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

摘要

"F-free" CeOBiS$_2$ single crystals have successfully grown, thoroughlyeliminating 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 thewell-developed plane. Single crystal X-ray structural analysis clearly revealedthat the CeOBiS$_2$ crystallizes with a space group $P4/nmm$ (with latticeparameters of $a$ = 4.0189(6) {\AA}, $c$ = 13.573(2) {\AA}). The bond valencesum estimation and X-ray photoelectron spectroscopy analysis showed that thechemical state of Ce was in the mixed valence of Ce$^{3+}$ and Ce$^{4+}$. Thesingle crystals show superconductivity with zero resistivity at $\sim$1.3 K.This is the first conclusive evidence of superconductivity driven by Ce valencefluctuation in surely non-doped CeOBiS$_2$. The superconducting transitiontemperature was enhanced up to $\sim$3.8 K by applying hydrostatic pressure.