AQP Seminar: On the origin of the electric field effect in superconducting nanoconstrictions

Aalto Quantum Physics -seminaari (Micronova). Puhuja: Prof. Yuri Pashkin (Department of Physics, Lancaster University)

Several recent publications in highly ranked journals report experiments on superconducting nanoconstrictions, in which the supercurrent was controlled by the electric field produced by a nearby gate. Besides observing the effect in superconducting structures made of Al, Ti and V, the effect was also observed in proximised Cu nanostructures. The authors make a strong claim that the electric field effect does exist in superconductors. Interestingly, it was not predicted by any of the theories developed so far and has never been confirmed by other experimental groups.

We have performed an experiment proving that the observed effect can be attributed to Joule heating caused by the Fowler-Nordheim emission at intense electric fields. We used a superconducting quarter-wave coplanar resonator connected to the ground plane through a nanoconstriction. Besides observing the clear effect of the gate voltage on the resonance frequency, which could in principle be interpreted as the field effect, we also see a drastic increase of the leakage current with the gate voltage. We explain our observations by the local heating of the superconductor that leads to the reduction of the critical current and hence to the increase of the inductance of the constriction. As a results, the resonance frequency decreases. This interpretation is further confirmed by the noise measurements.