PHASE-TRANSITIONS IN INDIVIDUAL SUBMICROMETER SUPERCONDUCTORS

The properties of a superconductor are expected to change radically wh en its size becomes comparable to that of the Cooper pairs, the quasip articles responsible for superconductivity. The effect of such confine ment is well understood for the case of the suppression of superconduc tivity by magnetic fields (which gives rise to so-called Little-Parks oscillations of the phase boundary)(1-4). But little is known about wh at happens in small superconductors in the zero-resistance state, whic h cannot be probed by resistance measurements. Here we apply a new tec hnique of ballistic Hall magnetometry(5) to study the magnetization of individual superconducting discs of diameters down to 100 nm. The sup erconducting state of these discs is found to be qualitatively differe nt from both macroscopic and microscopic(6) superconductors, with nume rous phase transitions whose character changes rapidly with size and t emperature. This exotic behaviour is due to size quantization of the C ooper-pair motion and resulting transitions between discrete states of the superconducting Bose condensate in a magnetic field.