arXiv subjects

S. M. Souliou

Publications and source records attributed to S. M. Souliou.

14 recordsLinked to original sources

arXiv2026

Interplay of Charge and Magnetic Orders in SmNiC$_2$ Mediated by Electron-Phonon Interaction

We investigate the interplay between charge density wave (CDW) instabilities and ferromagnetism in SmNiC$_2$ using diffuse and inelastic x-ray scattering together with Raman spectroscopy. We identify a soft acoustic phonon driving the incommensurate CDW (I-CDW) and uncover a second Kohn anomaly at the wave vector of the commensurate CDW (C-CDW) stabilized in other $R$NiC$_2$ members ($R=$ rare earth). The marked softening of both phonons and their contrasting evolution with temperature reveal a competition between the two ordering tendencies. Alongside pronounced anomalies in the temperature dependence of the zone center and soft phonons, we observe the collective amplitude mode of the CDW, which collapses abruptly as ferromagnetism sets in. Surprisingly, the Kohn anomalies persist in the ferromagnetic state despite the degradation of Fermi-surface nesting conditions. Our experimental findings, supported by ab initio calculations, highlight the central role of the electron-phonon interaction in driving the CDW formation and tuning the balance between competing charge and magnetic orders.

cond-mat.str-el↗

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↗

arXiv2025

Resonant x-ray scattering study of charge-density wave correlations in YBa$_{2}$Cu$_{3}$O$_{6+x}$ under uniaxial stress

We report a comprehensive study of the uniaxial stress response of charge-density-wave (CDW) correlations in detwinned single crystals of the high temperature superconductor YBa$_2$Cu$_3$O$_{6+x}$ (YBCO$_{6+x}$) with $0.40 \leq x \leq 0.93$ (hole-doping levels $0.072 \leq p \leq 0.168$) by means of Cu $L_3$-edge resonant energy-integrated x-ray scattering (REXS). We show that the influence of uniaxial stress is strongly doping dependent: the quasi-two-dimensional CDW is enhanced by in-plane uniaxial stress in a wide hole doping range ($0.45 \leq x \leq 0.80$), but only barely affected in the most underdoped and optimally doped samples ($x = 0.40$ and 0.93), where the CDW correlation length is minimal. A stress-induced three-dimensionally long-range ordered (3D) CDW was observed only in YBCO$_{6.50}$ and YBCO$_{6.67}$. The temperature dependence of the 3D CDW clearly indicates a strong competition with superconductivity. Based on the systematic strain-, doping-, and temperature-dependent REXS measurements reported here, we discuss the relationship between charge order and superconductivity in YBCO$_{6+x}$ and other cuprates.

cond-mat.supr-con↗

arXiv2023

Doping-Induced Electronic and Structural Phase Transition in the Bulk Weyl Semimetal Mo1-xWxTe2

A comprehensive study of the electronic and structural phase transition from 1T` to Td in the bulk Weyl semimetal Mo1-xWxTe2 at different doping concentrations has been carried out using time-of-flight momentum microscopy (including circular and linear dichroism), X-ray photoelectron spectroscopy (XPS), X-ray photoelectron diffraction (XPD), X-ray diffraction (XRD), angle-resolved Raman spectroscopy, transport measurements, density functional theory (DFT) and Kikuchi pattern calculations. High-resolution angle-resolved photoemission spectroscopy (ARPES) at 20 K reveals surface electronic states, which are indicative of topological Fermi arcs. Their dispersion agrees with the position of Weyl points predicted by DFT calculations based on the experimental crystal structure of our samples determined by XRD. Raman spectroscopy confirms the inversion symmetry breaking for the Td -phase, which is a necessary condition for the emergence of topological states. Transport measurements show that increasing the doping concentration from 2 to 9% leads to an increase in the temperature of the phase transition from 1T` to Td from 230 K to 270 K. Magnetoresistance and longitudinal elastoresistance show significantly increased values in the Td -phase due to stimulated inter-pocket electron backscattering. The results demonstrate the close relationship between electronic properties and elastic deformations in MoTe2.

cond-mat.mtrl-sci↗

arXiv2022

Soft-phonon and charge-density-wave formation in nematic BaNi$_2$As$_2$

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi$_2$As$_2$, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, $Q_{I-CDW}$. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around $Q_{I-CDW}$. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complex pattern of the I-CDW satellites observed in the current and earlier studies and settles the debated unidirectional nature of the I-CDW. The phonon instability and its reciprocal space position is well captured by our ${ab}$ ${initio}$ calculations. These however indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.% and suggest alternative origins of the I-CDW.

cond-mat.str-el↗

arXiv2022

Elastoresistivity in the incommensurate charge density wave phase of BaNi$_{\textrm{2}}$(As$_{\textrm{1-x}}$P$_{\textrm{x}}$)$_{\textrm{2}}$

Electronic nematicity, the breaking of the crystal lattice rotational symmetry by the electronic fluid, is a fascinating quantum state of matter. Recently, BaNi$_2$As$_2$ has emerged as a promising candidate for a novel type of nematicity triggered by charge fluctuations. In this work, we scrutinize the electronic nematicity of BaNi$_2$(As$_{1-x}$P$_x$)$_2$ with $0 \leq x \leq 0.10$ using electronic transport measurements under strain. We report a large $B_{1g}$ elastoresistance coefficient that is maximized at a temperature slightly higher than the first-order triclinic transition, and that corresponds to the recently discovered tetragonal-to-orthorhombic transition. The reported elastoresistance does not follow the typical Curie-Weiss form observed in iron-based superconductors but has a much sharper temperature dependence with a finite elastoresistance onsetting only together with a strong enhancement of the incommensurate charge density wave of the material. Consequently, the $B_{1g}$ elastoresistance and the associated orthorhombic distortion appears here as a property of this incommensurate charge density wave. Finally, we report and track the hysteretic behavior seen in the resistance versus strain sweeps and interpret its origin as the pinning of orthorhombic domains. Our results revise the understanding of the interplay between nematicity, charge density waves and structural distortions in this material.

cond-mat.str-el↗

arXiv2021

Absence of temperature-dependent phonon anomalies in Sr2IrO4 and Sr3Ir2O7

Following previous works reporting an anomalous behavior of several zone-center optical phonons across the magnetic transition of square lattice iridates Sr2IrO4 and Sr3Ir2O7, we have investigated the lattice dynamics as a function of momentum in these materials by means of high-resolution inelastic x-ray scattering (IXS). The observed phonon energies and scattering intensities across the Brillouin zone are in excellent agreement with ab-initio lattice dynamical calculations based on non-magnetic density-functional-perturbation theory (DFPT). Our results do not evidence any renormalization of the phonons at finite momentum across the magnetic transition of Sr2IrO4. The only anomalous behavior was detected for the in-plane polarized longitudinal-acoustic phonon branch in Sr3Ir2O7, which anomalously softens towards low temperatures and might be related to anisotropic negative thermal expansion in this compound. No anomalies related to potential charge ordering were observed.

cond-mat.str-el↗

arXiv2020

Inelastic X-ray scattering studies of phonon dispersions in superconductors at high pressures

Electron-phonon interaction is of central importance for the electrical and heat transport properties of metals, and is directly responsible for charge-density-waves or (conventional) superconducting instabilities. The direct observation of phonon dispersion anomalies across electronic phase transitions can provide insightful information regarding the mechanisms underlying their formation. Here, we review the current status of phonon dispersion studies in superconductors under hydrostatic and uniaxial pressure. Advances in the instrumentation of high resolution inelastic X-ray scattering beamlines and pressure generating devices allow these measurements to be performed routinely at synchrotron beamlines worldwide.

cond-mat.supr-con↗

arXiv2018

Light-Induced Metastable State in Charge-Ordered YBa$_2$Cu$_3$O$_{6+x}$

We report temporal changes in Raman-scattering spectra of detwinned YBa$_2$Cu$_3$O$_{6+x}$ single crystals under exposure to red laser light polarized along the crystallographic $a$-axis. A recent publication by Bakr et al. (Phys. Rev. B 88 214517 (2013)) identified new Raman-active modes that appear upon cooling below $T \sim 200$ K, and attributed these modes to charge ordering phenomena observed in x-ray scattering and nuclear magnetic resonance experiments on the same materials. Here we report that the intensities of these modes depend not only on temperature and oxygen content, but also on the cumulative photon dose absorbed by the YBa$_2$Cu$_3$O$_{6+x}$ samples. The light-induced changes in the Raman spectra exhibit a stretched-exponential time dependence, with a characteristic time that increases strongly upon cooling. They also depend strongly on doping and are most pronounced for $x \sim 0.6$. In fully illuminated samples, the mode intensities exhibit a monotonic temperature dependence indicative of a second-order phase transition. The findings indicate a metastable state generated by light-induced rearrangement of the oxygen dopants. We hypothesize that the new Raman phonons are associated with a three-dimensional charge-ordered state induced by light illumination, analogous to a state that was recently observed by resonant x-ray scattering in oxygen-disordered YBa$_2$Cu$_3$O$_{6+x}$ films.

cond-mat.supr-con↗

arXiv2017

Rapid Suppression of the Charge Density Wave in YBa$_2$Cu$_3$O$_{6.6}$ under Hydrostatic Pressure

We report on the effects of hydrostatic pressure (HP) on the charge density wave observed in underdoped cuprates. We studied YBa$_2$Cu$_3$O$_{6.6}$ ($T_c$=61 K) using high-resolution inelastic x-ray scattering (IXS), and reveal an extreme sensitivity of the phonon anomalies related to the charge density wave (CDW) order to HP. The amplitudes of the normal state broadening and superconductivity induced phonon softening at Q$_{CDW}$ rapidly decrease as HP is applied, resulting in the complete suppression of signatures of the CDW below $\sim$1 GPa. Additional IXS measurements on YBa$_2$Cu$_3$O$_{6.75}$ demonstrate that this very rapid effect cannot be explained by pressure-induced modification of the doping level and highlight the different role of external pressure and doping in tuning the phase diagram of the cuprates. Our results provide new insights into the mechanisms underlying the CDW formation and its interplay with superconductivity.

cond-mat.supr-con↗

arXiv2016

Soft phonon driven orbital order in CaMn7O12

We use variable-temperature x-ray thermal diffuse scattering and inelastic scattering to investigate the lattice dynamics in single crystals of multiferroic CaMn7O12 which undergo a series of orbital and magnetic transitions at low temperatures. Upon approaching the charge and orbital ordering temperature To=250 K from above, we observe intense diffuse scattering features and a pronounced optical phonon softening centered around the superstructure reflections of the incommensurately modulated structure. The phonon anomaly appears well above To and continuously increases upon cooling, following a canonical power-law temperature dependence that confirms the transition at To to be of second order and related to a soft-phonon lattice instability. Microscopic mechanisms for the incommensurate charge and orbital ordering based on competing interactions and on momentum dependent electron-phonon coupling could both account for the observed extended momentum width of the phonon softening. Our results highlight the importance of the lattice interactions in the physics of this magnetically induced ferroelectric system.

cond-mat.str-el↗

arXiv2014

Pressure-induced phase transition and superconductivity in YBa2Cu4O8

We investigate the pressure and temperature dependence of the lattice dynamics of the underdoped, stoichiometric, high temperature superconductor YBa2Cu4O8 by means of Raman spectroscopy and ab initio calculations. This system undergoes a reversible pressure-induced structural phase transition around 10 GPa to a collapsed orthorhombic structure, that is well reproduced by the calculation. The coupling of the B1g-like buckling phonon mode to the electronic continuum is used to probe superconductivity. In the low pressure phase, self-energy effects through the superconducting transition renormalize this phonon, and the amplitude of this renormalization strongly increases with pressure. Whereas our calculation indicates that this mode's coupling to the electronic system is only marginally affected by the structural phase transition, the aforementioned renormalization is completely suppressed in the high pressure phase, demonstrating that under hydrostatic pressures higher than 10 GPa, superconductivity in YBa2Cu4O8 is greatly weakened or obliterated.

cond-mat.supr-con↗

arXiv2013

Lattice dynamical signature of charge density wave formation in underdoped YBa2Cu3O6+x

We report a detailed Raman scattering study of the lattice dynamics in detwinned single crystals of the underdoped high temperature superconductor YBa2Cu3O6+x (x=0.75, 0.6, 0.55 and 0.45). Whereas at room temperature the phonon spectra of these compounds are similar to that of optimally doped YBa2Cu3O6.99, additional Raman-active modes appear upon cooling below ~170-200 K in underdoped crystals. The temperature dependence of these new features indicates that they are associated with the incommensurate charge density wave state recently discovered using synchrotron x-ray scattering techniques on the same single crystals. Raman scattering has thus the potential to explore the evolution of this state under extreme conditions.

cond-mat.supr-con↗

arXiv2013

Giant phonon anomalies and central peak due to charge density wave formation in YBa$_2$Cu$_3$O$_{6.6}$

The electron-phonon interaction is a major factor influencing the competition between collective instabilities in correlated-electron materials, but its role in driving high-temperature superconductivity in the cuprates remains poorly understood. We have used high-resolution inelastic x-ray scattering to monitor low-energy phonons in YBa$_2$Cu$_3$O$_{6.6}$ (superconducting $\bf T_c = 61$ K), which is close to a charge density wave (CDW) instability. Phonons in a narrow range of momentum space around the CDW ordering vector exhibit extremely large superconductivity-induced lineshape renormalizations. These results imply that the electron-phonon interaction has sufficient strength to generate various anomalies in electronic spectra, but does not contribute significantly to Cooper pairing. In addition, a quasi-elastic "central peak" due to CDW nanodomains is observed in a wide temperature range above and below $\bf T_c$, suggesting that the gradual onset of a spatially inhomogeneous CDW domain state with decreasing temperature is a generic feature of the underdoped cuprates.

cond-mat.supr-con↗