Second-order Structural Transition in Superconductor La$_3$Co$_4$Sn$_{13}$

TL;DR

This study investigates a second-order structural transition in the superconductor La$_3$Co$_4$Sn$_{13}$ at 152 K, combining experimental measurements and computational analysis to understand its nature and electronic properties.

Contribution

The paper identifies and characterizes a second-order structural transition in La$_3$Co$_4$Sn$_{13}$ using combined experimental and theoretical approaches, revealing phonon instability and Fermi surface features.

Findings

Transition at 152 K is second-order with a specific wave vector.

Lattice dynamics show imaginary phonon frequencies indicating instability.

Fermi surface analysis suggests similarities with related compounds.

Abstract

The quasi-skutterudite superconductor La$_3$Co$_4$Sn$_{13}$ undergoes a phase transition at $T^*=152$ K. By measuring the temperature dependence of heat capacity, electrical resistivity, and the superlattice reflection intensity using X-ray, we explore the character of the phase transition at $T^*$. Our lattice dynamic calculations found imaginary phonon frequencies around the ${\bf M}$ point, when the high temperature structure is used in the calculations, indicating that the structure is unstable at the zero temperature limit. The combined experimental and computational results establish that $T^*$ is associated with a second-order structural transition with $\bf{q}$=(0.5, 0.5, 0) (or the ${\bf M}$ point). Further electronic bandstructure calculations reveal Fermi surface sheets with low curvature segments, which allow us to draw qualitative comparison with both Sr$_3$Ir$_4$Sn$_{13}$ and Sr$_3$Rh$_4$Sn$_{13}$ in which similar physics has been discussed recently.