Observation of the Josephson effect in the heavy-fermion superconductor CeIrIn₅ above T_c

The Josephson effect for a heavy-fermion superconductor CeIrIn₅, which has two characteristic temperatures T₀ and T_c, has been investigated for CeIrIn₅-Cu-Nb junctions, where T₀ and T_c are the temperature of the transition to the zero-resistivity state and the bulk, thermodynamic transition temperature, respectively. For all the junctions fabricated on the (110) and (001) planes of CeIrIn₅, the temperature below which the Josephson effect is observed is near T₀ = 0.8 K and much higher than T_c = 0.3 K. This result suggests that at least the surface of CeIrIn₅ is in a superconducting state below T₀, in which phase coherence between CeIrIn₅ and Nb is possible.