High pressure effects on non-fluorinated BiS2-based superconductors La${}_{1-x}$M${}_x$OBiS${}_2$ (M = Ti and Th)

Abstract

Layered \textit{Ln}OBiS${}_2$ compounds with \textit{Ln} = La, Ce, Pr, Nd, and Yb can be rendered conducting and superconducting via two routes, substitution of F for O or the tetravalent ions Ti, Zr, Hf, and Th for trivalent \textit{Ln} ions. Electrical resistivity measurements on non-fluorinated La${}_{0.80}$Ti${}_{0.20}$OBiS${}_2$ and La${}_{0.85}$Th${}_{0.15}$OBiS${}_2$ superconductors were performed between $\sim$1.5 K and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature $T_c$, which is $\sim$2.9 K at ambient pressure, gradually increases with pressure to 3.2-3.7 K at $\sim$1 GPa, above which it is suppressed and the superconducting transitions become very broad. Measurements of the normal state electrical resistivity of the two compounds reveal discontinuous changes of the resistivity as a function of pressure at $\sim$0.6 GPa. Surprisingly, above 1.3 GPa, semiconducting-like behavior reappears in La${}_{0.80}$Ti${}_{0.20}$OBiS${}_2$. This study reveals a new high-pressure phase of La${}_{1-x}$$M$${}_x$OBiS${}_2$ containing the tetravalent ions $M$ = Ti, Th which does not favor superconductivity. In contrast, application of pressure to fluorinated LaO${}_0.5$F${}_0.5$BiS${}_2$ produces an abrupt tetragonal-monoclinic transition to a metallic phase with an enhanced $T_c$. These results demonstrate that the response of the normal and superconducting properties of LaOBiS${}_2$-based compounds depends strongly on the atomic site where the electron donor ions are substituted.

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