Magnetic field penetration depth of an organic superconductor: Evidence for anisotropic superconductivity of gapless nature

Abstract

The temperature dependence of the magnetic-field penetration depth \ensuremath{\lambda} of the organic superconductor \ensuremath{\kappa}-[bis(ethylenedithio)-tetrathiafulvalene${]}_{2}$Cu(NCS${)}_{2}$ has been determined by means of complex susceptibility measurements for single crystals. \ensuremath{\lambda} was found to follow a power law, \ensuremath{\lambda}(T)/\ensuremath{\lambda}(0)-1\ensuremath{\propto}(T/${\mathit{T}}_{\mathit{c}}$${)}^{2}$, at low temperatures instead of the exponential behavior expected from BCS theory. This result is considered as the first evidence for anisotropic superconductivity with nodes of the gap parameter on the Fermi surface in this material.