Specific heat of CeRhIn5: the pressure-driven transition from antiferromagnetism to heavy-fermion superconductivity

CeRhIn₅ has an unusual transition at a critical pressure of ~15 kbar. Specific-heat data show a gradual change in the zero-field 'magnetic' anomaly from one typical of antiferromagnetic (AF) ordering at ambient pressure to one that is more characteristic of a Kondo-singlet ground state at 21 kbar. However, at 15 kbar there is a discontinuous change from an AF ground state to a superconducting ground state, with evidence of a weak thermodynamic first-order transition. For pressures near 12 kbar, there is a pressure-dependent, second-order transition. The low-energy excitations above 15 kbar are characteristic of superconductivity with line nodes in the energy gap, and with extended gapless behaviour at intermediate pressures less than 21 kbar.