Soft phonons and structural phase transitions in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$

TL;DR

This study investigates the soft phonon behavior associated with the structural phase transition in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ using neutron scattering, revealing details about phonon softening, phase coexistence, and transition characteristics.

Contribution

It provides detailed neutron scattering analysis of phonon softening and phase coexistence during the LTO to LTT transition in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$, highlighting the nature of the transition.

Findings

The TO-mode softens and approaches zero energy at T_d2=62 K.

Phonon energy saturates below 50 K after rapid hardening.

Significant change in in-plane phonon dispersion at T_d2.

Abstract

Soft phonon behavior associated with a structural phase transition from the low-temperature-orthorhombic (LTO) phase ($Bmab$ symmetry) to the low-temperature-tetragonal (LTT) phase ($P4_{2}/ncm$ symmetry) was investigated in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ using neutron scattering. As temperature decreases, the TO-mode at $Z$-point softens and approaches to zero energy around $T_{\rm d2}=62$ K, where the LTO -- LTT transition occurs. Below $T_{\rm d2}$, the phonon hardens quite rapidly and it's energy almost saturates below 50 K. At $T_{\rm d2}$, the energy dispersion of the soft phonon along in-plane direction significantly changes while the dispersion along out-of-plane direction is almost temperature independent. Coexistence between the LTO phase and the LTT phase, seen in both the soft phonon spectra and the peak profiles of Bragg reflection, is discussed in context of the order of structural phase transitions.