Superconductivity in undoped CaFe2As2 single crystals

TL;DR

This study reports the discovery of superconductivity below 25 K in undoped CaFe2As2 single crystals induced by internal strain from rapid quenching, with structural phase transition around 80 K.

Contribution

It demonstrates that internal strain from quenching can induce superconductivity in undoped CaFe2As2, revealing a new pathway for superconductivity in this material.

Findings

Superconductivity observed below 25 K in quenched CaFe2As2.

Collapsed tetragonal phase transition occurs around 80 K.

Internal strain is key to inducing superconductivity.

Abstract

Single crystals of undoped CaFe2As2 were grown by a FeAs self-flux method, and the crystals were quenched in ice-water rapidly after high temperature growth. The quenched crystal undergoes a collapsed tetragonal structural phase transition around 80 K revealed by the temperature dependent X-ray diffraction measurements. Superconductivity below 25 K was observed in the collapsed phase by resistivity and magnetization measurements. The isothermal magnetization curve measured at 2 K indicates that this is a typical type-II superconductor. For comparison, we systematically characterized the properties of the furnace cooled, quenched, and post-annealed single crystals, and found strong internal crystallographic strain existing in the quenched samples, which is the key for the occurrence of superconductivity in the undoped CaFe2As2 single crystals.