Subphases in the superconducting state of CeIrIn_{5} revealed by low-temperature c-axis heat transport

Abstract

Low-temperature (down to ∼50 mK) thermal conductivity measurements with the heat flow direction along the interplane tetragonal c axis, κ_{c}, were used to study the superconducting state of heavy fermion CeIrIn_{5}. 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 H_{c2} 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, κ_{c} reveals nonmonotonic evolution with temperature and magnetic field suggesting subphase boundary in the superconducting state. The characteristic temperature T_{kink}∼ 0.07 K of the subboundary is well within the domain of bulk superconductivity T_{c}∼ 0.4 K and H_{c2}∼ 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 D_{4h} 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.