Persistent currents in superconducting nanorings

Superconductor-insulator-superconductor-insulator-superconductor (SIS'IS) tunnel nanostructures with the central electrode S' in a shape of a loop have been fabricated. It has been shown that at low temperatures at a fixed voltage bias V the tunnel current oscillates in perpendicular magnetic field B with period ΔΦ few times exceeding the superconducting flux quantum. The normalized magnitude of oscillations reaches its maximum close to the quadruple superconducting energy gap 4Δ. At higher and lower biases the oscillations are effectively damped. Low energy ion sputtering has been used to progressively reduce the cross-section of the line forming the loop-shaped 'island'. It has been found that at very narrow linewidths the magnitude of oscillations decreases, and their shape starts to deviate from the saw-tooth behavior typical for wider samples. The effect is associated with manifestation of quantum phase slips which remove the energy level crossing at the degeneracy points