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| - Giant superconducting fluctuation and anomalous semiconducting normal state in NdO 1−xF xBi 1−yS 2 single crystals
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| - We report the successful growth of the $\text{NdO}_{1-x}\text{F}_x\text{Bi}_{1-y}\text{S}_2$ single crystals and prove the intrinsic superconductivity. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at $T_c = 4.83\ \text{K}$ . Measurements of excess conductivity and the in-plane angle-dependent resistance reveal a giant superconducting fluctuation far above Tc (extending to $2T_c\text{--}4T_c$ ). This is supported by the Nernst and the scanning tunneling measurements. Analysis based on the anisotropic Ginzburg-Landau theory gives a very large anisotropy $\gamma=\sqrt{m_c/m_{ab}}\approx30\text{--}50$ . Two gap features with magnitudes of about $3.5 \pm 0.3\ \text{meV}$ and $7.5 \pm 1\ \text{meV}$ were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition yielding a huge ratio $2\Delta^{1}_s/k_BT_c=16.8$ , and the gapped feature remains up to 20−30 K. 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.
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