Subphases in the superconducting state of CeIrIn5 revealed by low-temperature c-axis heat transport

H. Shakeripour, M. A. Tanatar, C. Petrovic, and Louis Taillefer

Abstract

Low-temperature (down to ∼50 mK) thermal conductivity measurements with the heat flow direction along the interplane tetragonal c axis, κc, were used to study the superconducting state of heavy fermion CeIrIn5. Measurements were performed in the magnetic fields both parallel to the heat flow direction H∥c, and transverse to it H∥a. Interplane heat conductivity in H∥c configuration shows negligible initial increase with magnetic field and a rapid rise on approaching Hc2 from below, similar to the expectations for the superconducting gap without line nodes. This observation is in stark contrast to monotonic increase found in the previous in-plane heat transport measurements. In the configuration with the magnetic field breaking the tetragonal symmetry of the lattice H∥a, κc reveals nonmonotonic evolution with temperature and magnetic field suggesting subphase boundary in the superconducting state. The characteristic temperature Tkink∼ 0.07 K of the subboundary is well within the domain of bulk superconductivity Tc∼ 0.4 K and Hc2∼ 1.0 T. These results are consistent with a superconducting gap with an equatorial line node and polar point nodes, a gap symmetry of the D4h point group, for which magnetic field along the tetragonal plane breaks the degeneracy of the multicomponent order parameter and induces a phase transition with nodal topology change.