Unexpected Giant Superconducting Fluctuation and Anomalous Semiconducting Normal State in NdO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$Bi$_{\mathrm{1-y}}$S$_{2}$ Single Crystals

Experiments on single crystals of BiS$_{\mathrm{2}}$-based superconductors are highly desired. We report the successful growth of the NdO$_{\mathrm{1-x}}$F$_{\mathrm{x}}$Bi$_{\mathrm{1-y}}$S$_{\mathrm{2}}$ single crystals. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at Tc $=$ 4.83 K, while an unexpected giant superconducting fluctuation appears at temperatures as high as 2-4 kBTC. Analysis based on the anisotropic Ginzbaug-Landau theory gives an anisotropy $\gamma = \sqrt {m_{c} /m_{ab} } \approx 30\sim 45$. Two gap features with magnitudes of about 3.5 $\pm$ 0.3meV and 7.5 $\pm$ 1 meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition yielding a huge ratio $2\Delta_{s}^{1} /k_{B} T_{c} =$16.8, the larger gap remains up to about 26 K. The normal state recovered by applying a high magnetic field shows an anomalous semiconducting behavior. All these suggest that the superconductivity in this newly discovered superconductor cannot be formatted into the BCS picture.