Effect of pressure cycling on Iron: Signatures of an electronic instability and unconventional superconductivity

TL;DR

This study investigates how pressure cycling affects iron's electronic structure and superconductivity, revealing a possible quantum critical point and signs of unconventional superconductivity driven by ferromagnetic fluctuations.

Contribution

It provides direct crystallographic evidence of phase transition directions and links electronic instability to unconventional superconductivity in iron under pressure.

Findings

Superconductivity persists between 13 and 31 GPa.

Electronic instability occurs around 19 GPa, indicating a quantum critical point.

Resistivity exhibits a 5/3 power law exponent suggesting ferromagnetic fluctuation-mediated pairing.

Abstract

High pressure electrical resistivity and x-ray diffraction experiments have been performed on Fe single crystals. The crystallographic investigation provides direct evidence that in the martensitic $bcc \rightarrow hcp$ transition at 14 GPa the $\lbrace 110\rbrace_{bcc}$ become the $\lbrace 002\rbrace_{hcp}$ directions. During a pressure cycle, resistivity shows a broad hysteresis of 6.5 GPa, whereas superconductivity, observed between 13 and 31 GPa, remains unaffected. Upon increasing pressure an electronic instability, probably a quantum critical point, is observed at around 19 GPa and, close to this pressure, the superconducting $T_{c}$ and the isothermal resistivity ($0<T<300\,$K) attain maximum values. In the superconducting pressure domain, the exponent $n = 5/3$ of the temperature power law of resistivity and its prefactor, which mimics $T_{c}$, indicate that ferromagnetic fluctuations may provide the glue for the Cooper pairs, yielding unconventional superconductivity.