Unexpected Giant Superconducting Fluctuation and Anomalous Semiconducting Normal State in NdO1-xFxBi1-yS2 Single Crystals

Abstract

The BiS2-based superconductors were discovered recently. The superconductivity has been proved by many other groups. Since the previous experiments were all done on polycrystalline samples, therefore there remains a concern whether the superconductivity is really derived from the materials intrinsically or from some secondary phases. Experiments on single crystals are highly desired. In this paper, we report the successful growth of the NdO1-xFxBi1-yS2 single crystals. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at about 5 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 of 30-45. Two gap features with magnitudes of about 3.5+-0.3 meV and 7.5+-1 meV were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition at about 5 K yielding a huge ratio 2Delta_s/kBTc =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 theory.

21 pages, 6 figures

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• Compositional and temperature evolution of crystal structure of new thermoelectric compound LaOBiS2-xSex
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• BCS-like superconductivity in NdO${}_{1-x}$F${}_x$BiS${}_2$ ($x$ = 0.3 and 0.5) single crystals
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• Anisotropic two-gap superconductivity and the absence of a Pauli paramagnetic limit in single-crystalline LaO${}_{0.5}$F${}_{0.5}$BiS${}_2$
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• Synthesis, Crystal Structure, and Physical Properties of New Layered Oxychalcogenide La2O2Bi3AgS6
• Superconducting properties of novel BiSe${}_2$-based layered LaO${}_{1-x}$F${}_x$BiSe${}_2$ single crystals
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• Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS${}_2$-superconductors
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• Universal phonon softening in the pseudogap state of Tl${}_2$Ba${}_2$Ca${}_{n-1}$Cu${}_n$O${}_{2n+4+\delta }$
• Growth of (Cu,C)Ba${}_2$Ca${}_2$Cu${}_3$O${}_{9\pm \delta }$ thin films on flexible Hastelloy tapes
• Two-Dimensional Phase-Fluctuating Superconductivity in Bulk-Crystalline NdO${}_{0.5}$F${}_{0.5}$BiS${}_2$
• Investigation of in-plane anisotropy of c-axis magnetoresistance for BiCh2-based layered superconductor NdO0.7F0.3BiS2
• The upper critical field in the BiCh2-based superconductors CeOBiS1.7Se0.3 and PrO0.5F0.5BiS2-xSex (x = 0, 0.3)
• Superconducting properties of BiS2 based superconductor NdO1-xFxBiS2 (x=0 to 0.9)