Giant superconducting fluctuation and anomalous semiconducting normal state in NdO _1−x F _x Bi _1−y S ₂ single crystals

Abstract

The BiS2-based superconductors were discovered recently. The\nsuperconductivity has been proved by many other groups. Since the previous\nexperiments were all done on polycrystalline samples, therefore there remains a\nconcern whether the superconductivity is really derived from the materials\nintrinsically or from some secondary phases. Experiments on single crystals are\nhighly desired. In this paper, we report the successful growth of the\nNdO1-xFxBi1-yS2 single crystals. Resistive and magnetic measurements reveal\nthat the bulk superconducting transition occurs at about 5 K, while an\nunexpected giant superconducting fluctuation appears at temperatures as high as\n2-4 kBTC. Analysis based on the anisotropic Ginzbaug-Landau theory gives an\nanisotropy of 30-45. Two gap features with magnitudes of about 3.5+-0.3 meV and\n7.5+-1 meV were observed by scanning tunneling spectroscopy. The smaller gap is\nassociated with the bulk superconducting transition at about 5 K yielding a\nhuge ratio 2Delta_s/kBTc =16.8, the larger gap remains up to about 26 K. The\nnormal state recovered by applying a high magnetic field shows an anomalous\nsemiconducting behavior. All these suggest that the superconductivity in this\nnewly discovered superconductor cannot be formatted into the BCS theory.\n