Holographic description of finite-size effects in strongly coupled superconductors

Abstract

Despite its fundamental and practical interest, the understanding of mesoscopic effects in strongly coupled superconductors is still limited. Here we address this problem by studying holographic superconductivity in a disk and a strip of typical size $\ensuremath{\ell}$. For $\ensuremath{\ell}<{\ensuremath{\ell}}_{c}$, where ${\ensuremath{\ell}}_{c}$ depends on the chemical potential and temperature, we have found that the order parameter vanishes. The superconductor-metal transition at $\ensuremath{\ell}={\ensuremath{\ell}}_{c}$ is controlled by mean-field critical exponents, which suggests that quantum and thermal fluctuations induced by finite-size effects are suppressed in holographic superconductors. Intriguingly, the effective interactions that bind the order parameter increase as $\ensuremath{\ell}$ decreases. Most of these results are consistent with experimental observations in Pb nanograins at low temperature and are qualitatively different from the ones expected in a weakly coupled superconductor.