arXiv subjects

D. Bessas

Publications and source records attributed to D. Bessas.

6 recordsLinked to original sources

arXiv2026

Effect of glass stability on the low frequency vibrations of vapor deposited glasses

Ultra-stable glasses prepared from the physical vapor deposition of organic molecules present a very low density of two-level states, the kind of glass defects that determine their peculiar low temperature thermal properties. Numerical simulations suggest that quasi-localized harmonic vibrational modes emerge in the soft regions associated with two-level states. However, the connection between the low frequency vibrational modes and the local structural instabilities of glasses remains unexplained. Here we exploit a recently developed spectrograph for nuclear resonant analysis of inelastic X-ray scattering to probe the density of vibrational states of amorphous thin films of ultra-stable and conventional glasses down to an exceptionally low frequency of $\sim 70$ GHz. We show that the glass stability does not affect the harmonic vibrational modes at the lowest frequencies, despite a reduction of almost an order of magnitude in the density of two-level states. At the same time, the vibrational modes at higher frequencies, around the boson peak maximum, are extremely sensitive to the glass stability. Although we cannot exclude the possible existence of quasi-localized modes in glasses, we show that their presence is not strictly necessary to describe the measured density of low frequency vibrations. The experimental developments here presented pave the way to the solution to the long-standing debate on the low frequency vibrations in glasses.

cond-mat.dis-nn↗

arXiv2025

Hybridization of lattice and charge order excitations in a superconducting cuprate

The ubiquitous tendency of superconducting cuprates to form charge density waves (CDWs) has reignited interest in the nature of their electron-phonon interaction and its role in shaping their phase diagrams. While pronounced dispersion anomalies were reported in several phonon branches, their precise connection to charge order and superconductivity remains unresolved. Here, using high-resolution inelastic x-ray scattering under low temperature and high magnetic field, we uncover a striking phonon renormalization in YBa$_2$Cu$_3$O$_{6+x}$. It appears along a reciprocal space trajectory connecting the wave vectors of a short-range 2D-CDW, emerging above the superconducting transition, and a long-range 3D-CDW, appearing only when superconductivity is strongly suppressed. The spectral changes are strongest around the wave vector of the 3D-CDW despite the fact that it is absent in our experimental conditions. Our findings challenge conventional phonon self-energy renormalization models, instead support a scenario in which low-energy phonons hybridize with dispersive CDW excitations and provide insights into the interplay between lattice vibrations and electronic correlations in high-temperature superconductors.

cond-mat.supr-con↗

arXiv2020

Lattice dynamics of endotaxial silicide nanowires

Self-organized silicide nanowires are considered as main building blocks of future nanoelectronics and have been intensively investigated. In nanostructures, the lattice vibrational waves (phonons) deviate drastically from those in bulk crystals, which gives rise to anomalies in thermodynamic, elastic, electronic, and magnetic properties. Hence, a thorough understanding of the physical properties of these materials requires a comprehensive investigation of the lattice dynamics as a function of the nanowire size. We performed a systematic lattice dynamics study of endotaxial FeSi$_2$ nanowires, forming the metastable, surface-stabilized $\alpha$-phase, which are in-plane embedded into the Si(110) surface. The average widths of the nanowires ranged from 24 to 3 nm, their lengths ranged from several $\mu$m to about 100 nm. The Fe-partial phonon density of states, obtained by nuclear inelastic scattering, exhibits a broadening of the spectral features with decreasing nanowire width. The experimental data obtained along and across the nanowires unveiled a pronounced vibrational anisotropy that originates from the specific orientation of the tetragonal $\alpha$-FeSi$_2$ unit cell on the Si(110) surface. The results from first-principles calculations are fully consistent with the experimental data and allow for a comprehensive understanding of the lattice dynamics of endotaxial silicide nanowires.

cond-mat.mtrl-sci↗

arXiv2020

Lattice dynamics and polarization-dependent phonon damping in $α$-phase FeSi$_{2}$ nanoislands

We determined the lattice dynamics of metastable, surface-stabilized $α$-phase FeSi$_2$ nanoislands epitaxially grown on the Si(111) surface with average heights and widths ranging from 1.5 to 20 nm and 18 to 72 nm, respectively. The crystallographic orientation, surface morphology and local crystal structure of the nanoislands were investigated by reflection high-energy electron diffraction, atomic force microscopy and X-ray absorption spectroscopy. The Fe-partial phonon density of states (PDOS), obtained by nuclear inelastic scattering, exhibits a pronounced damping and broadening of the spectral features with decreasing average island height. First-principles calculations of the polarization-projected Si- and Fe-partial phonon dispersions and PDOS enable the disentanglement of the contribution of the $xy$- and $z$-polarized phonons to the experimental PDOS. Modeling of the experimental data with the theoretical results unveils an enhanced damping of the $z$-polarized phonons for islands with average sizes below 10 nm. This phenomenon is attributed to the fact that the low-energy $z$-polarized phonons couple to the low-energy surface/interface vibrational modes. The thermodynamic and elastic properties obtained from the experimental data show a pronounced size-dependent behavior.

cond-mat.mtrl-sci↗

arXiv2018

Testing the influence of acceleration on time dilation using a rotating MÖssbauer absorber

The aim of the experiment series was to test the influence of acceleration on time dilation by measuring the relative spectral shift between the resonance spectra of a rotating Mossbauer absorber with acceleration anti-parallel and parallel to the direction of the incident beam. Based on the experiences and know-how acquired in our previous experiments, We collected data for rotation frequencies up to 510Hz in both directions of rotation and also used different slits. For each run with high rotation, we observed a stable statistically significant relative shift between the spectra of the two states with opposite acceleration. This indicates the influence of acceleration on time dilation. However, we found that this shift also depends on the choice of the slit, and on the direction of rotation. These new unexpected findings, resulting from the loss of symmetry in obtaining the resonant lines in the two states, could overshadow the relative shift due to acceleration. This loss of the symmetry is caused by the deflection of the radiative decay due to the Nuclear Lighthouse effect from the rotating Mossbauer absorber. We also found that it is impossible to keep the alignment (between the optical and the dynamical rotor systems) with accuracy needed for such experiment, for long runs, which resulted in the reduction of the accuracy of the observed relative shift. These prevent us to claim with certainty the influence of acceleration on time dilation using the currently available technology. An improved KB optics with focal spot of less than 1 micron to avoid the use of a slit and a more rigid mounting of the rotor system, are necessary for the success of such experiment. Hopefully, these findings together with the indispensable plan for a conclusive experiment presented in the paper, will prove useful to future experimentalists wishing to pursue such an experiment.

physics.acc-ph↗

arXiv2013

Lattice instabilities in bulk EuTiO3

The phase purity and the lattice dynamics in bulk EuTiO3 were investigated both microscopically, using X-ray and neutron diffraction, 151-Eu-Mössbauer spectroscopy, and 151-Eu nuclear inelastic scattering, and macroscopically using calorimetry, resonant ultrasound spectroscopy, and magnetometry. Furthermore, our investigations were corroborated by ab initio theoretical studies. The perovskite symmetry, Pm-3m, is unstable at the M- and R- points of the Brillouin zone. The lattice instabilities are lifted when the structure relaxes in one of the symmetries: I4/mcm, Imma, R-3c with relative relaxation energy around -25 meV. Intimate phase analysis confirmed phase purity of our ceramics. A prominent peak in the Eu specific density of phonon states at 11.5 meV can be modelled in all candidate symmetries. A stiffening on heating around room temperature is indicative of a phase transition similar to the one observed in SrTiO3, however, although previous studies reported the structural phase transition to tetragonal I4/mcm phase our detailed sample purity analysis and thorough structural studies using complementary techniques did not confirm a direct phase transition. Instead, in the same temperature range, Eu delocalization is observed which might explain the lattice dynamical instabilities.

cond-mat.str-el↗