High-Energy Anomaly in the Band Dispersion of the Ruthenate Superconductor

H. Iwasawa, Y. Yoshida, I. Hase, K. Shimada, H. Namatame, M. Taniguchi, and Y. Aiura

They perform ARPES [Angle Resolved Photoemission Spectroscopy] on strontium ruthenate [Sr2RuO4]. Some of the main results are shown below [the vertical scale is energy].

The key issue is understanding how the measured quasi-particle dispersion (left panel) differs from the band structure calculated from LDA [Local Density Approximation of Density Functional Theory (DFT)].

Where the two curves cross is the "high energy anomaly". This is very much related to "kinks" and "waterfalls" in the cuprates, as I discussed in an earlier post.

The spectrum is compared to a very simple model self energy (right panel) that is consistent with Fermi liquid theory and includes a "cut off" energy scale associated with the underlying interactions [bosons?, magnons?, electron-electron?] that are the origin of the self energy.

The solid black curve in the right panel above is from a theoretical calculation [self-consistent perturbation theory and DMFT on the relevant multi-band Hubbard model with Hunds rule coupling]. [A 2000 PRL by Liebsch and Lichtenstein].

It is very impressive that this agrees with the experiment.

**This shows how good both ARPES and Dynamical Mean-Field Theory are getting.**

I found some of the discussion of theory in the paper poor and confusing. For example, I failed to see how Zhang-Rice singlets are relevant to the ruthenates.

But the biggest concern was they make a big deal of the value of the energy of the anomaly and try and compare it to other energy scales such as the Hubbard U and the Hund's rule coupling J. This is simplistic. The whole of point of work described in my earlier post is that

**this energy scale is emergent**and does not have a simple relationship with the energy scales of the underlying interactions.

I would have also like to see a comparison with the recent LDA+DMFT calculations of Jernez Mravlje and collaborators that I discussed here.

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