Observation of Two Collapsed Phases in CaRbFe4As4

TL;DR

This study identifies pressure-induced phase transitions in CaRbFe4As4, revealing a half-collapsed tetragonal phase suppressing superconductivity and a fully-collapsed phase driven by As-As bonding, with significant electronic and structural changes.

Contribution

First observation of two distinct collapsed tetragonal phases in CaRbFe4As4 under pressure, combining experimental and theoretical insights into their structural and electronic nature.

Findings

Superconductivity is suppressed from 35K to below 2K with increasing pressure.

Identification of a fully-collapsed tetragonal phase at around 22 GPa.

Electronic properties change significantly across the phase transitions.

Abstract

We report the observation of the pressure-induced, fully-collapsed tetragonal phase of CaRbFe4As4 for P~ 22 GPa via high-pressure x-ray diffraction and magnetotransport measurements. The x-ray measurements, along with resistivity measurements, show that there is an initial half-collapsed tetragonal phase for 6 < P < 22 GPa, in which superconductivity is continuously suppressed from Tc= 35K at P= 3.1 GPa to Tc <2K for P >17.2 GPa, as well as signs of the fully-collapsed tetragonal phase near P=22 GPa. Density functional calculations suggest that both of these transitions are driven by increased As-As bonding, first across the Ca layer, and then at the second transition, across the Rb layer. Although electrical resistivity measurements in the fully-collapsed tetragonal phase do not show superconductivity, there is a change in the slope of both the Hall coefficient and the longitudinal resistance near 22 GPa, suggesting a strong correlation between the electronic and lattice degrees of freedom in this new iron-based superconductor.