Synthesis, crystal structure and magnetism of beta-Fe1.00(2)Se1.00(3) single crystals

TL;DR

This study reports the successful synthesis of high-purity beta-FeSe single crystals using a flux method, revealing defect-induced weak ferromagnetism coexisting with superconductivity and clarifying the role of Fe sites in electronic properties.

Contribution

First high-quality impurity-free beta-FeSe single crystals grown via LiCl/CsCl flux, providing insights into their magnetic and electronic structure.

Findings

Phase purity confirmed by synchrotron analysis

Weak ferromagnetism coexists with superconductivity below Tc

Interstitial Fe(2) site is unoccupied, with all density of states from in-plane Fe(1)

Abstract

Understanding iron based superconductors requires high quality impurity free single crystals. So far they have been elusive for beta-FeSe and extraction of intrinsic materials properties has been compromised by several magnetic impurity phases. Herein we report synchrotron - clean beta-FeSe superconducting single crystals grown via LiCl/CsCl flux method. Phase purity yields evidence for a defect induced weak ferromagnetism that coexists with superconductivity below Tc. In contrast to Fe1+yTe - based superconductors, our results reveal that the interstitial Fe(2) site is not occupied and that all contribution to density of states at the Fermi level must come from in-plane Fe(1).