Three-dimensional Dirac and Weyl semimetals have attracted widespread interest in condensed matter physics and material science. Here, based on the first-principle calculations and symmetry analysis, we report that ${\mathrm{Ag}}_2\mathrm{S}$ with $P{2}_1{2}_1{2}_1$ symmetry is a topological Dirac semimetal in the absence of spin-orbit coupling (SOC). Every Dirac point is composed of two Weyl points with the same chirality overlapping in the momentum space. After taking the SOC into consideration, each Dirac point is split into two Weyl points. The Weyl points with the opposite chirality are far from each other in the momentum space. Therefore, the Weyl pairs are not easy to annihilate and are robust in the ${\mathrm{Ag}}_2\mathrm{S}$ compound, which also leads to long Fermi arcs on material surfaces. The robustness of the Weyl points against the strain is discussed.

The interplay between topology and crystalline symmetries in materials can lead to a variety of topological crystalline insulator (TCI) states. Despite significant effort towards their experimental realization, so far only ${\mathrm{Pb}}_{1-x}{\mathrm{Sn}}_x\mathrm{Te}$ has been confirmed as a mirror-symmetry-protected TCI. Here, based on first-principles calculations combined with a symmetry analysis, we identify a rotational-symmetry-protected TCI state in the transition metal dipnictide ${RX}_2$ family, where $R=\mathrm{Ta}$ or Nb and $X=\mathrm{P}$, As, or Sb. Taking ${\mathrm{TaAs}}_2$ as an exemplar system, we show that its low-energy band structure consists of two types of bulk nodal lines in the absence of spin-orbit coupling (SOC) effects. Turning on the SOC opens a continuous band gap in the energy spectrum and drives the system into a $C_{2}T$-symmetry-protected TCI state. On the (010) surface, we show the presence of rotational-symmetry-protected nontrivial Dirac cone states within a local bulk energy gap of $\sim 300\mathrm{meV}$. Interestingly, the Dirac cones have tilted energy dispersion, realizing a type-II Dirac fermion state in a topological crystalline insulator. Our results thus indicate that the ${\mathrm{TaAs}}_2$ materials family provides an ideal setting for exploring the unique physics associated with type-II Dirac fermions in rotational-symmetry-protected TCIs.

Nodal-line semimetals are characterized by a kind of topologically nontrivial bulk-band crossing, giving rise to almost flat surface states. Notably, direct evidence of the surface states is still awaited. Here we theoretically study impurity effects in topological nodal-line semimetals based on the $T$-matrix method. It is found that some low energy states may be induced near the impurity site for a bulk impurity, while the visible resonant impurity state can only exist for certain impurity strength. For a surface impurity, a robust resonant impurity state exists in a wide parametric range of impurity strength. Such robust resonant state stems from the topologically protected weak dispersive surface states, which can be probed by scanning tunneling microscopy, providing a clear signature of the topological surface states in the nodal-line semimetals.

In the context of a systematic study of oxide materials with the muon spin spectroscopy ($\mu \mathrm{SR}$) technique, we report here on an investigation of paramagnetic ${\mathrm{Nd}}_2{\mathrm{O}}_3$. The question was whether the magnetism of ${\mathrm{Nd}}^{3+}$ has an influence on the observed signals. In ${\mathrm{Nd}}_2{\mathrm{O}}_3$, as in the other oxides, a weakly paramagnetic component is observed besides the pure diamagnetic fraction. The paramagnetic part is assigned to a transient state formed between the initial atomic and the final bound muonium configuration. In addition, a fast relaxing signal ($\lambda \sim 7\mu {\mathrm{s}}^{-1}$) with 10% to 20% fraction is seen in longitudinal field. Contrary to this general behavior of the oxide materials, in the present magnetic compound, a resonancelike structure is seen in the temperature range around 40 K. We assign it tentatively to a dynamical process related to the population of the first excited Kramers doublet of the ${\mathrm{Nd}}^{3+}$ ion at 2.6 meV.

End-to-end correlated bound states are investigated in superconductor-semiconductor hybrid nanowires at zero magnetic field. Peaks in subgap conductance are independently identified from each wire end, and a cross-correlation function is computed that counts end-to-end coincidences, averaging over thousands of subgap features. Strong correlations in a short, 300-nm device are reduced by a factor of 4 in a long, 900-nm device. In addition, subgap conductance distributions are investigated, and correlations between the left and right distributions are identified based on their mutual information.

The electrodynamics of Weyl semimetals is an extension of Maxwell's theory where in addition to field strength tensor $F_{\mu \nu }$, an axion field enters the theory which is parametrized by a four-vector $b^{\mu }=(b_0,\mathit{b})$. In tilted Weyl materials (TWMs) an additional set of parameters $\mathit{\zeta }=({\zeta }_x,{\zeta }_y,{\zeta }_z)$ enters the theory that can be encoded into the metric of the spacetime felt by electrons in TWMs. This allows an extension of Maxwell's electrodynamics that describes electric and magnetic fields in TWMs, and tilted Dirac materials (TDMs) that correspond to $b^{\mu }=0$. The tilt parameter $\mathit{\zeta }$ appearing as an off-diagonal metric matrix element, mixing time and space components, which mingles $\mathit{E}$ and $\mathit{B}$ fields, whereby the inhomogeneous Maxwell's equations are modified. As an example of the application of the electrodynamics of TWMs, we study the surface plasmon polariton (SPP) in these systems. The peculiarity of the SPP on the surface of TWMs or TDMs is that it is not merely the propagation of electromagnetic modes on the surface of a conductor. It also describes the propagation of electromagnetic waves at the interface of two different spacetime geometries. In the case of TDMs, we find a characteristic dependence of the SPP spectrum on the tilt parameter $\zeta$ which can be used to map $\zeta$ from SPP measurements. In the case of TWMs, depending on whether the interface with vacuum supports a Fermi arc or not, and whether the propagation direction is along the Fermi arc or transverse to it, we find many unusual spectral features for SPP modes. These include (1) surface plasmons with much higher frequency than bulk plasmon frequency, (2) soft SPP modes at short length scales, (3) a tilt-controlled SPP window beyond which SPP modes are unstable, (4) a kink in the SPP dispersion, (5) uniform group velocity near the “horizon” ($\zeta =1$), and (6) possible negative group velocity. Our detailed study of the dependence of SPP spectra on the arrangements of three vectors $(\mathit{b},\mathit{q},\mathit{\zeta })$, the first two of which are at our control, can be utilized to map the tilt characteristics and Fermi arc characteristics from SPP measurements.

We present a micrometer-scale-resolution stress imaging method with millimeter field-of-view for diamonds containing a thin surface layer of nitrogen-vacancy (NV) centers. In this method, we reconstruct stress tensor elements over a two-dimensional field of view from NV optically detected magnetic resonance (ODMR) spectra. We use this technique to study how stress inhomogeneity affects NV magnetometry performance and show how NV $M_{z,\kappa }$ imaging is a useful and direct way to assess these effects. This tool for mapping stress in diamond will aid optimization of NV-diamond sensing, with wide-ranging applications in the physical and life sciences.

The concept of quantum memory plays an incisive role in the quantum information theory. As confirmed by the several recent rigorous mathematical studies, the quantum memory inmate in the bipartite system ${\rho }_{AB}$ can reduce the uncertainty about part $B$, after measurements done on part $A$. In the present work, we extend this concept to systems with a spin-orbit coupling and introduce the notion of spin-orbit quantum memory. We self-consistently explore the Uhlmann fidelity, the pre- and the post-measurement entanglement entropy, and the post-measurement conditional quantum entropy of the system with spin-orbit coupling and show that measurement performed on the spin subsystem decreases the uncertainty of the orbital part. The uncovered effect enhances with the strength of the spin-orbit coupling. We study the concept of macroscopic realism introduced by Leggett and Garg [Phys. Rev. Lett. 54, 857 (1985)] and observe that POVM measurements done on the system under the particular protocol are noninvasive. For the extended system, we perform quantum Monte Carlo calculations and consider the reshuffling of the electron densities due to an external electric field.

We show that ac-driven skyrmion lattices in a weak pinning channel confined by regions of strong pinning exhibit edge transport carried by skipping orbits while skyrmions in the bulk of the channel undergo localized orbits with no net transport. The magnitude of the edge currents can be controlled by varying the amplitude and frequency of the ac drive or by changing the ratio of the Magnus force to the damping term. We identify a localized phase in which the orbits are small and edge transport is absent, an edge transport regime, and a fluctuating regime that appears when the ac drive is strong enough to dynamically disorder the skyrmion lattice. We also find that in some cases, multiple rows of skyrmions participate in the transport due to a drag effect from the skyrmion-skyrmion interactions. The edge currents are robust for finite disorder and should be a general feature of skyrmions interacting with confined geometries or inhomogeneous disorder under an ac drive. We show that similar effects can occur for skyrmion lattices at interfaces or along domain boundaries for multiple coexisting skyrmion species. The edge current effect provides a new method to control skyrmion motion, and we discuss the connection of these results with recent studies on the emergence of edge currents in chiral active matter systems and gyroscopic metamaterials.

We performed first-principles calculations of the spin-orbit effect appearing in the electronic structure of the well-known ferroelectric perovskite oxide ${\mathrm{PbTiO}}_3$ and analyzed the results within group-theory-derived models. We evidenced some non-negligible linear Rasbha spin splittings of the unoccupied $p$ bands of Pb atoms and occupied $p$ bands of oxygen atoms. Our calculations also show that a cubic spin splitting is present for the unoccupied $d_{xy}$ bands of Ti atoms. All these spin-orbit effects lead to complex spin textures reversible by switching the electric polarization, which could be used for future spinorbitronic applications. Our results also demonstrate how applying epitaxial strain could be envisaged to tune these properties by changing the relative energy of some of the bands or the magnitude of the linear/cubic coefficients describing the spin splitting.

Landau levels (LL) have been predicted to emerge in systems with Dirac nodal points under applied nonuniform strain. We consider two-dimensional (2D), $d_{xy}$ singlet (2D-S), and three-dimensional (3D) $p_x\pm i{p}_y$ equal-spin triplet (3D-T) superconductors. We demonstrate the topological bound state and spinful Majorana nature of the bulk gapless zeroth LLs in the singlet and triplet cases, respectively. Strain along certain directions can induce two topologically distinct phases in the bulk, with zeroth LLs localized at the interface. These modes are unstable toward ferromagnetism for 2D-S cases. True Majorana fermions in 3D-T allow for more exotic possibilities.

Abrikosov vortices contain magnetic fields and circulating currents that decay at a short range $\lambda \sim 100$ nm. However, vortices can induce Josephson phase shifts at a long range $r\sim \mu \mathrm{m}\gg \lambda$. Mechanisms of this puzzling phenomenon are not clearly understood. Here we present a systematic study of vortex-induced phase shifts in planar Josephson junctions. We make two key observations: (i) The cutoff effect: Although vortex-induced phase shift is a long-range phenomenon, it is terminated by the junction and does not persist beyond it. (ii) A linear to superlinear crossover with a rapid upturn of the phase shift occurs upon approaching a vortex to a junction. The crossover occurs at a vortex-junction distance comparable to the penetration depth. Together with theoretical and numerical analysis this allows unambiguous identification of two distinct and independent mechanisms. The short range $r\lesssim \lambda$ mechanism is due to circulating vortex currents inside a superconducting electrode without involvement of magnetic fields. The long range $r\gg \lambda$ mechanism is due to stray magnetic fields outside electrodes without circulating vortex currents. We argue that understanding of controlling parameters of vortex-induced Josephson phase shift can be used for development of novel compact cryoelectronic devices.

We employed small-angle neutron scattering to demonstrate that the magnetic skyrmion lattice can be realized in bulk chiral magnets as a thermodynamically stable state at temperatures much lower than the ordering temperature of the material. This is in the regime where temperature fluctuations become completely irrelevant to the formation of the topologically nontrivial magnetic texture. In this attempt we focused on the model helimagnet MnSi, in which the skyrmion lattice was previously well characterized and shown to exist only in a very narrow phase pocket close to the Curie temperature of 29.5 K. We revealed that large uniaxial distortions caused by the crystal-lattice strain in MnSi result in stabilization of the skyrmion lattice in magnetic fields applied perpendicular to the uniaxial strain at temperatures as low as 5 K. To study the bulk chiral magnet subjected to a large uniaxial stress, we have utilized micrometer-sized single-crystalline inclusions of MnSi naturally found inside single crystals of the nonmagnetic material ${\mathrm{Mn}}_{11}{\mathrm{Si}}_{19}$. The reciprocal-space imaging allowed us to unambiguously identify the stabilization of the skyrmion state over the competing conical spin spiral.

We formulate a general framework for addressing both odd- and even-frequency superconductivity in Dirac semimetals and demonstrate that the odd-frequency or the Berezinskii pairing can naturally appear in these materials because of the chirality degree of freedom. We show that repulsive frequency-dependent interactions favor the Berezinskii pairing while an attractive electron-electron interaction allows for the BCS pairing. In the case of Dirac and Weyl semimetals at the charge neutrality point, both the conventional BCS and odd-frequency Berezinskii pairings require critical coupling. Since these pairings could originate from physically different mechanisms, our findings pave the way for controlling the realization of the Berezinskii superconductivity in topological semimetals. We also present the density of states with several cusplike features that can serve as an experimentally verifiable signature of the odd-frequency gap.

Domain boundaries and other twin boundaries in crystalline materials are receiving increasing interest. They can carry unique functional properties, which in many cases are absent in the surrounding bulk material. One such property of domain boundaries can be their electric polarity. Phenomenological insight in the polarity of domain boundaries was so far based either on the knowledge of the order parameter and the form of Landau-Ginzburg free energy functional, or on the knowledge of the symmetry of the domain boundaries. In the present work we show on the concrete examples of potassium thiocyanate (KSCN) and lacunar spinel crystals that the concept of the primary order parameter can help to find the layer group describing the maximal possible symmetry of a given domain boundary. A combination of layer group and order parameter symmetries is then employed to clarify the nature of the polarity of domain boundaries.

We investigated the effect of disorder in a gapped crystalline system by introducing a class of local quantities for an energy band, which is referred to as a band correlation function (BCF) and is the sum of correlation functions for all eigenstates of the band. We demonstrated that the BCFs are robust in the presence of a disorder if the band gap does not collapse. The eigenstate set of an energy band can be almost completely mapped onto the perturbed eigenstate set, referred to as quasiclosed mapping, when it is sufficiently isolated from other bands. A randomly perturbed system may exhibit some features of translational symmetry, which we demonstrated by numerically simulating the pumping process for a one-dimensional Rice-Mele model with disorders in hopping strength and on-site potential. The quantized pumping charge was robust against dynamic disorder. In addition, we investigated a dimerized one-dimensional Kitaev model to illustrate the universality of our finding. The result indicates the possibility of measuring the topological invariant of an experimental system with imperfections.

A very strong magnetoelastic effect in the ${\mathrm{CeCo}}_{1-x}{\mathrm{Fe}}_x\mathrm{Si}$ alloys is reported. The strength of the magnetostrictive effect can be tuned upon changing $x$. The moderate low-temperature linear magnetostriction observed at low Fe concentrations becomes very large $(\frac{\mathrm{\Delta }L}{L}\left(16T,2K\right)=3\times {10}^{-3})$ around the critical concentration $x_c\approx 0.23$ at which the long-range antiferromagnetic order vanishes. Upon increasing doping through the nonmagnetic region $(x>x_c)$, the magnetostriction strength gradually weakens again. The interplay between magnetic order and the Kondo screening appears to cause an enhanced valence susceptibility slightly changing the Ce ions valence, ultimately triggering the large magnetostriction observed around the critical concentration. Previous studies of the evolution of the lattice parameters with $x$ as well as magnetization and x-ray absorption spectroscopy measurements support this hypothesis.

The effects of spin-orbit coupling and symmetry breaking at the interface between a ferromagnet and heavy metal are particularly important for spin-based information storage and computation. Recent discoveries suggest they can create novel chiral spin structures (e.g., skyrmions), which have often been identified through the appearance of the bump/dip features of Hall signals, the so-called topological Hall effect (THE). In this work, however, we present an unusual anomalous Hall effect (UAHE) in $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers and demonstrate that the features extremely similar to THE can be generated without involving any chiral spin structures. Low-temperature magnetic force microscopy has been used to explore the magnetic field-dependent behavior of spin structures, and the UAHE as a function of magnetic field does not peak near the maximal density of magnetic bubbles. Our results unambiguously evidence that the UAHE in $\mathrm{M}{\mathrm{n}}_x\mathrm{Ga}/\mathrm{Pt}$ bilayers shows no correlation with chiral spin structures but is driven by the modified interfacial properties, indicating a wealth of underlying and interesting physics.

When spin ice systems undergo a sudden thermal quench, they have been shown to enter long-lived metastable states where the monopole excitations form so-called noncontractible pairs [Phys. Rev. Lett. 104, 107201 (2010)]. While the nature of these states is well understood, the dynamical mechanisms underpinning their formation remain largely unexplored and are the subject of this study. We find that the long-range tail of the Coulomb interactions between monopoles plays a central role by suppressing the monopole-assisted decay of noncontractible pairs with respect to monopole–antimonopole annihilation. In conjunction with low final quench temperatures, where the system enters a nonhydrodynamic regime in which the monopoles effectively move at terminal velocity in the direction of the local force acting on them, the interactions lead to a metastable plateau that persists in the thermodynamic limit. This is a remarkable phenomenon, reminiscent of jamming and some instances of glassiness: A transient modification of the short-time dynamics of the system allows it to enter a metastable state whose lifetime can easily be astronomically large at (experimentally relevant) low temperatures. We demonstrate this using Monte Carlo simulations and mean field population dynamics theory, and we provide an analytical understanding of the mechanisms at play. When the interactions between monopoles are truncated to finite range, the metastable plateau reduces to a finite size effect. We derive the finite size scaling behavior of the density of noncontractible pairs in the metastable plateau for both short- and long-range interactions and discuss the experimental implications of our results.

Motivated by recent experiments of exceptional accuracy, we study numerically the spin-glass dynamics in a film geometry. We cover all the relevant time regimes, from picoseconds to equilibrium, at temperatures at and below the 3D critical point. The dimensional crossover from 3D to 2D dynamics, which starts when the correlation length becomes comparable to the film thickness, consists of four dynamical regimes. Our analysis, based on a renormalization group transformation, finds consistent the overall physical picture employed by Orbach and co-workers in the interpretation of their experiments.