Attojoule calorimetry of mesoscopic superconducting loops

Abstract

We report the first experimental evidence of nontrivial thermal behavior of the simplest mesoscopic system - a superconducting loop. By measuring the specific heat C of an array of 450,000 noninteracting aluminum loops with very high accuracy of ~20 fJ/K, we show that the loops go through a periodic sequence of phase transitions (with period of an integer number of magnetic flux quanta) as the magnetic flux threading each loop is increased. The transitions are well described by the Ginzburg-Landau theory and are accompanied by discontinuities of C of only several thousands of Boltzmann constants k_B.