Non-magnetic impurity effects in charge-density-wave superconductors

Abstract

The effects of non-magnetic impurity scattering in the coexistence state of charge-density wave (CDW) and superconductivity orderings are investigated theoretically. Using a mean-field description of the coexistence state, the single-impurity problem is solved exactly by means of the scattering t-matrix. A 'particle' and a 'hole' impurity bound state are shown to form inside the energy gap of that region of the Fermi surface affected by the CDW ordering. The excitation energies of the two bound states are predicted to be strongly influenced by an external applied magnetic field. The modified electronic density of states in the presence of a single non-magnetic impurity is calculated. For finite concentration of non-magnetic impurities the bound states form impurity bands but do not lead to gapless superconductivity. The transition temperature for superconductivity in the coexistence state is shown to be raised by non-magnetic impurities and an analytic expression for this change, valid to linear order in the impurity concentration, is derived. The theory is expected to be applicable to CDW superconductors such as the layered compound 2H-NbSe2. Tunnelling experiments provide an ideal probe for testing these predictions.