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It is well known that phonons can overscreen the bare Coulomb electron-electron repulsion, turning it into the effective attraction that binds the Cooper pairs responsible for BCS superconductivity. Here, we use a simple lattice model to prove that the counterpart of this is also possible, whereby phonons overscreen the bare electron-hole attraction and may turn it repulsive at short distances, driving exciton dissociation in certain regions of the parameter space. We argue that this phonon-mediated short-range screening plays an important role in the physics of organic solar cell materials (and other materials with strong electron-phonon coupling) and could point the way to new strategies for optimizing their efficiencies., Comment: 11 pages, 6 figures

In a recent paper (Phys. Rev. E {\bf 104}, 014103 (2021) ) M. Baggioli and A. Zaccone formulate a theoretical description of the specific heat of liquids by using Debye's expression for the specific heat of solids and inserting a density of states (DOS) which they claim to represent the instantaneous-normal-mode (INM) spectrum of a liquid. However, the quantum-mechanical procedure of Debye cannot be used for a classical liquid and the authors' formula for the INM spectrum does not represent the known INM spectra of simple liquids. Furthermore, the derivation of this formula from their model equation of motion is mathematically in error. Finally experimental test of the teory for the specific heat of {\it liquids} is performed by fitting the data of {\it supercritical fluids}. \new{To our opinion,} these and a lot of other inconsistencies render this work not suitable for studying the specific heat of liquids., Comment: 3 pages, 1 figure

A disciplinary profile of a country is defined as the versor whose components are the numbers of articles produced in a given discipline divided the overall production of the country. Starting from the Essential Science Indicators (ESI) schema of classification of subject area, we obtained the yearly disciplinary profiles of a worldwide graph, where on each node sits a country, in the two time intervals [1980-1988] and [1992-2017], the fall of the Berlin Wall being the watershed. We analyse the empirical pairwise cross-correlation matrices of the time series of disciplinary profiles. The contrast with random matrix theory proves that, beyond measurement noise, the empirical cross-correlation matrices bring genuine information. Arising from the Shannon theorem as the least-structured model consistent with the measured pairwise correlations, the stationary probability distribution of disciplinary profiles can be described by a Boltzmann distribution related to a generalized $n_d$-dimensional Heisenberg model. The set of network interactions of the Heisenberg model have been inferred and to it they have been applied two clusterization methods, hierarchical clustering and principal component analysis. On a geopolitical plane this allow to obtain a characterization of the worldwide bilateral interactions based on physical modeling. A simple geopolitical analysis reveals the consistency of the results obtained and gives a boost to deeper historical analysis. In order to obtain the optimal set of pairwise interactions we used a Pseudo-Likelihood approach. We analytically computed the Pseudo-Likelihood and its gradient. The analytical computations deserve interest in whatever inference Bayesian problem involving a $n_d$-dimensional Heisenberg model.

We show that the presented in Phys.Rev.B, v.101, 214312 (2020) theoretical expressions for longitudinal current spectral function $C^L(k,\omega)$ and dispersion of collective excitations are not correct. Indeed, they are not compatible with the continuum limit and $C^L(k,\omega\to 0)$ contradicts the continuity equation., Comment: Submitted to Phys.Rev.B

Acousticlike excitations in topologically disordered media at mesocale/nanoscale present anomalous features with respect to the Debye's theory. The so-called Rayleigh scattering manifests in a strong increase of the attenuation of the acousticlike excitations and a softening of the phase velocity with respect to its continuum limit value. Mean field models developed in the random media theory framework can successfully predict the occurrence, at the proper length scale, of Rayleigh scattering. The overall attenuation in the Rayleigh region is, however, underestimated. In the framework of random media theory we developed an analytical model, which permits a quantitative description of the acousticlike excitations in topological glasses in the whole first pseudo-Brillouin zone. The underestimation of the Rayleigh scattering is avoided and, importantly, the model allows to account also for the polarization properties of the acousticlike excitations. In a three-dimensional medium an acoustic wave is characterized by its phase velocity, intensity, and polarization. Rayleigh scattering emphasizes how the topological disorder affects the first two properties. The topological disorder is, however, expected to influence also the third one. In common with the Rayleigh scattering, hallmarks possibly related to the mixing of polarizations have been traced in different classes of amorphous solids at nanoscale. The quantitative theoretical approach developed permits to demonstrate how the mixing of polarizations generates a distinctive feature in the dynamic structure factor of amorphous solids. The modeling capability of the proposed mean field theory is tested on glassy 1-octyl-3-methylimidazolium chloride, whose spatial distribution of the elastic moduli is well assessed and can be experimentally characterized. Contrast between theoretical and experimental features for the selected glass reveals an excellent agreement. The mean field approach we present retains a certain degree of generality and can be possibly extended to different stochastic media or different wave fields.

The work aims to show that small system sizes in numerical simulations turns out to be useful for investigating the Debye spectrum in glasses.

We present the results of an experiment aimed at studying the archetypal properties of the aluminum bulk plasmon at an organic/metal interface. Electron-electron coincidence spectroscopy is used to determine the contribution of aluminum bulk plasmon decay to the ionization of a thin copper phthalocyanine film. The latter directly depends on the amplitude of the bulk plasmon electric field (generated in the metal substrate) protruding inside the molecular overlayer. The emission of low-energy electrons from the clean substrate is dominated by plasmon-assisted ionization events. These events are not observed when the molecules are adsorbed onto the surface. Our findings suggest that, for the considered system, the bulk plasmon wave is confined within the medium in which it is generated and the interaction of the plasmon field with electrons located in the molecular overlayer does not lead to the emission of low-energy electrons.

We investigated the mechanisms of secondary electron (SE) emission from Be(0001) by impact of 100 and 150 eV electrons. We made use of (e, 2e) spectroscopy to disentangle the different SE production mechanisms. We observed a large increase in the SE yield when the energy loss of the primary electron equals the characteristic energy of volume and surface plasmons. The line shape of the SE spectrum associated with plasmon excitation reveals that one relevant emission mechanism corresponds to direct single-particle excitation in which the plasmon energy and momentum are transferred to a valence band electron of the solid. The contributions to the SE yield associated with surface and volume plasmon excitation are comparable in the case of specular geometry, where the projectile momentum is mainly transferred perpendicular to the surface. On the contrary, the emission of SEs associated with surface plasmon excitation is significantly enhanced when the exchanged momentum lies close to the surface plane and electrons are emitted from Be surface state. This reflects the increased sensitivity to surface modes of the latter geometry. Finally, the coupling between the direct ionization channel and the plasmon-assisted one results in a resonant increase of the secondary emission.

Anderson localization of light is traditionally described in analogy to electrons in a random potential. Within this description, the random potential depends on the wavelength of the incident light. For transverse Anderson localization, this leads to the prediction that the distribution of localization lengths---and, hence, its average---strongly depends on the wavelength. In an alternative description, in terms of a spatially fluctuating electric modulus, this is not the case. Here, we report on an experimentum crucis in order to investigate the validity of the two conflicting theories using optical samples exhibiting transverse Anderson localization. We do not find any dependence of the observed average localization radii on the light wavelength. We conclude that the modulus-type description is the correct one and not the potential-type one. We corroborate this by showing that in the derivation of the traditional potential-type theory, a term in the wave equation has been tacitly neglected. In our new modulus-type theory, the wave equation is exact. We check the consistency of the new theory with our data using the nonlinear sigma model. We comment on the consequences for the general case of three-dimensional disorder.

Measurements of several metallic glasses under strain reveal that the materials relieve stress through a two-step process that has previously been seen only in “softer” glasses.

The experimental results relevant for the understanding of the microscopic dynamics in liquid metals are reviewed, with special regards to the ones achieved in the last two decades. Inelastic Neutron Scattering played a major role since the development of neutron facilities in the sixties. The last ten years, however, saw the development of third generation radiation sources, which opened the possibility of performing Inelastic Scattering with X rays, thus disclosing previously unaccessible energy-momentum regions. The purely coherent response of X rays, moreover, combined with the mixed coherent/incoherent response typical of neutron scattering, provides enormous potentialities to disentangle aspects related to the collectivity of motion from the single particle dynamics. If the last twenty years saw major experimental developments, on the theoretical side fresh ideas came up to the side of the most traditional and established theories. Beside the raw experimental results, therefore, we review models and theoretical approaches for the description of microscopic dynamics over different length-scales, from the hydrodynamic region down to the single particle regime, walking the perilous and sometimes uncharted path of the generalized hydrodynamics extension. Approaches peculiar of conductive systems, based on the ionic plasma theory, are also considered, as well as kinetic and mode coupling theory applied to hard sphere systems, which turn out to mimic with remarkable detail the atomic dynamics of liquid metals. Finally, cutting edges issues and open problems, such as the ultimate origin of the anomalous acoustic dispersion or the relevance of transport properties of a conductive systems in ruling the ionic dynamic structure factor are discussed., 53 pages, 41 figures, to appear in "The Review of Modern Physics". Tentatively scheduled for July issue

""\\"Low-energy photoemission spectroscopy in the photon energy range between 5.5 and 21 eV is used to investigate the valence electronic structure of Yb polycrystalline films. With the lowering of the photon energy below 11 eV, additional spectral features, nearly absent at higher photon energy, become predominant, in particular a peak located right at the Fermi level. With the help of an ab initio theoretical calculation, we interpret the peak as due to the joint influence of filled and empty densities of states, identifying a p-band contribution close to the Fermi level, whose intensity is strongly enhanced at certain photon energies corresponding to modulation of the empty d band.\\"""

A structural investigation of the (1\ifmmode\times\else\texttimes\fi{}1)-Bi/GaAs(110) interface by grazing-incidence x-ray diffraction is presented. The (1\ifmmode\times\else\texttimes\fi{}1) structure has been prepared by depositing 1 ML of bismuth at room temperature on a GaAs(110) substrate grown by molecular-beam epitaxy. The structural model for the (1\ifmmode\times\else\texttimes\fi{}1) symmetry chemisorption of Bi and the in-plane structural parameters are obtained and compared with previous experimental results. We find that the epitaxial geometry corresponds to a shifted epitaxial continued layer structure (ECLS), contrary to the almost perfect ECLS geometry of the Sb/GaAs(110) interface. We determine a 2.85-\AA{} Bi-Bi bonding length, lower than the bulk Bi value of 3.1 \AA{}, and consistent with the presence of a compression of the Bi atoms within the overlayer.

The absence of sharp structures in the core-valence-valence Auger line shapes of partially filled bands has severely limited the use of electron spectroscopy in magnetic crystals and other correlated materials. Here by a novel interplay of experimental and theoretical techniques we achieve a combined understanding of the Photoelectron, Auger %$M_{23}M_{45}M_{45}$ and Auger-Photoelectron Coincidence Spectra (APECS) of CoO. This is a prototype antiferromagnetic material in which the recently discovered Dichroic Effect in Angle Resolved (DEAR) APECS reveals a complex pattern in the strongly correlated Auger line shape. A calculation of the \textit{unrelaxed} spectral features explains the pattern in detail, labeling the final states by the total spin. The present theoretical analysis shows that the dichroic effect arises from a spin-dependence of the angular distribution of the photoelectron-Auger electron pair detected in coincidence, and from the selective power of the dichroic technique in assigning different weights to the various spin components. Since the spin-dependence of the angular distribution exists in the antiferromagnetic state but vanishes at the N\'eel temperature, the DEAR-APECS technique detects the phase transition from its local effects, thus providing a unique tool to observe and understand magnetic correlations in such circumstances, where the usual methods (neutron diffraction, specific heat measurements) are not applicable., Comment: Accepted by: Physical Review Letters

New inelastic X-ray scattering experiments have been performed on liquid lithium in a wide wavevector range. With respect to the previous measurements, the instrumental resolution, improved up to 1.5 meV, allows to accurately investigate the dynamical processes determining the observed shape of the the dynamic structure factor, $S(Q,\omega)$. A detailed analysis of the lineshapes shows the co-existence of relaxation processes with both a slow and a fast characteristic timescales, and therefore that pictures of the relaxation mechanisms based on a simple viscoelastic model must be abandoned., Comment: 5 pages, 4 .PS figures

The off-equilibrium dynamics of a monatomic Lennard-Jones glass is investigated after sudden isothermal density jumps (crunch) from well equilibrated liquid configurations towards the glassy state. The generalized fluctuation-dissipation relation has been studied and the temperature dependence of the violation factor m is found in agreement with the one step replica symmetry breaking scenario, i.e. at low temperature m(T) is found proportional to T up to an off-equilibrium effective temperature T_eff, where m(T_eff)=1. We report T_eff as a function of the density and compare it with the glass transition temperatures T_g as determined by equilibrium calculations., Comment: 4 pages,4 figures

The sound attenuation in the THz region is studied down to T 16 K in glassy glycerol by inelastic x-ray scattering. At striking variance with the decrease found below 100 K in the GHz data, the attenuation in the THz range does not show any T dependence. This result (i) indicates the presence of two different attenuation mechanisms, active, respectively, in the high- and low-frequency limits, (ii) demonstrates the nondynamic origin of the attenuation of THz sound waves, and confirms a similar conclusion obtained in SiO2 glass by molecular dynamics, and (iii) supports the low-frequency attenuation mechanism proposed by Fabian and Allen [ Phys. Rev. Lett. 82, 1478 (1999)].

High-resolution, inelastic x-ray scattering measurements of the dynamic structure factor S(q,omega) of liquid water in the THz frequency range have been performed as a function of wave vector q (1-7 nm(-1)) and temperature T (273-473 K), using pressure (0-1.5 kbar) to keep the density at rho approximately 1 g/cm(3). We show that, for qor=2 nm(-1), the S(q,omega) spectra can be consistently explained in terms of a hydrodynamic formalism which includes a viscoelastic, q-independent contribution to the memory function for the density fluctuations. This allows us to extract values for the infinite frequency sound velocity and for the structural relaxation time which are found to compare favorably with those obtained using techniques which probe a lower-frequency range. As a consequence, the atomic dynamics in water is shown to have a homogeneous character down to a length scale of approximately 3 nm. At q2 nm(-1), we find that the viscoelastic contribution to the memory function becomes q dependent. Thus this work provides a view on the evolution of the collective dynamics of water across the q region where the continuum approximation inherent in the hydrodynamic formulation begins to fail. The physical consequences of such a result are discussed in some detail.

The dynamic structure factor Ssq, vd of o-terphenyl, obtained by Brillouin light scattering (q o 0.04 nm21) in the 1 to 30 GHz range, shows signatures of relaxation phenomena at temperatures well within the glassy phase. Using a phenomenological ansatz for the density fluctuations memory function and Ssq, vd data obtained by inelastic x-ray scattering at high momentum transfer (q ­ 1 6 nm21), we describe this relaxational dynamics in terms of two distinct processes: (i) one in the 10211 s range and slightly temperature dependent, and (ii) an “instantaneous” one, representing the microscopic transient and giving rise to a T-independent contribution to the Brillouin linewidth. [S0031-9007(99)08497-5]

A simple model to investigate the long time dynamics of glass-formers is presented and applied to study a Lennard-Jones system in supercooled and glassy phases. According to our model, the point representing the system in the configurational phase space performs harmonic vibrations around (and activated jumps between) minima pertaining to a connected network. Exploiting the model, in agreement with the experimental results, we find evidence for: i) stretched relaxational dynamics; ii) a strong T-dependence of the stretching parameter; iii) breakdown of the Stokes-Einstein law., Comment: 4 pages (Latex), 4 eps figures

The temperature dependence of the acoustic dispersion relation, $\ensuremath{\Omega}(Q,T)$, has been determined across the liquid-glass transition in o-terphenyl using inelastic x-ray scattering in the $1--6{\mathrm{nm}}^{\ensuremath{-}1}$ momentum transfer ( $Q$) range. From this the infinite frequency sound velocity ${v}_{\ensuremath{\infty}}(T)$ can be obtained. In the glass, ${v}_{\ensuremath{\infty}}(T)$ compares well with existing light scattering determinations, while in the liquid its value can be used to discriminate among different phenomenological models previously proposed to infer ${v}_{\ensuremath{\infty}}(T)$ from low temperature data. Moreover, the Debye-Waller factors ( ${f}_{Q}$) in two different $Q$ regimes ( $Q\ensuremath{\approx}0$ and $Q\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}2.5{\mathrm{nm}}^{\ensuremath{-}1}$) are determined and found to be mutually consistent.

the general temperature trends. The analysis is based on the existence of fast ( b) and slow (a) relaxation processes in the liquid. A phenomenological description of a two step relaxation function is proposed, and the phenomenological parameters @relaxation times, t a and t b , relative weight, f ˜ (T), etc.# of the two relaxation processes are derived. As a consequence of the two step relaxation process, we propose to revise the concept of ‘‘infinite frequency’’ ( c‘) sound velocity, introducing two characteristic sound velocities, one which applies if vt b@1, c‘ , and one, c‘a , which describes the elastic response of the liquid at frequency higher than 1/ t a but smaller than 1/t b . Within the framework of a two step relaxation process, the meaning of ‘‘nonergodicity’’ parameter of mode-coupling theory and its relation with f ˜ (T) are also discussed. The weight f ˜ (T) is found to decrease monotonically from near 1 to near 0.4 between Tg and 1.5Tg . The comparison of our findings with the predictions of the mode-coupling theory shows qualitative accord in several important points. @S1063-651X~97!07212-7# PACS number~s!: 64.70.Pf