Lattice dynamical properties of superconducting SrPt$_3$P studied via inelastic x-ray scattering and density functional perturbation theory

TL;DR

This study combines inelastic x-ray scattering and density functional perturbation theory to analyze the lattice dynamics of SrPt$_3$P, revealing strong electron-phonon coupling and discrepancies between experimental and theoretical phonon energies.

Contribution

It provides new insights into the phonon spectrum and electron-phonon interactions in SrPt$_3$P, highlighting limitations in current theoretical models.

Findings

Strong electron-phonon coupling with λ ~ 2

Observed phonon softening from room to low temperatures

No spectral changes across the superconducting transition

Abstract

We present a study of the lattice dynamical properties of superconducting SrPt$_3$P ($T_c = 8.4$ K) via high-resolution inelastic x-ray scattering (IXS) and ab initio calculations. Density functional perturbation theory including spin-orbit coupling (SOC) results in enhanced electron-phonon coupling (EPC) for the optic phonon modes originating from the Pt(I) atoms, with energies $\sim 5$ meV, resulting in a large EPC constant $\lambda \sim 2$. An overall softening of the IXS powder spectra occurs from room to low temperatures, consistent with the predicted strong EPC and with recent specific-heat experiments ($2\Delta_0 / k_{\mathrm{B}}T_c \sim 5$). The low-lying phonon modes observed in the experiments are approximately 1.5 meV harder than the corresponding calculated phonon branch. Moreover, we do not find any changes in the spectra upon entering the superconducting phase. We conclude that current theoretical calculations underestimate the energy of the lowest band of phonon modes indicating that the coupling of these modes to the electronic subsystem is overestimated.