Superconductivity at 44 K in K intercalated FeSe system with excess Fe

TL;DR

This paper reports the discovery of a new superconducting phase at 44 K in K intercalated FeSe with excess Fe, achieved through precise crystal growth, structural analysis, and thermodynamic measurements, revealing a novel high-temperature superconductivity mechanism.

Contribution

The study introduces a new high-Tc superconducting phase at 44 K in K intercalated FeSe with excess Fe, supported by structural and first-principles analysis, and uncovers a correlation between Tc and FeSe layer separation.

Findings

Superconducting transition at 44 K confirmed by resistivity and ESR.

Coexistence of two phases responsible for 30 K and 44 K Tc.

Large c-axis (~18.10 Å) in the 44 K phase close to 122 structure.

Abstract

We report here that a new superconducting phase with much higher Tc has been found in K intercalated FeSe compound with excess Fe. We successfully grew crystals by precisely controlling the starting amount of Fe. Besides the superconducting (SC) transition at ~30 K, we observed a sharp drop in resistivity and a kink in susceptibility at 44 K. By combining thermodynamic measurements with electron spin resonance (ESR), we demonstrate that this is a new SC transition. Structural analysis unambiguously reveals two phases coexisting in the crystals, which are responsible respectively for the SC transitions at 30 and 44 K. The structural experiments and first-principles calculations consistently indicate that the 44 K SC phase is close to a 122 structure, but with an unexpectedly large c-axis of 18.10 {\AA}. We further find a novel monotonic dependence of the maximum Tc on the separation of neighbouring FeSe layers.