Theoretical models of tunneling in high-Tc superconductors

Many results were published on the determination of the energy gap of high temperature superconductors from tunneling experiments. For the Y-Ba-Cu-O system the situation seems to be complicated. Gap values from 12 up to 20 meV were often reported, but there are much smaller values of about 1 up to 9 meV and higher values and multiple structures from 40 to 100 meV reported, too. The results depend on the material and the way they were obtained. The determination of the energy gap out of the conductance-voltage characteristic offers some problems. Only the ratio of th conductance in the superconducting state to the normal conductance is directly connected to the density of states in the material. The obtained curves of normalized conductance can not be described by a simple BCS density of states with a singularity at the gap energy Δ. There are different models to explain the structures connected with energy gap. In this paper the authors give a short view of three of them--the multi-gap model, the Coulomb effect model and the proximity effect model