SLAC-Pub-6407 August 1994 (SSRL-m) Electronic Structure and Photoemission Studies of Late Transition-Metal Oxides -- Mott Insulators and High-Temperature Superconductors Z.-X. Shen and D.S. Dessau Department of Applied Physics Stanford University Stanford, CA 94305-4055 and Stanford Linear Accelerator Center Stanford Synchrotron Radiation Laboratory Stanford, CA 94309 Submitted to Physics Reports * Work supported in part by the Department of Energy Contract DE-AC03-76SF00515 (SLAC) 0. Abstract......................................................................................................................1 I. Introduction ...............................................................................................................3 II. Brief Review of Experimental Techniques ............................................................8 A. Photoemission ................................................................................................8 1. General description ..............................................................................8 2. Angle-resolved photoemission.............................................................9 3. Resonance photoemission ....................................................................11 4. Some photoemission terminologies: EDC, CIS and CFS ....................12 5. Related Experimental techniques.........................................................13 6. Theoretical description of the photoemission process .........................14 B. Sample surface preparation ............................................................................15 III. Photoemission from Late Transition-Metal Oxides ............................................18 A. Pedagogical description of the Mott insulators..............................................18 B. Angle-integrated photoemission of CuO and Cu2O.......................................21 1. Experimental results and the first order explanation ...........................21 2. More Subtle Issues ...............................................................................23 3. Results of cluster and impurity model calculations .............................25 C. Angle-integrated photoemission from other Mott insulators .........................26 1. Experimental data of NiO and their interpretation...............................26 2. Results from CoO, FeO and MnO .......................................................30 3. Role of the magnetic interactions.........................................................31 D. Angle-resolved photoemission from Mott insulators.....................................33 1. Results from NiO .................................................................................33 2. Results from CoO and MnO ................................................................41 E Closing remarks...............................................................................................42 IV. Photoemission Studies of the Normal State of Cuprate Superconductors ........44 A. Ground state properties and issues to be addressed .......................................44 ii B. Samples and Sample quality issues ................................................................49 C. Basic framework of the electronic structure..................................................49 D. Angle-integrated photoemission data from the whole valence band. ............50 1. Evidence for strong correlation effects ................................................50 2. Low energy excitation state .................................................................52 3. Important experimental issues regarding the earlier data ....................53 E. Angle-resolved photoemission data from near the Fermi level......................56 1. Overview.............................................................................................56 a. Notation of the high symmetry points of the Brillouin zones of the cuprate supercondutors ............................................57 b. Results of model calculations of the electronic structure of the cuprates ..............................................................................58 c. Two-dimensionality ................................................................62 2. Rules and methods to determine the Fermi surface from photoemission ..........................................................................................63 3. Results from Bi2Sr 2CaCu2O8+ δ..........................................................64 4. Results from Bi2Sr 1.9Pr .1 CuO 6+ δ........................................................74 5. Results from YBa2Cu3O7-δ and YBa2Cu4O8 .....................................75 6. Results from n-type superconductors..................................................85 7. Comparison of band and Fermi surface mappings from the different families of the cuprate superconductors....................................88 8. Doping behavior of the cuprates as studied by angle-resolved photoemission ..........................................................................................93 9. Symmetry information obtained from the polarization of the incident photon beam. ..............................................................................97 10. Analysis of the quasi-particle line shape...........................................100 11. Information of Fermi surface from other techniques ........................108 iii 12. State of affairs and implications........................................................109 V. Photoemission Studies of the Superconducting State ...........................................111 A. Background: conventional theory and the superconducting gap of conventional superconductors..............................................................................112 B. Angle-integrated photoemission from Bi2Sr 2CaCu2O8+ δ..............................115 C. Angle-resolved photoemission from Bi2Sr 2CaCu2O8+ δ................................117 1. Anomalous spectral weight transfer...................................................119 2. k-dependence and symmetry of the superconducting gap ..................130 D. Results from YBa2Cu3O7-δ............................................................................137 E. Comparison to superconducting state measurements by other techniques.............................................................................................................139 VI. Summary and Concluding Remarks.....................................................................142 VII Addendum: .............................................................................................................151 VIII Acknowledgements:..............................................................................................153 IX. References..............................................................................................................154 iv Abstract: Stimulated by the discovery of high-temperature superconductivity, the electronic structure of late 3d transition-metal oxides is presently one of the most extensively studied subject in condensed matter physics. In this review, we hope to summarize the progress we have made and the problems we are facing. The emphasis of the review is on the latest angle-resolved photoemission studies that have provided much insight towards the understanding of these materials. This includes the recent experiments from transition-metal mono-oxides, normal state electronic structure and Fermi surface mapping of Bi2Sr2CaCu2O8+ δ, Bi2Sr 2CuO6+ δ, YBa2Cu3O7-x, YBa2Cu4O8, Nd2- xCexCuO4, and the superconducting gap of Bi2Sr 2CaCu2O8+ δ. For the transition-metal mono-oxide, we discuss the experimental manifestation of the four aspects of the electronic structure that made these Mott-Hubbard insulators so interesting. This include the large Coulomb interaction U (on the cation sites), the charge transfer as a result of strong hybridization, the energy dispersion in the crystal lattice, and the multiplet and magnetic splittings. For the high-temperature superconductors, we concentrate on the low energy excitations, the topology of the Fermi surface in the normal state, and the superconducting gap. Angle-resolved photoemission data show that the oxide superconductors have well defined Fermi surfaces. The volume of the Fermi surface at high doping regime appear to be consistent with the results of band calculations. A striking feature of the low energy excitations is the presence of some very flat bands (due to a saddle point sigularity in the band structure) which lie near the Fermi energy in p- type compounds near their optimal doping levels for superconductivity. The corresponding flat bands are well below the Fermi energy in n-type cuprates. The energy position of these flat bands is expected to have wide ranging effects on the physical 1 properties of these materials, including the temperature dependence of the resistivity and the superconducting transition temperature. High-resolution photoemission has also been successfully applied to the study of the superconducting gap in Bi2Sr 2CaCu2O8+ δ. While the presently attainable energy resolution is poorer than that of many other spectroscopies, photoemission has the advantage that it is k-resolved as well as being very direct. This unique capability has enabled recent photoemission experiments to reveal the highly anisotropic nature of the superconducting gap in the a-b plane. This suggests the possibility of a detailed experimental determination