Gaplessness and properties of layered superconductors: Application to high-Tccuprates

The presence of different low-dimensional structural units along with a short coherence length leads to the formation of a two-gap spectrum, that is to a double-peak structure in the superconducting density of states (e.g., Y-Ba-Cu-O). The presence of magnetic impurities leads to a gapless state. As opposed to the usual one-gap isotropic model, the region of gaplessness is extended, so that one can observe the effect of gaplessness without a noticeable shift in ${\mathit{T}}_{\mathit{c}}$. Strong anisotropy of the pairing interaction within the planes of a layered superconductor also leads to a similar extension of the region of gapless behavior in the presence of magnetic impurities. This gaplessness has a great impact on the spectroscopy of the cuprates, particularly for nonstoichiometric compounds. Experimental data are discussed in detail.