Photoemission studies of high critical transition temperature superconductors: Doping dependences, Fermi surfaces, superconducting gaps, and metal-superconductor interfaces

Angle-resolved photoemission spectroscopy is one of the most powerful probes of the electronic structure of a solid since it gives direct information about the energy and momentum-dependent occupied density-of-states. We have applied angle-resolved photoemission with very high energy resolution ({dollar}sim{dollar}35 meV) to the study of the recently discovered high-temperature superconductors. This high energy resolution enables studies of the very low energy excitations (such as the superconducting energy gap) which contain the most important and interesting physics of the system.; In our studies we have concentrated primarily on single crystalline samples of {dollar}rm Bisb2Srsb2CaCusb2Osb{lcub}8+delta{rcub}{dollar} (with different values of {dollar}delta{dollar}), because these appear to give the highest quality cleaved surfaces of any of the high-temperature superconductors. We have studied the nature of the doping behavior, normal state, superconducting state, and the metal-superconductor interface. Among the key observations that will be discussed in this thesis are: (1) Clear evidence for a chemical potential shift as a function of doping exists for metallically doped samples of Bi2212. (2) A clear Fermi surface due to states of primarily Cu-O dp{dollar}sigma{dollar} character exists for Bi2212. The density of states at the Fermi level N(E{dollar}sb{lcub}rm F{rcub}{dollar}) is anisotropic in k-space, with a very high N(E{dollar}sb{lcub}rm F{rcub}{dollar}) near the M point of the Brillouin zone. (3) As the sample goes superconducting, not only is there spectral weight transfer from the gap region to the pile-up as in the conventional BCS theory of superconductivity, but near the M point there is also some spectral-weight transfer from higher binding-energies in the form of a dip in the spectral lineshape. (4) The superconducting gap has an unusually large k-space anisotropy. The k-space dependence of this anisotropy appears consistent with d-wave pairing theories of superconductivity. (5) If a proximity-effect induced gap exists in gold overlayers on top of high T{dollar}sb{lcub}rm c{rcub}{dollar} substrates, then the gap is small ({dollar}<{dollar}5 meV).