Thermally induced gaplessness and Fermi arcs in a "s-wave" magnetic superconductor

An electron system with pre-existing local moments and an effective electron-electron attraction can exhibit simultaneous magnetic and superconducting order. Increasing the magnetic coupling weakens pairing and the ground state loses superconductivity at a critical coupling. In the vicinity of the critical coupling magnetic order dramatically modifies the quasiparticle dispersion in the superconductor, creating low energy spectral weight and significant gap anisotropy in the notional 's-wave' state. Using a Monte Carlo approach to the Hubbard-Kondo lattice problem we establish a thermal phase diagram, for varying magnetic coupling, that corresponds qualitatively to the borocarbide superconductors. In addition to the superconducting and magnetic transition temperatures, we identify two new thermal scales in this nominal s-wave system. These are associated, respectively, with crossover from gapped to gapless superconductivity, and from an anisotropic (nodal) 'Fermi surface' at low temperature, through a Fermi arc regime, to an isotropic Fermi surface at high temperature. Some of the spectral effects are already visible in the Ho and Er based borocarbides, others can be readily tested.