# 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.

## Key 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.

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Source: https://tomesphere.com/paper/1902.00472