Genuine superconducting ground state in κ-(BEDT-TTF)2Cu[N(CN)2]Br: an understanding after decade of controversy

We overview the state of knowledge regarding the symmetry of the superconducting order parameter in layered organic superconductors based on κ-(BEDT-TTF). A remnant intrinsic disorder at low temperatures in nominally pure single crystals emerges as one of decisive key ingredients that influences the value and the temperature dependence of the penetration depth, while leaving the transition temperature unchanged. However, the microscopic origin of this influence remains still unclear.