Figure 1

Temperature dependence of electric resistivity $\rho (T)$ under pressure up to 8.0 GPa. (a) Resistivity in the temperature range from 4.2 to 300 K. The pressure was increased in the sequence of 0.1 MPa, 1.5, 3.0, 4.0, 5.0, 6.5, to 8.0 GPa. We remeasured $\rho (T)$ at 1.5 GPa after pressure was decreased from 8.0 GPa to confirm the reversibility with respect to the variation of pressure. The characteristic temperature $T^{*}$ is indicated by the arrows. Superconductivity is observed at $P\le 5.0\mathrm{G}\mathrm{P}\mathrm{a}$. (b) An expanded plot of $\rho (T)$ in the range $4.2<T<40\mathrm{K}$ at $P\le 5.0\mathrm{G}\mathrm{P}\mathrm{a}$. The solid lines are the best fits with the phenomenological equation (1) using the parameters listed in Table .Reuse & Permissions

Figure 2

Pressure dependence of electric resistivity $\rho (T)$. The open and closed circles represent $\rho (10\mathrm{K}\mathrm{)}$ and $\rho (300\mathrm{K}\mathrm{)}$, respectively. Each point was taken from the experimental data shown in Fig. 1a. Both $\rho (10\mathrm{K}\mathrm{)}$ and $\rho (300\mathrm{K}\mathrm{)}$ basically increase monotonically with pressure, although an inflection point is seen in $\rho (10\mathrm{K}\mathrm{)}$ between 5.0 and 6.5 GPa.Reuse & Permissions

Figure 3

The superconducting transition temperature $T_{\mathrm{c}}$ and the characteristic temperature $T^{*}$ plotted as functions of pressure. The solid and broken lines are guides to the eyes. Superconductivity vanishes at $P_{\mathrm{S}\mathrm{I}}$, where $T^{*}$ has the minimum value and the metal-insulator phase boundary (hatched) is expected to lie.Reuse & Permissions

Figure 4

Pressure dependence of Raman spectra at 300 K. Each spectrum is shifted vertically to avoid the overlaps. There is neither appearance of a new phonon peak nor indication of peak splitting. The relative change in the intensity among the peaks is arbitrary since the diameter of laser beam ($\sim 2\mu \mathrm{m}$) is almost the same as the grain size of the single-crystalline $\mathrm{C}\mathrm{u}\mathrm{R}{\mathrm{h}}_{\mathrm{2}}{\mathrm{S}}_{\mathrm{4}}$ and the position of the focused single crystal in the sample possibly changes very slightly by compression.Reuse & Permissions