Anomalous Little-Parks oscillations and field enhanced superconductivity in mesoscopic loops

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Abstract
Quantum interference phenomena have been studied in superconducting Al loops with dimensions smaller than the temperature dependent coherence length ξ(T). Besides the classical Little-Parks (LP) effect, anomalous resistance oscillations have been observed whose amplitude shows an unusual magnetic field and current dependence at low flux quanta. The magnetic field (H) - critical temperature (Tc) phase boundary has been measured for different transport currents jt. A clear field enhancement of Tc is observed at low current densities. The amplitude of the field enhancement of Tc is strongly influenced by the transport current through the loop. Higher jt values induce a transition from the Tc(H) boundary with LP oscillations to a new regime without oscillations. This transition is governed by the interplay between the characteristic superconducting length scale ξ(T), the loop circumference 2πr and the de Broglie wavelength λD ∝ (1/jt), tuned by the transport current.