Crossover from impurity-controlled to granular superconductivity in (TMTSF)₂ClO₄

Using a proper cooling procedure, a controllable amount of nonmagnetic structural disorder can be introduced at low temperature in (TMTSF)₂ClO₄. Here we performed simultaneous measurements of transport and magnetic properties of (TMTSF)₂ClO₄ in its normal and superconducting states, while finely covering three orders of magnitude of the cooling rate around the anion ordering temperature. Our result reveals, with increasing density of disorder, the existence of a crossover between homogeneous defect-controlled d-wave superconductivity and granular superconductivity. At slow cooling rates, with small amount of disorder, the evolution of superconducting properties is well described with the Abrikosov-Gorkov theory, providing further confirmation of non-s-wave pairing in this compound. In contrast, at fast cooling rates, zero resistance and diamagnetic shielding are achieved through a randomly distributed network of superconducting puddles embedded in a normal conducting background and interconnected by proximity effect coupling. The temperature dependence of the ac complex susceptibility reveals features typical for a network of granular superconductors. This makes (TMTSF)₂ClO₄ a model system for granular superconductivity where the grain size and their concentration are tunable within the same sample.