Finite-Temperature $\textit{ab initio}$ Structural Optimization of the Bilayer Nickelate Superconductor La$_3$Ni$_2$O$_7$

TL;DR

This paper introduces a first-principles finite-temperature structural optimization method incorporating anharmonic phonon effects, applied to La$_3$Ni$_2$O$_7$ to map its phase diagram and explore superconductivity implications.

Contribution

The authors develop an improved, efficient first-principles framework for finite-temperature structural optimization that accounts for anharmonic vibrational effects, applicable to systems with changing primitive cell sizes.

Findings

Mapped the pressure-temperature phase diagram of La$_3$Ni$_2$O$_7$

Found a negative slope of the phase boundary (~-60 K / GPa)

Provided insights into how crystal symmetry affects superconductivity

Abstract

We develop a first-principles framework for finite-temperature structural optimization that incorporates vibrational contributions to the free energy through anharmonic phonon theory. We extend and further improve the efficiency of the recent approach, enabling its application to systems in which the size of the primitive cell changes across structural phase transitions. Applying this framework to La$_3$Ni$_2$O$_7$, we establish its pressure-temperature phase diagram and find that the slope of the phase boundary between the high-symmetry and low-symmetry phases is negative, with a magnitude of approximately -60 K / GPa. The present results provide a theoretical foundation for discussing how changes in crystal symmetry influence the emergence of superconductivity.