Soft phonons and structural phase transition in superconducting Ba0.59K0.41BiO3

TL;DR

This study investigates phonon softening and a structural phase transition in superconducting Ba0.59K0.41BiO3 using neutron scattering, revealing a cubic to tetragonal transition without direct impact on superconductivity.

Contribution

It provides detailed neutron scattering evidence of phonon softening and structural transition in Ba0.59K0.41BiO3, including new observations of superstructure peaks.

Findings

Phonon energies decrease with temperature leading to a cubic-tetragonal transition.

Weak superstructure peaks indicate additional modulation components.

No significant change in phonon properties related to superconductivity.

Abstract

We have observed a softening of phonons and a structural phase transition in a superconducting Ba0.59K0.41BiO3 (Tc = 31 K) single crystal using elastic and inelastic neutron scattering measurements. The soft phonon occurs for the [111] transverse acoustic mode at the zone boundary. The phonon energies in this vicinity are found to continuously decrease with decreasing temperature from above room temperature to 200 K, where a structural phase transition from cubic to tetragonal symmetry occurs. The overall results are consistent with previous data that reported phonon softening and a (0.5, 0.5, 0.5) type superstructure in several Ba1-xKxBiO3 systems. However, we also find weak (0.5, 0.5, 0) type superstructure peaks that reveal an additional component to the modulation. No significant change related to the superconductivity was observed for the soft phonon energies or linewidths.