is characterized by vortex attraction. We have found that the phase separation of the vortex lattice into an irregular domain structure takes place via a process showing strong similarity to spinodal decomposition. While pinning disorders the domain morphology, the characteristic length scale of the domain structure is governed by an interplay of field distortion energy and domain surface tension. Finally, geometric barriers in the disk-shaped samples provoke an inhomogeneous distribution of domains on the macroscopic scale.

Fermion-induced quantum critical points (FIQCPs) were recently discovered at the putatively first-order transitions between two-dimensional (2D) Dirac semimetals and the Kekule valence bond solids on the honeycomb lattice by sign-free quantum Monte Carlo simulations [Nat. Commun. 8, 314 (2017)]. Here, we investigate possible FIQCPs in 3D topological Weyl semimetals at a $Z_3$ symmetry-breaking transition that is putatively first-order according to the Landau criterion. We construct a lattice model featuring 3D double-Weyl fermions (monopole charges $\pm 2$), and we show that $Z_3$ nodal-nematic transitions occur under finite Hubbard interaction. Furthermore, using renormalization-group analysis, we identify such a transition as a genuine FIQCP where the cubic terms are irrelevant and an enlarged U(1) symmetry emerges at low energy. We further discuss quantum critical behaviors and experimental signatures of such FIQCPs in 3D double-Weyl semimetals.

In contrast to the seminal weak localization prediction of a noncritical Hall constant ($R_H$) at the Anderson metal-insulator transition (MIT), $R_H$ in quite a few real disordered systems exhibits both a strong $T$ dependence and critical scaling near their MIT. Here we investigate these issues in detail within a nonperturbative “strong localization” regime using cluster-dynamical mean-field theory (CDMFT). We uncover (i) clear and unconventional quantum-critical scaling of the Gell-Mann law, or $\gamma$ function for magnetotransport, finding that $\gamma \left(g_{xy}\right)=\frac{d\left[\text{log}\left(g_{xy}\right)\right]}{d\left[\text{log}\right(T\left)\right]}\simeq \log \left(g_{xy}\right)$ over a wide range spanning the continuous MIT, very similar to that seen for the longitudinal conductivity, and (ii) strongly $T$ dependent and clear quantum critical scaling in both transverse conductivity and $R_H$ at the MIT. We show that these surprising results are in comprehensive and very good accord with signatures of a novel Mott-like localization in NbN near the MIT, providing substantial support for our “strong” localization view.

According to the extensive theoretical and experimental investigations, it is widely accepted that the long-range Coulomb interaction is too weak to generate a dynamical excitonic gap in graphene with a perfect Dirac cone. We study the impact of the deformation of a Dirac cone on dynamical gap generation. When a uniaxial strain is applied to graphene, the Dirac cone is made elliptical on the equal-energy plane, and the fermion velocity becomes anisotropic. The applied uniaxial strain has two effects: It decreases the mean value of fermion velocity; it increases the velocity anisotropy. After solving the Dyson-Schwinger gap equation, we show that dynamical gap generation is promoted by the former effect but suppressed by the latter. For suspended graphene, we find that the system undergoes an excitonic insulating transition when the strain is roughly 7.34%. We also solve the gap equation in case the Dirac cone is tilted, which might be realized in the organic material $\alpha -{(\mathrm{BEDT}-\mathrm{TTF})}_2{\mathrm{I}}_3$ and find that the tilt of the Dirac cone can suppress dynamical gap generation. It turns out that the geometry of the Dirac cone plays an important role in the formation of excitonic pairing.

We report a magnetotransport study of Dirac semimetal $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ under high pressure. The Shubnikov–de Haas oscillations are measured at different pressures to reveal the evolution of the Fermi surface. A sudden change in the phase factor of the oscillations occurs at 1.3 GPa along with the unanticipated shrinkage of the Fermi surface, which is well below the structure transition point from the tetragonal to the monoclinic structure ($\sim 2.5\mathrm{GPa}$). High-pressure x-ray diffraction also reveals an anisotropic compression of the $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ lattice around a similar pressure. Through the first-principles calculations, we find that such axial compression along the $c$ direction will shift the Dirac nodes towards the Brillouin zone center and will eventually introduce a finite energy gap. Our result unveils a possible topological phase transition in pressurized $\mathrm{C}{\mathrm{d}}_3\mathrm{A}{\mathrm{s}}_2$ without structure transition.

Using the tight-binding approach, we investigate the electronic properties of bilayer phosphorene (BLP) quantum dots (QDs) in the presence of perpendicular electric and magnetic fields. Since BLP consists of two coupled phosphorene layers, it is of interest to examine the layer-dependent electronic properties of BLP QDs, such as the electronic distributions over the two layers and the so-produced layer-polarization features, and to see how these properties are affected by the magnetic field and the bias potential. We find that in the absence of a bias potential only edge states are layer polarized while the bulk states are not, and the layer-polarization degree (LPD) of the unbiased edge states increases with increasing magnetic field. However, in the presence of a bias potential both the edge and bulk states are layer polarized, and the LPD of the bulk (edge) states depends strongly (weakly) on the interplay of the bias potential and the interlayer coupling. At high magnetic fields, applying a bias potential renders the bulk electrons in a BLP QD to be mainly distributed over the top or bottom layer, resulting in layer-polarized bulk Landau levels (LLs). In the presence of a large bias potential that can drive a semiconductor-to-semimetal transition in BLP, these bulk LLs exhibit different magnetic-field dependences, i.e., the zeroth LLs exhibit a linearlike dependence on the magnetic field while the other LLs exhibit a square-root-like dependence.

We investigate the emergence and consequences of odd-frequency spin-triplet $s$-wave pairing in superconducting hybrid junctions at the edge of a two-dimensional topological insulator without any magnetism. More specifically, we consider several different normal-superconductor hybrid systems at the topological insulator edge, where spin-singlet $s$-wave superconducting pairing is proximity induced from an external conventional superconductor. We perform fully analytical calculations and show that odd-frequency mixed spin-triplet $s$-wave pairing arises due to the unique spin-momentum locking in the topological insulator edge state and the naturally nonconstant pairing potential profile in hybrid systems. Importantly, we establish a one-to-one correspondence between the local density of states (LDOS) at low energies and the odd-frequency spin-triplet pairing in NS, NSN, and SNS junctions along the topological insulator edge; at interfaces the enhancement in the LDOS can directly be attributed to the contribution of odd-frequency pairing. Furthermore, in SNS junctions we show that the emergence of the zero-energy LDOS peak at the superconducting phase $\varphi =\pi$ is associated purely with odd-frequency pairing in the middle of the junction.

We investigate the electronic properties of $N$-layer black phosphorus by means of an analytical method based on a recently proposed tight-binding Hamiltonian involving 14 hopping parameters. The method provides simple and accurate general expressions for the Hamiltonian of $N$-layer phosphorene, which are suitable for the study of electronic transport and optical properties of such systems, and the results show the features that emerge as the number of layers increases. In addition, we show that the $N$-layer problem can be translated into $N$ effective monolayer problems in the long wavelength approximation and, within this analytical picture, we obtain expressions for the energy gap and the effective masses for electrons and holes along the $N$-layer black phosphorus plane directions as a function of the number of layers, as well as for the Landau levels as a function of perpendicular magnetic field.

Recently, thermal Hall effect has been observed in the paramagnetic state of volborthite, which consists of distorted kagome layers with $S=1/2$ local moments. Despite the appearance of magnetic order below $1\mathrm{K}$, the response to external magnetic field and unusual properties of the paramagnetic state above $1\mathrm{K}$ suggest possible realization of exotic quantum phases. Motivated by these discoveries, we investigate possible spin liquid phases with fermionic spinon excitations in a nonsymmorphic version of the kagome lattice, which belongs to the two-dimensional crystallographic group $p2gg$. This nonsymmorphic structure is consistent with the spin model obtained in the density functional theory calculation. Using projective symmetry group analysis and fermionic parton mean field theory, we identify twelve distinct ${\mathbb{Z}}_2$ spin liquid states, four of which are found to have correspondence in the eight Schwinger boson spin liquid states we classified earlier. We focus on the four fermionic states with bosonic counterpart and find that the spectrum of their corresponding root $U\left(1\right)$ states features spinon Fermi surface. The existence of spinon Fermi surface in candidate spin liquid states may offer a possible explanation of the finite thermal Hall conductivity observed in volborthite.

In the present work, we provide results from specific heat, magnetic susceptibility, dielectric constant, ac conductivity, and electrical polarization measurements performed on the lacunar spinel $\mathrm{Ga}{\mathrm{V}}_4\mathrm{S}{\mathrm{e}}_8$. With decreasing temperature, we observe a transition from the paraelectric and paramagnetic cubic state into a polar, probably ferroelectric state at 42 K followed by magnetic ordering at 18 K. The polar transition is likely driven by the Jahn-Teller effect due to the degeneracy of the ${\mathrm{V}}_4$ cluster orbitals. The excess polarization arising in the magnetic phase indicates considerable magnetoelectric coupling. Overall, the behavior of $\mathrm{Ga}{\mathrm{V}}_4\mathrm{S}{\mathrm{e}}_8$ in many respects is similar to that of the skyrmion host $\mathrm{Ga}{\mathrm{V}}_4{\mathrm{S}}_8$, exhibiting a complex interplay of orbital, spin, lattice, and polar degrees of freedom. However, its dielectric behavior at the polar transition markedly differs from that of the Jahn-Teller-driven ferroelectric $\mathrm{Ge}{\mathrm{V}}_4{\mathrm{S}}_8$, which can be ascribed to the dissimilar electronic structure of the Ge compound.

The proximity-induced superconducting state in the three-dimensional topological insulator HgTe has been studied using electronic transport of a normal metal-superconducting point contact as a spectroscopic tool (Andreev point-contact spectroscopy). By analyzing the conductance as a function of voltage for various temperatures, magnetic fields, and gate voltages, we find evidence, in equilibrium, for an induced order parameter in HgTe of 70 µeV and a niobium order parameter of 1.1 meV. To understand the full conductance curve as a function of applied voltage we suggest a non-equilibrium-driven transformation of the quantum transport process where the relevant scattering region and equilibrium reservoirs change with voltage. This change implies that the spectroscopy probes the superconducting correlations at different positions in the sample, depending on the bias voltage.

Electron-spin momentum locking together with background magnetic textures can significantly alter the electron transport properties. We investigate theoretically the electron transport at the interface between a topological insulator and a magnetic insulator with magnetic skyrmions on the top. In contrast to the conventional topological Hall effect in normal metals, the skyrmions yield an additional contribution to the anomalous Hall conductivity even in the absence of in-plane magnetic texture, arising from the phase factor characteristic of Dirac electrons acquired at the skyrmion boundary.

We investigate the ballistic tunneling transport properties of a monolayer of black phosphorus under a magnetic barrier. The conductance of the system depends strongly on the orientation of the magnetic barrier, which is suppressed maximally when the magnetic barrier is oriented along the armchair direction. The mechanism relies on the highly anisotropic energy dispersion of phosphorene and the magnetic-barrier-induced suppression of available phase space for transmission. The magnetoresistance is enhanced by the reduction of the band gap under the same effective mass components.

Plasmon excitations in free-standing graphene and graphene/hexagonal boron nitride (hBN) heterostructure are studied using linear-response time-dependent density functional theory within the random phase approximation. Within a single theoretical framework, we examine both the plasmon dispersion behavior and lifetime (linewidth) of Dirac and $\pi$ plasmons on an equal footing. Particular attention is paid to the influence of the hBN substrate and the anisotropic effect. Furthermore, a model-based analysis indicates that the correct dispersion behavior of $\pi$ plasmons should be ${\omega }_{\pi }\left(q\right)=\sqrt{E_g^2+\beta q\phantom{{}^l}}$ for small $q$'s, where $E_g$ is the band gap at the $M$ point in the Brillouin zone, and $\beta$ is a fitting parameter. This model is radically different from previous proposals, but in good agreement with our calculated results from first principles.

With unique distorted 1T structure and the associated in-plane anisotropic properties, mono- and few-layer $\mathrm{Re}{X}_2$ ($X=\mathrm{S},\mathrm{Se}$) have recently attracted particular interest. Based on experiment and first-principles calculations, we investigate the fracture behavior of $\mathrm{Re}{X}_2$. We find that the cleaved edges of $\mathrm{Re}{X}_2$ flakes usually form an angle of $\sim {120}^{\circ }$ or $\sim {60}^{\circ }$. In order to understand such phenomenon, we perform comprehensive investigations on the uniaxial tensile stress-strain relation of monolayer and multilayer $\mathrm{Re}{X}_2$ sheets. Our numerical calculation shows that the particular cleaved edges of $\mathrm{Re}{X}_2$ flakes are caused by unique anisotropic ultimate tensile strengths and critical strains. We also calculate the stress-strain relation of $\mathrm{WT}{\mathrm{e}}_2$, which explains why their cleaved edges are not corresponding to the principle axes. Our proposed mechanism about the fracture angle is also supported by the calculated cleavage energies and surface energies for different edge surfaces.

We study thermal rectification (TR) in a selectively restructured graphene by performing deviational phonon Monte Carlo (MC) simulations with frequency-dependent phonon transport properties obtained from first principles. The restructuring is achieved by introducing vacancy defects in a portion of graphene. The defects significantly change phonon transport properties, resulting in a modulation of temperature dependence of thermal conductivity. With this modulated temperature dependence, we predict TR ratio through a Fourier's-law-based iterative scheme (FIS), where heat flow through the system is analyzed by solving the Fourier's law of heat conduction with spatially varying temperature-dependent thermal conductivity. To identify structure parameters for maximal TR ratio, we investigate the influence of defect size, volume percentage of defects, and system (consisting of defective and nondefective regions) length through FIS analysis. As a result, we find that the TR ratio is mainly a function of length of defective and nondefective regions and volume percentage of defect, and it is mostly independent of defect size. A longer (of the order of 10 μm) nondefective side, coupled to a shorter (of the order of 100 nm) defective side, can lead to large TR ratios. Finally, MC simulation for the restructured graphene (full system) is performed to verify the predictions from FIS analysis. The full system calculations give similar trends but with enhanced TR ratios up to 70% for the temperature range of 200–500 K.