We report a comprehensive study of lattice dynamics of the Dirac node-line semimetal ZrSiS single crystal by Raman spectroscopy and first-principles calculations. The weak covalent bonding between ZrSiS layers is confirmed by the absence of low-frequency shear or breathing Raman modes down to $15\mathrm{c}{\mathrm{m}}^{-1}$. All six Raman-active optical phonons are identified at 300 K, whose energies and symmetries match our phonon-dispersion calculations and polarized Raman measurements. The thermodynamic stability is verified from 77 to 300 K, and, with increasing temperature, sizable softening of Raman modes is observed with broadened profiles. The first-order temperature coefficients are found to be linearly dependent on temperature. Furthermore, using multiple excitation laser wavelengths of 488, 514.5, 568, 647, and 785 nm, we find that three out-of-plane Raman modes are all nondispersive, and their normalized intensity resonances at different laser energies can be attributed to the different interband transitions. Our work provides detailed information of ZrSiS lattice vibrations, as well as the coupling between ZrSiS lattice vibrations and its electronic states.

We analyze and compare the structural, dynamical, and electronic properties of liquid water next to prototypical metals including Pt, graphite, and graphene. Our results are built on Born-Oppenheimer molecular dynamics (BOMD) generated using density functional theory (DFT) which explicitly include van der Waals (vdW) interactions within a first principles approach. All calculations reported use large simulation cells, allowing for an accurate treatment of the water-electrode interfaces. We have included vdW interactions through the use of the optB86b-vdW exchange correlation functional. Comparisons with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional are also shown. We find an initial peak, due to chemisorption, in the density profile of the liquid water-Pt interface not seen in the liquid water-graphite interface, liquid water-graphene interface, nor interfaces studied previously. To further investigate this chemisorption peak, we also report differences in the electronic structure of single water molecules on both Pt and graphite surfaces. We find that a covalent bond forms between the single water molecule and the Pt surface but not between the single water molecule and the graphite surface. We also discuss the effects that defects and dopants in the graphite and graphene surfaces have on the structure and dynamics of liquid water.

We report on spin-orbit torque measurements in ferrimagnetic ${\mathrm{Gd}}_{21}{\left({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}\right)}_{79}$ films with bulk perpendicular magnetic anisotropy and thicknesses up to $30\mathrm{nm}$. The dampinglike and fieldlike torques have been quantified using second harmonic Hall voltage detection. Both torques show an inverse linear dependence on the thickness that is indicative of the interfacial nature of the torques. The spin-Hall angle remains constant over the thickness range of 10 to $30\mathrm{nm}$ ${\mathrm{Gd}}_{21}{\left({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}\right)}_{79}$. Remarkably, we find that this interfacial torque is able to switch a $30\text{−}\mathrm{nm}$-thick ${\mathrm{Gd}}_{21}{\left({\mathrm{Fe}}_{90}{\mathrm{Co}}_{10}\right)}_{79}$ film with a reasonably large thermal stability of $\approx 100k_{B}T$.

We study the effect of critical pairing fluctuations on the electronic properties in the normal state of a clean superconductor in three dimensions. Using a functional renormalization group approach to take the non-Gaussian nature of critical fluctuations into account, we show microscopically that in the BCS regime, where the inverse coherence length is much smaller than the Fermi wave vector, critical pairing fluctuations give rise to a nonanalytic contribution to the quasiparticle damping of order $T_c\sqrt{\text{Gi}}ln(80/\text{Gi})$, where the Ginzburg-Levanyuk number $\text{Gi}$ is a dimensionless measure for the width of the critical region. As a consequence, there is a temperature window above $T_c$ where the quasiparticle damping due to critical pairing fluctuations can be larger than the usual $T^2$ Fermi liquid damping due to noncritical scattering processes. On the other hand, in the strong coupling regime where $\text{Gi}$ is of order unity, we find within the Gaussian approximation that the quasiparticle damping due to critical pairing fluctuations is proportional to the temperature. Moreover, we show that in the vicinity of the critical temperature $T_c$ the electronic density of states exhibits a fluctuation-induced pseudogap. We also use functional renormalization group methods to derive and classify various types of higher-order scattering processes induced by the pairing interaction in Fermi systems close to the superconducting instability.

We study the dynamics of two initially entangled qubits, where each one interacts locally and independently of the other, with a plasmonic nanostructure. By considering two different cases for the qubits, two identical two-level systems and two identical V-type quantum systems, where one two-level transition plays the role of the qubit while the third level acts as an “umbrella level”, we study the corresponding entanglement dynamics for several pure and mixed initial states. As the plasmonic nanostructure we take a two-dimensional lattice of metal-coated dielectric nanoparticles. The presence of this nanostructure leads to highly suppressed spontaneous emission rates of the individual quantum systems, as well as to highly anisotropic spontaneous decay rates for orthogonal dipole matrix elements due to the anisotropic Purcell effect, leading to quantum interference in spontaneous emission. Both of the effects can be used for significantly prolonging the time evolution of entanglement for several system parameters.

We investigate low-temperature transport through single InAs quantum wells and broken-gap InAs/GaSb double quantum wells. Nonlocal measurements in the regime beyond bulk pinch-off confirm the presence of edge conduction in InAs quantum wells. The edge resistivity of $1–2\mathrm{k}\mathrm{\Omega }/\mu \mathrm{m}$ is of the same order of magnitude as edge resistivities measured in the InAs/GaSb double quantum well system. Measurements in a tilted magnetic field suggests an anisotropy of the conducting regions at the edges with a larger extent in the plane of the sample than normal to it. Finger-gate samples on both material systems shine light on the length dependence of the edge resistance with the intent to unravel the nature of edge conduction in InAs/GaSb coupled quantum wells.

We demonstrate the existence of the excited state of an exciton-polariton in a semiconductor microcavity. The strong coupling of the quantum well heavy-hole exciton in an excited $2s$ state to the cavity photon is observed in nonzero magnetic field due to surprisingly fast increase of Rabi energy of the $2s$ exciton-polariton in magnetic field. This effect is explained by a strong modification of the wave function of the relative electron-hole motion for the $2s$ exciton state.

Multi-Weyl semimetals are a kind of topological phase of matter with discrete Weyl nodes characterized by multiple monopole charges, in which the chiral anomaly, the anomalous nonconservation of an axial current, occurs in the presence of electric and magnetic fields. Electronic transport properties related to the chiral anomaly in the presence of both electromagnetic fields and axial electromagnetic fields in multi-Weyl semimetals are systematically studied. It has been found that the anomalous Hall conductivity has a modification linear in the axial vector potential from inhomogeneous strains. The axial electric field leads to an axial Hall current that is proportional to the distance of Weyl nodes in momentum space. This axial current may generate chirality accumulation of Weyl fermions through delicately engineering the axial electromagnetic fields even in the absence of external electromagnetic fields. Therefore this work provides a nonmagnetic mechanism of generation of chirality accumulation in Weyl semimetals and might shed new light on the application of Weyl semimetals in the emerging field of valleytronics.

We propose the creation of a two-dimensional topological semimetal in a semiconductor artificial lattice with triangular symmetry. An in-plane magnetic field drives a quantum phase transition between the topological insulating and topological semimetal phases. The topological semimetal is characterized by robust band-touching points which carry quantized Berry flux and edge states which terminate at the band-touching points. The topological phase transition is predicted to occur at magnetic fields $\sim 4\text{T}$ in high mobility GaAs artificial lattices, and can be detected via the anomalous behavior of the edge conductance.

We demonstrate the fabrication of strain-free and widely adjustable GaAs quantum-dot molecules (QDMs) by filling of droplet etched nanoholes in AlGaAs. Gate-voltage dependent optical spectra of highly asymmetric QDMs exhibit anticrossings that clearly indicate strong coupling with delocalized molecule states. Furthermore, indirect excitons are observed that are related to recombinations of excited-state electrons and ground-state holes both located in different dots. Simple numerical simulations reproduce the electric-field dependent energy shifts of direct and indirect transitions and predict their radiative lifetimes. The visibility of excited-state indirect excitons even for strong off-resonant energy detuning indicates the presence of a phonon bottleneck which suppresses the relaxation of excited electrons into lower levels.

The critical properties of the single-crystalline semiconducting ferromagnet ${\mathrm{Cr}}_2{\mathrm{Ge}}_2{\mathrm{Te}}_6$ were investigated by bulk dc magnetization around the paramagnetic to ferromagnetic phase transition. Critical exponents $\beta =0.200\pm 0.003$ with a critical temperature $T_c=62.65\pm 0.07$ K and $\gamma =1.28\pm 0.03$ with $T_c=62.75\pm 0.06$ K are obtained by the Kouvel-Fisher method whereas $\delta =7.96\pm 0.01$ is obtained by a critical isotherm analysis at $T_c=62.7$ K. These critical exponents obey the Widom scaling relation $\delta =1+\gamma /\beta$, indicating self-consistency of the obtained values. With these critical exponents the isotherm $M\left(H\right)$ curves below and above the critical temperatures collapse into two independent universal branches, obeying the single scaling equation $m=f_{\pm }\left(h\right)$, where $m$ and $h$ are renormalized magnetization and field, respectively. The determined exponents match well with those calculated from the results of the renormalization group approach for a two-dimensional Ising system coupled with a long-range interaction between spins decaying as $J\left(r\right)\approx r^{-(d+\sigma )}$ with $\sigma =1.52$.

The properties are considered in detail of a nonlocal (integral) equation for the superconducting gap parameter, which is obtained by a coarse-graining procedure applied to the Bogoliubov-de Gennes (BdG) equations over the whole coupling-versus-temperature phase diagram associated with the superfluid phase. It is found that the limiting size of the coarse-graining procedure, which is dictated by the range of the kernel of this integral equation, corresponds to the size of the Cooper pairs over the whole coupling-versus-temperature phase diagram up to the critical temperature, even when Cooper pairs turn into composite bosons on the BEC side of the BCS-BEC crossover. A practical method is further implemented to solve numerically this integral equation in an efficient way, which is based on a novel algorithm for calculating the Fourier transforms. Application of this method to the case of an isolated vortex, throughout the BCS-BEC crossover and for all temperatures in the superfluid phase, helps clarifying the nature of the length scales associated with a single vortex and the kinds of details that are in practice disposed off by the coarse-graining procedure on the BdG equations.

Motivated by the recent studies of out-of-time-order correlation functions and the holographic duality, we propose the quantum critical point conjecture, which is stated as: For a many-body quantum system with a quantum phase transition, the Lyapunov exponent extracted from the out-of-time-order correlators will exhibit a maximum around the quantum critical region. We first demonstrate that the Lyapunov exponent is well defined in the one-dimensional Bose-Hubbard model with the help of the out-of-time-order correlation–Rényi-entropy theorem. We then support the conjecture by numerically computing the out-of-time-order correlators. We also compute the butterfly velocity, and propose an experiment protocol of measuring this correlator without inverting the Hamiltonian.

The force between a sharp scanning probe tip and a surface can drive a graphene flake over crystalline substrates. The recent design of particular patterns of structural defects on a graphene surface allows us to propose an alternative approach for controlling the motion of a graphene flake over a graphene substrate. The thermally induced motion of a graphene flake is controlled by engineering topological defects in the substrate. Such defect regions lead to an inhomogeneous energy landscape and are energetically unfavorable for the motion of the flake, and will invert and scatter graphene flakes when they are moving toward the defect line. Engineering the distribution of these energy barriers results in a controllable trajectory for the thermal motion of the flake without using any external force. We predict superlubricity of the graphene flake for motion along and between particular defect lines. This Rapid Communication provides insights into the frictional forces of interfaces and opens a route to the engineering of the stochastic motion of a graphene flake over any crystalline substrate.

The antiferromagnetic spin-one chain is considerably one of the most fundamental quantum many-body systems, with symmetry-protected topological order in the ground state. Here, we present results for its dynamical spin structure factor at finite temperatures, based on a combination of exact numerical diagonalization, matrix-product-state calculations, and quantum Monte Carlo simulations. Open finite chains exhibit a subgap band in the thermal spectral functions, indicative of localized edge states. Moreover, we observe the thermal activation of a distinct low-energy continuum contribution to the spin spectral function with an enhanced spectral weight at low momenta and its upper threshold. This emerging thermal spectral feature of the Haldane spin-one chain is shown to result from intraband magnon scattering due to the thermal population of the single-magnon branch, which features a large bandwidth-to-gap ratio. These findings are discussed with respect to possible future studies on spin-one chain compounds based on inelastic neutron scattering.

The spin-1 Haldane chain is an example of the symmetry-protected-topological (SPT) phase in one dimension. Experimental realization of the spin chain materials usually involves both the uniaxial-type, $D{\left(S^z\right)}^2$, and the rhombic-type, $E[{\left(S^x\right)}^2-{\left(S^y\right)}^2]$, single-ion anisotropies. Here, we provide a precise ground-state phase diagram for a spin-1 Haldane chain with these single-ion anisotropies. Using quantum numbers, we find that the ${\mathbb{Z}}_2$ symmetry breaking phase can be characterized by double degeneracy in the entanglement spectrum. Topological quantum phase transitions take place on particular paths in the phase diagram, from the Haldane phase to the large-$E_x$, large-$E_y$, or large-$D$ phases. The topological critical points are determined by the level spectroscopy method with a newly developed parity technique in the density matrix renormalization group [Phys. Rev. B 86, 024403 (2012)], and the Haldane-large-$D$ critical point is obtained with an unprecedented precision, ${(D/J)}_c=0.9684713\left(1\right)$. Close to this critical point, a small rhombic single-ion anisotropy $\left|E\right|/J\ll 1$ can destroy the Haldane phase and bring the system into a $y$-Néel phase. We propose that the compound $\left[\mathrm{Ni}\left({\mathrm{HF}}_2\right){\left(3\text{−}\mathrm{Clpy}\right)}_4\right]{\mathrm{BF}}_4$ is a candidate system to search for the $y$-Néel phase.

The series of compounds $A{\mathrm{Ag}}_2\mathrm{Cr}{\left[{\mathrm{VO}}_4\right]}_2$, with $A=\mathrm{Ag}$, K, or Rb, is layered S = 3/2 triangular-lattice (TL) systems in which the magnetic exchange interactions between ${\mathrm{Cr}}^{3+}\left(3d^3\right)$ ions are mediated by nonmagnetic ${\left[{\mathrm{VO}}_4\right]}^{3-}$ entities. Here, the relative orientation of the vanadate is altered with respect to the TL as a function of the $A$ site, which corresponds to an induced symmetry change of the $\left[{\mathrm{CrO}}_6\right]$ complex. All members of this series of compounds belong to the class of frustrated TL antiferromagnets. We find that the distorted TL ($A=\mathrm{Ag}$) exhibits collinear antiferromagnetic long-range order (LRO) at $T_N\approx 10$ K, whereas the high-symmetry cases ($A=\mathrm{K}$, Rb) evade LRO in zero field down to 0.03 K, the lowest temperature of our experiments. The latter members of the series belong to the undistorted TL and are candidates for spin-liquid ground states presumably not related to Ising anisotropy or dimerization.