Phonon spectra in CaFe2As2 and Ca0.6Na0.4Fe2As2: Measurement of the pressure and temperature dependence and comparison with ab-initio and shell model calculations

TL;DR

This study investigates how pressure and temperature affect phonon spectra in CaFe2As2 and Ca0.6Na0.4Fe2As2, revealing minimal changes under pressure but notable softening with temperature, and compares experimental results with theoretical models.

Contribution

It provides detailed measurements of phonon spectra under varying conditions and compares them with ab-initio and shell model calculations, enhancing understanding of phonon behavior in these superconductors.

Findings

No significant phonon spectrum change in Ca0.6Na0.4Fe2As2 up to 5 kbar pressure.

Phonon softening observed in CaFe2As2 with decreasing temperature.

Phonon spectra suggest interaction with electron or spin fluctuations.

Abstract

We report the pressure and temperature dependence of the phonon density-of-states in superconducting Ca0.6Na0.4Fe2As2 (Tc=21 K) and the parent compound CaFe2As2, using inelastic neutron scattering. We observe no significant change in the phonon spectrum for Ca0.6Na0.4Fe2As2 at 295 K up to pressures of 5 kbar. The phonon spectrum for CaFe2As2 shows softening of the low-energy modes by about 1 meV when decreasing the temperature from 300 K to 180 K. There is no appreciable change in the phonon density of states across the structural and anti-ferromagnetic phase transition at 172 K. These results, combined with our earlier temperature dependent phonon density of states measurements for Ca0.6Na0.4Fe2As2, indicate that the softening of low-energy phonon modes in these compounds may be due to the interaction of phonons with electron or short-range spin fluctuations in the normal state of the superconducting compound as well as in the parent compound. The phonon spectra are analyzed with ab-initio and empirical model calculations giving partial densities of states and dispersion relations.