We introduce and study the dynamical probes of band-structure topology in the postquench time evolution of quantum many-body systems initialized in mixed states. Our construction generalizes the notion of dynamical quantum phase transitions (DQPTs), a real-time counterpart of conventional equilibrium phase transitions in quantum dynamics, to finite temperatures and generalized Gibbs ensembles. The nonanalytical signatures hallmarking these mixed-state DQPTs are found to be characterized by observable phase singularities manifesting in the dynamical formation of vortex-antivortex pairs in the interferometric phase of the density matrix. Studying quenches in Chern insulators, we find that changes in the topological properties of the Hamiltonian can be identified in this scenario, without ever preparing a topologically nontrivial or low-temperature initial state. Our observations are of immediate relevance for current experiments aimed at realizing topological phases in ultracold atomic gases.

A detailed analytical and numerical study of the spin wave modes of the free layer of a nanopillar spin torque nano-oscillator (STNO) has been performed as a function of the magnetostatic interaction between the free and the fixed magnetic layers. Results for higher frequency normal modes show that the magnetostatic interaction does not appreciably affect the spin wave frequencies and the critical current densities, due to more relevance of the exchange interaction in these modes. For lower frequency normal modes we observe a decrease in frequency and in the critical current density for auto-oscillations when the strength of the magnetostatic interaction between the layers increases, an effect that may be appreciable.

We report the results of magnetization measurements with the magnetic field applied along the $c$ axis on superconducting $\mathrm{L}{\mathrm{a}}_{1.9}\mathrm{C}{\mathrm{a}}_{1.1}\mathrm{C}{\mathrm{u}}_2{\mathrm{O}}_{6+\delta }$ single crystals processed under ultrahigh oxygen pressure. Strong fluctuation effects were found in both low- and high-field regimes. Scaling analysis of the high-field magnetization data near the critical temperature $(T_c=53.5\mathrm{K})$ region reveals the characteristics of critical fluctuation behavior of quasi-two-dimensional (2D) superconductivity, described by Ginzburg-Landau theory using the lowest Landau level approximation. Low-field magnetic susceptibility data can be successfully explained by the Lawrence-Doniach model for a quasi-2D superconductor, from which we obtained the $ab$ plane Ginzburg-Landau coherence length of this system, ${\xi }_{ab}\left(0\right)=11.8\pm 0.9\AA$. The coherence length along the $c$ axis, ${\xi }_c\left(0\right)$, is estimated to be about 1.65 Å, which is in between those of 2D cuprate systems, such as $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{C}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{10}$ and $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{r}}_2\mathrm{CaC}{\mathrm{u}}_2{\mathrm{O}}_8$, and quasi-three-dimensional (3D) cuprate systems, such as overdoped $\mathrm{L}{\mathrm{a}}_{2-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Cu}{\mathrm{O}}_4$ and $\mathrm{YB}{\mathrm{a}}_2\mathrm{C}{\mathrm{u}}_3{\mathrm{O}}_{7-\delta }$. Our studies suggest a strong interplay among the fluctuation effects, dimensionalities, and the ratios of the interlayer Cu-O plane spacing, $s$, to the c-axis coherence lengths. A high $s/{\xi }_c\left(0\right)$ was observed in the high-pressure oxygenated $\mathrm{L}{\mathrm{a}}_{1.9}\mathrm{C}{\mathrm{a}}_{1.1}\mathrm{C}{\mathrm{u}}_2{\mathrm{O}}_{6+\delta }$, and that apparently drives this system to behave more like a quasi-2D superconductor.

We theoretically investigate pump-probe optical responses in the two-dimensional extended Hubbard model describing cuprates by using a time-dependent Lanczos method. At half filling, pumping generates photoinduced absorptions inside the Mott gap. A part of low-energy absorptions is attributed to the independent propagation of photoinduced holons and doublons. The spectral weight just below the Mott gap increases with decreasing the on-site Coulomb interaction $U$. We find that the next-nearest-neighbor Coulomb interaction $V_1$ enhances this $U$ dependence, indicating the presence of biexcitonic contributions formed by two holon-doublon pairs. Photopumping in hole-doped systems also induces spectral weights below remnant Mott-gap excitations, being consistent with recent experiments. The induced weights are less sensitive to $V_1$ and may be related to the formation of a biexcitonic state in the presence of hole carriers.

The phase diagram of the quantum dimer model on the hexagonal (honeycomb) lattice is computed numerically, extending on earlier work by Moessner, Sondhi, and Chandra. The different ground state phases are studied in detail using several local and global observables. In addition, we analyze imaginary-time correlation functions to determine ground state energies as well as gaps to the first excited states. This leads in particular to a confirmation that the intermediary so-called plaquette phase is gapped. On the technical side, we describe an efficient world-line quantum Monte Carlo algorithm with improved cluster updates that increase acceptance probabilities by taking account of potential terms of the Hamiltonian during the cluster construction. The Monte Carlo simulations are supplemented with variational computations.

Recent transport experiments have revealed the activation of longitudinal magnetoresistance of Weyl semimetals in the quantum limit, suggesting the breakdown of chiral anomaly in a strong magnetic field. Here we provide a general mechanism for gapping the zeroth chiral Landau levels applicable for both Dirac and Weyl semimetals. Our result shows that the zeroth Landau levels anticross when the magnetic axis is perpendicular to the Dirac/Weyl node separation and when the inverse magnetic length $l_B^{-1}$ is comparable to the node separation scale $\mathrm{\Delta }k$. The induced bulk gap increases rapidly beyond a threshold field in Weyl semimetals, but has no threshold and is nonmonotonic in Dirac systems due to the crossover between $l_B^{-1}>\mathrm{\Delta }k$ and $l_B^{-1}<\mathrm{\Delta }k$ regions. We also find that the Dirac and possibly Weyl systems host counterpropagating edge states between the zeroth Landau levels, leading to a state with metallic side walls and zero Hall conductance.

The theoretical description of the effect of the electric field $F$ on the hopping mobility $\mu$ belongs to the not-yet-resolved problems related to charge transport in disordered materials. An often proposed solution is to simulate hopping transport via sites placed on regular grids and to fit the results by phenomenological equations. This approach currently dominates the theoretical research of hopping transport in organic disordered semiconductors. We show that the dependence $\mu \left(F\right)$ in the case of regular grids can drastically differ from that in systems with spatial disorder. While $\mu$ increases with $F$ on lattices, it can decrease in random systems with the same material parameters. Moreover, the material parameters responsible for the dependence $\mu \left(F\right)$ on lattices differ from those responsible for $\mu \left(F\right)$ in spatially disordered systems, which makes lattice models inappropriate for studying the field dependence of the hopping mobility.

The impact of a heavy doping on oxygen diffusion at 350 ${}^{\circ }\mathrm{C}-700{}^{\circ }\mathrm{C}$ is widely discussed in literature, however, the retardation/enhancement mechanisms remains unclear at that temperature range. In this paper, we study the impact of heavy doping on the oxygen diffusion coefficient in silicon by using density functional theory calculations. While it is known that the lowering of temperature induces a switch in the diffusion mechanism from monomer mediated diffusion to dimer one, we have discovered that the reported enhanced oxygen diffusion in $p$-doped silicon is driven by a switch back from the dimer to monomer. We base our claim on extensive calculations of both pre-exponential factors and activation energies in various doping and stress conditions. We show that the stress has a negligible effect and we attribute the switch back to monomer diffusion at low temperatures in $p$-doped materials, to a charge assisted mechanism that reduces the migration energy of the monomer of 0.4 eV, while the diffusion rate is kept high thanks to the pre-exponential factor. We also provide comparisons to $n$-doped and isovalent cases.

We apply the recently developed partition-free time-dependent Landauer-Büttiker (TD-LB) formalism to the study of periodically driven transport in graphene nanoribbons (GNRs). When an ac driving is applied, this formalism can be used to prove generic conditions for the existence of a nonzero dc component of the net current (pump current) through the molecular device. Time-reversal symmetry breaking in the driving field is investigated and found to be insufficient for a nonzero pump current. We then derive explicit formulas for the current response to a particular biharmonic bias. We calculate the pump current through different GNR configurations and find that the sign and existence of a nonzero pump current can be tuned by simple alterations to the static parameters of the TD bias. Furthermore, we investigate transient currents in different GNR configurations. We find a selection rule of even and odd harmonic response signals depending on a broken dynamical inversion symmetry in the bias.

The Sachdev-Ye-Kitaev (SYK) model is a concrete model for a non-Fermi liquid with maximally chaotic behavior in $(0+1)$ dimensions. In order to gain some insights into real materials in higher dimensions where fermions could hop between different sites, here we consider coupling a SYK lattice by constant hopping. We call this the dispersive SYK model. Focusing on $(1+1)$-dimensional homogeneous hopping, by either tuning the temperature or the relative strength of the random interaction (hopping) and constant hopping, we find a crossover between a dispersive metal to an incoherent metal, where the dynamic exponent $z$ changes from 1 to $\infty$. We study the crossover by calculating the spectral function, charge density correlator, and the Lyapunov exponent. We further find the Lyapunov exponent becomes larger when the chemical potential is tuned to approach a van Hove singularity because of the large density of states near the Fermi surface. The effect of the topological nontrivial bands is also discussed.

We compute from first principles the effective interaction parameters appropriate for a low-energy description of the rare-earth nickelate ${\mathrm{LuNiO}}_3$ involving the partially occupied $e_g$ states only. The calculation uses the constrained random-phase approximation and reveals that the effective on-site Coulomb repulsion is strongly reduced by screening effects involving the oxygen-$p$ and nickel-$t_{2g}$ states. The long-range component of the effective low-energy interaction is also found to be sizable. As a result, the effective on-site interaction between parallel-spin electrons is reduced down to a small negative value. This validates effective low-energy theories of these materials that were proposed earlier. Electronic structure methods combined with dynamical mean-field theory are used to construct and solve an appropriate low-energy model and explore its phase diagram as a function of the on-site repulsion and Hund's coupling. For the calculated values of these effective interactions, we find that in agreement with experiments, ${\mathrm{LuNiO}}_3$ is a metal without disproportionation of the $e_g$ occupancy when considered in its orthorhombic structure, while the monoclinic phase is a disproportionated insulator.

Population distributions and transition fluxes of the $A$ exciton in bulk GaN are theoretically analyzed using rate equations of states of the principal quantum number $n$ up to 5 and the continuum. These rate equations consist of the terms of radiative, electron-collisional, and phononic processes. The dependence of the rate coefficients on temperature is revealed on the basis of the collisional-radiative model of hydrogen plasma for the electron-collisional processes and theoretical formulation using Fermi's “golden rule” for the phononic processes. The respective effects of the variations in electron, exciton, and lattice temperatures are exhibited. This analysis is a base of the discussion on nonthermal equilibrium states of carrier-exciton-phonon dynamics. It is found that the exciton dissociation is enhanced even below 150 K mainly by the increase in the lattice temperature. When the thermal-equilibrium temperature increases, the population fluxes between the states of $n>1$ and the continuum become more dominant. Below 20 K, the severe deviation from the Saha-Boltzmann distribution occurs owing to the interband excitation flux being higher than the excitation flux from the $1S$ state. The population decay time of the $1S$ state at 300 K is more than ten times longer than the recombination lifetime of excitons with kinetic energy but without the upper levels $(n>1$ and the continuum). This phenomenon is caused by a shift of population distribution to the upper levels. This phonon-exciton-radiation model gives insights into the limitations of conventional analyses such as the ABC model, the Arrhenius plot, the two-level model $(n=1$ and the continuum), and the neglect of the upper levels.

One of the remarkable capabilities of ultrashort polarized laser pulses is the generation of laser-induced periodic surface structures (LIPSS). The origin of this phenomenon is largely attributed to the interference of the incident laser wave and surface electromagnetic wave that creates a periodic absorption pattern. Although, commonly, LIPSS are produced by repetitive irradiation of the same area by multiple laser pulses in the regime of surface melting and resolidification, recent reports demonstrate the formation of LIPSS in the single-pulse irradiation regime at laser fluences well above the ablation threshold. In this paper, we report results of a large-scale molecular dynamics simulation aimed at providing insights into the mechanisms of single-pulse ablative LIPSS formation. The simulation performed for a Cr target reveals an interplay of material removal and redistribution in the course of spatially modulated ablation, leading to the transient formation of an elongated liquid wall extending up to $\sim 600$ nm above the surface of the target at the locations of the minima of the laser energy deposition. The upper part of the liquid wall disintegrates into droplets while the base of the wall solidifies on the time scale of $\sim 2$ ns, producing a $\sim 100$-nm-tall frozen surface feature extending above the level of the initial surface of the target. The properties of the surface region of the target are modified by the presence of high densities of dislocations and vacancies generated due to the rapid and highly nonequilibrium nature of the melting and resolidification processes. The insights into the LIPSS formation mechanisms may help in designing approaches for increasing the processing speed and improving the quality of the laser-patterned periodic surface structures.

We present experimental evidence for a quantum phase transition in the easy-axis $S=3/2$ anisotropic quasi-one-dimensional ferromagnet ${\mathrm{CoCl}}_2·2{\mathrm{D}}_2\mathrm{O}$ in a transverse field. Elastic neutron scattering shows that the magnetic order parameter vanishes at a transverse critical field ${\mu }_0{H}_c=16.05$(4) T, while inelastic neutron scattering shows that the gap in the magnetic excitation spectrum vanishes at the same field value, and reopens for $H>H_c$. The field dependence of the order parameter and the gap are well described by critical exponents $\beta =0.45\pm 0.09$ and $z\nu$ close to $1/2$, implying that the quantum phase transition in ${\mathrm{CoCl}}_2·2{\mathrm{D}}_2\mathrm{O}$ differs significantly from the textbook version of a $S=1/2$ Ising chain in a transverse field. We attribute the difference to weak but finite three-dimensionality of the magnetic interactions.

We report a study of magnetism and magnetic transitions of hexagonal ${\mathrm{ErMnO}}_3$ single crystals by magnetization, specific heat, and heat transport measurements. Magnetization data show that the $c$-axis magnetic field induces three magnetic transitions at 0.8, 12, and 28 T. The specific heat shows a peak at 2.2 K, which is due to a magnetic transition of ${\mathrm{Er}}^{3+}$ moments. For low-$T$ thermal conductivity $\left(\kappa \right)$, a clear diplike feature appears in the $\kappa \left(H\right)$ isotherm at 1–1.25 T for $H\parallel ab$, while in the case of $H\parallel c$, a steplike increase is observed at 0.5–0.8 T. The transition fields in $\kappa \left(H\right)$ are in good agreement with those obtained from magnetization, and the anomaly of $\kappa$ can be understood by a spin-phonon scattering scenario. The natures of magnetic structures and corresponding field-induced transitions at low temperatures are discussed.

We have studied the thickness and angular dependence of the magnetocurrent of hot electrons in a magnetic tunnel transistor (MTT) with crossed magnetic anisotropies. In a first step, we show that the magnetocurrent increases with ferromagnetic layer thickness as for MTTs with collinear magnetic configurations. The maximum magnetocurrent value is obtained to be 85%, which is close to the theoretical maximum value of 100% for MTTs with crossed magnetic configurations. In a second step, we demonstrate that we are able to reproduce both current vs field direction and current vs field intensity measurements in a framework taking into account a reduced number of magnetic parameters and a simple cosine dependence of the hot-electron current on the angle between magnetizations.

Superconductivity in noncentrosymmetric ${\mathrm{LaNiC}}_2$ is expected to be induced by electron-phonon interactions due to its lack of magnetic instabilities. The non-Bardeen-Cooper-Schrieffer (BCS) behaviors found in this material call into question the long-standing idea that relates unconventional superconductivity with magnetic interactions. Here we report magnetic penetration-depth measurements in a high-purity single crystal of ${\mathrm{LaNiC}}_2$ at pressures up to 2.5 GPa and temperatures down to 0.04 K. At ambient pressure and below $0.5T_c$ the penetration depth goes as $T^4$ for the in-plane and $T^2$ for the out-of-plane component, firmly implying the existence of point nodes in the energy gap and the unconventional character of this superconductor. The present study also provides evidence of magnetism in ${\mathrm{LaNiC}}_2$ by unraveling a pressure-induced antiferromagnetic phase inside the superconducting state at temperatures below 0.5 K, with a quantum critical point around ambient pressure. The results presented here maintain a solid base for the notion that unconventional superconductivity only arises near magnetic order or fluctuations.

It is of interest to determine the exit angle of a vortex from a superconductor surface, since this affects the intervortex interactions and their consequences. Two ways to determine this angle are to image the vortex magnetic fields above the surface, or the vortex core shape at the surface. In this work we evaluate the field $\mathit{h}(x,y,z)$ above a flat superconducting surface $x,y$ and the currents $\mathit{J}(x,y)$ at that surface for a straight vortex tilted relative to the normal to the surface, for both the isotropic and anisotropic cases. In principle, these results can be used to determine the vortex exit tilt angle from analyses of magnetic field imaging or density of states data.

We present the results of the investigation of magnetic properties of the ${\mathrm{Er}}_2{\mathrm{Ti}}_{2-x}{\mathrm{Sn}}_x{\mathrm{O}}_7$ series. For small doping values, the ordering temperature decreases linearly with $x$, while the moment configuration remains the same as in the $x=0$ parent compound. Around $x=1.7$ doping level, we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time, we present evidence of a long-range order (LRO) in ${\mathrm{Er}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$ ($x=2.0$) below $T_N=130$ mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to $x=0$. We discuss our results in the framework of a possible quantum phase transition occurring close to $x=1.7$.