Specific heat of CeRhIn5: Pressure-driven evolution of the ground state from antiferromagnetism to superconductivity

Abstract

Resistivity measurements on CeRhIn5 have suggested an unusual “first-order-like” transition from antiferromagnetism to superconductivity at a critical pressure Pc, ∼15 kbar: At pressures below Pc the magnetic ordering temperature is approximately independent of pressure. At Pc antiferromagnetism disappears abruptly and is replaced by superconductivity, with a critical temperature that is also approximately independent of pressure. Here we report measurements of the low-temperature specific heat of CeRhIn5 at pressures to 21 kbar, and for 21 kbar in magnetic fields to 70 kOe. They confirm, by measurement of a bulk thermodynamic property, the unusual relation between magnetism and superconductivity, and permit an estimate of the discontinuity in entropy at Pc. They also give insight into the natures of the antiferromagnetic and superconducting states and their changes with pressure: With increasing pressure the zero-field specific-heat anomaly changes from one typical of antiferromagnetic ordering at ambient pressure to one more characteristic of the formation of a Kondo singlet ground state at 21 kbar. The change in general shape of the anomaly is gradual, but at Pc, where the data suggest a weak thermodynamic first-order transition, there is a discontinuous change from an antiferromagnetic ground state to a superconducting ground state. Below Pc the quasiparticle density of states increases and the spin-wave stiffness decreases with increasing pressure. At Pc the low-energy magnetic excitations disappear and are replaced by excitations that are characteristic of superconductivity with line nodes in the energy gap. The quasiparticle density of states is continuous at Pc, but decreases with increasing pressure, to zero at 21 kbar. These features suggest the possibility of superconductivity with d-wave pairing and, at intermediate pressures, “extended gaplessness.” At 21 kbar except for the line nodes, the Fermi surface is fully gapped. The specific-heat data also show the existence of a second-order transition in the 11–12-kbar region, where features in the magnetic susceptibility and resistivity have been observed.

Source-reported materials — not catalogue approval

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