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

Superconducting properties under high magnetic fields for the BiS2-based layered superconductor LaO0.5F0.5BiS2 (polycrystalline sample prepared using the high-pressure synthesis method) are investigated. The anisotropy of the upper critical field is discussed by analyzing the temperature dependence of the temperature derivative of resistivity. The temperature dependence of the upper critical field shows an anomalous behavior with a characteristic magnetic field of 8 T. We reveal that the anomalous behavior is caused by the existence of three kinds of upper critical field μ0HC2max, μ0HC2mid, and μ0HC2min in LaO0.5F0.5BiS2. The lowest upper critical field μ0HC2min is regarded as μ0Hc2, where the superconducting states of the grains with an orientation of H||c are suppressed. The μ0HC2max and μ0HC2mid are regarded as μ0Hc2, where the superconducting states of the grains with an orientation of H||ab are suppressed. The difference between μ0HC2max and μ0HC2mid could be explained by the anisotropy of superconducting states within the Bi-S plane. The estimated anisotropy parameter of the upper critical fields is about 7.4 for the high-pressure-synthesized LaO0.5F0.5BiS2 polycrystalline sample. Since this value is clearly lower than the anisotropy parameter of over 30 observed in previously reported LaO0.5F0.5BiS2 single crystals grown under ambient pressure, we conclude that a larger anisotropy of superconductivity is not essential for the enhancement of Tc and lower anisotropy might be important for a higher Tc in LaO1-xFxBiS2.