Temperature-Dependent Anisotropy and Two-Band Superconductivity Revealed by Lower Critical Field in Organic Superconductor κ-(BEDT-TTF)₂CuN(CN)₂Br

Resistivity and magnetization have been measured at different temperatures and magnetic fields in organic superconductors κ-(BEDT-TTF)₂CuN(CN)₂Br. 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 Γ is strongly dependent on temperature. It is realized that Γ is quite large (above 20) near T_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_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_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.