How to read between the lines of electronic spectra: Tackling the pseudogap from fluctuation diagnostics to large embedded quantum clusters

One of the paradigmatic phenomena of the unconventional cuprate superconductors is their so-called pseudogap regime. Upon cooling, electronic states are suppressed in certain directions in reciprocal space as experimentally observed, for instance, in angle-resolved photoemission spectroscopy (ARPES) or the suppression of the Knight shift in nuclear magnetic resonance (NMR). While calculations and measurements of single-particle spectral properties often offer the most direct route to study correlated electron systems, their origin, at the same time, may stay elusive.

In my talk I will first introduce a computational technique for identifying the dominant two-particle scattering processes controlling the shape of the one-particle spectral functions and, in some cases, of the physical response of the system, the so-called “fluctuation diagnostics” [1,2]. Second, I will show results from applying this technique to the pseudogap regime in the square lattice Hubbard model, where short-ranged spin fluctuations are identified as the driving force of the momentum-selective depletion of quasiparticle states. Eventually, I will connect these findings to cutting-edge results from large real-space quantum embedding [3], allowing for non-uniform charge densities, as well as to numerically exact diagrammatic Monte Carlo techniques [4].

[1] O. Gunnarsson, T. Schäfer, J. LeBlanc, E. Gull, J. Merino, G. Sangiovanni, G. Rohringer, and A. Toschi, Phys. Rev. Lett. 114, 236402 (2015).

[2] T. Schäfer and A. Toschi, J. Phys.: Condens. Matter 33, 214001 (2021).

[3] M. Meixner, H. Menke, M. Klett, S. Heinzelmann, S. Andergassen, P. Hansmann, and T. Schäfer, SciPost Physics 16, 059 (2024).

[4] F. Šimkovic, R. Rossi, A. Georges, and M. Ferrero, Science 385, 6715 (2024).