We study the ground state ordering of quadrupolar ordered $S=1$ magnets as a function of spin dilution probability $p$ on the triangular lattice. In sharp contrast to the ordering of $S=1/2$ dipolar Néel magnets on percolating clusters, we find that the quadrupolar magnets are quantum disordered at the percolation threshold, $p=p^{*}$. Further we find that long-range quadrupolar order is present for all $p<p^{*}$ and vanishes first exactly at $p^{*}$. Strong evidence for scaling behavior close to $p^{*}$ points to an unusual quantum criticality without fine tuning that arises from an interplay of quantum fluctuations and randomness.

FeCrAs displays an unusual electrical response that is neither metallic in character nor divergent at low temperatures, as expected for an insulating response, and therefore it has been termed a “nonmetal metal.” The anomalous resistivity occurs for temperatures below $\sim 900$ K. We have carried out neutron scattering experiments on powder and single crystal samples to study the magnetic dynamics and critical fluctuations in FeCrAs. Magnetic neutron diffraction measurements find ${\mathrm{Cr}}^{3+}$ magnetic order setting in at $T_N=115\mathrm{K}\sim 10$ meV with a mean-field critical exponent. Using neutron spectroscopy we observe gapless, high velocity, magnetic fluctuations emanating from magnetic positions with propagation wave vector ${\vec{q}}_0=(\frac{1}{3},\frac{1}{3})$, which persists up to at least 80 $\mathrm{meV}\sim 927$ K, an energy scale much larger than $T_N$. Despite the mean-field magnetic order at low temperatures, the magnetism in FeCrAs therefore displays a response which resembles that of itinerant magnets at high energy transfers. We suggest that the presence of stiff high-energy spin fluctuations extending up to a temperature scale of $\sim 900$ K is the origin of the unusual temperature dependence of the resistivity.

The temperature-dependent magnetization dynamics in ferrimagnetic TbFeCo alloys with various compositions of Tb is investigated by the pump-probe time-resolved magneto-optical Kerr effect (TR-MOKE) in different geometries. It is shown that, for the case when a magnetic field is applied noncollinearly to the easy axis at room temperature, the decrease of the MOKE signal occurring at several tens of picoseconds (ps) after a rapid demagnetization within a few hundred femtoseconds (fs) is in fact caused by a highly damped precessional motion with precession lifetime shorter than the precession period. This is demonstrated by reducing the Tb content to 6% in atomic ratio and measuring TR-MOKE at elevated temperatures. This has the dual effect of reducing the damping constant allowing the observation of more precession cycles and of weakening the exchange interaction between Tb and FeCo, making the precession oscillations more pronounced. The results give important insight into the ultrafast spin dynamics of rare-earth-doped transition-metal alloys, and the remarkable impact of Tb on the damping in such alloy systems.

The dynamic structure factor of superfluid ${}^4\mathrm{He}$ has been investigated at very low temperatures by inelastic neutron scattering. The measurements combine different incoming energies resulting in an unprecedentedly large dynamic range with excellent energy resolution, covering wave vectors $Q$ up to 5 ${\AA }^{-1}$ and energies $\omega$ up to 15 meV. A detailed description of the dynamics of superfluid ${}^4\mathrm{He}$ is obtained from saturated vapor pressure up to solidification. The single-excitation spectrum is substantially modified at high pressures, as the maxon energy exceeds the roton-roton decay threshold. A highly structured multiexcitation spectrum is observed at low energies, where clear thresholds and branches have been identified. Strong phonon emission branches are observed when the phonon or roton group velocities exceed the sound velocity. The spectrum is found to display strong multiexcitations whenever the single excitations face disintegration following Pitaevskii's type $a$ or $b$ criteria. At intermediate energies, an interesting pattern in the dynamic structure factor is observed in the vicinity of the recoil energy. All these features, which evolve significantly with pressure, are in very good agreement with the dynamic many-body calculations, even at the highest densities, where the correlations are strongest.

The transition metal dichalcogenide ($M{X}_2$, where $M=\mathrm{Mo}$, W and $X=\mathrm{S}$, Se, Te) monolayers are of high interest for semiconducting applications at the nanoscale level; this interest is due to both their direct band gaps and high charge mobilities. In this regard, an in-depth understating of the related Schottky barrier heights, associated with the incorporation of $M{X}_2$ sheets into novel low-dimensional metal-semiconductor junctions, is of crucial importance. Herein, we generate and provide analysis of the Schottky barrier heights behavior to account for the metal-induced gap states concept as its explanation. In particular, the present investigations concentrate on the estimation of the charge neutrality levels directly by employing the primary theoretical model, i.e., the cell-averaged Green's function formalism combined with the complex band structure technique. The results presented herein place charge neutrality levels in the vicinity of the midgap; this is in agreement with previous reports and analogous to the behavior of three-dimensional semiconductors. The calculated canonical Schottky barrier heights are also found to be in agreement with other computational and experimental values in cases where the difference between electronegativities of the semiconductor and metal contact is small. Moreover, the influence of the spin-orbit effects is herein considered and supports that Schottky barrier heights have metal-induced gap state-derived character, regardless whether spin-orbit coupling interactions are considered. The results presented within this report constitute a direct and vital verification of the importance of metal-induced gap states in explaining the behavior of observed Schottky barrier heights at $M{X}_2$-metal junctions.

Recent experimental measurements for near-field radiative heat transfer between two bodies have been able to approach the gap distance within $2\mathrm{nm}$, where the contributions of Coulomb fluctuation and electron tunneling are comparable. Using the nonequilibrium Green's function method in the $G_0{W}_0$ approximation, based on a tight-binding model, we obtain for the energy current a Caroli formula from the Meir-Wingreen formula in the local equilibrium approximation. Also, the Caroli formula is consistent with the evanescent part of the heat transfer from the theory of fluctuational electrodynamics. We go beyond the local equilibrium approximation to study the energy transfer in the crossover region from electron tunneling to Coulomb fluctuation based on a numerical calculation.

Light-induced coupling between bound and delocalized excitons in a single semiconductor thin film has been investigated. A planar defect with a negative $\delta$-function-type potential acts as a potential well for the center-of-mass motion of an exciton. In the nano-to-bulk size regime, enhanced radiative corrections of particular delocalized states attain the binding energy of the bound state, which leads to unconventional resonant states of the excitonic system. We demonstrate that such radiative couplings and their quantum interference enhance light absorption and asymmetric optical spectra around the bound level.

We show theoretically that two-dimensional direct-gap semiconductors with a valley degree of freedom, including monolayer transition-metal dichalcogenides and gapped bilayer graphene, have a longitudinal magnetoconductivity contribution that is odd in valley and odd in the magnetic field applied perpendicular to the system. Using a quantum kinetic theory we show how this valley-dependent magnetoconductivity arises from the interplay between the momentum-space Berry curvature of Bloch electrons, the presence of a magnetic field, and disorder scattering. We discuss how the effect can be measured experimentally and used as a detector of valley polarization.

We investigate electronic transport through a graphene n-p junction in the quantum Hall effect regime at high perpendicular magnetic field, when the filling factors in the $n$-doped and $p$-doped regions are fixed to 2 and $-2,$ respectively. We compute numerically the conductance $G$, the noise $Q$, and the Fano factor $F$ of the junction when inelastic effects are included along the interface in a phenomenological way, by means of fictitious voltage probes. Using a scaling approach, we extract the system coherence length $L_{\varphi }$ and describe the full crossover between the coherent limit ($W\ll L_{\varphi }$) and the incoherent limit ($W\gg L_{\varphi }$), $W$ being the interface length. While $G$ saturates at the value $e^2/h$ in the incoherent regime, $Q$ and $F$ are found to vanish exponentially for large length $W$. Corrections due to disorder are also investigated. Our results are finally compared to available experimental data.

A first-principles-based effective Hamiltonian is developed and used, along with direct ab initio techniques, to investigate finite-temperature properties of the system commonly coined the most complex perovskite, that is ${\mathrm{NaNbO}}_3$. Such simulations successfully reproduce the existence of seven different phases in its phase diagram. The decomposition of the total energy of this effective Hamiltonian into different terms, altogether with the values of the parameters associated with these terms, also allow us to shed some light into puzzling features of such a compound. Examples include revealing the microscopic reasons of why $R3c$ is its ground state and why it solely adopts in-phase tiltings at high temperatures versus complex nanotwins for intermediate temperatures. The results of the computations also call for a revisiting of the so-called $P,R$, and $S$ states, in the sense that an unexpected and previously overlooked inhomogeneous electrical polarization is numerically found in the $P$ state while complex tiltings associated with the simultaneous condensation of several $k$ points are predicted for the controversial $R$ and $S$ phases.

Intense femtosecond infrared laser pulses induce a nonequilibrium between thousands of Kelvin hot valence electrons and room-temperature ions in a germanium sample foil. The evolution of this exotic state of matter is monitored with time-resolved extreme ultraviolet absorption spectroscopy across the Ge $M_{2,3}$ edge ($\simeq 30\mathrm{eV}$) using the FERMI free-electron laser. We analyze two distinct regimes in the ultrafast dynamics in laser-excited Ge: First, on a subpicosecond time scale, the electron energy distribution thermalizes to an extreme temperature unreachable in equilibrium solid germanium; then, during the following picoseconds, the lattice reacts strongly altering the electronic structure and resulting in melting to a metallic state alongside a breakdown of the local atomic order. Data analysis, based on a hybrid approach including both numerical and analytical calculations, provides an estimation of the electron and ion temperatures, the electron density of states, the carrier-phonon relaxation time, as well as the carrier density and lattice heat capacity under those extreme nonequilibrium conditions. Related structural anomalies, such as the occurrence of a transient low-density liquid phase and the possible drop in lattice heat capacity are discussed.

$(\mathrm{Ba},\mathrm{Ca}){\mathrm{TiO}}_3$ and $\mathrm{Ba}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_3$ solid solutions are the building blocks of lead-free piezoelectric materials that attract a renewed interest. We investigate the properties of these systems by means of first-principles calculations, with a focus on the lattice dynamics and the competition between different ferroelectric phases. We first analyze the four parent compounds in order to compare their properties and their different tendency towards ferroelectricity. The core of our study is systematic characterization of the binary systems $(\mathrm{Ba},\mathrm{Ca}){\mathrm{TiO}}_3$ and $\mathrm{Ba}(\mathrm{Ti},\mathrm{Zr}){\mathrm{O}}_3$ within both the virtual crystal approximation and direct supercell calculations. In the case of Ca doping, we find a gradual transformation from $B$-site to $A$-site ferroelectricity due to steric effects that largely determines the behavior of the system. In the case of Zr doping, in contrast, the behavior is eventually dominated by cooperative Zr-Ti motions and the local electrostatics. In addition, our comparative study reveals that the specific microscopic physics of these solids sets severe limits to the applicability of the virtual crystal approximation for these systems.

${}^{55}\mathrm{Mn}$ NMR was used to perform the atomic-scale study of the anisotropic properties of ${\mathrm{Mn}}_5{\mathrm{Ge}}_3$/Ge(111) epitaxial films with thicknesses between 9 and 300 nm. The NMR spectra have been recorded as a function of strong external magnetic field applied in the film plane and perpendicular to it. Two ${}^{55}\mathrm{Mn}$ NMR resonances have been observed, corresponding to the two manganese sites $4d$ and $6g$, in the hexagonal $\mathrm{D}{8}_8$ structure; in zero field their frequency is centered around 207.5 and 428 MHz, respectively. The anisotropy of ${}^{55}\mathrm{Mn}$ hyperfine fields between the hexagonal $c$ direction and the $c$ plane at both Mn sites was evidenced and attributed to the anisotropic term due to the unquenched Mn orbital momentum. The anisotropy of the orbital contribution to hyperfine fields was determined as 1.52 T in the $4d$ site and up to 2.77 T in the $6g$ site. The $4d$ site reveals a quadrupolar interaction due to the strong electric field gradient: $V_{zz}=5.3\times {10}^{19}\text{V}/{\text{m}}^2$ in this site, which is shown to be oriented along the hexagonal $c$ axis.

We study the electric polarization of the kagome staircase $\mathrm{N}{\mathrm{i}}_3{\mathrm{V}}_2{\mathrm{O}}_8$ in magnetic fields up to 30 T and report a magnetoelectric memory effect controlled by bias electric fields. The explored ferroelectric phase in $19–24\mathrm{T}$ is electrically controlled, whereas the ferroelectric phase in $2–11\mathrm{T}$ exhibits unusual memory effects. We determine a characteristic critical magnetic field ${\mathit{H}}_3=11\mathrm{T}$, below which strong memory exists and the polarization is frozen even in opposite bias fields. But when magnetic fields exceed ${\mathit{H}}_3$, the frozen polarization is released and polarization reversal appears by tuning bias electric fields. We ascribe these phenomena to the pinning-depinning mechanism: nucleation and the accompanying pinning of chiral domain walls cooperatively induce the frozen behavior; the polarization reversal results from the depinning through the ferroelectrtic-to-paraelectric phase transition in high magnetic fields. Our experimental results reveal that the first-order phase transition plays an important role in these unusual memory effects.

Understanding (i) the emergence of diffusion from truly microscopic principles continues to be a major challenge in experimental and theoretical physics. At the same time, isolated quantum many-body systems have experienced an upsurge of interest in recent years. Since in such systems the realization of a proper initial state is the only possibility to induce a nonequilibrium process, understanding (ii) the largely unexplored role of the specific realization is vitally important. Our work reports a substantial step forward and tackles the two issues (i) and (ii) in the context of typicality, entanglement as well as integrability and nonintegrability. Specifically, we consider the spin-1/2 XXZ chain, where integrability can be broken due to an additional next-nearest neighbor interaction, and study the real-time and real-space dynamics of nonequilibrium magnetization profiles for a class of pure states. Summarizing our main results, we show that signatures of diffusion for strong interactions are equally pronounced for the integrable and nonintegrable case. In both cases, we further find a clear difference between the dynamics of states with and without internal randomness. We provide an explanation of this difference by a detailed analysis of the local density of states.

The field-induced quantum phase transitions in the disorder-free and disordered samples of the spin ladder material ${\left({\mathrm{CH}}_3\right)}_2{\mathrm{CHNH}}_3\mathrm{Cu}{\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_3$ are studied using magnetic calorimetry and magnetic neutron diffraction on single crystal samples. Drastically different critical indexes and correlation lengths in the high field phase are found for two different orientations of the applied field. It is argued that for a field applied along the crystallographic $c$ axis, as in previous studies [T. Hong et al., Phys. Rev. B 81, 060410 (2010)], the transition is best described as an Ising transition in random field and anisotropy, rather than as a magnetic Bose glass to Bose condensate transition.

Results of inelastic neutron scattering measurements are reported for two annealed compositions of ${\mathrm{Ca}(\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2,x=0.026$ and 0.030, which possess stripe-type antiferromagnetically ordered and superconducting ground states, respectively. In the AFM ground state, well-defined and gapped spin waves are observed for $x=0.026$, similar to the parent ${\mathrm{CaFe}}_2{\mathrm{As}}_2$ compound. We conclude that the well-defined spin waves are likely to be present for all $x$ corresponding to the AFM state. This behavior is in contrast to the smooth evolution to overdamped spin dynamics observed in ${\mathrm{Ba}(\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$, wherein the crossover corresponds to microscopically coexisting AFM order and SC at low temperature. The smooth evolution is likely absent in ${\mathrm{Ca}(\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{)}_2{\mathrm{As}}_2$ due to the mutual exclusion of AFM ordered and SC states. Overdamped spin dynamics characterize paramagnetism of the $x=0.030$ sample and high-temperature $x=0.026$ sample. A sizable loss of magnetic intensity is observed over a wide energy range upon cooling the $x=0.030$ sample, at temperatures just above and within the superconducting phase. This phenomenon is unique amongst the iron-based superconductors and is consistent with a temperature-dependent reduction in the fluctuating moment. One possible scenario ascribes this loss of moment to a sensitivity to the $c$-axis lattice parameter in proximity to the nonmagnetic collapsed tetragonal phase and another scenario ascribes the loss to a formation of a pseudogap.

Large-scale molecular dynamics simulations have been used to study the microstructure in Y-doped ${\mathrm{ZrO}}_2$. From simulations performed as a function of composition the dependence of microstructure on composition is quantified, showing how it is formed from two coexisting phases, and the transformation to the stabilized cubic form is observed at higher concentrations of yttrium and higher temperatures. The effect of composition and temperature on oxygen diffusion is also studied, showing strong correlations between microstructure and diffusion.

Antiferromagnetic insulators (AFIs) have attracted much interest from many researchers as promising candidates for use in ultrafast, ultralow-dissipation spintronic devices. As a fast method of reversing magnetization, precessional switching is realized when antiferromagnetic Néel orders $l=\left({\mathbf{s}}_1+{\mathbf{s}}_2\right)/2$ surmount the magnetic anisotropy or potential barrier in a given magnetic system, which is described well by the antiferromagnetic plane pendulum (APP) model. Here, we report that, as an alternative switching scenario, the direct coupling of an electric field with Dzyaloshinskii-Moriya (DM) interaction, which stems from spin-orbit coupling, is exploited for optimal switching. We derive the pendulum equation of motion of antiferromagnets, where DM torque is induced by a pulsed electric field. The temporal DM interaction is found to not only be in the form of magnetic torques (e.g., spin-orbit torque or magnetic field) but also modifies the magnetic potential that limits $l$ 's activity; as a result, appropriate controls (e.g., direction, magnitude, and pulse shape) of the induced DM vector realize deterministic reversal in APP. The results present an approach for the control of a magnetic storage device by means of an electric field.