We study the electrical conductance in single-mode quantum wires with Rashba spin-orbit interaction subjected to externally applied magnetic fields in the regime in which the ratio of spin-orbit momentum to the Fermi momentum is close to an odd integer, so that a combined effect of multi-electron interaction and applied magnetic field leads to a partial gap in the spectrum. We study how this partial gap manifests itself in the temperature dependence of the fractional conductance of the quantum wire. We use two complementing techniques based on bosonization: refermionization of the model at a particular value of the interaction parameter and a semiclassical approach within a dilute soliton gas approximation of the functional integral. We show how the low-temperature fractional conductance can be affected by the finite length of the wire, by the properties of the contacts, and by a shift of the chemical potential, which takes the system away from the resonance condition. We also predict an internal resistivity caused by a dissipative coupling between gapped and gapless modes.

We study the tunneling conductance of a ballistic normal metal/ferromagnet/spin-triplet superconductor junction using the extended Blonder-Tinkham-Klapwijk formalism as a model for a $c$-axis-oriented Au/${\mathrm{SrRuO}}_3\text{/}{\mathrm{Sr}}_2{\mathrm{RuO}}_4$ junction. We compare chiral $p$-wave (CPW) and helical $p$-wave (HPW) pair potentials, combined with ferromagnet magnetization directions parallel and perpendicular to the interface. For fixed ${\theta }_M$, where ${\theta }_M$ is a direction of magnetization in the ferromagnet measured from the $c$ axis, the tunneling conductances of CPW and HPW clearly show different voltage dependencies. It is found that the cases where the $d$ vector is perpendicular to the magnetization direction (CPW with ${\theta }_M=\pi /2$ and HPW with ${\theta }_M=0$) are identical. The obtained results serve as a guide to determine the pairing symmetry of the spin-triplet superconductor ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$.

We reexamined the rotation-induced effect observed in solid ${}^4\mathrm{He}$ by using a rigid two-frequency torsional oscillator (TO). The previous rotation experiments reported the rotation-induced suppression of the “nonclassical” TO response that was interpreted as evidence of irrotational bulk superfluidity in solid ${}^4\mathrm{He}$. However, the experiment employed a nonrigid TO that could amplify the elastic contribution in the TO response. Thus, it is important to clarify if the rotation-induced suppression of the TO response could be attributed to an unavoidable elastic effect. In our rigid TO, complicated nonlinear viscoelastic contributions are systematically eliminated. In addition, the TO operating at two different resonant frequencies allows us to decompose a possible superfluidlike frequency-independent contribution on period drop from that of the linear elastic overshoot effect. We found no substantial rotation-induced effect in the out-of-phase resonant mode unlike that found in the previous rotation experiments. It indicates that the previous rotation effect in the nonrigid TO cannot be attributed to the genuine supersolidity. According to the frequency analysis of the TO response, the frequency-dependent period drop, which can be attributed to the elastic overshoot effect, remains unaffected upon application of dc rotation. However, the frequency-independent superfluidlike contribution exhibits a strikingly different rotation effect that is currently inexplicable.

A Ginzburg-Landau free energy for a superconducting chiral $p$-wave order parameter is derived from a two-dimensional tight-binding lattice model with weak spin-orbit coupling included as a general symmetry-breaking field. Superconductivity is accounted for by a BCS-type nearest-neighbor opposite-spin interaction where we project the potential onto the $p$-wave irreducible representation of the square lattice symmetry group and assume this to be the dominating order. The resulting free energy contains kinetic terms that mix components of the order parameter as well as directional gradients—so-called mixed gradient terms—as a virtue of the symmetry of the order parameter. Spin-orbit coupling and electron-hole anisotropy lead to additional contributions to the coefficients of these terms, increasing the number of necessary phenomenological parameters by one compared to previous work and leading to an increase in the coefficient measuring Fermi-surface anisotropy for Rashba spin-orbit coupling in the continuum limit.

Resistivity $\rho$, thermopower $\mathcal{S}$, and thermal conductivity $\kappa$ were measured simultaneously on a set of CoFe alloy films. Variation of the Co content $x_{\mathrm{Co}}$ allows for a systematic tuning of the Fermi level through the band structure, and the study of the interplay between electronic and magnetic contributions to the transport coefficients. While band-structure and magnon effects in $\rho$ and $\kappa$ are rather weak, they turn out to be very significant in $\mathcal{S}$. A decomposition of $\mathcal{S}$ into Mott and magnon drag contributions results in a systematic evolution between the two limiting cases of pure Fe and pure Co. At low temperatures, we find an interesting sign change of the curvature of $\mathcal{S}\left(T\right)$ that indicates a corresponding sign change of the magnon drag.

The recent discovery of superconductivity in ${\mathrm{NaSn}}_2{\mathrm{As}}_2$ with a van der Waals layered structure raises immediate questions on its pairing mechanism and underlying electronic structure. Here, we present measurements of the temperature-dependent magnetic penetration depth $\lambda \left(T\right)$ in single crystals of ${\mathrm{NaSn}}_2{\mathrm{As}}_2$ down to $\sim 40$ mK. We find a very long penetration depth $\lambda \left(0\right)=960\mathrm{nm}$, which is strongly enhanced from the estimate of first-principles calculations. This enhancement comes from a short mean free path $\ell \approx 1.7\mathrm{nm}$, indicating atomic scale disorder possibly associated with the valence-skipping states of Sn. The temperature dependence of superfluid density is fully consistent with the conventional fully gapped $s$-wave state in the dirty limit. These results suggest that ${\mathrm{NaSn}}_2{\mathrm{As}}_2$ is an ideal material to study quantum phase fluctuations in strongly disordered superconductors with its controllable dimensionality.

We report inelastic light scattering results on the stoichiometric and fully ordered superconductor ${\mathrm{CaKFe}}_4{\mathrm{As}}_4$ as a function of temperature and light polarization. In the energy range between 10 and 315 ${\mathrm{cm}}^{-1}$ (1.24 and 39.1 meV) we observe the particle-hole continuum above and below the superconducting transition temperature $T_c$ and seven of the eight Raman active phonons. The main focus is placed on the analysis of the electronic excitations. Below $T_c$ all three symmetries projected with in-plane polarizations display a redistribution of spectral weight characteristic for superconductivity. The energies of the pair-breaking peaks in $A_{1g}$ and $B_{2g}$ symmetry are in approximate agreement with the results from photoemission studies. In $B_{1g}$ symmetry the difference between the normal and superconducting state is most pronounced, and the feature is shifted downwards with respect to those in $A_{1g}$ and $B_{2g}$ symmetry. The maximum peaking at 134 ${\mathrm{cm}}^{-1}$ (16.6 meV) has a substructure on the high-energy side. We interpret the peak at 134 ${\mathrm{cm}}^{-1}$ in terms of a collective Bardasis-Schrieffer (BS) mode and the substructure as a remainder of the pair-breaking feature on the electron bands. There is a very weak peak at 50 ${\mathrm{cm}}^{-1}$ (6.2 meV) which is tentatively assigned to another BS mode.

Radiation can drive the electrons in a material out of thermal equilibrium with the nuclei, producing hot, transient electronic states that modify the interatomic potential energy surface. We present a rigorous formulation of two-temperature molecular dynamics that can accommodate these electronic effects in the form of electronic-temperature-dependent force fields. Such a force field is presented for silicon, which has been constructed to reproduce the ab initio-derived thermodynamics of the diamond phase for electronic temperatures up to $2.5\mathrm{eV}$, as well as the structural dynamics observed experimentally under nonequilibrium conditions in the femtosecond regime. This includes nonthermal melting on a subpicosecond timescale to a liquidlike state for electronic temperatures above $\sim 1\mathrm{eV}$. The methods presented in this paper lay a rigorous foundation for the large-scale atomistic modeling of electronically driven structural dynamics with potential applications spanning the entire domain of radiation damage.

We employed x-ray absorption spectroscopy and x-ray magnetic circular dichroism to study the magnetic properties of single rare-earth (RE) atoms (Nd, Tb, Dy, Ho, and Er) adsorbed on the graphene/Ir(111) surface. The interaction of RE atoms with graphene results for Tb in a trivalent state with $4f^{n-1}$ occupancy, and in a divalent state with $4f^n$ occupancy for all other studied RE atoms ($n$ corresponds to the $4f$ occupancy of free atoms). Among the studied RE on graphene/Ir(111), Dy is the only one that shows magnetic hysteresis and remanence at 2.5 K. By comparing measured spectra and magnetization curves with multiplet calculations, we determine the energy diagram of the magnetic states and show for each element the magnetization reversal process that determines the timescale of its magnetic bistability.

We study the electronic structure of ${\mathrm{WTe}}_2$ thin films with different thicknesses. High-quality thin-film samples are obtained with carrier mobility up to 5000 ${\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which enables us to resolve the four main Fermi pockets from Shubnikov–de Haas (SdH) oscillations. Angle-resolved SdH oscillations show that the ${\mathrm{WTe}}_2$ thin films cross from three-dimensional to two-dimensional electronic systems at a thickness of $\sim 20$ nm. Using the field effect, the nature of the Fermi pockets in thin-film ${\mathrm{WTe}}_2$ is identified, and the evolution of SdH oscillation frequencies is traced over different sample thicknesses. It is found that the frequencies dramatically decrease at a thickness of approximately 12 nm, which indicates the onset of finite-size effects on the band structure. Our work pins down two critical length scales of the thickness-dependent electronic structure in ${\mathrm{WTe}}_2$ thin films.

The $t-J$ model is a standard model of strongly correlated electrons, often studied in the context of high-$T_c$ superconductivity. However, most studies of this model neglect three-site terms, which appear at the same order as the superexchange $J$. As these terms correspond to pair hopping, they are expected to play an important role in the physics of superconductivity when doped sufficiently far from half filling. We present a density matrix renormalization-group study of the one-dimensional $t-J$ model with the pair hopping terms included. We demonstrate that these additional terms radically change the one-dimensional ground-state phase diagram, extending the superconducting region at low fillings, while at larger fillings, superconductivity is completely suppressed. We explain this effect by introducing a simplified effective model of repulsive hardcore bosons.

Effects that are not captured by the generalized-gradient density-functional theory have a prominent effect on the structural binding and on the electronic and optical properties of reduced-dimensional and weakly bound materials. Here, we report the exfoliation energy of selenium and tellurium atomic chains with nonempirical van der Waals corrections, and their electronic and optical properties with the $GW$ and Bethe-Salpeter formalisms. The exfoliation energy is found to be within 0.547–0.719 eV/u.c. for the selenium atomic chain, and 0.737–0.926 eV/u.c. for the tellurium atomic chain (where u.c. stands for unit cell), depending on the approximation for the van der Waals interaction and the numerical tool chosen. The $GW$ electronic band gap turned out to be 5.22–5.47 (4.44–4.59) eV for the Se (Te) atomic chains, with the lowest bound obtained with the Godby-Needs (GN), and the upper bound to the Hybertsen-Louie (HL) plasmon-pole models (PPMs). The binding energy of the ground-state excitonic state ranges between 2.69 and 2.72 eV for selenium chains within the HL and GN PPM, respectively, and it turned out to be 2.35 eV for tellurium chains with both approximations. The ground-state excitonic wave function is localized within 50 Å along the axis for both types of atomic chains, and its energy lies within the visible spectrum: blue [2.50(GN)–2.78(HL) eV] for selenium and yellow-green [2.09(GN)–2.28(HL) eV] for tellurium, which could be useful for LED applications in the visible spectrum.

We study the temperature-dependent corrections to the conductance due to electron-electron ($e-e$) interactions in clean two-dimensional conductors, such as lightly doped graphene or other Dirac matter. We use semiclassical Boltzmann kinetic theory to solve the problem of collision-dominated transport between reflection-free contacts. Time-reversal symmetry and the kinematic constraints of scattering in two dimensions (2D) ensure that inversion-odd and inversion-even distortions of the quasiparticle distribution relax with parametrically different rates at low temperature. This entails the surprising result that at lowest temperatures the conductance of very long samples tends to the noninteracting, ballistic conductance, despite the relaxation of the quasiparticle distribution to a drifting equilibrium. The relative correction to the conductance depends on the ratio of relaxation rates of even and odd modes and scales as $\delta G/G_{\mathrm{ballistic}}\sim \left(T/{\varepsilon }_F\right)\sqrt{log\left(\frac{{\varepsilon }_F}{T}\right)}$, in stark contrast to the behavior in other dimensionalities. This holds generally in 2D systems with simply connected and convex but otherwise arbitrary Fermi surfaces, as long as $e-e$ scattering processes are dominant and umklapp scattering is negligible. These results are especially relevant to the bulk of wide and long suspended high-mobility graphene sheets.

AB-stacked bilayer graphene with a tunable electronic band gap in excess of the optical phonon energy presents an interesting active medium, and we consider such a theoretical possibility in this Rapid Communication. We argue the possibility of a highly resonant optical gain in the vicinity of the asymmetry gap. Associated with this resonant gain are strongly amplified plasmons, plasmons with negative group velocity and superluminal effects, as well as directional leaky modes.

We develop the theory of transverse magnetoresistance in layered quasi-two-dimensional (Q2D) metals. Using the Kubo formula and harmonic expansion, we calculate intralayer conductivity in a magnetic field perpendicular to conducting layers. The analytical expressions for the amplitudes and phases of magnetic quantum oscillations (MQO) and of the so-called slow oscillations (SO) are derived and applied to analyze their behavior as a function of several parameters: magnetic field strength, interlayer transfer integral, and Landau-level width. Both the MQO and SO of intralayer and interlayer conductivities have approximately opposite phase in weak magnetic field and the same phase in strong field. The amplitude of SO of intralayer conductivity changes sign at ${\omega }_c{\tau }_0=\sqrt{3}$. There are several other qualitative differences between magnetic oscillations of in-plane and out-of-plane conductivity. The results obtained are useful to analyze experimental data on magnetoresistance oscillations in various strongly anisotropic Q2D metals.

We find that the topological phase transition in a chiral ladder is characterized by dramatic signatures in many body entanglement entropy between the legs, close to half filling. The value of entanglement entropy for various fillings close to half filling is identical, at the critical point, but splays out on either side, thus showing a sharp signature at the transition point. A second signature is provided by the change in entanglement entropy when a particle is added (or subtracted) from half filling which turns out to be exactly $-log2$ in the trivial phase but zero in the topological phase. A microscopic understanding of tendencies to form singlets along the rungs in the trivial phase, and along the diagonals in the topological phase, is afforded by a study of concurrence. At the topological phase transition the magnitude of the derivative of the average concurrence of all the rungs shows a sharp peak. Also, at the critical point, the average concurrence is the same for various fillings close to half filling but splays out on either side, just like entanglement entropy. Entanglement entropy also offers a striking signature at the vortex-to-Meissner phase transition inherent in this system.

In this work we present a high-throughput first-principles study of elastic properties of bulk and monolayer materials mainly using the vdW-DF-optB88 functional. We discuss the trends on the elastic response with respect to changes in dimensionality. We identify a relation between exfoliation energy and elastic constants for layered materials that can help to guide the search for vdW bonding in materials. We also predicted a few novel materials with auxetic behavior. The uncertainty in structural and elastic properties due to the inclusion of vdW interactions is discussed. We investigated 11 067 bulk and 257 monolayer materials. Lastly, we found that the trends in elastic constants for bulk and their monolayer counterparts can be very different. All the computational results are made publicly available at easy-to-use websites: https://www.ctcms.nist.gov/∼knc6/JVASP.html and https://jarvis.nist.gov/. Our dataset can be used to identify stiff and flexible materials for industrial applications.

We examine instabilities of the plateau phases in the spin-$\frac{1}{2}$ kagome lattice antiferromagnet in an applied field by means of degenerate perturbation theory, and find some emergent supersolid phases below the $m=\frac{5}{9}$ plateau. The wave functions of the plateau phases in a magnetic field have the particular construction based on the building blocks of resonating hexagons and their surrounding sites. Magnon excitations on each of these blocks suffer from a kinetic frustration effect, namely, they cannot hop easily to the others since the hopping amplitudes through the two paths destructively cancel out with each other. The itinerancy is thus weakened, and the system is driven toward the strong coupling regime, which together with the selected paths allowed in real space bears a supersolid phase. This mechanism is contrary to that proposed in lattice Bose gases, where the strong competing interactions suppress with each other, allowing a small kinetic energy scale to attain the itinerancy. Eventually, we find a supersolid state in which the pattern of resonating hexagons is preserved from the plateau crystal state and only one third of the originally polarized spins outside the hexagons dominantly join the superfluid component, or equivalently, participate in the magnetization process.