Karl Franzens University Graz

Graz University of Technology 

Diagrammatic treatment of spatial correlations beyond an interacting reference system: the D-TRILEX approach
Dr. Evgeny Stepanov
Ecole Polytechnique, France
16:15 - 17:15 Tuesday 20 January 2026 TUG HS P2

Local approximations to electronic correlations, such as those provided by dynamical mean-field theory (DMFT), are a standard approach for addressing correlated multi-orbital systems. However, attempts to go beyond this local picture are often associated with expensive numerical calculations. To resolve this problem, we have developed the dual triply-irreducible local expansion (D-TRILEX) method [PRB 100, 205115 (2019); PRB 103, 245123 (2021); SciPost Phys. 13, 036 (2022)], which allows for a consistent diagrammatic treatment of leading non-local charge, spin, and orbital fluctuations beyond DMFT. Applying the D-TRILEX approach to the well-known ruthenate compound Sr2RuO4 resolved a long-standing discrepancy between theory and experiment regarding the strength and form of spin excitations in this material [arXiv:2412.14735]. We have shown that incorporating spatial magnetic fluctuations in D-TRILEX reduces the degree of correlations, destabilizing magnetic ordering and yielding a spin susceptibility that quantitatively matches experimental inelastic neutron scattering (INS) measurements. Additionally, we find that the spin excitations in Sr2RuO4 exhibit an orbitally-selective character, a feature that may be linked to a multi-component superconducting state.