Pressure-tuned enhancement of superconductivity and change of ground state properties in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystals

By using a hydrostatic pressure, we have successfully tuned the normal state property and superconductivity in ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystals. It is found that, with the increase of pressure, the original superconducting phase with $T_c\sim 3.5$ K can be tuned to a state with lower $T_c$, and then a new superconducting phase with $T_c\sim 6.5$ K emerges. Accompanied by this crossover, the normal state is switched from that with a low temperature resistivity upturning to a metallic one. Accordingly, the normal state resistivity also shows a nonmonotonic change with the external pressure. Furthermore, by applying a magnetic field, the new superconducting state under pressure with $T_c\sim 6.5$ K is suppressed, and the recovered normal state reveals a resistivity-upturning feature again. These results illustrate a nontrivial relationship between the normal state property and superconductivity in this newly discovered superconducting system.

Figure 1

(a) X-ray diffraction pattern for a ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ (nominal composition) single crystal. The inset shows the back Laue x-ray diffraction pattern of a ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystal. (b) Energy dispersive x-ray microanalysis spectrum taken on a ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystal. The inset shows the SEM photograph of the crystal with typical dimensions of about $1.5\times 0.75\times 0.04$ ${\mathrm{mm}}^3$.

Figure 2

(a) Temperature dependence of resistivity for sample 1 at various pressures in the temperature range 2 K to 300 K. The inset shows the magnetic susceptibility of a ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystal in an applied field of 10 Oe ($\parallel c$ axis) under the ambient pressure. Both the magnetic susceptibility measured in zero-field-cooled (ZFC) and field-cooled (FC) modes are shown. (b), (c) Enlarged views of the resistive transition in the temperature range 2 K to 10 K at various pressures for sample 1 and sample 2, respectively. The superconducting transitions are rather sharp at ambient and high pressures.

Figure 3

(a) Phase diagram of $T_c^{\mathrm{onset}}$ versus pressure for the two ${\mathrm{LaO}}_{0.5}{\mathrm{F}}_{0.5}{\mathrm{BiSe}}_2$ single crystals investigated here. The dark and red symbols represent the $T_c^{\mathrm{onset}}$ of sample 1 and sample 2, respectively. The filled symbols stand for the low $T_c$ phase; the open and crossed symbols stand for the high $T_c$ phase with and without zero resistance, respectively. (b) Resistivity at 8 K in the normal state at various pressures for sample 1 (filled squares) and sample 2 (filled circles).

Figure 4

(a) Temperature dependence of resistivity for sample 1 under a pressure of 2.04 GPa at various magnetic fields. The inset shows the enlarged view of a weak resistivity-upturning feature in the normal state under high magnetic fields. (b) Upper critical field determined by $T_c^{\mathrm{onset}}$ and 90% normal state resistivity ${\rho }_n$.