Analysis of the gap in high-temperature superconductors using photoemission spectroscopy

The discovery of the cuprate superconductors has led to intensive research activity aimed at discovering the mechanism of high temperature superconductivity. A key feature in the electronic structure of a superconductor is the energy gap in the excitation spectrum. While gaps have been detected in the cuprate superconductors, traditional techniques for measuring the gap often give confusing or inconsistent results. The high transition temperatures and correspondingly large gaps of the cuprate superconductors have allowed photoemission spectroscopy to be used for the first time as a technique for studying the superconducting gap. Advantages of photoemission include that it is a direct measure of the gap, the surface studied is by necessity a clean, well defined region with variable depth, and the gap can be measured as a function of position in k-space. The largest disadvantage of photoemission as a tool for studying the gap is that the best resolution available so far is only on the order of the gap size. This makes a determination of gap size difficult. We have used photoemission to study the behavior of the gap near the surface of Bi{dollar}sb2{dollar}Sr{dollar}sb2{dollar}CaCu{dollar}sb2{dollar}O{dollar}sb{lcub}8+delta{rcub}{dollar} and its variation as a function of k position. We have found that the gap does not diminish near the surface, an indication of the highly two dimensional nature of these materials. Our photoemission experiment has shown no evidence of a proximity effect in a metal overlayer. We have also found that the gap is anisotropic in k space within the planes themselves. In particular, by measuring how the gap varies with k, we can gain information on the symmetry of the gap function. Our data is consistent with a mixed s+id symmetry gap and possibly with a pure d wave gap. Determining the symmetry of the gap provides one of the most stringent experimental constraints on the theory of high T{dollar}sb{lcub}rm c{rcub}{dollar} superconductivity yet found. We compare our result with other experiments which measure the gap in order to attempt to provide a complete and consistent picture of what is presently known of the behavior of the superconducting gap in cuprate superconductors.