Excitation Spectrum and Effects of Normal Impurities on Heavy Electron Superconductors

Excited states in superconductors formed at normal impurities are investigated in the circumstance that the effective fermi energy is at most an order or magnitude larger than the superconducting gap and consequently the coherence length is roughly an order of magnitude larger than the lattice constant. It is discussed that the substantial spatial variation of the order parameter amplitude around an impurity easily occurs. The excitation energy of the bound state around the impurity extends down to a tenth of the gap in typical examples, which results in a new type of gapless-like behavior except at extremely low temperatures. The specific heat shows qualitatively different behavior from the BCS result and does not follow simple temperature dependence.