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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

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 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.

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.

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

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.

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

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 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.

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.

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

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.

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.

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

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

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.

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.

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 $\ensuremath{\mu}\mathrm{eV}$.

Optical charge state switching was previously observed in photoluminescence experiments for the divacancy defect in $4H$-SiC. The participating dark charge state could not be identified with certainty. We use constrained density-functional theory to investigate the mechanism of charge state conversion from the bright neutral charge state of the divacancy defect to the positive and negative charge states including corresponding recovery of the neutral charge state. While we can confirm that the positive charge state is dark, we do not find evidence that the negative charge state is dark. We compute similar absorption energies required for conversion of the neutral defect to both charge states. However, the formation of the positive charge state requires a series of excitations involving a 2-photon excitation, while the creation of the negative charge state is achieved through a single 2-photon process. Calculated absorption energies for the recovery of the neutral defect from the positive charge state fit the experimental value better than those from the negative charge state. Defect formation energies as a function of the Fermi energy show a very small Fermi energy range in which the negative charge state is most stable, while the positive charge state exhibits a wide stability range. Overall, our computational results give more support to the identification of the dark charge state as the positive over the negative charge state in the mechanism of optical charge state switching.

We present experimental results for the in-plane resistivity of the electron-doped cuprate superconductor $La_{2-x}Ce_xCuO_4$ above its transition temperature $T_c$ as a function of Ce doping x and temperature. For the doping x between 0.11 and 0.17, where $T_c$ varies from 30 K (x=0.11) to 5 K (x=0.17), we find that the resistivity shows an approximate $T^2$ behavior for all values of doping over the measurement range from 100 K to 400 K. The coefficient of the $T^2$ resistivity term decreases with increasing x following the trend in $T_c$. We analyze our data theoretically and posit that n-type cuprates are better thought of as strange metals. Although the quadratic temperature dependence appears to be in naive agreement with the Fermi liquid (FL) expectations, the fact that the measured resistivity is large and no phonon-induced linear-in-T resistivity manifests itself even at 400 K argue against a standard normal metal Fermi liquid picture being applicable. We discuss possible origins of the strange metal behavior.

The electronic stopping power of nickel for protons and alpha particles at velocities below and around the Fermi velocity has been obtained to high accuracy using time-dependent density functional theory. For the wide range of projectile velocities considered, we observed different regimes of electronic stopping due to the alternative participation of $s$- and $d$-band electrons. Despite the sharp discontinuity in the electronic density of states near the Fermi energy characteristic of the nickel band structure, we do not find an anomalous nonlinear electronic stopping power limit as a function of velocity. However, we find a crossover region above $v=0.15\phantom{\rule{0.28em}{0ex}}\mathrm{a}.\mathrm{u}.$ both for protons and alpha particles, related to the increase in participating host electrons and, in the case of alpha particles, to an increase of the charge state. We compare our calculated results with widely available experimental data and analyze the low velocity limits in the context of Lindhard's linear response theory and previous nonlinear density functional calculations. The comparison shows good accord with the lowest velocity experiments available. This may indicate that the adiabatic local density approximation is already a good theory to calculate electronic stopping power in materials at low velocity.

Transition metal dichalcogenide (TMD) monolayers are interesting materials in part because of their strong spin-orbit coupling. This leads to intrinsic spin-splitting of opposite signs in opposite valleys, so the valleys are intrinsically spin-polarized when hole-doped. We study spin response in a simple model of these materials, with an eye to identifying sharp collective modes (i.e, spin-waves) that are more commonly characteristic of ferromagnets. We demonstrate that such modes exist for arbitrarily weak repulsive interactions, even when they are too weak to induce spontaneous ferromagnetism. The behavior of the spin response is explored for a range of hole dopings and interaction strengths., Comment: 12 pages, 8 figures

We perform fully relativistic calculations of the energy loss channels for a charged particle traversing a single layer of graphene under oblique incidence in a setting pertinent to a Scanning Transmission Electron Microscope (STEM), where we distinguish between the energy deposited in graphene in the form of electronic excitations (Ohmic loss) and the energy emitted in the far field in the form of transition radiation (TR). Our formulation of the problem uses a definition of two in-plane, dielectric functions of graphene, which describe the longitudinal and transverse excitation processes that contribute separately to those two energy loss channels. Using several models for the electric conductivity of graphene as the input in those dielectric functions enables us to discuss the effects of oblique incidence on several processes in a broad range of frequencies, from the terahertz (THz) to the ultraviolet (UV). In particular, at the THz frequencies, we demonstrate that the nonlocal effect in the graphene?s conductivity is not important in the retarded regime, and we show that the longitudinal and transverse contributions to the emitted TR spectra exhibit strongly anisotropic angular patterns that are readily distinguishable in a cathodoluminescence measurement in a STEM. Moreover, we explore the possibility of exciting the so-called transverse mode in the optical response of graphene at the mid-infrared (MIR) range of frequencies by means of a fast charged particle under oblique incidence. Finally, we demonstrate that, besides the usual high-energy peaks in the longitudinal contribution to the Ohmic energy loss in the MIR to the UV frequency range, there may arise strongly directional features in the in-plane distribution of the transverse contribution to the Ohmic energy loss for an oblique trajectory, which could be possibly observed via momentum- and angle-resolved electron energy loss spectroscopy of graphene in STEM. Fil: Akbari, Kamran. University of Waterloo; Canadá Fil: Segui Osorio, Silvina Inda Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Centro Atómico Bariloche; Argentina Fil: Miskovic, Zoran L.. University of Waterloo; Canadá Fil: Gervasoni, Juana Luisa. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica; Argentina Fil: Arista, Nestor Ricardo. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro; Argentina. Comisión Nacional de Energía Atómica; Argentina

The compounds ${\mathrm{BaDy}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{BaHo}}_{2}{\mathrm{O}}_{4}$ are part of a family of frustrated systems exhibiting interesting properties, including spin-liquid-type ground states, magnetic-field-induced phases, and the coexistence of short- and long-range magnetic order, with dominant one-dimensional correlations, which can be described as Ising ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ zigzag chains along the $c$ axis. We have investigated polycrystalline samples of ${\mathrm{BaDy}}_{2}{\mathrm{O}}_{4}$ and ${\mathrm{BaHo}}_{2}{\mathrm{O}}_{4}$ with both neutron diffraction and neutron spectroscopy, coupled to detailed crystalline electric field calculations. The latter points to site-dependent anisotropic magnetism in both materials, which is corroborated by the magnetic structures we determined. The two systems show the coexistence of two different long-range orders: two double N\'eel $\ensuremath{\uparrow}\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\downarrow}$ orders in the $ab$ plane with propagation vectors ${\mathbf{k}}_{1}=(\frac{1}{2},0,\frac{1}{2})$ and ${\mathbf{k}}_{2}=(\frac{1}{2},\frac{1}{2},\frac{1}{2})$ for ${\mathrm{BaDy}}_{2}{\mathrm{O}}_{4}$, and two distinct arrangements of simple N\'eel $\ensuremath{\uparrow}\ensuremath{\downarrow}\ensuremath{\uparrow}\ensuremath{\downarrow}$ orders along the $c$ axis, both with the propagation vector ${\mathbf{k}}_{0}=(0,0,0)$ for ${\mathrm{BaHo}}_{2}{\mathrm{O}}_{4}$. The order for both wave vectors in ${\mathrm{BaDy}}_{2}{\mathrm{O}}_{4}$ occurs at ${T}_{\mathrm{N}}=0.48\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, while in ${\mathrm{BaHo}}_{2}{\mathrm{O}}_{4}$ the first order sets in at ${T}_{\mathrm{N}}\ensuremath{\sim}\phantom{\rule{3.33333pt}{0ex}}1.3\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and the second one has a lower ordering temperature of 0.84 K. Both compounds show extensive diffuse scattering which we successfully modeled with a one-dimensional axial next-nearest neighbor Ising (ANNNI) model. In both materials, strong diffusive scattering persists to temperatures well below where the magnetic order is fully saturated.

We present a general method to accelerate ab initio atomic structural relaxations. In this method, the conjugate gradient ab initio relaxation is preconditioned by a force field relaxation. This force field is constructed on-the-flight with the information of the ab initio forces and the current atomic configuration. At each ab initio relaxation step, the so-constructed force field is used to prerelax the system, with the relaxation direction as the preconditioned direction for the ab initio conjugated gradient method. The force field model and its parameters are rather general making this method applicable for a wide range of systems commonly used in material simulations. More than 80 different systems have been tested representing different cases in material simulations. Across the board, we find accelerations mostly over a factor of 2 and for some large systems with a factor of $6\ensuremath{\sim}9$. There is no case where the relaxation becomes worse. The code and lib for this method are provided, which can be used as a plugin in a standard ab initio atomic relaxation code.

The collective excitation spectrum of two-dimensional (2D) antimonene is calculated beyond the low energy continuum approximation. The dynamical polarizability is computed using a 6-orbitals tight-binding model that properly accounts for the band structure of antimonene in a broad energy range. Electron-electron interaction is considered within the random phase approximation. The obtained spectrum is rich, containing the standard intra-band 2D plasmon and a set of single inter-band modes. We find that spin-orbit interaction plays a fundamental role in the reconstruction of the excitation spectrum, with the emergence of novel inter-band branches in the continuum that interact with the plasmon., 8 pages, 9 figures, accepted by Phys. Rev. B

The XY pyrochlore Yb$_2$Ti$_2$O$_7$, with pseudo spin 1/2 at the Yb$^{3+}$ site, has been celebrated as potential host for the quantum spin ice state. The substitution of non-magnetic Ti with Pt gives Yb$_2$Pt$_2$O$_7$, a system with remarkably similar magnetic properties. The large nuclear gyromagnetic ratio ($\gamma_{n}/2 \pi = 9.15$~MHz/T) of $^{195}$Pt makes Yb$_2$Pt$_2$O$_7$ an ideal material for NMR investigation of its unconventional magnetic properties. Based on the $^{195}$Pt nuclear spin-lattice relaxation rate $1/T_1$ and the magnetic specific heat $C_{p}$ measured in a broad range of magnetic field $B_{ext}$, we demonstrate that the field-induced magnon gap linearly decreases with $B_{ext}$ but additional low energy mode of spin excitations emerge below $\sim 0.5$~T.

The dynamics of the phonon scattering caused by the guest-host interactions in fully filled skutterudites $(\mathrm{Yb},\phantom{\rule{0.16em}{0ex}}\mathrm{La},\phantom{\rule{0.16em}{0ex}}\mathrm{Ba})\mathrm{F}{\mathrm{e}}_{4}\mathrm{S}{\mathrm{b}}_{12}$ are studied by ab initio molecular dynamics. The characteristics of filler vibrations are found to be the origin of ultralow lifetimes for the filler-dominant modes. Under the phonon-phonon interaction diagram, the large amplitudes of the filler vibrations and the coupling between filler and framework phonon modes greatly increase the strength of phonon scatterings. The lattice thermal conductivities for the three skutterudites, however, are all overestimated comparing with the experimental results. The introduction of the resonant scattering is thus necessary to account for the deviations. Furthermore, by applying the wavelet-based analysis to the resonant scattering process, the time-frequency power spectrum shows clearly that the time-varying localized motions of the fillers periodically absorb the heat-carrying lattice phonons and emit them. This process is the origin of the resonant scattering mechanism in skutterudites, and strongly interferes with the propagation of phonons near the frequency range of the fillers.