Temperature-Dependent Anisotropy and Two-Band Superconductivity Revealed by Lower Critical Field in Organic Superconductor $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br
Huijing Mu1, Jin Si1, Qingui Yang2, Ying Xiang1, Haipeng Yang2*, and Hai-Hu Wen1*
1National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China 2College of Materials Science and Engineering, Shenzhen Key Laboratory of Polymer Science and Technology, Shenzhen University, Shenzhen 518060, China
Abstract：Resistivity and magnetization have been measured at different temperatures and magnetic fields in organic superconductors $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br. The lower critical field and upper critical field are determined, which allow to depict a complete phase diagram. Through the comparison between the upper critical fields with magnetic field perpendicular and parallel to the conducting $ac$-planes, and the scaling of the in-plane resistivity with field along different directions, we find that the anisotropy ${\varGamma}$ is strongly dependent on temperature. It is realized that ${\varGamma}$ is quite large (above 20) near $T_{\rm c}$, which satisfies the 2D model, but approaches a small value in the low-temperature region. The 2D-Tinkham model can also be used to fit the data at high temperatures. This is explained as a crossover from the orbital depairing mechanism in high-temperature and low-field region to the paramagnetic depairing mechanism in the high-field and low-temperature region. The temperature dependence of lower critical field, $H_{\rm c1} (T)$, shows a concave shape in wide temperature region. It is found that neither a single d-wave nor a single s-wave gap can fit the $H_{\rm c1} (T)$, however a two-gap model containing an s-wave and a d-wave can fit the data rather well, suggesting two-band superconductivity and an unconventional pairing mechanism in this organic superconductor.
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2023, Vol. 40 
