Anisotropic upper critical field of BiS2-based superconductor LaO0.5F0.5BiS2

Superconducting properties under high magnetic fields for the novel BiS2-based layered superconductor LaO0.5F0.5BiS2 (polycrystalline sample prepared using the high-pressure synthesis method) have been investigated. The anisotropy of the upper critical field was discussed by analyzing the temperature dependence of the temperature derivative of resistivity. The temperature dependence of upper critical field showed an anomalous behavior with a characteristic magnetic field of 8 T. We revealed that the anomalous behavior was caused by the existence of three kinds of upper critical field, HC2max, HC2mid and HC2min in LaO0.5F0.5BiS2. The lowest upper critical field HC2min is regarded as the Hc2 where the superconducting states of the grains with an orientation of H // c are suppressed. The HC2max and HC2mid are regarded as the Hc2 where the superconducting states of the grains with an orientation of H // ab are suppressed. The difference between the HC2max and HC2mid could be explained by the anisotropy of superconducting states within the Bi-S plane. The estimated anisotropy parameter of the upper critical fields was about 7.4 for the high-pressure-synthesized LaO0.5F0.5BiS2 polycrystalline sample. Since the value was clearly lower than the anisotropy parameter of over 30 observed in the previous report on the LaO0.5F0.5BiS2 single crystals grown under ambient pressure, we concluded that a larger anisotropy of superconductivity was not essential for the enhancement of Tc, and lower anisotropy might be important for a higher Tc in LaO1-xFxBiS2.