Superconductivity and its Enhancement under High Pressure in "F-free" Single Crystals of CeOBiS$_2$

TL;DR

This study reports the growth of F-free CeOBiS$_2$ single crystals, revealing superconductivity driven by Ce valence fluctuation, with transition temperature enhancement under high pressure, marking a significant advancement in understanding this material.

Contribution

First demonstration of superconductivity in non-doped CeOBiS$_2$ crystals driven by Ce valence fluctuation, with pressure-induced Tc enhancement.

Findings

Superconductivity observed at ~1.3 K in CeOBiS$_2$ crystals.

Superconducting transition temperature increased to ~3.8 K under high pressure.

Crystals confirmed to be F-free and have mixed Ce valence states.

Abstract

"F-free" CeOBiS$_2$ single crystals have successfully grown, thoroughly eliminating a concern about F-contamination by using a high-purity CsCl flux. The obtained crystals have a plate-like shape with a size of 1.0-3.0 mm in the well-developed plane. Single crystal X-ray structural analysis clearly revealed that the CeOBiS$_2$ crystallizes with a space group $P4/nmm$ (with lattice parameters of $a$ = 4.0189(6) {\AA}, $c$ = 13.573(2) {\AA}). The bond valence sum estimation and X-ray photoelectron spectroscopy analysis showed that the chemical state of Ce was in the mixed valence of Ce$^{3+}$ and Ce$^{4+}$. The single crystals show superconductivity with zero resistivity at $\sim$1.3 K. This is the first conclusive evidence of superconductivity driven by Ce valence fluctuation in surely non-doped CeOBiS$_2$. The superconducting transition temperature was enhanced up to $\sim$3.8 K by applying hydrostatic pressure.