Aim: Understanding how the band structure in NiO evolves and how it is experimentelly determined.

Manuscript to be discussed: Photoemission study on NiO: Sawatzky1994 - see also Ref. 2 of this paper to understand the model!

Modern numerical approaches to describe the data: Tjeng 2017

Overall question: Why is NiO not metallic? What is the difference between Ni and NiO? (you may remind yourself to the case of Ni PES spectroscopy here)

Guiding questions:

• How does the XPS and the BIS experiments work? Sketch the basic idea.
• Which information is in general gained by each of the two experiments?
• Explain Fig. 1 of Sawatzky's paper.
• Can you explain the bar diagram at the top of Fig.2? 
• Which are the main ingredients of the Cluster approach?
• For which aspects of Mott-Hubbard physics the Cluster approach is good and which are not covered well?
• What is meant when Swatzky speaks about a "d¹⁰L-peak"?
• What is meant on page 3  left column line 7 by "Ni 3d emission"?
• What do the authors means by "exciton" and "excitonic nature" of excitations?
• Why it is claimed that NiO is rather a charge-transfer than a Mott-insulator? How large is the correlation gap (and what may it be)?

Helpful textbooks:

• D.I. Khomskii: Basic Aspects of the Quantum Theory of Solids: Order and Elementary Excitations: Chapter 12!
• D.I. Khomskii: Transition metal compounds (online via heidi): Basics on charge-transfer vs. Mott-Hubbard insulators in chapter 5.
• P. Fazekas: Lecture notes on electron correlation and magnetism.

A few slides here.