Metallic crossover through the tilt-free transition in La$_3$Ni$_2$O$_7$ at high pressure and temperature

TL;DR

This study investigates the pressure and temperature-induced structural transition in La$_3$Ni$_2$O$_7$, revealing a crossover from a bad metal to a good metal associated with high-T$_c$ superconductivity and enhanced electron-phonon coupling.

Contribution

The paper combines Raman and infrared spectroscopies to map structural and electronic changes, identifying a new upper temperature limit of the Amam phase and linking structural transitions to electronic properties.

Findings

Structural transition from tilted Amam to untilted Fmmm phase near 10-15 GPa.

Emergence of high-T$_c$ superconductivity (~80 K) associated with the transition.

A two-order-magnitude increase in carrier density indicating metallization.

Abstract

La$_3$Ni$_2$O$_7$, a bilayer nickelate with Ruddlesden-Popper structure, undergoes a pressure-induced structural transition from a tilted Amam phase to an untilted Fmmm (or I4/mmm) phase near 10-15 GPa, concomitant with the emergence of high-T$_c$ superconductivity (T$_c$ $\sim$ 80 K). Despite intense interest, the phase boundaries and the impact of structural changes on the electronic properties remain unclear. Here, we combine high-pressure and high-temperature Raman and synchrotron-based infrared spectroscopies to map the structural and electronic evolutions. Raman measurements confirm the pressure-driven structural transition and reveal the emergence of Fano line shapes, indicating enhanced electron-phonon coupling. High-temperature data show analogous spectral signatures above 544 K, suggesting an unreported upper temperature limit of the Amam phase within the T-P phase diagram of this system. Infrared reflectivity measurements evidence a concomitant metallization, with a tremendous two-order-of-magnitude increase in carrier density, marking a crossover from a bad metal to a good metal. These results establish a unified picture of the structural transition and its strong coupling to the electronic properties.