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-T_c materials^5,6: 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/2_e. 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 I_cR_n values may point to a small tunnelling probability between the two superconductors, and hence to unconventional pairing in UPd₂Al₃