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We consider how electron-phonon interaction influences the insulator-metal transitions driven by doping in the strongly correlated system. Using the polaronic version of the generalized tight-binding method, we investigate a multiband two-dimensional model taking into account both Holstein and Su-Schrieffer-Heeger types of electron-lattice contributions. For adiabatic ratio of the hopping parameter and the phonon field energy, different types of band structure evolution are observed in a wide electron-phonon parameter range. We demonstrate the relationship between transition features and such properties of the system as the polaron and bipolaron crossovers, pseudogap behavior of various origin, orbital selectivity, and the redistribution of the spectral weight due to the electron-phonon interaction., Comment: 6 pages with 4 figures

We propose a mechanism to suppress heating in periodically driven many-body quantum systems by employing sufficiently long-range interactions and experimentally relevant initial conditions. The mechanism is robust to local perturbations and does \emph{not} rely on disorder or high driving frequencies. Instead, it makes use of an approximate fragmentation of the many-body spectrum of the non-driven system into bands, with band gaps that grow with the system size. We show that when these systems are driven, there is a regime where \emph{decreasing} the driving frequency \emph{decreases} heating and entanglement build-up. This is demonstrated numerically for a prototypical system of spins in one dimension, but the results can be readily generalized to higher dimensions., Comment: 5 pages, 4 figures

A critical parameter in characterizing the properties of single-domain nanoparticles is their magnetic relaxation time. It must be known, for example, to estimate the anisotropy from magnetization versus temperature measurements. The time it takes for the magnetization to relax also determines the behavior of particles in various oscillating applied fields, which is critically important for their application in magnetic particle imaging and hyperthermia treatment. However, an analytic expression for this relaxation time has been generally missing. Brown's [Phys. Rev. 130, 1677 (1963)] famous result is only valid for the easy anisotropy axes of each particle in the ensemble aligned along the external field direction and overestimates the relaxation time. Despite this overestimation, this expression is most commonly used to extract magnetic nanoparticle parameters such as anisotropy energy from magnetometry data. Here, we use Brown's formalism to derive a different, simple, approximate relaxation time expression that is valid for randomly aligned easy axes. Using parameters appropriate for magnetite, we compare our results to other results in the literature and with stochastic Landau-Lifshitz-Gilbert simulations to show that our result is more accurate across a range of applied field strengths and temperatures. We note that several analytic expressions for the relaxation time do a reasonable job, as long as one uses a full calculation for the attempt time, rather than the commonly used estimate ${\ensuremath{\tau}}_{0}\ensuremath{\sim}1$ ns.

We present a rigorous quasinormal mode approach to describe the strong coupling behavior between a monolayer of ${\mathrm{MoSe}}_{2}$ and a single gold nanoparticle. The onset of strong coupling, described through a classical spectral mode splitting (analog of vacuum Rabi splitting) is quantified by computing the full three-dimensional hybrid quasinormal modes of the combined structure, allowing one to accurately model light-matter interactions without invoking the usual phenomenological theories of strong coupling. We explore the hybrid quasinormal modes as a function of gap size and temperature, and find spectral splittings in the range of around 80--110 meV, with no fitting parameters for the material models. We also show how the hybrid modes exhibit Fano-like resonances and quantify the complex poles of the hybrid modes as well as the Purcell factor resonances from embedded dipole emitters. The Rabi splitting is found to be larger at elevated temperatures for very small gap separations between the metal nanoparticle and the monolayer, but smaller at elevated temperatures for larger gaps. We also show how these spectral splittings can differ qualitatively from the actual complex poles of the hybrid quasinormal modes.

Although the contact theory between rough surfaces is designed for adhesion energies ≳100mJ/m2, microsystems are controlled by much weaker adhesion ≲100μJ/m2, which is critical for their operation. The weakest adhesion is related to the omnipresent fluctuation-induced dispersion forces. We develop a theory for such a weak adhesion emphasizing that the adhesion energy as a function of the average distance separating the bodies is almost entirely defined by the dispersion interaction. This dependence can be evaluated using the Lifshitz theory, but the effects of contact interaction or plastic deformations give only small contribution to the adhesion. Such a behavior is explained by a specific roughness of the deposited thin films used in microtechnologies. The films deposited on cold substrates have a much larger number of high asperities than is predicted by the Gaussian distribution and the contact occurs over a few asperities with heights much larger than the root-mean-square roughness. Finally, we discuss application of the effect for more precise determination of the distance upon contact, which is crucial for precise measurements of the dispersion forces especially at short separations in the range 5

The indirect band gap of the diamondlike silicon does not allow direct travel of electrons between valence band and conduction band edges, which has limited its application and performance in optoelectronic devices. Searching new silicon allotropes with highly tunable or direct band gap becomes more and more urgent with the increasing demand on clean energy. Here, we predict a silicon allotrope, $h10$-Si, which is an indirect-gap semiconductor with a band gap of about 0.96 eV. Furthermore, the electronic and optical properties exhibit a strong dependence on strain. In particular, the indirect band gap of the predicted silicon allotrope can be switched to a direct band gap at a uniaxial tensile strain of about 8% and the light absorption can be tuned continuously in a wide range of photon energy. Besides, the single-layer counterpart of this allotrope is confirmed to be dynamically stable and more stable in energy than silicene, endowing it with potential applications in nanoscale devices.

Using terahertz spectroscopy, we investigate resonant transmission through a dielectric slab with a conducting layer of varied mobility on one side. We observe the well-known Fabry-P\'erot resonances in the transmission. Two fundamentally different regimes of the electrodynamic response of the system are established. In the clean limit, when the radiation frequency exceeds the carrier relaxation rate, $\ensuremath{\omega}\ensuremath{\tau}\ensuremath{\gg}1$, the system exhibits a plasmonic response. As a result, there is a continuous shift in the Fabry-P\'erot resonances with respect to the system parameters. By contrast, in the dirty regime, when $\ensuremath{\omega}\ensuremath{\tau}\ensuremath{\ll}1$, there is an abrupt $\ensuremath{\pi}$ shift in the resonance frequency, as the renormalized film resistance matches the impedance of free space. We find that under this condition, Fabry-P\'erot oscillations disappear entirely over a broad frequency range. Such a matching effect might be of interest in various terahertz applications, for example, involving broadband filters or absorbers.

The material Sr${}_{2}$RuO${}_{4}$ is notorious for the difficulty in reconciling all experimental characterizations of its superconducting order parameter. In this theoretical study, the authors explore what superconducting order parameters are most favorable from the perspective of spin-fluctuation mediated pairing, taking into account the multiorbital nature of the system and its sizable spin-orbit coupling. They find that longer-range Coulomb interactions induce a near-degeneracy between nodal $s$-wave and ${d}_{x\phantom{\rule{0}{0ex}}y}$ superconductivity, suggesting a ground-state condensate with time-reversal symmetry breaking of the form $s+i\phantom{\rule{0}{0ex}}{d}_{x\phantom{\rule{0}{0ex}}y}$.

We numerically study crossed Andreev reflection (CAR) in a topological insulator nanowire T-junction where one lead is proximitized by a superconductor. We perform realistic simulations based on the 3D BHZ model and compare the results with those from an effective 2D surface model, whose computational cost is much lower. Both approaches show that CAR should be clearly observable in a wide parameter range, including perfect CAR in a somewhat more restricted range. Furthermore, it can be controlled by a magnetic field and is robust to disorder. Our effective 2D implementation allows to model systems of micronsize, typical of experimental setups, but computationally too heavy for 3D models., 10 pages, 8 figures

We apply the cluster expansion (CE) method to determine the effective cluster interactions (ECIs) from a simple energetic model that depends on both local and global composition. This model is defined by the site energies of random solid solutions of a one-dimensional alloy Co-Pt. We explore how these local and global dependencies are reflected on the cluster interactions. The energies of the structures are not well reproduced with concentration-independent interactions. Moreover, the interactions have a larger range than the energetic model which is limited to the nearest neighbors. This problem does not seem to have been addressed until now. By fitting the ECIs on the site energies we suggest a mean-field type weighting of the excess variables present in large clusters size. We show that the site energy formalism controls the clusters size required for CE convergence and their concentration dependence. Finally, we take advantage of the site energy formalism to describe the elastic and chemical effects that control the thermodynamics of the alloy as a function of the ECIs.

In this paper, we measure the bilinear interlayer magnetic coupling ${J}_{1}$ between two Co layers coupled across a Ru spacer layer over a wide range of spacer layer thicknesses from 0.4 to 3.4 nm and temperatures from 5 to 300 K. These measurements are fit using the interface-reflection interlayer magnetic coupling model in order to determine coupling strengths and electron Fermi velocities within the spacer layer in the direction perpendicular to the film interface for each of the critical spanning vectors. We find that there is a significant contribution to ${J}_{1}$ from several different critical spanning vectors, all with different periods of oscillation with respect to the spacer layer thickness. The results indicate that there is likely no exponential superexchangelike contribution to coupling in our samples. The nonoscillatory antiferromagnetic coupling bias of ${J}_{1}$ seen in thinner Ru spacer layers can be explained solely by a linear combination of oscillatory Ruderman-Kittel-Kasuya-Yosida-like coupling from several different critical spanning vectors, all with different periods of oscillation. The experimentally determined electron Fermi velocities are found to be within the range expected from theoretical calculations. The results also indicate that the interface-reflection model is capable of describing the bilinear interlayer exchange coupling in our samples over the entire range of spacer layer thicknesses and temperatures measured in this paper.

The low temperature acoustic properties of bulk metallic glasses measured over a broad range of frequencies rigorously test the predictions of the standard tunneling model. The strength of these experiments and their analyses is mainly based on the interaction of the tunneling states with conduction electrons or quasiparticles in the superconducting state. A new series of experiments at kHz and GHz frequencies on the same sample material essentially confirms previous measurements and their discrepancies with theoretical predictions. These discrepancies can be lifted by considering more correctly the line widths of the dominating two-level atomic-tunneling systems. In fact, dephasing caused or mediated by interaction with conduction electrons may lead to particularly large line widths and destroy the tunneling sytems' two-level character in the normal conducting state.

Solid-state superconducting heterostructures can provide a possible source of entangled electrons for the purpose of quantum-information processing, and shot noise can indicate when this occurs. Shot noise cross-correlations in a ballistic NSNSN (normal-superconductor-normal-superconductor-normal) is normally negative for electron currents, but becomes positive for entangled electrons that result from the breakup of Cooper pairs. A one-to-one correspondence is found between the energies of the quasibound states and regions of positivity in the cross-correlated shot noise. The quasibound states of the NSNSN system are associated with poles of the NSNSN scattering matrix. We find that regions of positive cross-correlated shot noise distributions, and the associated emission of entangled electrons, exist over a wide range of system sizes and in the presence of multiple quasibound states. We also find that, at the quasibound-state energies, the Andreev approximation is not adequate to describe the key physical processes in the NSNSN device.

We show with a recently devised extended first-principles molecular dynamics method that calculated Hugoniots of poly-$\ensuremath{\alpha}$-methylstyrene agree well with precision experimental results of Kritcher et al. [Nature (London) 584, 51 (2020)] and D\"oppner et al. [Phys. Rev. Lett. 121, 025001 (2018)]. The deviation is smaller than 0.8%. This agreement does not sensitively rely on the approximations in the employed first-principles methods as long as underlying physics are well described, as illustrated in the calculation of equation of state for polystyrene covering the warm dense regime. These results may stimulate a broad range of quantitative investigations on warm dense matter that were not thought possible before, and may thus afford a new prospect to the field of inertial confinement fusion and high-energy-density physics.

In spin-transport experiments with spin currents propagating through an antiferromagnetic (AFM) material, the antiferromagnet is mainly treated as a passive spin conductor not generating nor adding any spin current to the system. The spin current transmissivity of the AFM NiO is affected by magnetic fluctuations, peaking at the N\'eel temperature and decreasing by lowering the temperature. To study the role of antiferromagnetism in local and nonlocal spin-transport experiments, we send spin currents through NiO of various thicknesses placed on ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$. The spin currents are injected either electrically or by thermal gradients and measured at a wide range of temperatures and magnetic field strengths. The transmissive role is reflected in the sign change of the local electrically injected signals and the decrease in signal strength of all other signals by lowering the temperature. The thermally generated signals, however, show an additional upturn below $100\phantom{\rule{4pt}{0ex}}\mathrm{K}$ that is unaffected by an increased NiO thickness. A change in the thermal conductivity could affect these signals. The temperature and magnetic field dependence are similar to those for bulk NiO, indicating that NiO itself contributes to thermally induced spin currents.

In this work, we investigate the possibilities of reaching wide-angle perfect absorption with Huygens' metasurfaces possessing both electric and magnetic responses. We analyze the role of the anisotropy of the polarizability tensor of the unit cell and reveal the conditions for the optimal angular stability in symmetric absorption as well as polarization independence. Then we approximate the desirable behavior with a Huygens' metasurface whose reflectance is low in a wide frequency range while the absorption is total around a single resonance frequency. Reaching of the optimal angular stability was numerically demonstrated and the corresponding wide-angle and wide-band properties were experimentally checked.

We analyze the thermoelectric cooling properties of a Corbino device in the quantum Hall regime on the basis of experimental data of electrical conductance. We focus on the cooling power and the coefficient of performance within and beyond linear response. Thermovoltage measurements in this device reported in {\em Phys. Rev. Applied, {\bf 14} 034019 (2020)} indicated that the transport takes place in the diffusive regime, without signatures of effects due to the electron-phonon interaction in a wide range of temperatures and filling factors. In this regime, the heat and charge currents by electrons can be described by a single transmission function. We infer this function from experimental data of conductance measurements and we calculate the cooling power and the coefficient of performance for a wide range of filling factors and temperatures, as functions of the thermal and electrical biases. We predict an interesting cooling performance in several parameter regimes., 10 pages, 8 figures

We numerically examine the depinning and sliding dynamics of a Wigner crystal in the presence of quenched disorder and a magnetic field. In the disorder-free limit, the Wigner crystal Hall angle is independent of crystal velocity, but when disorder is present, we find that Hall angle starts near zero at the depinning threshold and increases linearly with increasing drive before reaching a saturation close to the disorder free value at the highest drives. The drive dependence is the result of a side jump effect produced when the charges move over pinning sites. The magnitude of the side jump is reduced at the higher velocities. The drive dependent Hall angle is robust for a wide range of disorder parameters and should be a generic feature of classical charges driven in the presence of quenched disorder and a magnetic field., Comment: 6 pages, 4 postscript figures

We report an inelastic neutron scattering study on the magnetic excitations of ${\mathrm{Pr}}_{2}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ ($\ensuremath{\delta}\ensuremath{\sim}0.24\ifmmode\pm\else\textpm\fi{}0.01$) at $T=10$ K. Spin stripe ordering becomes pronounced below $\ensuremath{\sim}220$ K with an incommensurability $\ensuremath{\epsilon}\ensuremath{\approx}0.346$, and a strong influence of interstitial oxygen is identified on establishing a long-range spin stripe ordering. Apart from the Goldstone modes emerging from the magnetic satellites (${\mathbf{q}}_{m}$), multiple homologous modes are observed along the spin stripe modulation separated by $\mathrm{\ensuremath{\Delta}}{\mathbf{q}}_{m}\ensuremath{\approx}0.076$ in reciprocal lattice units, which is interpreted by the internal periodicity of the long-range ordered discommensurated spin stripes.

The current-induced motion of transverse magnetic domain walls (DWs) in a multisegmented Co/Ni nanowire is investigated numerically. We find that the phase diagram current pulse length magnitude presents a rare diversity of behaviors depending on the segment's length and material parameters. We show that by changing only the pulse shape, in a range of parameters we obtain the controlled motion of the DW with or without polarity change. The polarity change arises in the simplest case from the birth and propagation of an antivortex along the width of the nanowire. The antivortex can be displaced over long distances depending on the pulse characteristics and boundary conditions. The systematic motion of the DW with polarity flip is found to be stable at room temperature. Moreover, by modifying the material parameters through alloying, the phase diagram can be engineered, decreasing the depinning current and paving the way for storage or logic applications.

Plasmonic sensors based on metallic nanorings benefit from resonances covering a wide spectral range and homogeneous cavity fields. Here, we explore the potential for nanorings in active plasmonics by examining the tunability of plasmon resonances due to external magnetic fields. Within the electrostatic approximation, we compute plasmon resonances and their shifts in magnetic fields for both solid and planar nanorings. In particular, solid nanorings of circular, elliptical, and disk-shaped cross sections are critically examined and compared to planar rings. Overall, we find that magnetoplasmon shifts in nanorings are greatly reduced compared to standard nanoparticles. However, flat geometries are found to be preferable and allow for relatively large shifts.

We report a resonant elastic x-ray scattering study of a thin-film sample of ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$, which has one of the highest ferrimagnetic transition temperatures among ordered double perovskites. We found resonantly enhanced magnetic Bragg peaks at $\mathbf{Q}=$ (odd, odd, odd) at both the rhenium ${L}_{2}$ and ${L}_{3}$ edges, which coincide with the structural Bragg peaks of ${\mathrm{Sr}}_{2}{\mathrm{CrReO}}_{6}$. By analyzing the widths of these Bragg peaks, we extracted very different structural and magnetic correlation lengths. The former is about 15 nm, while the latter is constrained by the instrumental resolution to be at least 90 nm. We argue that a finite structural correlation length is consistent with the existence of antiphase nanodomains in our sample. On the other hand, a much larger magnetic correlation length indicates that the magnetic correlation extends far beyond the boundaries of individual domains and is consistent with strong antiferromagnetic coupling between different antiphase domains. Last, from the azimuthal dependence of the magnetic intensity, we show that the magnetic moments lie perpendicular to the $c$ axis, which explains the earlier bulk magnetization data.

LaCrGe$_3$ has attracted attention as a paradigm example of the avoidance of ferromagnetic (FM) quantum criticality in an itinerant magnet. By combining thermodynamic, transport, x-ray and neutron scattering as well as $\mu$SR measurements, we refined the temperature-pressure phase diagram of LaCrGe$_3$. We provide thermodynamic evidence (i) for the first-order character of the FM transition when it is suppressed to low temperatures and (ii) for the formation of new phases at high pressures. From our microscopic data, we infer that short-range FM ordered clusters exist in these high-pressure phases. These results suggest that LaCrGe$_3$ is a rare example, which fills the gap between the two extreme limits of avoided FM quantum criticality in clean and strongly disordered metals., Comment: 7 pages, 4 figures plus Supplemental Information

We study the ground state of a system with an interface between $\nu=4$ and $\nu=3$ in the quantum Hall regime. Far from the interface, for a range of interaction strengths, the $\nu=3$ region is fully polarized but $\nu=4$ region is locally a singlet. Upon varying the strength of the interactions and the width of the interface, the system chooses one of two distinct edge/interface phases. In phase A, stabilized for wide interfaces, spin is a good quantum number, and there are no gapless long-wavelength spin fluctuations. In phase B, stabilized for narrow interfaces, spin symmetry is spontaneously broken at the Hartree-Fock level. Going beyond Hartree-Fock, we argue that phase B is distinguished by the emergence of gapless long-wavelength spin excitations bound to the interface, which can, in principle, be detected by a measurement of the relaxation time $T_2$ in nuclear magnetic resonance., Comment: 9 pages 10 figures including supplemental material

Motivated by the poor understanding of the applicability of new exchange-correlation (XC) functionals to warm dense matter (WDM), we designed and performed multiple-shock reverberation compression experiments on dense krypton to evaluate explicitly the implications of recently derived XC functionals. The equation of states of krypton up to 155 GPa and 45 000 K, which ranges from an initial dense gaseous state up to the insulator-metal transition regime, were determined accurately. It is found that the experimental data are better reproduced by the strongly constrained and appropriately normed (SCAN) XC functional compared to the conventional Perdew-Burke-Ernzerhof and Van der Waals (vdW) DF1 functionals, elucidating that the introduction of the kinetic energy density and the intermediate-range vdW interaction is decisive. However, the incorporation of long-range interactions into the SCAN ($\mathrm{SCAN}+\mathrm{rVV}10$ XC functional) results in a noticeably stiffer prediction due to an overestimation of the density and internal energy of the system at low densities and temperatures. Our evaluation of the Karasiev-Sjostrom-Dufty-Trickey free-energy functional experimentally validates the XC thermal effect in the WDM regime, verifies the previous predictions, and sheds light on a direction for future theoretical efforts. Finally, a phase diagram of krypton is given, which provides a clear picture for understanding the thermophysical behavior of krypton in a wider temperature-pressure range.

Relaxor ferroelectrics are of great scientific and technological significance as they exhibit large and unusual responses to external stimuli. Their hallmark features are broadness and frequency dispersion of the peak in the temperature dependence of the dielectric constant. Both are believed to originate from the dynamics of polar nanoregions. We apply first-principles-based molecular dynamics to resolve the gigahertz electric response of dynamically poled $\mathrm{Ba}({\mathrm{Ti}}_{1\ensuremath{-}x},{\mathrm{Zr}}_{x}){\mathrm{O}}_{3}$ ferroelectric relaxors that remained overlooked so far. We find that (i) the hallmark relaxor features continue to persist even in the dynamically poled structures, but do not necessarily originate from the polar nanoregions dynamics; (ii) dynamically poled samples exhibit polarization aging which leads to the frequency dependence of both remnant polarization and the Curie temperature; (iii) ``dynamical'' nature of the latter naturally explains the frequency dependence of the dielectric susceptibility maximum in dynamically poled $\mathrm{Ba}({\mathrm{Ti}}_{1\ensuremath{-}x},{\mathrm{Zr}}_{x}){\mathrm{O}}_{3}$; and (iv) incorporation of the polarization aging into dielectric susceptibility expression explains its enhancing contribution in an intuitive way.

We present time-dependent density functional molecular dynamics (TDDFT-MD) simulations with an adiabatic approximation to the exchange-correlation potential for hyperthermal protons (${\mathrm{H}}^{+}$) with initial kinetic energies in the range of 2--50 eV impinging on the fcc-hollow site and the on-top site of an Al(111) surface. The surface is modeled by a finite-size cluster and the results are generalized to ${\mathrm{H}}^{+}$-Al surface scattering. From the simulation, neutralization distances are determined and the time development of the kinetic energy and the electronic excitation energy are derived. The results can be rationalized on the basis of the ground-state potential energy surface and the ${\mathrm{H}}^{+}$-Al(111) interaction potential. Furthermore, the difference in initial kinetic energy between ${\mathrm{H}}^{+}$ and ${\mathrm{H}}^{0}$ projectiles required to yield identical exit velocities is derived. Notably, this difference changes sign within the studied range of kinetic energies. This is traced back to the neutralization distances and the difference between both the ground-state potential energy surface and the ${\mathrm{H}}^{+}$-Al(111) interaction potential at those distances.

Artificial hyperbolic metamaterials (HMM) are perspective for the nonlinear optical applications, their exciting functionality being due to the hyperbolic dispersion induced by a strong shape anisotropy. Here we study the second harmonic generation (SHG) in HMM formed by arrays of silver nanorods in anodic alumina. In the hyperbolic dispersion regime, giant SHG is observed associated with the fulfillment of the phase-matching conditions supported by the epsilon-near-pole resonance at the SHG wavelength, and an increase in the pump field in the epsilon-near-zero (ENZ) spectral range. We predict a strong increase in the SHG efficiency for metamaterials with the ENZ resonance at the second harmonic frequency.

Despite recent transport studies of ABC-stacked multilayer graphene systems revealed various unusual quantum phenomena which arise from the nearly flat electronic bands, their quantum tunneling properties have rarely been addressed. Here we investigate the local tunneling characteristics of a gapped ABC-stacked trilayer graphene (TLG) and report the experimental observation of the nearly flat band induced atomic-site-dependent negative differential conductivity (NDC, characterized by a current drop with increasing voltage) via scanning tunneling spectroscopy (STS) measurements. We show that strong NDC emerges in the gap region next to a sharp STS peak induced by the very flat low-energy dispersion of ABC TLG. The NDC is found to mainly reside on one atomic sublattice of the surface layer due to the strong sublattice and layer localization of the nearly flat bands. The observed NDC behavior is explained by the tunnel-diode mechanism, namely, the coexistence of a sharp flat-dispersion STS peak in which tunneling is strongly enhanced and a subsequent gap region in which tunneling is forbidden. Furthermore, we also find that a local defect could effectively switch off the NDC over a large spatial range. Our result highlights a quantum tunneling effect unique to the graphene-based nearly flat band system and expands the potential application scope of ABC TLG.

Considerable attention has been paid recently to $\mathrm{Fe}{\mathrm{Te}}_{2}{\mathrm{O}}_{5}\mathrm{Cl}$ due to reduced dimensionality and frustration in the magnetic subsystem, succession of phase transitions, and multiferroicity. The efforts to grow its selenite sibling resulted in the mixed halide compound $\mathrm{Fe}({\mathrm{Te}}_{1.5}{\mathrm{Se}}_{0.5}){\mathrm{O}}_{5}\mathrm{Cl}$, which was found crystallizing in a different structural type and possessing properties drastically different from those of a parent system. Its magnetic subsystem features weakly coupled ${\mathrm{Fe}}^{3+}\ensuremath{-}{\mathrm{Fe}}^{3+}$ dimers showing the regime of short-range correlations at ${T}_{\mathrm{M}}\ensuremath{\sim}70\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and long-range order at ${T}_{\mathrm{N}}=22\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. In a magnetically ordered state, sizable spin-orbital interactions lead to a small canting of ${\mathrm{Fe}}^{3+}$ moments. Magnetic dipole-dipole interactions contribute significantly to the experimentally observed orientation of magnetization easy axis in the $ac$ plane. The first principles calculations of leading exchange interactions were found in agreement with measurements of thermodynamic properties and Raman spectroscopy.

We investigate diffusion of excitation in one- and two-dimensional lattices with random onsite energies and deterministic long-range couplings (hopping) inversely proportional to the distance. Three regimes of diffusion are observed in strongly disordered systems: ballistic motion at short time, standard diffusion for intermediate times, and a stationary phase (saturation) at long times. We propose an analytical solution valid in the strong-coupling regime which explains the observed dynamics and relates the ballistic velocity, diffusion coefficient, and asymptotic diffusion range to the system size and disorder strength via simple formulas. We show also that in the long-time asymptotic limit of diffusion from a single site the occupations form a heavy-tailed power-law distribution.

As a potential high-efficiency XUV light source, high-order harmonic generation (HHG) in solids contains the dynamic information of electrons. The interactions among many particles in solids are ubiquitous and, to some extent, modulate the motion of electrons. However, the role of electron-electron interaction (EEI) effect in solid HHG has not been well understood. In this paper, we study the laser-intensity-dependent EEI effect under the given laser pulse durations. By comparing the finite chain model with the infinite extension model, we find that, if the electric field is strong enough, the induced electric field caused by EEI is nontrivial for the finite chain model. We also find that EEI only can enhance HHG spectra in a limited frequency range under some specific intensities of the laser field in both models, and the EEI effect is related to the configuration of the system. This study can provide a reference for whether EEI can be ignored when simulating the interaction between one-dimensional solids and lasers.

The layered van der Waals ferromagnetic ${\mathrm{Fe}}_{3}{\mathrm{GeTe}}_{2}$ harbors an unconventional interplay between topology and magnetism, leading to a large anomalous Hall conductivity at low temperatures. Here, we investigate the temperature dependence of its charge dynamics and reveal that upon entering the ferromagnetic state at ${T}_{C}\ensuremath{\sim}200$ K and further lowering the temperature there is the onset of a gradual spectral weight reshuffling from the mid-infrared range towards far- as well as near-infrared frequencies. This twofold spectral weight transfer indicates the important role of the Hund's coupling as primary source for electronic correlations and signals an incoherent-coherent crossover at low temperatures. Our findings also convey the electronic environment, based on nodal-line topological states, favoring the large anomalous Hall conductivity.

The anisotropy and the temperature dependence of the spin-wave stiffness constants of the permanent magnet ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$ are determined in the range 160--440 K using single-crystal inelastic neutron scattering technique. It is found that the deviation from isotropic spin-wave dispersion, between the $a$ and the $c$ axis of the tetragonal structure, is small and eventually the anisotropy decreases when the temperature increases. The softening of the stiffness constants can be described by a linear temperature variation with an average rate of about $\ensuremath{-}0.46$ meV ${\AA{}}^{2}/\mathrm{K}$. These results are compared with the most recent atomistic and multiscale calculations.

Electron transport across a tunneling barrier is governed by intricate and diverse causes such as interface conditions, material properties, and device geometries. Here, by measuring the shot noise, we investigate electron transport in a (Ga,Mn)As/GaAs-based spin Esaki diode junction over a wide range of bias voltage. The asymmetric electronic band profile across the junction allows us to tune the types of tunneling process. By changing the bias voltage in a single device, we successively address the conventional direct tunneling, the excess current conduction through the mid-gap localized states, and the thermal excitation current conduction. These observations lead to a proper comparison of the bias dependent Fano factors. While the Fano factor is unity for the direct tunneling, it is pronouncedly reduced in the excess current region. Thus, we have succeeded in evaluating several types of conduction process with the Fano factor in a single junction.

A numerical approach, based on a local comparative projectional analysis onto symmetry modes of elementary structural units of glasses and their crystalline counterparts, is developed and used for the analysis of atomic vibrations in glasses in terms of phonons for the entire frequency range. The results of such an analysis shed light on the origin of the modes in the controversial low-frequency range, including the boson-peak region. Comparative analyses undertaken for glass, lattice-glass, and polycrystalline models reveal the role of different types of disorder in atomic vibrations.

Collective excitations in liquids are important for understanding liquid dynamical and thermodynamic properties. Gapped momentum states (GMS) are a notable feature of liquid dynamics predicted to operate in the transverse sector of collective excitations. Here, we combine inelastic neutron scattering experiments, theory and molecular dynamics modelling to study collective excitations and GMS in liquid Ga in a wide range of temperature and $k$-points. We find that all three lines of enquiry agree for the longitudinal sector of liquid dynamics. In the transverse sector, the experiments agree with theory, modelling as well as earlier X-ray experiments at larger $k$, whereas theory and modelling agree in a wide range of temperature and $k$-points. We observe the emergence and development of the $k$-gap in the transverse sector which increases with temperature and inverse of relaxation time as predicted theoretically., 11 pages, 8 figures Reference to theoretical equations used in version 1 (arXiv:2005.00470v1) is corrected

The ability to generate transversely propagating elastic waves on free-standing semiconductor films in the 10+ GHz frequency range is a critical element in the effort to achieve phonon-based coupling to on-chip electronic devices. The results of this joint experimental and theoretical study show that such phonons are created by the excitation of free-standing optically subwavelength thickness GaAs films with highly focused femtosecond optical pulses. With sufficiently high strain levels, this approach could become enabling for applications in nano-optomechanics.

The reflectivity of noble metals can dramatically change upon ultrashort laser irradiation due to the heating of $d$ electrons to high temperatures. Knowledge of the resulting transient optics is fundamental for determining and controlling the absorbed energy. In this work, high-accuracy self-reflectivity measurements are performed and interpreted using a theory for the space-resolved and time-resolved energy absorption and dissipation. The combined approach yields the reflectivity of gold in dependence on the incident laser fluence for a wide spectral range.

Two separate gate electrodes with opposite gate voltages can drive an AB-stacked bilayer graphene (BLG) under them into different quantum valley Hall states. And between the two topological domains there appears a one-dimensional boundary, dubbed the topological channel (TC). By means of the Lanczos method, we theoretically study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in such a TC of BLG. We find that the RKKY interaction is long-range with an ${R}^{\ensuremath{-}1}$ decay rate as the distance $R$ between two magnetic impurities increases, whereas the slowest decay rate in any graphene structure is ${R}^{\ensuremath{-}2}$ according to previous literature. Moreover, we also find that the strength and sign of the RKKY interaction in the TC can be readily controlled by altering the gate voltage or carrier doping. Such a tunable and ${R}^{\ensuremath{-}1}$ decaying RKKY interaction implies the possibility of establishing and controlling a long-range magnetic ordering state in a dilute magnetic impurity-doped BLG.

The interfacial Dzyaloshinskii-Moriya interaction (iDMI) is of great interest in thin-film magnetism because of its ability to stabilize chiral spin textures. It can be quantified by investigating the frequency nonreciprocity of oppositely propagating spin waves. However, as the iDMI is an interface interaction, the relative effect reduces when the films become thicker, making quantification more difficult. Here, we utilize all-electrical propagating spin-wave spectroscopy to disentangle multiple contributions to spin wave frequency nonreciprocity to determine the iDMI. This is done by investigating nonreciprocities across a wide range of magnetic layer thicknesses (from 4 to 26 nm) in Pt/Co/Ir, Pt/Co/Pt, and Ir/Co/Pt stacks. We find the expected sign change in the iDMI when inverting the stack order and a negligible iDMI for the symmetric Pt/Co/Pt. We additionally extract a difference in surface anisotropies and find a large contribution due to the formation of different crystalline phases of the Co, which is corroborated using nuclear magnetic resonance and high-resolution transmission-electron-microscopy measurements. These insights will open up avenues to investigate, quantify, and disentangle the fundamental mechanisms governing the iDMI, and pave a way toward engineered large spin-wave nonreciprocities for magnonic applications.

A density functional theory based semiempirical van der Waals (vdW) method with dielectric functions being incorporated is developed for two-dimensional materials. The coefficients of interatomic pairwise potentials are derived from atomic polarizabilities obtained via a Clausius-Mossotti relation dedicated for layered crystals. The method not only can efficiently describe the dispersion energy for a range of planar graphene-like materials at nearly the same accuracy as the adiabatic connection fluctuation-dissipation theorem, but also rationalizes experimentally measured relative interfacial strengths of heterostructures and interlayer registry of hexagonal boron nitride that have plagued other vdW methods.

International audience; The melting curve of gold has been measured up to 110 GPa using laser-heated diamond anvil cells and synchrotron x-ray diffraction techniques. Accurate pyrometry temperature measurements and a homogeneous heating of the gold sample were achieved by implementing a sample assembly consisting of two boron-doped diamond cupped disks sandwiching the gold sample. In the investigated pressure range, the fcc solid gold remains stable up to melting. A clear structural signature of bulk melting is observed. Ab initio molecular dynamics simulations within the two-phase approach give a melting curve in good agreement with the experimental one. We discuss the validity of calculations based on the Lindemann criteria of melting which have been up to now used to obtain the melting line of Au in the 100 GPa range.

Terahertz radiation is demonstrated to generate phonon-induced resistance oscillations in two-dimensional electron gases. This effect dominates over microwave-induced resistance oscillations in the low terahertz range, but could not be detected in the gigahertz regime. Possible mechanisms for phonon generation are discussed.