Soft phonon and the central peak at the cubic-to-tetragonal phase transition in SrTiO$_3$

TL;DR

This study investigates the origin of the central peak observed in neutron scattering during the cubic-to-tetragonal phase transition in SrTiO$_3$, using inelastic x-ray scattering to analyze soft phonon behavior and local crystal properties.

Contribution

It provides the first inelastic x-ray scattering analysis comparing two differently grown SrTiO$_3$ samples, revealing the local origin of the central peak and its relation to anisotropic correlation lengths.

Findings

Central peak intensity varies by a factor of four between samples.

Soft phonon evolution is similar in both samples.

Local crystal properties influence the central peak.

Abstract

The continuous displacive phase transition in SrTiO$_3$ near $T_c \approx 105$ K features a central elastic peak in neutron scattering investigations at temperatures above $T_c$, i.e., before the corresponding soft phonon mode is overdamped upon cooling. The origin of this central peak is still not understood. Here, we report an inelastic x-ray scattering investigation of the cubic-to-tetragonal phase transition in SrTiO$_3$. We compare quantitatively measurements of the soft phonon mode on two differently grown samples and discuss the findings regarding results from thermodynamic and transport probes such as specific heat and thermal conductivity. Furthermore, we use inelastic x-ray scattering to perform elastic scans with both high momentum- and milli-electronvolt energy-resolution and, thus, be able to separate elastic intensities of the central peak from low-energy quasielastic phonon scattering. Our results indicate that the evolution of the soft mode is similar in both samples though the intensities of the central peak differ by a factor of four. Measurements revealing anisotropic correlation lengths on cooling towards $T_c$, indicate that local properties of the crystals to which collective lattice excitations are insensitive are likely at the origin of the central elastic line in SrTiO$_3$.