We present an extensive powder and single-crystal neutron scattering investigation of the crystallographic structure and magnetic order of the frustrated metallic $f$-electron magnet $\mathrm{U}{\mathrm{Ni}}_4\mathrm{B}$. We carry out a full refinement of the crystallographic structure and conclude that the low-temperature lattice symmetry is orthorhombic [space group Pmm2; cell parameters: $a=6.963\left(4\right)$, $b=14.793\left(9\right)$, $c=17.126\left(8\right)$ Å]. We determine the magnetically ordered structure, concluding that below $T_{\mathrm{N}}=19.5\mathrm{K}$, the material undergoes a transition into a partially ordered antiferromagnetic state. The magnetic structure is consistent with the existence of toroidal order in this material. We further test the proposal of a second magnetic transition occurring at $330\mathrm{mK}$, concluding that the thermodynamic anomalies observed at these temperatures do not reflect modifications of the magnetic structure. Our study provides a consistent picture of the interrelationship of structural and magnetic properties in the frustrated magnet $\mathrm{U}{\mathrm{Ni}}_4\mathrm{B}$ previously unresolved.

The angular momentum of an electron is characterized well by pseudospin with $J=\frac{3}{2}$ in the presence of strong spin-orbit interactions. We study theoretically the Josephson effect of superconductors in which two such $J=\frac{3}{2}$ electrons form a Cooper pair. Within even-parity symmetry class, pseudospin-quintet pairing states with $J=2$ can exist as well as pseudospin-singlet states with $J=0$. We focus especially on the Josephson selection rule among these even-parity superconductors. We find that the selection rule between quintet states is more severe than that between spin-triplet states formed by two $S=\frac{1}{2}$ electrons. The effects of a pseudospin-active interface on the selection rule are discussed as well as those of odd-frequency Cooper pairs generated by pseudospin-dependent band structures.

Parafermions are exotic quasiparticles with non-Abelian fractional statistics that could be exploited to realize universal topological quantum computing. Here, we study the scrambling of quantum information in one-dimensional parafermionic chains, with a focus on ${\mathbb{Z}}_3$ parafermions in particular. We use the generalized out-of-time-ordered correlators (OTOCs) as a measure of the information scrambling and introduce an efficient method based on matrix product operators to compute them. With this method, we compute the OTOCs for ${\mathbb{Z}}_3$ parafermions chains up to 200 sites for the entire early growth region. We find that, in stark contrast to the dynamics of conventional fermions or bosons, the information scrambling light cones for parafermions can be both symmetric and asymmetric, even for inversion-invariant Hamiltonians involving only hopping terms. In addition, we find a deformed light cone structure with a sharp peak at the boundary of the parafermion chains in the topological regime, which gives unambiguous evidence of the strong zero modes at infinite temperature.

The phenomenon of super scattering was previously found to arise in massless pseudospin-1 two-dimensional Dirac material systems with a flat band. Here we report the phenomenon of super skew scattering of massive pseudospin-1 quasiparticles, which does not arise in the corresponding massless system. In particular, the scatterer is electrically generated with a certain geometric shape, and the mass is induced by gap opening between the Dirac cones. Even for a circular scatterer, the occurrence of resonant states inside it can induce a sizable anomalous Hall current, which is associated with the gap opening. The striking finding is that a significant reduction in the scatterer size and/or the potential height does nothing to weaken the skew scattering and, for certain resonant states, even tends to strengthen the scattering. This phenomenon of super skew scattering in Dirac materials with a flat band is in stark contrast to the scattering of massive pseudospin-$\frac{1}{2}$ quasiparticles from the same configuration, where skew scattering is significantly weaker and a reduction in the scatterer strength can quickly diminish it. The phenomenon is established analytically for the case of a circular scatterer in the framework of continuum Hamiltonian, and is found to be robust for an elliptical scatterer, which is solved numerically by adopting the multiple-multipole method to massive pseudospin-1 scattering. Calculations of the electronic transport properties in the Lieb lattice system reveal the occurrence of a large anomalous Hall current as well, paving the way for experimental observation and test of super skew scattering. Because of the “skew” nature that is absent in massless pseudospin-1 systems, the phenomenon of super skew scattering in massive systems can be exploited for applications in novel electronic or photonic Hall devices.

We demonstrate that the nuclear spin fluctuations lead to the electric current noise in mesoscopic samples of organic semiconductors, which show the pronounced magnetoresistance in weak magnetic fields. For the bipolaron and electron-hole mechanisms of organic magnetoresistance, the current noise spectrum consists of the high-frequency peak related to the nuclear spin precession in the Knight field of the charge carriers and the low-frequency peak related to the nuclear spin relaxation. The shape of the spectrum depends on the external magnetic and radio frequency fields, which allows one to prove the role of the nuclei in the magnetoresistance experimentally.

We study the thermal effects on the frictional properties of atomically thin sheets. We simulate a simple model based on the Prandtl-Tomlinson model that reproduces the layer dependence of friction and strengthening effects seen in atomic force microscope experiments. We investigate sliding at a constant speed as well as reversing direction. We also investigate contact aging: the changes that occur to the contact when the sliding stops completely. We compare the numerical results to analytical calculations based on Kramers rates. We find that there is a slower than exponential contact aging that weakens the contact and that we expect will be observable in experiments. We discuss the implications for sliding as well as aging experiments.

We comparatively study the possible topological charge pump (TCP) in the Dice lattice by considering two types of time-dependent mass terms of the Dirac electrons, the $S_z$ and $U$ term, which are taken as the pumping potentials. It is shown that the former $S_z$ pumping potential preserving the electron-hole symmetry of the system leads to a vanishing charge pump, whereas the later $U$ mass term breaking the electron-hole symmetry can pump out an integer number of charges in a pumping cycle, even the Berry phases of Dirac electrons in both cases are the same vanishing, $\gamma =0$. The topological interface state (TIS), due to the flat bands bridging the two pumping regions not from the Dirac-cone bands, can account for the TCP found in the $U$-mass pumping case and oppositely, there is no TIS due to the flat bands in the $S_z$ pumping case. Our findings imply that the flat band in the Dice lattice may contribute to the electron transport under appropriate settings.

Topological Dirac semimetals are a class of semimetals that host symmetry-protected Dirac points near the Fermi level, which arise due to a band inversion of the conduction and valence bands. In this work, we study the less explored class of noncentrosymmetric topological Dirac semimetals in three dimensions. We identify the noncentrosymmetric crystallographic point groups required to stabilize fourfold degenerate band crossings and derive model Hamiltonians for all distinct types of band inversions allowed by symmetry. Using these model Hamiltonians, which emphasize the physical nature of the allowed couplings, we establish the generic electronic phase diagram noncentrosymmetric Dirac semimetals and show that it generically includes phases with coexistent Weyl point nodes or Weyl line nodes. In particular, for one specific type of band inversion in sixfold symmetric systems we show that Weyl line nodes are always present. Based on first-principles calculations, we predict that ${\mathrm{BiPd}}_2{\mathrm{O}}_4$ is a noncentrosymmetric Dirac semimetal under 20 Gpa pressure and hosts topological type II Dirac points on the fourfold rotation axis. Furthermore, we propose that the hexagonal polar alloy ${\mathrm{LiZnSb}}_x{\mathrm{Bi}}_{1-x}$ realizes a Dirac semimetal with coexistent Weyl points. Interestingly, the emergence and location of the Weyl points is highly tunable and can be controlled by the alloy concentration $x$. More generally, our results not only establish band-inverted noncentrosymmetric systems as a broad and versatile class of topological semimetals, but also provide a framework for studying the quantum nonlinear Hall effect and nonlinear optical properties in the Dirac semimetals.

We report experimental and theoretical evidence of strong electron-plasmon interaction in $n$-doped single-layer ${\mathrm{MoS}}_2$. Angle-resolved photoemission spectroscopy measurements reveal the emergence of distinctive signatures of polaronic coupling in the electron spectral function. Calculations based on many-body perturbation theory illustrate that electronic coupling to two-dimensional carrier plasmons provides an exhaustive explanation of the experimental spectral features and their energies. These results constitute compelling evidence of the formation of plasmon-induced polaronic quasiparticles, suggesting that highly doped transition-metal dichalcogenides may provide a new platform to explore strong-coupling phenomena between electrons and plasmons in two dimensions.

Recently, defect production was investigated during nonunitary dynamics due to non-Hermitian Hamiltonian. By ramping up the non-Hermitian coupling linearly in time through an exceptional point, defects are produced in much the same way as approaching a Hermitian critical point. A generalized Kibble-Zurek scaling accounted for the ensuing scaling of the defect density in terms of the speed of the drive and the corresponding critical exponents. Here we extend this setting by adding the recycling term and considering the full Lindbladian time evolution of the problem with quantum jumps. We find that by linearly ramping up the environmental coupling in time, and going beyond the steady-state solution of the Liouvillian, the defect density scales linearly with the speed of the drive for all cases. This scaling is unaffected by the presence of exceptional points of the Liouvillian, which can show up in the transient states. By using a variant of the adiabatic perturbation theory, the scaling of the defect density is determined exactly from a set of algebraic equations. Our study indicates the distinct sensitivity of the Lindbladian time evolution to exceptional points corresponding to steady states and transient states.

Nodal ring semimetals are a class of topological material characterized by a one-dimensional circular region of band crossing in momentum space. The presence of spin-orbit coupling, whether extrinsic or intrinsic, may change the parent nodal ring phase to a Weyl semimetal, Dirac semimetal, or topological insulator child phase. We investigate second harmonic generation and circular photogalvanic effect in the mid-infrared region of nodal ring materials where spin-orbit coupling produces a Weyl semimetal child phase (such as in $\mathrm{Zr}{\mathrm{Te}}_5$ and $\mathrm{Ca}{\mathrm{P}}_3$). Spin-orbit coupling breaks the symmetries protecting the nodal ring, inducing a nontrivial Berry curvature which gives rise to colossal photocurrents up to the order of ${10}^3 \mu \mathrm{A}/{\text{V}}^2$ at the interband harmonic. Our results are found to be rather robust to parameters such as Fermi level, residual scattering rate, and the number of Weyl points. However, decreasing temperature tends to destroy the harmonic peaks and changing the nodal ring radius drastically alters the harmonic condition, shifting the peak frequency. Equivalent calculations and experiments have been carried out for intrinsic Weyl semimetals such as TaAs where the photocurrents calculated and observed were at least one order of magnitude smaller, highlighting that the parent nodal ring phase enhances these optical nonlinear phenomena.

Nonlinear terahertz conductivity spectra of charges confined in semiconductor nanostructures were calculated using a semiclassical Monte Carlo method. The confinement-induced nonlinear response per charge carrier is much stronger than the intrinsic nonlinearity of common bulk semiconductors and more than 20 times stronger than in graphene, which has been considered as a material with one of the highest terahertz nonlinearities. Moderate intensities of the terahertz radiation are thus sufficient to achieve efficient frequency mixing or high-harmonics generation. Enclosing the nanostructures into metallic nanoslits concentrates the electric field into the semiconductor and thus easily provides nonlinear terahertz signal strength comparable to the linear one.

Electron-phonon interactions are fundamentally important physical processes responsible for many key discoveries in condensed matter physics and material sciences. Herein, by exploiting the scattering-type scanning near-field optical microscope (s-SNOM) excited with a femtosecond infrared (IR) laser, we explored the strong coupling between IR phonons in few-layer graphene (FLG) with ultrahot electrons, which are heated up by the intense laser field enhanced by the s-SNOM tip. More specifically, we found that the intensity of the phonon resonance can be tuned systematically by varying the laser power that controls the electron temperature. Furthermore, the high spatial resolution of s-SNOM allows us to map the local phonon characteristics at sharp boundaries and nanostructures. Our findings offer insights into the intriguing physics behind the electron-phonon interactions in nonequilibrium conditions and open a pathway for manipulating phonons with optical means.

Domain wall motion and lattice strain dynamics of ferroelectrics at resonance were simultaneously measured by combining high-power burst excitation and in situ high-energy x-ray diffraction. The increased loss at high vibration velocity was directly related to the increased domain wall motion, driven by dynamic mechanical stress. A general relationship between the microstructural strain contributions and macroscopic electromechanical behavior was established, allowing the prediction of high-power stability of ferroelectric materials. The results indicate that the materials' stability during high-power drive is predominantly related to the basic chemical composition, while the piezoelectric hardening mechanisms mainly influence the small-signal behavior.

Many-body localization (MBL) behavior is analyzed in an extended Bose-Hubbard model with quasiperiodic infinite-range interactions. No additional disorder is assumed. The analysis of the level statistics and of the eigenstates' entanglement entropy shows that a significant fraction of eigenstates becomes localized as the strength of the global interactions is increased. This behavior scales differently depending on the choice of the thermodynamic limit. The system is asymptotically ergodic by scaling the interaction strength so as to keep the energy extensive. On the other hand, the MBL regime seems to be stable if one allows for a superextensive scaling of the energy. We argue that our findings can be experimentally verified in many-body cavity quantum electrodynamics setups by means of the so-called quench spectroscopy. The latter can allow one, amongst others, to reveal the presence of a mobility edge.

Using the spin Seebeck effect (SSE), we study the propagation distance of thermally induced spin currents inside a magnetic insulator thin film in the short-range regime. We disambiguate spin currents driven by temperature and chemical potential gradients by comparing the SSE signal before and after adding a heat-sinking capping layer on the same device. We report that the measured spin decay behavior near the heat source is well accounted for by a diffusion model where the magnon diffusion length is in submicron range, in other words, two orders of magnitude smaller than previous estimates inferred from the long-range behavior. Our results highlight the caveat in applying a diffusive theory to describe thermally generated magnon transport, where a single decay length may not capture the behavior on all length scales.

Topological superconductivity is an exotic phase of matter in which the fully gapped superconducting bulk hosts gapless Majorana surface states protected by topology. Intercalation of copper, strontium, or niobium between the quintuple layers of the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ increases the carrier density and leads to superconductivity that is suggested to be topological. Here we study the electronic structure of strontium-intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ using angle-resolved photoemission spectroscopy and Shubnikov–de Haas oscillations. Despite the apparent low Hall number of $\sim 2\times {10}^{19}{\mathrm{cm}}^{-3}$, we show that the Fermi surface has the shape of an open cylinder with a larger carrier density of $\sim {10}^{20}{\mathrm{cm}}^{-3}$. We suggest that superconductivity in intercalated ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ emerges with the appearance of a quasi-two-dimensional open Fermi surface.

A comprehensive computational analysis of the stable accommodations of an atom in the Lennard-Jones (LJ) face-centered-cubic (fcc) and hexagonal-closest-packed (hcp) lattices is presented. For a wide range of the guest-host LJ interaction parameters and the number of host atoms removed, $N$, global optimizations are performed to find the minimum energy structures. Stable accommodations are determined using the convex hull of the energy dependence on $N$. The most stable ground accommodations are mapped onto the guest-host LJ parameter space for the hosts representing the rare-gas solids from Ne to Xe. The radial distribution function analysis is used to classify the structures found as originating from the guest placements in the lattice voids or at the node. Ten generic accommodations are identified for each lattice. Comparing to mostly polyhedral structures in fcc, the hcp ones have generally lower symmetry, which should affect the band shapes of atomic absorption spectra. Despite the well-known wrong prediction of higher hcp phase stability in the LJ model, the cases of strong accommodation preferences in one phase or another are identified. Being generic, the obtained results can be useful for a qualitative interpretation of atomic matrix isolation experiments.