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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.

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.

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.

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.

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.

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 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.

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.

A method is presented for quantifying quantum-mechanical exchange effects within the dielectric theory of electron scattering, the prevailing model for electron energy losses in condensed-matter systems. The approach utilizes a uniquely symmetric view of direct and exchange scattering events along with a generalized interference phase to account physically for the reduced scattering rates required by the Pauli exclusion of final-state fermions with corresponding spin, energy, and momentum. It is found that existing implementations either neglect or substantially overestimate the impact of exchange interference for condensed materials with broad optical loss spectra due to mathematical approximations or physical assumptions which are largely avoidable. The results suggest that the impact of exchange may alter inelastic scattering cross sections by between 10% and 20% for incident energies below 200 eV, a critical energy range for current investigations in electron and photoelectron spectroscopies.

Direct current in clean semiconductors and metals was recently shown to obey the laws of hydrodynamics in a broad range of temperatures and sample dimensions. However, the determination of frequency window for hydrodynamic phenomena remains challenging. Here, we reveal a phenomenon being a hallmark of high-frequency hydrodynamic transport, the Cerenkov emission of plasmons by drifting Dirac electrons. The effect appears in a hydrodynamic regime only due to reduction of plasmon velocity by electron-electron collisions below the velocity of carrier drift. To characterize the Cerenkov effect quantitatively, we analytically find the high-frequency nonlocal conductivity of drifting Dirac electrons across the hydrodynamic-to-ballistic crossover. We find the growth rates of hydrodynamic plasmon instabilities in two experimentally relevant setups: parallel graphene layers and graphene covered by subwavelength grating, further showing their absence in ballistic regime. We argue that the possibility of Cerenkov emission is linked to singular structure of nonlocal conductivity of Dirac materials and is independent on specific dielectric environment.

Nodal-line semimetals are characterized by a kind of topologically nontrivial bulk-band crossing, giving rise to almost flat surface states. Yet, a direct evidence of the surface states is still lacking. Here we study theoretically impurity effects in topological nodal-line semimetals based on the T-matrix method. It is found that for a bulk impurity, some in-gap states may be induced near the impurity site, while the visible resonant impurity state can only exist for certain strength of the impurity potentials. For a surface impurity, robust resonant impurity states exist in a wide range of impurity potentials. Such robust resonant states stem from the topological protected weak dispersive surface states, which can be probed by scanning tunneling microscopy, providing a strong signature of the topological surface states in the nodal-line semimetals., Comment: 7 pages, 5 figures

\textit{In situ} high-pressure Brillouin light scattering experiments along loading-unloading paths are used to investigate the compressibility of vitreous silica. An accurate equation of state is obtained below \SI{9}{GPa} using sound velocities corrected for dispersion. Conversely, huge inelastic effects are observed in the range \SIrange{10}{60}{GPa}, unveiling the reversible transformation from the fourfold-coordinated structure to the sixfold one. We find that the associated density changes fully correlate with the average Si coordination number. Decompression curves from above \SI{20}{GPa} reveal abrupt backward coordination changes around \SIrange{10}{15}{GPa} and significant hysteresis. Further, contrary to common wisdom, the residual densification of recovered silica samples can be figured out from the pressure cycles., 5 pages, 4 figures, revised version

We present the derivation and application of an analytical effective potential that is able to describe, in a simple way, the interaction of many electrons confined in low-dimensional structures of realistic size. The effective potential takes into account the contribution of both the electron-electron interaction inside the nanostructure and the quantum confinement by a surrounding material. With this model, we explore the electronic distribution in quantum wells, wires, and dots in the full range from doped to high-doped concentrations. We also use this effective potential to explicitly determine the parameters that trigger the formation of the Wigner molecule in quantum wires. The comparison with experimental data reported in the literature shows the accuracy and reliability of this potential model.

We study the electronic contribution to the nonlinear optical response of the $\ensuremath{\alpha}\text{\ensuremath{-}}{T}_{3}$ model. This model is an interpolation between a graphene $(\ensuremath{\alpha}=0)$ and dice $(\ensuremath{\alpha}=1)$ lattice. Using a second-quantized formalism, we calculate the first- and third-order responses for a range of $\ensuremath{\alpha}$ and chemical potential values as well as considering a band gap in the first-order case. Conductivity quantization is observed for the first-order, while higher-order harmonic generation is observed in the third-order response with the chemical potential determining which applied field frequencies both quantization and harmonic generation occur at. We observe a range of experimentally accessible critical fields between ${10}^{2}--{10}^{6}$ V/m with dynamics depending on $\ensuremath{\alpha},\ensuremath{\mu}$, and the applied field frequency. Our results suggest an $\ensuremath{\alpha}\text{\ensuremath{-}}{T}_{3}$-like lattice could be an ideal candidate for use in terahertz devices.

The presence of defects in the narrow gap semiconductors GaSb and InSb affects their dopability and hence applicability for a range of optoelectronic applications. Here, we report hybrid density functional theory (DFT)-based calculations of the properties of intrinsic point defects in the two systems, including spin-orbit coupling effects, which influence strongly their band structures. With the hybrid DFT approach adopted, we obtain excellent agreement between our calculated band dispersions and structural, elastic, and vibrational properties and available measurements. We compute point defect formation energies in both systems, finding that antisite disorder tends to dominate, apart from in GaSb under certain conditions, where cation vacancies can form in significant concentrations. Calculated self-consistent Fermi energies and equilibrium carrier and defect concentrations confirm the intrinsic n- and p-type behavior of both materials under anion-rich and anion-poor conditions. Moreover, by computing the compensating defect concentrations due to the presence of ionized donors and acceptors, we explain the observed dopability of GaSb and InSb.

Recent investigations have found that the strongly constrained and appropriately normed (SCAN) meta-GGA exchange-correlation functional significantly over-magnetizes elemental Fe, Co, and Ni solids. For the paradigmatic case, bcc Fe, the error relative to experiment is $\gtrsim 20 \%$. Comparative analysis of magnetization results from SCAN and its \textit{deorbitalized} counterpart, SCAN-L, leads to identification of the source of the discrepancy. It is not from the difference between Kohn-Sham (SCAN-L) and generalized Kohn-Sham (SCAN) procedures. The key is the iso-orbital indicator $\alpha$ (the ratio of the local Pauli and Thomas-Fermi kinetic energy densities). Its \textit{deorbitalized} counterpart, $\alpha_L$, has more dispersion in both spin channels with respect to magnetization in an approximate region between 0.6 Bohr and 1.2 Bohr around an Fe nucleus. The overall effect is that the SCAN switching function evaluated with $\alpha_L$ reduces the energetic disadvantage of the down channel with respect to up compared to the original $\alpha$, which in turn reduces the magnetization. This identifies the cause of the SCAN magnetization error as insensitivity of the SCAN switching function to $\alpha$ values in the approximate range $0.5 \lesssim \alpha \lesssim 0.8$ and oversensitivity for $\alpha \gtrsim 0.8$., Comment: 5 pages, 5 figures, revised version

We study the dynamic long range interaction induced topological photonic bands and edge modes in a one-dimensional (1D) array of strongly dispersive gyromagnetic resonant cylinders. We propose a 1D topological model for such a dispersive gyromagnetic system and demonstrate that the dynamic long-range interaction can lead to localized topological edge modes, while the quasistatic interaction alone does not. Different from the conventional Su-Schrieffer-Heeger model that has only nearest-neighbor interactions, we find that the normal modes of the system coupled strongly to the photon mode of the background medium and the dynamic effects create a different band gap. Our results indicate the importance of the dynamic long-range interactions on the band structures and topological edge modes in strongly dispersive gyromagnetic systems.

Employing elastic and inelastic neutron scattering (INS) techniques, we report on the microscopic properties of the ferromagnetism in the dilute magnetic topological insulator ${({\mathrm{Bi}}_{0.95}{\mathrm{Mn}}_{0.05})}_{2}{\mathrm{Te}}_{3}$. Neutron diffraction of polycrystalline samples shows the ferromagnetic (FM) ordering is long range within the basal plane, and mainly two dimensional (2D) in character with short-range correlations between layers below ${T}_{\mathrm{C}}\ensuremath{\approx}13$ K. Remarkably, we observe gapped and collective magnetic excitations in this dilute magnetic system. The excitations appear typical of quasi-2D FM systems despite the severe broadening of short-wavelength magnons which is expected from the random spatial distribution of Mn atoms in the Bi planes. A detailed analysis of the INS provides the average values for exchange couplings which are consistent with reports of carrier-mediated interactions.

The rare-earth nitride ferromagnetic semiconductors owe their varying magnetic properties to the progressive filling of the $4f$ shell across the series. Recent electrical transport measurements on samarium nitride, including the observation of superconductivity, have been understood in terms of a contribution from a $4f$ transport channel. Band structure calculations generally locate an empty majority-spin $4f$ band within the conduction band although over a wide range of possible energies. Here we report optical reflection and transmission measurements on samarium nitride from 0.01 to 4 eV that demonstrate clearly that the $4f$ band forms the bottom of the conduction band. Results at the lowest energies show no free carrier absorption, indicating a semiconducting ground state, and support earlier conclusions based on transport measurements.

Wave transport through disordered materials is of broad interest in science and engineering. It has been generally believed that long-range transport cannot exist in low-dimensional disordered systems due to the Anderson localization of all states. However, in this work we report the anomalous transparency of phonon transport induced by cooperative effects of mass and force-constant disorders in completely random harmonic wires. In contrast to the full localization in mass-disordered wire, an anomalous transparent phonon state can emerge due to the intrinsic cooperation of disordered mass and force constants. We obtain the frequency of the transparent state, tuned by the disordered force constants and masses. Consequently, the phonon transport in the completely disordered system falls into three different regimes: the full localization, resonant, and sub-ballistic regimes. While there exists a single nonzero transparent frequency in the resonant regime, transparent states in a wide range of frequencies can be present in the sub-ballistic regime, which significantly enhances the thermal conductivity of disordered wires. These findings may open new doors to modulate thermal transport and design thermal devices, including thermal filter and high-thermal-conductivity materials.

Electric and magnetic resonances of dielectric particles have recently uncovered a range of exciting applications in steering of light at the nanoscale. Breaking of particle inversion symmetry further modifies its electromagnetic response giving rise to bianisotropy known also as magneto-electric coupling. Recent studies suggest the crucial role of magneto-electric coupling in realization of photonic topological metamaterials. To further unmask this fundamental link, we design and test experimentally one-dimensional array composed of dielectric particles with overlapping electric and magnetic resonances and broken mirror symmetry. Flipping over half of the meta-atoms in the array, we observe the emergence of interface states providing photonic realization of the celebrated Jackiw-Rebbi model. We trace the origin of these states to the fact that local modification of particle bianisotropic response affects its effective coupling with the neighboring meta-atoms which provides a promising avenue to engineer topological states of light., 5 pages, 5 figures

The effects of rhenium doping in the range 0 to 10 atomic percent on the static and dynamic magnetic properties of Fe65Co35 thin films have been studied experimentally as well as with first principles electronic structure calculations focusing on the change of the saturation magnetization and the Gilbert damping parameter. Both experimental and theoretical results show that the saturation magnetization decreases with increasing Re doping level, while at the same time Gilbert damping parameter increases. The experimental low temperature saturation magnetic induction exhibits a 29 percent decrease, from 2.31 T to 1.64 T, in the investigated doping concentration range, which is more than predicted by the theoretical calculations. The room temperature value of the damping parameter obtained from ferromagnetic resonance measurements, correcting for extrinsic contributions to the damping, is for the undoped sample 0.0027, which is close to the theoretically calculated Gilbert damping parameter. With 10 atomic percent Re doping, the damping parameter increases to 0.0090, which is in good agreement with the theoretical value of 0.0073. The increase in damping parameter with Re doping is explained by the increase in density of states at Fermi level, mostly contributed by the spin-up channel of Re. Moreover, both experimental and theoretical values for the damping parameter are observed to be weakly decreasing with decreasing temperature., 13 pages, 8 figures, 5 tables, Materials for spin-logic circuits

We studied the magnetic phase diagram of the rare-earth manganites series ${\mathrm{Gd}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ (GCMO) over the full concentration range based on density functional theory. GCMO has been shown to form solid solutions. We take into account this disordered character by adapting special quasi-random structures at different concentration steps. The magnetic phase diagram is mainly described by means of the magnetic exchange interactions between the Mn sites, and Monte Carlo simulations were performed to estimate the corresponding transition temperatures. They agree very well with recent experiments. The hole-doped region $xl0.5$ shows a strong ferromagnetic ground state, which competes with A-type antiferromagnetism at higher Ca concentrations $xg0.6$.

A Falicov-Kimball model which thermodynamically reduces the local Coulomb interaction of particles to attraction or repulsion is studied within the dynamical mean-field theory. In the strong interaction regime a fractionalization of particles into charge and spin objects, the physical properties of which are different from the whole particles, is observed in both high- and low-temperature phases. At high temperature and strong interaction the single-particle density of states opens an excitation gap, but the charge compressibility and the spin susceptibility exhibit the features of gapless excitations. The low-temperature phase has a long-range order, and the single-particle spectra are always gapped, while the charge and spin excitations are gapless in the strong interaction regime. In the fractionalized long-range ordered phase both the charge compressibility and the spin susceptibility are universal scaling functions of temperature.

Surface and core are two essential but distinct structural parts of a nanowire---but their individual effects on overall thermal expansion coefficient of a nanowire have never been quantified. Here we present an average bond-length based framework to determine the effects of the surface and core regimes of $3C$-SiC nanowires on their effective volumetric thermal expansion coefficient over a wide range of temperatures. Our results suggest that the surface and core atoms exhibit differential anharmonic response at finite temperatures, which makes the surface regime exhibit disparate expansion behavior compared to the core. While at lower temperatures the differential anharmonicity is negligible, at temperatures higher than the room temperature there is a pronounced differential anharmonicity in the nanowire. Furthermore, temperature-dependent expansion coefficients of the nanowire and the surface and core regimes qualitatively follow the behavior of the bulk---but they vary substantially quantitatively, with the maximum coefficient exhibited by the surface at higher temperatures. The diameter-dependent expansion coefficients follow inverse power laws with their exponents varying from 0.95 to 2.5. In thinner nanowires the expansion coefficient is controlled by an intricate combination of mass inertia and bond stiffness at the surface and core, whereas the expansion of thicker nanowires is dominated by the anharmonic motion of the core atoms alone. The surface effects saturate with increasing diameter, but the core effects decay nonlinearly with increasing diameter and approaches the bulk value as $d\ensuremath{\rightarrow}\ensuremath{\infty}$.

In this work we present the eigenspectra of a novel species of Wannier excitons when exposed to crossed electric and magnetic fields. In particular, we compute the eigenenergies of giant-dipole excitons in ${\mathrm{Cu}}_{2}\mathrm{O}$ in crossed fields. In our theoretical approach, we calculate the excitonic spectra within both an approximate as well as a numerically exact approach for arbitrary field configurations. We verify that stable bound excitonic giant-dipole states are only possible in the strong magnetic field limit, as this is the only regime providing sufficiently deep potential wells for their existence. Comparing both analytic as well as numerical calculations, we obtain excitonic giant-dipole spectra with level spacings in the range of 0.6--100 $\ensuremath{\mu}\mathrm{eV}$.