We study the competing charge-density-wave and superconducting order in the attractive Hubbard model under a voltage bias, using steady-state nonequilibrium dynamical mean-field theory. We show that the charge-density wave is suppressed in a current-carrying nonequilibrium steady state. This effect is beyond a simple Joule-heating mechanism and a “supercooled” metallic state is stabilized at a nonequilibrium temperature lower than the equilibrium superconducting $T_c$. On the other hand, a current-carrying superconducting state is dissipationless and thus not subject to the same nonthermal suppression, and can therefore nucleate out of the supercooled metal, e.g., in a resistive switching experiment. The fact that an electric current can change the relative stability of different phases compared to thermal equilibrium, even when a system appears locally thermal due to electron-electron scattering, provides a general perspective to control intertwined orders out of equilibrium.

Relativistic interaction between an electron's spin and orbital angular momentum has provided an efficient mechanism to control the magnetization of nanomagnets. Extensive research has been done to understand and to improve spin-orbit-interaction-driven torques generated by nonmagnets by applying electric current. In this work, we show that heat current in a nonmagnet can also couple to its spin-orbit interaction to produce torque on the adjacent ferromagnet. Hence, this work provides a platform to study spin-orbito-caloritronic effects in heavy metal/ferromagnet bilayers.

Transition-metal dichalcogenide monolayers are interesting materials in part because of their strong spin-orbit coupling. This leads to intrinsic spin splitting of opposite signs in opposite valleys, so the valleys are intrinsically spin polarized when hole doped. We study spin response in a simple model of these materials, with an eye to identifying sharp collective modes (i.e., spin waves) that are more commonly characteristic of ferromagnets. We demonstrate that such modes exist for arbitrarily weak repulsive interactions, even when they are too weak to induce spontaneous ferromagnetism. The behavior of the spin response is explored for a range of hole dopings and interaction strengths.

Nelson et al. [Phys. Rev. B 95, 054118 (2017)] recently reported first-principles calculations on the behavior of group-II difluorides (${\mathrm{BeF}}_2, {\mathrm{MgF}}_2$, and ${\mathrm{CaF}}_2$) under high-pressure and low- and high-temperature conditions. The calculations were based on ab initio random structure searching and the quasiharmonic approximation (QHA). Here, we point out that, despite the inestimable value of such calculations at high-pressure and low-temperature conditions, the high-$P$ high-$T$ phase diagram proposed by Nelson et al. for ${\mathrm{CaF}}_2$ is not in qualitative agreement with the results of previous ab initio molecular-dynamics simulations, nor does it agree with the existing body of experimental data. Therefore, we conclude that the QHA-based approach employed by Nelson et al. cannot be applied reliably to the study of phase boundaries involving superionic phases. This conclusion is further corroborated by additional ab initio calculations performed in the superionic compounds ${\mathrm{SrF}}_2, {\mathrm{BaF}}_2, {\mathrm{Li}}_3\mathrm{OCl}$, and AgI.

Cazorla et al. (preceding Comment) criticize our recent results on the high-$PT$ phase diagram of ${\mathrm{CaF}}_2$ [Phys. Rev. B 95, 054118 (2017)]. According to our analysis, Cazorla et al. have not converged their calculations with respect to simulation cell size, undermining the Comment's conclusions about both the high-$T$ behavior of the $P\overline{6}2m-{\mathrm{CaF}}_2$ polymorph, and the use of the QHA in our work. As such, we take this opportunity to emphasize the importance of correctly converging molecular-dynamics simulations to avoid finite-size errors. We compare our quasiharmonic phase diagram for ${\mathrm{CaF}}_2$ with currently available experimental data, and we find it to be entirely consistent and in qualitative agreement with such data. Our prediction of a superionic phase transition in $P\overline{6}2m-{\mathrm{CaF}}_2$ (made on the basis of the QHA) is shown to be accurate, and we argue that simple descriptors, such as phonon frequencies, can offer valuable insight and predictive power concerning superionic behavior.

We study the effective spin-orbital model that describes the magnetism of $4d^1$ or $5d^1$ Mott insulators in ideal tricoordinated lattices. In the limit of vanishing Hund's coupling, the model has an emergent SU(4) symmetry, which is made explicit by means of a Klein transformation on pseudospin degrees of freedom. Taking the hyperhoneycomb lattice as an example, we employ parton constructions with fermionic representations of the pseudospin operators to investigate possible quantum spin-orbital liquid states. We then use variational Monte Carlo (VMC) methods to compute the energies of the projected wave functions. Our numerical results show that the lowest-energy quantum liquid corresponds to a zero-flux state with a Fermi surface of four-color fermionic partons. In spite of the Fermi surface, we demonstrate that this state is stable against tetramerization. A combination of linear flavor wave theory and VMC applied to the complete microscopic model also shows that this liquid state is stable against the formation of collinear long-range order.

Lattice thermal conductivity is a sensitive probe of distortions of the translational invariance of crystalline materials. Crossplane thermal conductivity, $\kappa \left(T\right)$, was studied by the 3ω technique in superlattices, comprising orthorhombic charge/orbital ordered manganite ${\mathrm{Pr}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{MnO}}_3$ and cubic ${\mathrm{SrTiO}}_3$. The manganite/titanite superlattices show a peak in $\kappa \left(T\right)$ just above the charge order transition temperature $T_{\mathrm{CO}}\sim 240\mathrm{K}$ with a pronounced thermal hysteresis. This ordering peak in $\kappa \left(T\right)$ is successively suppressed with decreasing layer thickness. Our results demonstrate a minor effect of the interface density on $\kappa$, primarily visible below $T_{\mathrm{CO}}$. In contrast, much higher changes in $\kappa \left(T\right)$ evolve close to $T_{\mathrm{CO}}$, due to interface-induced effects on charge and orbital ordering. Our results suggest that the ordering peak in $\kappa \left(T\right)$ is a result of lattice softening above the phase transition which is modified by thickness-dependent misfit strain.

We propose a simple modification of the density matrix renormalization-group (DMRG) method in order to tackle strongly disordered quantum spin chains. Our proposal, akin to the idea of the adaptive time-dependent DMRG, enables us to reach larger system sizes in the strong disorder limit by avoiding most of the metastable configurations, which hinder the performance of the standard DMRG method. We benchmark our adaptive method by revisiting the random antiferromagnetic XXZ spin-1/2 chain for which we compute the random-singlet ground-state average spin-spin correlation functions and von Neumann entanglement entropy. We then apply our method to the bilinear-biquadratic random antiferromagnetic spin-1 chain tuned to the antiferromagnet and gapless highly symmetric SU(3) point. We find the new result that the mean correlation function decays algebraically with the same universal exponent $\varphi =2$ as the spin-1/2 chain. We then perform numerical and analytical strong-disorder renormalization-group calculations, which confirm this finding and generalize it for any highly symmetric $\mathrm{SU}(N$) random-singlet state.

Topological properties of graphene with nanoholes are theoretically investigated in detail. It is revealed that two types of hole arrangements, honeycomb array and triangular array, give rise to two topologically distinct states. We give a concise two-step strategy for choosing array structures, where two types of Brillouin zone folding, corner-to-corner or corner-to-center, play crucial roles. The topological nontriviality is confirmed by interface states at a domain wall between the honeycomb and triangular hole arrays, which show a cross-shaped band structure resembling helical edge states in quantum spin Hall states. We also diagnose the topology of the states from the view point of the topological crystalline insulator and show that the parity index against the $C_2$ rotation is a key quantity. These findings indicate that nanotechnology-based geometrical design and manipulation are promising in exploration of topological phases of matter.

The last two decades have witnessed various experiments reporting very unusual magnetic properties of ensembles of gold nanoparticles surrounded by organic ligands, including ferromagnetic, paramagnetic, or (large) diamagnetic responses. These behaviors are at odds with the small diamagnetic response of macroscopic gold samples. Here we theoretically investigate the possibility that the observed unusual magnetism in capped gold nanoparticles is of orbital origin. Employing semiclassical techniques, we calculate the orbital component to the zero-field susceptibility of individual as well as ensembles of metallic nanoparticles. While the result for the orbital response of individual nanoparticles can exceed by orders of magnitude the bulk Landau susceptibility in absolute value, and can be either diamagnetic or paramagnetic depending on nanoparticle size, we show that the magnetic susceptibility of a noninteracting ensemble of nanoparticles with a smooth size distribution is always paramagnetic at low magnetic fields. In particular, we predict that the zero-field susceptibility follows a Curie-type law for small nanoparticle sizes and/or low temperatures. The calculated field-dependent magnetization of an ensemble of diluted nanoparticles is shown to be in good agreement with existing experiments that yielded a large paramagnetic response. The width of the size distribution of the nanoparticles is identified as a key element for the quantitative determination of the orbital response.

Electric circuits are known to realize topological quadrupole insulators. We explore electric circuits made of capacitors and inductors forming the breathing kagome and pyrochlore lattices. They are known to possess three phases (trivial insulator, higher-order topological insulator, and metallic phases) in the tight-binding model, where the topological phase is characterized by the emergence of zero-energy corner states. A topological phase transition is induced by tuning continuously the capacitance, which is possible by using variable capacitors. It is found that the two-point impedance yields huge resonance peaks when one node is taken at a corner in the topological phase. It is a good signal to detect a topological phase transition. We also show that the topological corner resonance is robust against the randomness of capacitance and inductance. Furthermore, the size of the electric circuits can be quite small to realize the topological phase together with topological phase transitions.

Graphite under high magnetic field exhibits consecutive metal-insulator (MI) transitions as well as reentrant insulator-metal (IM) transitions in the quasiquantum limit at low temperature. In this paper, we identify the low-$T$ insulating phases as excitonic insulators with spin nematic orderings. We first point out that graphite under the relevant field regime is in the charge neutrality region, where electron and hole densities compensate each other. Based on this observation, we introduce interacting electron models with electron pocket(s) and hole pocket(s) and enumerate possible umklapp scattering processes allowed under the charge neutrality. Employing effective boson theories for the electron models and renormalization group analyses for the boson theories, we show that there exist critical interaction strengths above which the umklapp processes become relevant and the system enters excitonic insulator phases with long-range order of spin superconducting phase fields (“spin nematic excitonic insulator”). We argue that when a pair of electron and hole pockets gets smaller in size, a quantum fluctuation of the spin superconducting phase becomes larger and destabilizes the excitonic insulator phases, resulting in the reentrant IM transitions. We also show that an odd-parity excitonic pairing between the electron and hole pockets reconstructs surface chiral Fermi arc states of electron and hole into a 2-dimensional helical surface state with a gapless Dirac cone. We discuss field and temperature dependencies of in-plane resistance by surface transport via these surface states.

We investigate the effect of electron-phonon coupling on low-temperature phases in metallic single-wall carbon nanotubes. We obtain low-temperature phase diagrams of armchair and zigzag type nanotubes with screened interactions with a weak-coupling renormalization group approach. In the absence of electron-phonon coupling, two types of nanotubes have similar phase diagrams. A $D$-Mott phase or $d$-wave superconductivity appears when the on-site interaction is dominant, while a charge-density wave or an excitonic insulator phase emerges when the nearest neighbor interaction becomes comparable to the on-site interaction. The electron-phonon coupling, treated by a two-cutoff scaling scheme, leads to different behavior in two types of nanotubes. For strong electron-phonon interactions, phonon softening is induced and a Peierls insulator phase appears in armchair nanotubes. We find that this softening of phonons may occur for any intraband scattering phonon mode. On the other hand, the effect of electron-phonon coupling is negligible for zigzag nanotubes. The distinct behavior of armchair and zigzag nanotubes against lattice distortion is explained by analysis of the renormalization group equations.

We study controlled spin transport through a two-dimensional (2D) spin-orbit coupled quantum ring of finite width. Complete analytical expressions for the energy spectrum and wave function of a 2D ring are obtained in the presence of both an external magnetic field and Rashba spin-orbit interaction. These results have been used to investigate the magnetoconductance of the ring in which an interface electric field leads to the Rashba mechanism. Nonperiodicity of the ground-state angular momentum transition with respect to the external magnetic field is observed for a 2D ring which is in contrast to the perfectly periodic transition spectrum of the one-dimensional (1D) ring. Our analytical model has successfully reproduced the salient features of the magnetoconductance of the ring. In 1D as well as 2D ring geometries, the probability amplitude for the transfer of electron spin, polarized in the direction of an external magnetic field, is found to be maximum irrespective of the polarization of the incoming spin. Since the spin polarization of the transmitted electron changes continuously with Rashba coupling, this system can act as a tunable spin switch electrically controlled by gate voltage.

Specially designed instrumentation for electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope makes it possible to probe very low-loss excitations in matter with a focused electron beam. Here we study the nanoscale interaction of fast electrons with optical phonon modes in silica. In particular, we analyze the spatial dependence of EEL spectra in two geometrical arrangements: a free-standing truncated slab of silica and a slab with a junction between silica and silicon. In both cases, we identify different loss channels, involving polaritonic and nonpolaritonic contributions to the total electron energy loss, and we obtain the corresponding energy-filtered maps. Furthermore, we present a comparison of the theoretical simulations for a silica-silicon junction with experimental results, and we discuss the spatial resolution attainable from the energy-filtered map considering optical phonon excitations in a conventional experimental arrangement.

This work studies the guided localized electromagnetic waves propagating along finite and infinite chains of parallel thin resonant wires periodically arranged over an interface of a dielectric half-space. Both issues of guided wave propagation in an infinite chain and resonant excitation of hybridized eigenmodes of a finite chain are solved analytically and numerically. The effect of changing the modes' resonant frequency order is observed as the chain approaches the interface closer than its periodicity staying inside the dielectric. This effect, called mode hopping, has been directly demonstrated by solving the problem of a finite chain excited by a plane wave using Pocklington's system of integral equations. Moreover, the effect was further related to the dispersion properties of the corresponding infinite array. The analytically described mode-hopping effect was verified numerically and experimentally.

Liquid cesium (l-Cs) sound velocity at high densities was investigated along a 500-K isotherm using high-pressure picosecond acoustics measurements. At 2.0 GPa, the liquid sound velocity goes through a maximum versus pressure without any change on the reflectivity and interferometry acoustic signals. Upon further compression, a softening of the l-Cs viscoelastic properties is observed from 2.0 up to 4.0 GPa, pressure at which the reflectometric signal is abruptly reversed whereas the interferometric signal remains qualitatively the same. This anomalous behavior could be related to an electronic transformation within the l-Cs state, which here again could reflect what happens at lower temperature within the solid state. If so, such liquid-liquid transition may be driven by the progressive collapse of the $6s$ electronic orbital onto the $5d$ ones. Above 4.0 GPa, the l-Cs sound velocity starts again to increase as commonly expected upon compression.

The low-energy magnetic excitation from the highly Ca-doped quasi-one-dimensional magnet $\mathrm{SrC}{\mathrm{a}}_{13}\mathrm{C}{\mathrm{u}}_{24}{\mathrm{O}}_{41}$ was studied in the magnetic ordered state by using inelastic neutron scattering. We observed the gapless spin-wave excitation, dispersive along the $a$ and $c$ axes but nondispersive along the $b$ axis. Such excitations are attributed to the spin wave from the spin-chain sublattice. Model fitting to the experimental data gives the nearest-neighbor interaction $J_c$ as 5.4 meV and the interchain interaction $J_a=4.4\mathrm{meV}$. $J_c$ is antiferromagnetic and its value is close to the nearest-neighbor interactions of the similar edge-sharing spin-chain systems such as $\mathrm{CuGe}{\mathrm{O}}_3$. Comparing with the hole-doped spin chains in $\mathrm{S}{\mathrm{r}}_{14}\mathrm{C}{\mathrm{u}}_{24}{\mathrm{O}}_{41}$, which shows a spin gap due to spin dimers formed around Zhang-Rice singlets, the chains in $\mathrm{SrC}{\mathrm{a}}_{13}\mathrm{C}{\mathrm{u}}_{24}{\mathrm{O}}_{41}$ show a gapless excitation in this paper. We ascribe such a change from gapped to gapless excitations to holes transferring away from the chain sublattice into the ladder sublattice upon Ca doping.

We introduce a method to carry out zero-temperature calculations within density functional theory (DFT) but without relying on the Born-Oppenheimer (BO) approximation for the ionic motion. Our approach is based on the finite-temperature many-body path-integral formulation of quantum mechanics by taking the zero-temperature limit and treating the imaginary-time propagation of the electronic variables in the context of DFT. This goes beyond the familiar BO approximation and is limited from being an exact treatment of both electrons and ions only by the approximations involved in the DFT component. We test our method in two simple molecules, ${\mathrm{H}}_2$ and benzene. We demonstrate that the method produces a difference from the results of the BO approximation which is significant for many physical systems, especially those containing light atoms such as hydrogen; in these cases, we find that the fluctuations of the distance from its equilibrium position, due to the zero-point motion, is comparable to the interatomic distances. The method is suitable for use with conventional condensed-matter approaches and currently is implemented on top of the periodic pseudopotential code siesta.