On the superconducting gap structure of high-temperature superconductors by STM/STS

Abstract

The tunneling spectra observed by STM on the HTSC's have been converging in terms of the superconducting gap energy values: 2 Δ/ k B T c=6 to 9. There are, however, still disagreements between the details of various results. Both d- and s-wave-like features have been claimed for the superconducting gap structure. Furthermore, both flat and V-shaped structures have been observed in the background conductance curve outside the superconducting gap. The major differences among various reports seem to be attributable to the difference in the top layer and in the carrier density of the sample. It is noted that those who observe on the BiO topmost plane in a Bi2212 crystal or on the CuO-chain plane of YBCO seem to be claiming the d-wave, and those who observe on different top planes the s-wave symmetry.