High-pressure effects on nonfluorinated ${\mathbf{BiS}}_{\mathbf{2}}$-based superconductors ${\mathbf{La}}_{\mathit{1}-x}{\mathit{M}}_x{\mathbf{OBiS}}_{\mathbf{2}}$ ($\mathit{M}=\mathbf{Ti}$ and Th)

Layered ${\mathrm{LnOBiS}}_2$ (where Ln represents a lanthanide element) compounds with 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 Ln ions. Electrical resistivity measurements on nonfluorinated ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$ and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_2$ superconductors were performed between $\sim 1.5$ and 300 K and under pressure up to 2.4 GPa. For both compounds, the superconducting transition temperature $T_c$, which is $\sim 2.9\mathrm{K}$ at ambient pressure, gradually increases with pressure to 3.2–3.7 K at $\sim 1\mathrm{GPa}$, 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 in the resistivity as a function of pressure at $\sim 0.6\mathrm{GPa}$. Surprisingly, above 1.3 GPa, semiconductinglike behavior reappears in ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$. This paper reveals a new high-pressure phase of ${\mathrm{La}}_{1-x}{M}_x{\mathrm{OBiS}}_2$ containing the tetravalent ions $M=\mathrm{Ti}$, Th which does not favor superconductivity. In contrast, application of pressure to fluorinated ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiS}}_2$ 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 ${\mathrm{LaOBiS}}_2$-based compounds depends strongly on the atomic site where the electron donor ions are substituted.

Figure 1

Temperature $T$ dependence of electrical resistivity $\rho$, normalized to its normal-state value at 4.2 K, below 4 K for ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$ (upper panel) and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_2$ (lower panel). The numbers and symbols with different colors in both panels are used to indicate the pressures listed in the lower panel. Most of the $\rho \left(T\right)$ data (represented by circles) were obtained upon increasing pressure. Crosses and the $\downarrow$ on the right side of the number 8 in this figure as well as in the following figures throughout this paper are used to represent data taken upon decreasing pressure.

Figure 2

Superconducting transition temperature $T_c$ vs pressure for ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$ (upper panel) and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_2$ (lower panel). The temperature at which $\rho$ drops to 99%, 90%, 50%, and 10% of its normal-state value above the superconducting transition is indicated by symbols $T_c^{99\%},T_c^{90\%},T_c^{50\%}$, and $T_c^{10\%}$, respectively. The lengths of the vertical lines represent the difference between $T_c^{99\%}$ and $T_c^{90\%}$ and are measures of the broadening of the superconducting transition under pressure. The crosses represent data taken upon decreasing pressure. The solid and dashed lines are guides to the eye for the experimental data and the corresponding extrapolations, respectively.

Figure 3

Temperature $T$ dependence of the electrical resistivity $\rho$, normalized to the value of $\rho$ at 280 K, ($\rho /{\rho }_{280}{}_{\mathrm{K}}$) of ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$ (upper panel) and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_2$ (lower panel) at various pressures. The pressure corresponding to each ($\rho /{\rho }_{280}{}_{\mathrm{K}}$) curve is indicated in Fig. 1 with the same color. The upper inset: enlargement of the $T$ dependence of $\rho /{\rho }_{280}{}_{\mathrm{K}}$ where the anomalies in $\rho \left(T\right)$ can be observed. The orange curve represents the data obtained at 0.24 GPa during the pressure releasing cycle. The lower inset: temperature at which the anomaly in $\rho \left(T\right)$ occurs, ($T_a$) vs pressure ($P$). The black arrows in the figure indicate the location of the anomalies in the resistivity.

Figure 4

Evolution of the slopes of $\rho \left(T\right)$ at (a) $\sim 10\mathrm{K}$ and at (b) $\sim 250\mathrm{K}$ with increasing pressure for ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$ (red circles) and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_2$ (blue squares), respectively. The open circles indicate that the values are negative, suggesting semiconductinglike behavior. The dashed lines are guides to the eye.

Figure 5

Normal-state electrical resistivity $\rho$ at 4.2 K for ${\mathrm{La}}_{0.80}{\mathrm{Ti}}_{0.20}{\mathrm{OBiS}}_2$ (red circles) and ${\mathrm{La}}_{0.85}{\mathrm{Th}}_{0.15}{\mathrm{OBiS}}_2$ (blue squares) under pressure. The dashed lines are linear fits of ${\rho }_{4.2\mathrm{K}}$ vs $P$ data, whose slopes change at $\sim 0.6\mathrm{GPa}$ (indicated by the arrows) for both compounds.