Superconductivity is an example of a macroscopic quantum state: a state in which macroscopic properties including electrical and thermal conductivity are governed by quantum mechanics rather than classical mechanics. Below a temperature, called the critical temperature, superconductors undergo a phase transition where resistance disappears and the electrons in the superconducting state are locked together with the same phase. However, very close to the critical temperature the phase coherence of the superconductor is stressed and begins to fluctuate. This paper studies superconducting phase coherence in micron-scale aluminum rings. We do this by applying a magnetic field to the rings which induces a current. We then use our magnetic sensor to measure the moment of the current response. We find that rings with specific physical parameters lose their phase coherence even at temperatures where they remain superconducting. We developed a mathematical model which predicts this effect and checked it with a scanning magnetic flux sensor called a SQUID.

The figure shows a sketch of the SQUID measurement device which we used to measure the magnetic response of superconducting rings.

Julie A. Bert, Nicholas C. Koshnick, Hendrik Bluhm and Kathryn A. Moler, Physical Review B 84 134523 (2011). Full Text