Coherence of the superconducting wavefunction between the heavy-fermion superconductor UPd2AI3 and niobium

Abstract

HEAVY-fermion superconductors (in which the charge carriers have an effective mass ∼100 times the free electron mass) have been the subject of intense study during the past decade1, in part because of the suggestion that some of these materials may exhibit non-conventional pairing mechanisms2. Here we report a demonstration of quantum coherence of the superconducting wavefunction between the conventional superconductor niobium and the recently discovered3 heavy-fermion superconductor UPd2Al3, which is a potential candidate for a non-conventional pairing mechanism4. The experimental method is similar to our earlier work on high-Tc materials5,6: we use a small pointed rod of UPd2Al3 to bridge the gap in an almost closed niobium ring. We observe persistent currents in the composite ring, and trapped flux, which is in discrete quantum states separated by the flux quantum h/2e. Although the observation of phase coherence between UPd2Al3 and niobium may not constrain the nature of the pairing in UPd2Al3, we also observe Josephson-like current-voltage characteristics at the junction, but with very small products of critical current and normal-state resistance. If other possible causes can be eliminated, these small IcRn values may point to a small tunnelling probability between the two superconductors, and hence to unconventional pairing in UPd2Al3