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T. Schäfer:
"The physics underlying electronic spectra: from parquet decomposition to flluctuation diagnostics";
Vortrag: Seminar College de France, Paris, France (eingeladen); 09.01.2017.

Spectroscopic measurements are invaluable tools for analyzing strongly
correlated electron systems. On the experimental side, techniques which access the system's one-particle properties (e.g. spectral functions) are often complemented by two-particle experiments (like measuring momentum-dependent response functions), in order to provide insight into the physical mechanisms originating the observed one-particle spectra.
In my talk, I will introduce two techniques for identifying which physical processes eventually determine a given theoretical prediction for the spectral function. Both of them are based on the (exact) Dyson-Schwinger equation of motion. I will show that one method, the parquet decomposition, can be successfully applied in the weak-coupling regime, but fails approaching intermediate interaction strengths, where interesting physical phenomena, like the pseudogap in the Hubbard model, appear [1]. The breakdown of the parquet decomposition can be traced back to divergences in (irreducible) vertices, recently associated to the non-perturbative precursor effects of the Mott-Hubbard transition and closely related to the non-uniqueness of the Luttinger-Ward functional [2,3,4]. To overcome
these intrinsic limitations, I will introduce a second technique, the fluctuation diagnostics, based on the full vertex, which does not show any (low-frequency) divergences [5]. Applying the fluctuation diagnostics to the two-dimensional Hubbard model, I will show that, there, spin-
fluctuations can be unambiguously identified being responsible for opening the pseudogap [5,6].
[1] O. Gunnarsson, T. Schäfer, J. P. F. LeBlanc, J. Merino, G. Sangiovanni,
G. Rohringer, and A. Toschi, Phys. Rev. B 93, 245102 (2016).
[2] T. Schäfer, G. Rohringer, O. Gunnarsson, S. Ciuchi, G. Sangiovanni, and A.
Toschi, Phys. Rev. Lett. 110, 246405 (2013).
[3] T. Schäfer, S. Ciuchi, M. Wallerberger, P. Thunström, O. Gunnarsson, G.
Sangiovanni, G. Rohringer, and A. Toschi, Phys. Rev. B 94, 235108 (2016).
[4] E. Kozik, M. Ferrero, and A. Georges, Phys. Rev. Lett. 114, 156402 (2015).
[5] O. Gunnarsson, T. Schäfer, J. P. F. LeBlanc, E. Gull, J. Merino, G. Sangiovanni, G. Rohringer, and A. Toschi, Phys. Rev. Lett. 114, 236402 (2015).
[6] W. Wu, M. Ferrero, A. Georges, and E. Kozik, arXiv:1608.08402 (2016).