Topological insulators (TIs) are exciting materials, which exhibit unprecedented properties, such as helical spin-momentum locking, which leads to large torques for magnetic switching and highly efficient spin current detection. Here we explore the compound YPtBi, an example from the class of half-Heusler materials, for which the typical band inversion of topological insulators was predicted. We prepared this material as thin films by conventional cosputtering from elementary targets. By in situ time-of-flight momentum microscopy, a Dirac conelike surface state with a Dirac point $\simeq 300$ meV below the Fermi energy was observed, in agreement with electronic structure-photoemission calculations. Only little additional spectral weight due to other states was observed at $E_F$, which corroborates the identification of the topologically protected surface state and is highly relevant for spintronics applications.

Tilting of the Dirac cone has an important effect on the optical response of a type-I and -II Weyl semimetal. The linear in photon energy $\left(\mathrm{\Omega }\right)$ interband background, characteristic of Dirac and Weyl semimetals, is modified as the tilt parameter $w$ is increased from the $w=0$ case. For $w<1$ (type I) and chemical potential $\mu$, the changes are confirmed to the photon region $\mathrm{\Omega }<\frac{2\mu }{1-w}$ but for $w>1$ (type II, overtilted) changes persist to the highest energy. While for $\mathrm{\Omega }>>\frac{2\mu }{w-1}$ a quasilinear law is still obtained, its slope is modified from the value for $w=0$ and now does not extrapolate to the origin in the limit $\mathrm{\Omega }\to 0$ as it would for $w<1$. Rather, it cuts the vertical axis at a finite value. As the chemical potential is increased, optical weight is lost in the interband background. It is transferred to the intraband (Drude) response and a sum rule applies for the case $w<1$. In addition, the magnitudes of dc electrical and thermal conductivity are changed with increasing $w$ but their dependence on temperature $\left(T\right)$ and chemical potential $\left(\mu \right)$ are not. The Wiedemann-Franz law remains unaltered, as is the thermopower. For the overtilted case, there are important alterations of variations with $T$ and $\mu$.

We characterize nonperturbatively the Rényi entropies of degree $n=2,3,4$, and 5 of three-dimensional, strongly coupled many-fermion systems in the scale-invariant regime of short interaction range and large scattering length, i.e., in the unitary limit. We carry out our calculations using lattice methods devised recently by us. Our results show the effect of strong pairing correlations on the entanglement entropy, which modify the subleading behavior for large subsystem sizes (as characterized by the dimensionless parameter $x=k_F{L}_A$, where $k_F$ is the Fermi momentum and $L_A$ the linear subsystem size), but leave the leading order unchanged relative to the noninteracting case. Moreover, we find that the onset of the subleading asymptotic regime is at surprisingly small $x\simeq 2–4$. We provide further insight into the entanglement properties of this system by analyzing the spectrum of the entanglement Hamiltonian of the two-body problem from weak to strong coupling. The low-lying entanglement spectrum displays clear features as the strength of the coupling is varied, such as eigenvalue crossing and merging, a sharp change in the Schmidt gap, and scale invariance at unitarity. Beyond the low-lying component, the spectrum appears as a quasicontinuum distribution, for which we present a statistical characterization; we find, in particular, that the mean shifts to infinity as the coupling is turned off, which indicates that that part of the spectrum represents nonperturbative contributions to the entanglement Hamiltonian. In contrast, the low-lying entanglement spectrum evolves to finite values in the noninteracting limit. The scale invariance of the unitary regime guarantees that our results are universal features intrinsic to three-dimensional quantum mechanics and represent a well-defined prediction for ultracold atom experiments, which were recently shown to have direct access to the entanglement entropy.

By using first-principles calculations, we propose that WC-type ZrTe is a new type of topological semimetal (TSM). It has six pairs of chiral Weyl nodes in its first Brillouin zone, but it is distinguished from other existing TSMs by having an additional two paris of massless fermions with triply degenerate nodal points as proposed in the isostructural compounds TaN and NbN. The mirror symmetry, threefold rotational symmetry, and time-reversal symmetry require all of the Weyl nodes to have the same velocity vectors and locate at the same energy level. The Fermi arcs on different surfaces are shown, which may be measured by future experiments. It demonstrates that the “material universe” can support more intriguing particles simultaneously.

We show that two spatially separated semiconductor quantum dots under resonant and continuous-wave excitation can be strongly entangled in the steady state, thanks to their radiative coupling by mutual interaction through the normal modes of a photonic crystal dimer. We employ a quantum master equation formalism to quantify the steady-state entanglement by calculating the system negativity. Calculations are specified to consider realistic semiconductor nanostructure parameters for the photonic crystal dimer–quantum dots coupled system, determined by a guided-mode expansion solution of Maxwell equations. Negativity values of the order of 0.1 ($20\%$ of the maximum value) are shown for interdot distances that are larger than the resonant wavelength of the system. It is shown that the amount of entanglement is almost independent of the interdot distance, as long as the normal mode splitting of the photonic dimer is larger than their linewidths, which becomes the only requirement to achieve a local and individual qubit addressing. Considering inhomogeneously broadened quantum dots, we find that the steady-state entanglement is preserved as long as the detuning between the two quantum dot resonances is small when compared to their decay rates. The steady-state entanglement is shown to be robust against the effects of pure dephasing of the quantum dot transitions. We finally study the entanglement dynamics for a configuration in which one of the two quantum dots is initially excited and find that the transient negativity can be enhanced by more than a factor of two with respect to the steady-state value. These results are promising for practical applications of entangled states at short time scales.

A tight-binding model of 8-$Pmmn$ borophene, a two-dimensional boron crystal, is developed. We confirm that the crystal hosts massless Dirac fermions and the Dirac points are protected by symmetry. Strain is introduced into the model, and it is shown to induce a pseudomagnetic field vector potential and a scalar potential. The dependence of the potentials on the strain tensor is calculated. The physical effects controlled by the pseudomagnetic field are discussed.

It is now well established that the paramagnetic-to-ferromagnetic transition in the magnetocaloric $\mathrm{La}{\left(\mathrm{FeSi}\right)}_{13}$ is a cooperative effect involving spin, charge, and lattice degrees of freedom. However, the influence of this correlated behavior on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of $\mathrm{LaF}{\mathrm{e}}_x\mathrm{M}{\mathrm{n}}_y\mathrm{S}{\mathrm{i}}_z$ samples, with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature, and the spin-wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced $d$-band electronic interactions at play. The Sommerfeld coefficient is found to be $90–210\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}$, unusually high compared to that expected from band-structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin-wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly and provide a basis for future theoretical development.

A low-frequency resistance noise study in highly underdoped thick films of ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ $(x=0.07$ and 0.08) reveals slow, correlated dynamics and breaking of ergodicity near the superconducting transition of the Berezinskii-Kosterlitz-Thouless type. The observed correlated behavior is strongly suppressed by disorder.

A quantitative description of Coulomb interactions is developed for two-dimensional superconducting materials, enabling us to compare intrinsic with external screening effects, such as those due to substrates. Using the example of a doped monolayer of ${\mathrm{MoS}}_2$ embedded in a tunable dielectric environment, we demonstrate that the influence of external screening is limited to a length scale, bounded from below by the effective thickness of the quasi-two-dimensional material and from above by its intrinsic screening length. As a consequence, it is found that unconventional Coulomb-driven superconductivity cannot be induced in ${\mathrm{MoS}}_2$ by tuning the substrate properties alone. Our calculations of the retarded Morel-Anderson Coulomb potential ${\mu }^{*}$ reveal that the Coulomb interactions, renormalized by the reduced layer thickness and the substrate properties, can shift the onset of the electron-phonon driven superconducting phase in monolayer ${\mathrm{MoS}}_2$ but do not significantly affect the critical temperature at optimal doping.

Using a combination of low-temperature nanocalorimetry and x-ray diffraction we identify three temperature regimes characterized by distinct Na ordering patterns (low temperature up to 290 K, intermediate 290–340 K, and high above 340 K). Through freezing-in of these patterns we establish the two key roles sodium intercalation plays in the formation of the magnetic ground state: supplying the proper electron count for in-plane ferromagnetic interaction and through the 3D sodium ordering providing the interplane antiferromagnetic exchange path.

Magnetic skyrmions have been receiving growing attention as potential information storage and magnetic logic devices since an increasing number of materials have been identified that support skyrmion phases. Explorations of artificial frustrated systems have led to new insights into controlling and engineering new emergent frustration phenomena in frustrated and disordered systems. Here, we propose a skyrmion spin ice, giving a unifying framework for the study of geometric frustration of skyrmion crystals (SCs) in a nonfrustrated artificial geometrical lattice as a consequence of the structural confinement of skyrmions in magnetic potential wells. The emergent ice rules from the geometrically frustrated SCs highlight a novel phenomenon in this skyrmion system: emergent geometrical frustration. We demonstrate how SC topology transitions between a nonfrustrated periodic configuration and a frustrated icelike ordering can also be realized reversibly. The proposed artificial frustrated skyrmion systems can be annealed into different ice phases with an applied current-induced spin-transfer torque, including a long-range ordered ice rule obeying ground state, as-relaxed random state, biased state, and monopole state. The spin-torque reconfigurability of the artificial skyrmion ice states, difficult to achieve in other artificial spin ice systems, is compatible with standard spintronic device fabrication technology, which makes the semiconductor industrial integration straightforward.

We calculate the electronic spin susceptibility and spin-lattice relaxation rate in a singlet superconductor near a pair-breaking surface, or in a domain wall of the order parameter. We directly link the presence of high-density Andreev bound states in the inhomogeneous region, combined with coherence factors, to the enhancement of the susceptibility above the normal state's value for certain $\mathbf{q}$ vectors. Besides the dominant peak at ferromagnetic vector $q=0$, we find significant enhancement of antiferromagnetic correlations at vectors $q\lesssim 2k_f$, with $\mathbf{q}$ along the domain wall in an $S$-wave superconductor, and across the domain wall in $D$-wave (nodes along the wall). These features are destroyed by applying a moderate Zeeman field that splits the zero-energy peak. We solve Bogoliubov-de Gennes equations in momentum space and discuss the deviation of our results from the lattice models investigated previously. Large enhancement of the spin-lattice relaxation rate $T_1^{-1}$ at the domain wall provides a clear signature of the quasiparticle bound states, and is in good agreement with recent experiment in organic superconductor $\kappa \text{−}{\left(\mathrm{BEDT}\text{−}\mathrm{TTF}\right)}_2\mathrm{Cu}{\left(\mathrm{NCS}\right)}_2$.

Relaxation times employed to study electron transport in metals are typically taken to be constants and obtained empirically. Here, we use fully ab initio calculations to compute the electron-phonon relaxation times of Cu, Ag, and Au and find that they vary significantly on the Fermi surface, with values from $\sim 15$ to 45 fs that are correlated with the Fermi surface topology. We compute room-temperature resistivities in excellent agreement with experiment by combining GW quasiparticle band structures, Wannier-interpolated band velocities, and ab initio relaxation times. We introduce an importance sampling scheme to speed up the convergence of resistivity and transport calculations.

Symmetrization of topologically ordered wave functions is a powerful method for constructing new topological models. Here we study wave functions obtained by symmetrizing quantum double models of a group $G$ in the projected entangled pair states (PEPS) formalism. We show that symmetrization naturally gives rise to a larger symmetry group $\tilde{G}$ which is always non-Abelian. We prove that by symmetrizing on sufficiently large blocks, one can always construct wave functions in the same phase as the double model of $\tilde{G}$. In order to understand the effect of symmetrization on smaller patches, we carry out numerical studies for the toric code model, where we find strong evidence that symmetrizing on individual spins gives rise to a critical model which is at the phase transitions of two inequivalent toric codes, obtained by anyon condensation from the double model of $\tilde{G}$.

Photoinduced magnetization (PIM) effect of ${\mathrm{Hg}}_{1-x}{\mathrm{Mn}}_x\mathrm{Te}$ provides an attractive solution for realizing the quantum anomalous Hall effect in quantum wells with a light field. In this paper, the PIM effect of $p$-type ${\mathrm{Hg}}_{0.74}{\mathrm{Mn}}_{0.26}\mathrm{Te}$ single crystal was investigated by power-, polarization- and temperature-dependent photoluminescence (PL) measurements in both reflection and transmission geometries. Giant Zeeman splitting and polarization of PL spectra were observed without an external magnetic field evolving with excitation-power density of the pumping laser and temperature, which were accounted for by the PIM effect. The occurrence of the PIM was qualitatively understandable by the carrier-mediated mean-field theory known as the Zener model. The results indicate that infrared PL measurements with enhanced sensitivity and signal-to-noise ratio can serve as a convenient pathway for clarifying the PIM effect of semimagnetic semiconductors.

We present an ab initio study of the electronic stopping power of protons in copper over a wide range of proton velocities $v=0.02–10\mathrm{a}.\mathrm{u}.$ where we take into account nonlinear effects. Time-dependent density functional theory coupled with molecular dynamics is used to study electronic excitations produced by energetic protons. A plane-wave pseudopotential scheme is employed to solve the time-dependent Kohn-Sham equations for a moving ion in a periodic crystal. The electronic excitations and the band structure determine the stopping power of the material and alter the interatomic forces for both channeling and off-channeling trajectories. Our off-channeling results are in quantitative agreement with experiments, and at low velocity they unveil a crossover region of superlinear velocity dependence (with a power of $\sim 1.5)$ in the velocity range $v=0.07–0.3\mathrm{a}.\mathrm{u}.$, which we associate to the copper crystalline electronic band structure. The results are rationalized by simple band models connecting two separate regimes. We find that the limit of electronic stopping $v\to 0$ is not as simple as phenomenological models suggest and it is plagued by band-structure effects.

Electron transport is studied in surface oxidized single-crystal copper thin films with a thickness of up to 5.6 nm by applying density functional theory and density functional tight binding methods to determine electron transport properties within the ballistic regime. The variation of the electron transmission as a function of film thickness as well as the different contributions to the overall electron transmission as a function of depth into the the films is examined. Transmission at the oxidized copper film surfaces is found to be universally low. Films with thickness greater than 2.7 nm exhibit a similar behavior in local transmission per unit area with depth from the film surface; transmission per unit area initially increases rapidly and then plateaus at a depth of approximately 0.35–0.5 nm away from the surface, dependent on surface facet. Unstrained films tend to exhibit a higher transmission per unit area than corresponding films under tensile strain.

Towards the goal of realizing topological phases in thin films of correlated oxide and heterostructures, we propose here a quantum anomalous Hall insulator (QAHI) in ultrathin films of double perovskites based on mixed $3d-5d$ or $3d-4d$ transition-metal ions, grown along the [111] direction. Considering the specific case of ultrathin ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$, we present a theoretical analysis of an effective Hamiltonian derived from first principles. We establish that a strong spin-orbit coupling at the Re site, $t_{2g}$ symmetry of the low-energy $d$ bands, polarity of its [111] orientation of perovskite structure, and mixed $3d-5d$ chemistry results in room temperature magnetism with a robust QAHI state of Chern number $C=1$ and a large band gap. We uncover and highlight a nonrelativistic orbital Rashba-type effect in addition to the spin-orbit coupling, that governs this QAHI state. With a band gap of $\sim 100$ meV in electronic structure and magnetic transition temperature $T_c\sim 300\mathrm{K}$ estimated by Monte Carlo simulations, our finding of the QAHI state in ultrathin ${\mathrm{Ba}}_2{\mathrm{FeReO}}_6$ is expected to stimulate experimental verification along with possible practical applications of its dissipationless edge currents.