We study the chiral anomaly in disordered Weyl semimetals, where the broken translational symmetry prevents the direct application of Nielsen and Ninomiya's mechanism and disorder is strong enough that quantum effects are important. In the weak disorder regime, there exist rare regions of the random potential where the disorder strength is locally strong, which gives rise to quasilocalized resonances and their effect on the chiral anomaly is unknown. We numerically show that these resonant states do not affect the chiral anomaly only in the case of a single Weyl node. At energies away from the Weyl point, or with strong disorder where one is deep in the diffusive regime, the chiral Landau level itself is not well defined and the semiclassical treatment is not justified. In this limit, we analytically use the supersymmetry method and find that the Chern-Simons term in the effective action which is not present in nontopological systems gives rise to a nonzero average level velocity which implies chiral charge pumping. We numerically establish that the nonzero average level velocity serves as an indicator of the chiral anomaly in the diffusive limit.

The impact of surface morphology on the electronic structure of topological crystalline insulators is studied theoretically. As an example, the structure of topologically protected electronic states on a (001) (Pb,Sn)Se surface with terraces of atomic height is modeled. Within the envelope function model it is shown that valley mixing, the phenomenon responsible for the peculiar “double Dirac cone” shape of the surface state dispersion, depends crucially on the structure of the surface. By varying the width and the number of atomic layers in the terraces, a comprehensive explanation of recent experimental findings, i.e., the emergence of one-dimensional states bound to odd-height atomic step edges [P. Sessi et al., Science 354, 1269 (2016)] as well as the collapse of “double Dirac cone” structure on a rough surface [C. Polley et al., ACS Nano 12, 617 (2018)], is achieved. This approach allows us also to determine topological indices characterizing terraces and their interfaces. In the (001) surface of (Pb,Sn)Se the adjacent terraces turn out to be described by different values of the winding number topological invariant.

The zero-bias anomaly at low temperatures, originated by the Kondo effect when an electric current flows through a system formed by a spin-$1/2$ quantum dot and two metallic contacts is theoretically investigated. In particular, we compare the width of this anomaly $2T_{\mathrm{NE}}$ with that of the Kondo resonance in the spectral density of states $2T_K^{\rho }$, obtained from a Fano fit of the corresponding curves and also with the Kondo temperature $T_K^G$ defined from the temperature evolution of the equilibrium conductance $G\left(T\right)$. In contrast to $T_K^G$ and $2T_K^{\rho }$, we found that the scale $2T_{\mathrm{NE}}$ strongly depends on the asymmetry between the couplings of the quantum dot to the leads while the total hybridization is kept constant. While the three scales are of the same order of magnitude, $2T_{\mathrm{NE}}$ and $T_K^{\rho }$ agree only in the case of large asymmetry between the different tunneling couplings of the contacts and the quantum dot. On the other hand, for similar couplings, $T_{\mathrm{NE}}$ becomes larger than $T_K^{\rho }$, reaching the maximum deviation, of the order of $30\%$, for identical couplings. The fact that an additional parameter to $T_{\mathrm{NE}}$ is needed to characterize the Kondo effect, weakening the universality properties, points that some caution should be taken in the usual identification in experiments of the low temperature width of the zero-bias anomaly with the Kondo scale. Furthermore, our results indicate that the ratios $T_{\mathrm{NE}}/T_K^G$ and $T_K^{\rho }/T_K^G$ depend on the range used for the fitting.

Both the isothermal Helmholtz energy and the adiabatic internal energy of condensed matter exhibit minima at zero pressure. As a consequence, the leading term in a density expansion of these two thermodynamic potentials is quadratic in density displacement. When the quadratic term dominates, the material behaves symmetrically in response to isotropic compressive and tensile forces. From an atomistic viewpoint, compressing or stretching atomic bonds to the same degree in a symmetric material increases a corresponding thermodynamic potential by the same amount. The quadratic term contributes to the pressure as a simple cubic equation of state (EOS). Among 29 metals and 17 inorganic solids surveyed, only metallic gold satisfies this simple EOS to the highest measured compressions (>40%). Other solids such as Pt and NaCl also follow this simple EOS to significant compressions and all materials should follow it at low compressions/dilations. A thermodynamic protocol is proposed to extract the bulk modulus $B_0$ as well as higher order modulus pressure coefficients,$B_1,B_2,\mathrm{and}{B}_3,$ from least squares smoothed data without taking derivatives or by appealing to an EOS model. Reliable and unbiased experimental values of these higher order modulus coefficients have been obtained for 46 solids. These moduli are strongly correlated and satisfy the relationship $B_0^2{B}_3\simeq -2B_0{B}_1{B}_2$. This relation obtains exactly from an adiabatic EOS based on the Mie (m-n) potential and to a high degree of approximation for the well-known Birch-Murnaghan and Vinet EOS. Truncating the thermodynamic expansion at the fifth order term with only the pressure coefficient $B_1$ as a parameter, this approximate thermodynamic EOS models experimental values of $B_2\text{and}{B}_3$ better than either the third order Birch-Murnaghan or Vinet EOS.

The relation between the nonadiabaticity parameter $\beta$ and the damping parameter $\alpha$ is investigated in permalloy-based microdisks. In order to determine $\beta$, high-resolution imaging of the current-induced vortex-core displacement is performed using scanning electron microscopy with polarization analysis. The materials properties of the films are varied via rare-earth Dy doping, leading to a greatly enhanced damping, while retaining the same spin configuration for the confined vortex state. A clear trend to much higher nonadiabaticity values is seen for the higher doping levels and an averaged value of $\beta =(0.29\pm 0.15)\times {10}^{-2}$ is determined for $1.73\%$ Dy doping, compared to $(0.067\pm 0.014)\times {10}^{-2}$ which is extracted for pure permalloy. This is supportive of a similar scaling of $\beta$ and $\alpha$ in this system, pointing to a common origin of the spin relaxation which is at the heart of nonadiabatic transport and the dissipation of angular momentum that provides damping, in line with theoretical calculations.

We consider topological Hall effect (THE) in thin ferromagnetic films due to electron scattering on nanoscale magnetic skyrmions in the presence of the strong external magnetic field. We account for the effect of the magnetic field on a skyrmion structure and describe the hallmarks of THE differentiating it from ordinary and anomalous Hall effects. We have found that, although in typical ferromagnets the variation of magnetic field substantially affects the skyrmion size, THE changes rather weakly remaining quite robust. Therefore, the magnitude of THE is primarily determined by the skyrmions sheet density $n_{sk}$, i.e., for PdFe/Ir(111) it is comparable to the magnitude of the ordinary Hall effect (OHE) at $n_{sk}={10}^{11}{\mathrm{cm}}^{-2}$. The sign of THE is opposite to that of OHE for skyrmions with positive vorticity, while for antiskyrmions the signs are the same.

We study the photoinduced magnetic transition in an itinerant magnet described by the double-exchange model, in which conduction electrons couple with localized spins through the ferromagnetic (FM) Hund coupling. It is shown that intense light applied to the FM ground state induces an antiferromagnetic (AFM) order, in contrast to the AFM-to-FM transition due to the photocarrier injection. In particular, we focus on the mechanism for instability of the FM structure by the light irradiation. The magnon spectrum in the Floquet state is formulated on the basis of the perturbative expansion of the Floquet Green's function. The magnon dispersion shows softening at momentum $(\pi ,\pi )$ in the square lattice with increasing the light amplitude, implying photoinduced AFM instability. This result is mainly attributed to a nonequilibrium electron distribution, which promotes low-energy Stoner excitations. The transient optical conductivity spectra characterized by interband excitations and Floquet sidepeaks are available to identify the photoinduced AFM state.

We introduce configuration space as a natural representation for calculating the mechanical relaxation patterns of incommensurate two-dimensional (2D) bilayers. The approach can be applied to a wide variety of 2D materials through the use of a continuum model in combination with a generalized stacking fault energy for interlayer interactions. We present computational results for small-angle twisted bilayer graphene and molybdenum disulfide (${\mathrm{MoS}}_2$), a representative material of the transition-metal dichalcogenide family of 2D semiconductors. We calculate accurate relaxations for ${\mathrm{MoS}}_2$ even at small twist-angle values, enabled by the fact that our approach does not rely on empirical atomistic potentials for interlayer coupling. The results demonstrate the efficiency of the configuration space method by computing relaxations with minimal computational cost. We also outline a general explanation of domain formation in 2D bilayers with nearly aligned lattices, taking advantage of the relationship between real space and configuration space. The configuration space approach also enables calculation of relaxations in incommensurate multilayer systems.

A theoretical investigation of the ultrafast processes and dynamics of the excited carriers upon irradiation of GaAs with femtosecond pulsed lasers is performed in conditions that induce material damage and eventually surface modification of the heated solid. A parametric study is followed to correlate the produced transient carrier density with the damage threshold for various pulse duration values ${\tau }_p$ (it increases as $\sim {\tau }_p^{0.053\pm 0.011}$ at relatively small values of ${\tau }_p$ while it drops for pulse durations of the order of some picoseconds) based on the investigation of the fundamental multiscale physical processes following femtosecond laser irradiation. Moreover, fluence values for which the originally semiconducting material demonstrates a metallic behaviour are estimated. It is shown that a sufficient number of carriers in the conduction band are produced to excite surface-plasmon waves that upon coupling with the incident beam and a fluid-based surface modification mechanism lead to the formation of subwavelength periodic structures orientated perpendicularly to the laser beam polarization. Experimental results for the damage threshold and the frequencies of induced periodic structures show a good agreement with the theoretical predictions.

The hydration and ammonization of $\mathrm{Zr}{\mathrm{W}}_2{\mathrm{O}}_8$ is known to lead to positive and negative thermal expansion behavior, respectively. We report ab initio calculations to understand this anomalous behavior. We identify the crucial low-energy phonon modes involving translations, rotations, and distortions of $\mathrm{W}{\mathrm{O}}_4$ and $\mathrm{Zr}{\mathrm{O}}_6$ polyhedra, which lead to negative thermal expansion in $\mathrm{Zr}{\mathrm{W}}_2{\mathrm{O}}_8$ in pure and ammoniated forms; however, the rotation and distortion motions get inhibited on hydration and lead to positive thermal expansion. We demonstrate that the thermal expansion coefficient could be tailored by engineering the phonon dynamics of a material.

We study magnetic textures realized in noncentrosymmetric Kondo lattice models in which localized magnetic moments weakly interact with itinerant electrons subject to Rashba and Dresselhaus spin-orbit couplings. By virtue of state-of-the-art numerical simulations as well as variational calculations, we uncover versatile multiple-$Q$ orderings under a zero magnetic field, which are found to originate in the instabilities of the Fermi surface whose spin degeneracy is lifted by the spin-orbit couplings. In the case with equally strong Rashba and Dresselhaus spin-orbit couplings, which is known to realize a persistent spin helix in semiconductor quantum wells, we discover a sextuple-$Q$ magnetic ordering with a checkerboardlike spatial pattern of the spin scalar chirality. In the presence of either Rashba or Dresselhaus spin-orbit coupling, we find another multiple-$Q$ ordering, which is distinct from skyrmion crystals discussed under the same symmetry. Our results indicate that the cooperation of the spin-charge and spin-orbit couplings brings about richer magnetic textures than those studied within effective spin models. The situations would be experimentally realized, e.g., in noncentrosymmetric heavy-fermion compounds and heterostructures of spin-orbit-coupled metals and magnetic insulators.

The phase diagram of cubic helimagnets near the critical temperature is obtained from a Landau-Ginzburg model, including fluctuations to Gaussian level. The free energy is evaluated via a saddle-point expansion around the local minima of the Landau-Ginzburg functional. The local minima are computed by solving the Euler-Lagrange equations with appropriate boundary conditions, preserving manifestly the full nonlinearity that is characteristic of skyrmion states. It is shown that the fluctuations stabilize the skyrmion lattice in a region of the phase diagram close to the critical temperature, where it becomes the equilibrium state. A comparison of this approach with previous computations performed with a different approach (truncated Fourier expansion of magnetic states) is given.

We report a giant barocaloric effect in the spin-crossover compound $\left[\mathrm{Fe}{\left(\mathrm{hyptrz}\right)}_3\right]{\mathrm{A}}_2·{\mathrm{H}}_2\mathrm{O}$ with $\mathrm{hyptzr}=4$-(3′-hydroxypropyl)-1,2,4-triazole and $\mathrm{A}=4$-chlorobenzenesulfonate. The origin of the giant barocaloric effect comes from the strong coupling between the lattice vibration and the electronic states $t_{2g}^6\left(S=0\right)$ and $\left(t_{2g}^4{e}_g^2\right)\left(S=2\right)$ of low and high spin in $\mathrm{F}{\mathrm{e}}^{+2}{\mathrm{N}}_6$ cores, respectively. Our theoretical and indirect experimental results lead to the barocaloric potential $\mathrm{\Delta }{S}_T\sim 56\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$ and $\mathrm{\Delta }{T}_{\mathrm{ad}}\sim 10\mathrm{K}$, upon pressure changes $\mathrm{\Delta }P=0.9$ kbar, which can easily be obtained experimentally. The giant barocaloric in $\left[\mathrm{Fe}{\left(\mathrm{hyptrz}\right)}_3\right]{\mathrm{A}}_2·{\mathrm{H}}_2\mathrm{O}$ leads to colossal refrigerant capacity of $\mathrm{RC}=1223\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}$ in a wide temperature span around ($T=273\mathrm{K}$), the freezing temperature of the water.

We explore a method for regulating 2+1D quantum critical points in which the ultraviolet cutoff is provided by the finite density of states of particles in a magnetic field rather than by a lattice. Such Landau-level quantization allows for numerical computations on arbitrary manifolds, like spheres, without introducing lattice defects. In particular, when half-filling a Landau level with $N=4$ electron flavors, with appropriate interaction anisotropies in flavor space, we obtain a fully continuum regularization of the $\mathrm{O}\left(5\right)$ nonlinear sigma model with a topological term, which has been conjectured to flow to a deconfined quantum critical point. We demonstrate that this model can be solved by both infinite density-matrix renormalization group (DMRG) calculations and sign-free determinantal quantum Monte Carlo. DMRG calculations estimate the scaling dimension of the $\mathrm{O}\left(5\right)$ vector operator to be in the range ${\mathrm{\Delta }}_V\sim 0.55–0.7$, depending on the stiffness of the nonlinear sigma model. Future Monte Carlo simulations will be required to determine whether this dependence is a finite-size effect or further evidence for a weak first-order transition.

A key property of Majorana zero modes is their protection against local perturbations. In the standard picture, this protection is guaranteed by a high degree of spatial nonlocality of the Majoranas, namely a suppressed wave-function overlap, in the topological phase. However, a careful characterization of resilience to local noise goes beyond mere spatial separation and must also take into account the projection of wave-function spin. By considering the susceptibility of a given zero mode to different local perturbations, we find the relevant forms of spin-resolved wave-function overlaps that measure its resilience. We quantify these overlaps and study their dependence with nanowire parameters in several classes of experimentally relevant configurations. These include nanowires with inhomogeneous depletion and induced pairing, barriers, and quantum dots. Smooth inhomogeneities have been shown to produce near-zero modes, so-called pseudo-Majoranas, below the critical Zeeman field in the bulk. Surprisingly, their resilience is found to be comparable or better than that of topological Majoranas in realistic systems. We further study how accurately their overlaps can be estimated using a purely local measurement on one end of the nanowire, accessible through conventional transport experiments. In uniform nanowires, this local estimator is remarkably accurate. In inhomogeneous cases, it is less accurate but can still provide reasonable estimates for potential inhomogeneities of the order of the superconducting gap. We further analyze the zero-mode wave-function structure, spin texture, and spectral features associated with each type of inhomogeneity. All our results highlight the strong connection between internal wave-function degrees of freedom, nonlocality, and protection in smoothly inhomogeneous nanowires.

A translation invariant one-dimensional system of spinless fermions with a finite-range attraction experiences a quantum phase transition to a phase-separated state. While being a conventional Luttinger liquid for a small interaction strength, spinless fermions form a droplet with the size smaller than the available one-dimensional volume when the interaction strength exceeds some critical value. A particularly remarkable signature of the droplet formation is the change in the lower edge of the many-body excitation spectrum. In the homogeneous phase, it has a Luttinger-liquid shape of periodic arcs on top of the shallow parabolic dispersion of the center of mass. When the interaction strength is increased, the arcs disappear completely as soon as the droplet is formed. We perform an exact diagonalization study of this system with the focus on the signatures of the quantum phase transition and the droplet properties. The one-particle and density-density correlation functions, the pressure, the sound velocity, and the droplet density are examined. The value of the critical interaction strength obtained from numerical data reasonably agrees with a simple mean-field analytical estimate. Due to the boson-fermion correspondence valid in one dimension, our results also hold for hard-core bosons with a finite-range attraction.