Angle-Resolved Photoemission Study on High Temperature Superconductors

Angle-resolved photoemission spectroscopy (ARPES) is used to study the nature of normal and superconducting states in rm Bi_2Sr_2CaCu_2O _8 (Bi2212). Measurements of the momentum dependence of the superconducting gap in Bi2212 shows that the gap, being highly anisotropic, has a non-trivial momentum dependence: it is large in the vicinity of the | M point where the extended saddle points are located, becoming smaller towards Gamma -X(Y), or (pi, pi) directions. It was also found that the gap has an extended region of nodes, being almost zero over a range of approximately +/-10^circ about the (pi, pi) directions. From the normal-state measurements of this material, we found only one CuO_2 band related feature. All other spectral features can be ascribed either to umklapps from the superlattice or to "shadow bands". No evidence is found for bilayer splitting. We show some evidence that the "dip feature" in the spectrum below T _{rm c} arises not from bilayer splitting, but rather from many body effects. In order to further understand angle-resolved photoemission, we found that within a spectral function A({ bf k}, omega) interpretation of angle-resolved photoemission, the intensity obeys the sum rule int domega f(omega)A({ bf k},omega) = n({bf k}). Consequently the integrated intensity of a spectral feature in ARPES at {bf k}_ {rm F} is independent of temperature. This has been proven from our ARPES data in Bi2212. We also demonstrate the possibility of measuring n(k) using ARPES. More implications of the experimental results will be discussed.