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.Comment: 8 pages, 6 figures. Invited review submitted to Superconductor Science and Technology, Focus issue on Hydride & High-Pressure Superconductors. References in figure caption fixed. Hyperlinks adde

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.Comment: 12 pages, 11 figure

Elastoresistivity in the incommensurate charge density wave phase of BaNi₂(As₁₋ₓPₓ)₂

Electronic nematicity, the breaking of the crystal lattice rotational symmetry by the electronic fluid, is a fascinating quantum state of matter. In this work, using electronic transport under strain we investigate the electronic nematicity of BaNi$_2$(As$_{1−x}$P$_x$)$_2$, a candidate system for charge-induced nematicity. We report a large B$_{1g}$ elastoresistance coefficient that is maximized at the tetragonal-to-orthorhombic transition temperature, that slightly precedes the first-order triclinic transition. An hysteretic behavior is observed in the resistance versus strain sweeps and interpreted as the pinning of orthorhombic domains. Remarkably, the elastoresistance only onsets together with a strong enhancement of the incommensurate charge density wave of the material, strongly suggesting that this electronic instability is uniaxial in nature and drive the orthorhombic transition. The absence of sizeable elastoresistance above this electronic phase clearly contrasts dynamic and static electronic nematicity. Finally, the elastoresistance temperature dependence that strongly differs from the Curie-Weiss form of iron-based superconductors reveals major differences for the respective coupling of electronic nematicity to the lattice. Our results uncover an extremely strain-sensitive platform to study electronic anisotropy induced by a charge-density-wave instability

Soft-Phonon and Charge-Density-Wave Formation in Nematic BaNi₂As₂

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 are 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

Soft-phonon and charge-density-wave formation in nematic BaNi2_2As2_2

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave order (I-CDW) in BaNi$_2$As$_2$, a candidate system for charge-driven electronic nematicity. At low temperatures, the I-CDW sets in before a structural transition to a triclinic phase, within which it is suppressed and replaced by a commensurate CDW order (C-CDW). Intense diffuse scattering signal is observed around the modulation vector of the I-CDW, $Q_{I-CDW}$ already visible at room temperature and collapsing into superstructure reflections in the ordered state. 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 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

Using strain to uncover the interplay between two- and three-dimensional charge density waves in high-temperature superconducting YBa₂Cu₃O_y

Uniaxial pressure provides an efficient approach to control charge density waves in YBa2Cu3Oy. It can enhance the correlation volume of ubiquitous short-range two-dimensional charge-density-wave correlations, and induces a long-range three-dimensional charge density wave, otherwise only accessible at large magnetic fields. Here, we use x-ray diffraction to study the strain dependence of these charge density waves and uncover direct evidence for a form of competition between them. We show that this interplay is qualitatively described by including strain effects in a nonlinear sigma model of competing superconducting and charge-density-wave orders. Our analysis suggests that strain stabilizes the 3D charge density wave in the regions between disorder-pinned domains of 2D charge density waves, and that the two orders compete at the boundaries of these domains. No signatures of discommensurations nor of pair density waves are observed. From a broader perspective, our results underscore the potential of strain tuning as a powerful tool for probing competing orders in quantum materials