Crystal structure instability of FeSe grains: Formation of non-superconducting phase at the grain surface

TL;DR

This study reveals that small FeSe grains develop a non-superconducting monoclinic phase at their surface, leading to reduced superconductivity, highlighting the importance of grain size in FeSe-based superconductors.

Contribution

It demonstrates the correlation between grain size, surface phase formation, and superconducting properties in FeSe polycrystalline samples.

Findings

Large grains exhibit high critical current density.

Small grains show no superconductivity but magnetization hysteresis.

Non-superconducting monoclinic phase forms at small grain surfaces.

Abstract

We investigated the magnetization and crystal structure of FeSe polycrystalline samples with various grain sizes. For the samples with large grains, a large magnetic critical current density Jc was observed. For the samples with small grains, superconductivity signals were not observed; instead, magnetization hysteresis, which is not a result of superconductivity, was observed. In the X-ray diffraction pattern for the samples with small grains, broad additional peaks were observed, corresponding to the formation of the non-superconducting (monoclinic) Fe-Se phase at the FeSe grain surface. The crystal structure instability at the grain surface would be the reason for the low superconducting properties of the Fe-chalcogenide polycrystalline wires investigated thus far.