Two Superconducting Phases in CeRh1 xIrxIn5

As a conventional superconductor is cooled below Tc, a finite energy gap in the electronic density-of-states N(EF) opens over the entire Fermi surface. This gap to quasiparticle excitations produces an exponential temperature dependence of physical properties that depend on N(EF), eg. specific heat, thermal conductivity, and spin-lattice relaxation rate. In contrast, power-law dependences of these properties are found in superconducting heavy-fermion systems 1 as well as in cuprates, 2 ruthenates 3 and low-dimensional organics. 4 The existence of these power laws can be understood if the superconducting energy gap, instead of being everywhere finite, is zero on parts of the Fermi surface so that the excitation spectrum starts from zero energy. These qualitative departures from conventional behavior suggest that Cooper pairing may be mediated by excitations other than phonons. In each class of materials mentioned