Giant superconducting fluctuation and anomalous semiconducting normal state in NdO_1−xF_xBi_1−yS₂ single crystals

¹ Center for Superconducting Physics and Materials, National Laboratory of Solid State Microstructures and Department of Physics, National Center of Microstructures and Quantum Manipulation, Nanjing University Nanjing 210093, China
² National Laboratory for Superconductivity, Institute of Physics and National Laboratory for Condensed Matter Physics, Chinese Academy of Sciences - Beijing 100190, China

^(a) zhuxiyu@nju.edu.cn (corresponding author)
^(b) hhwen@nju.edu.cn (corresponding author)

Received: 3 April 2014
Accepted: 25 May 2014

Abstract

We report the successful growth of the single crystals and prove the intrinsic superconductivity. Resistive and magnetic measurements reveal that the bulk superconducting transition occurs at . Measurements of excess conductivity and the in-plane angle-dependent resistance reveal a giant superconducting fluctuation far above T_c (extending to ). This is supported by the Nernst and the scanning tunneling measurements. Analysis based on the anisotropic Ginzburg-Landau theory gives a very large anisotropy . Two gap features with magnitudes of about and were observed by scanning tunneling spectroscopy. The smaller gap is associated with the bulk superconducting transition yielding a huge ratio , 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.