Superconductivity in Uncollapsed Tetragonal LaFe2As2

TL;DR

This study demonstrates that uncollapsed tetragonal LaFe2As2 exhibits superconductivity at 12.1 K after annealing, with structural changes affecting its electronic properties and potential pairing mechanisms.

Contribution

It reveals the emergence of superconductivity in LaFe2As2 through structural transition from collapsed to uncollapsed tetragonal phase.

Findings

Superconductivity appears at 12.1 K in uncollapsed tetragonal La122.

Annealing induces a structural transition from collapsed to uncollapsed tetragonal.

The Fermi surface changes due to electron doping, affecting pairing mechanisms.

Abstract

We report synthesis, crystal structure and superconductivity in ThCr2Si2-type LaFe2As2 (La122). La122 was synthesized at 960{\deg}C for 1.5 h under a pressure of 3.4 GPa. An as-synthesized La122 (non-superconductor) had a collapsed tetragonal structure with a short c-axis length of 11.0144(4) {\AA} as observed in CaFe2As2 under pressure. The collapsed tetragonal transformed into an uncollapsed tetragonal by annealing the as-synthesized La122 at 500{\deg}C. The c-axis length remarkably extended to 11.7317(4) {\AA} and superconductivity emerged at 12.1 K in the uncollapsed tetragonal La122. A cylindrical hole-like Fermi-surface around the {\Gamma} point that plays an important role for an s+-wave paring in iron-based superconductors was missing in the uncollapsed tetragonal La122 due to heavily electron-doping. Superconductivity in La122 may be closely related to that induced in CaFe2As2 under pressure.