Giant superconducting fluctuation and anomalous semiconducting normal state in NdO_1-xFxBi_1-yS2 single crystals

We report the successful growth of the NdO1?xFxBi1?yS2 single crystals and prove the intrinsic superconductivity. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at T_c = 4.83 K. Measurements of excess conductivity and the in-plane angle-dependent resistance reveal a giant superconducting fluctuation far above T _c(extending to 2T_c4T_c). This is supported by the Nernst and the scanning tunneling measurements. Analysis based on the anisotropic Ginzburg-Landau theory gives a very large anisotropy γ = √m_c/m_a/b ≈ 30-50. Two gap features with magnitudes of about 3.5 ± 0.3meV and 7.5 ± 1meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition yielding a huge ratio 2Δ₁ ²/k_BT_c= 16.8, and the gapped feature remains up to 2030K. Another fascinating phenomenon is that the normal state recovered by applying a high magnetic field along the c-axis shows an anomalous semiconducting behavior. All these suggest that the superconductivity in this newly discovered superconductor may have an exotic reason which is beyond the BCS picture.