Riccardo Rossi - a pioneer in algorithms for strongly correlated systems, and expert in diagrammatic Monte Carlo and resummation theory.
We are pleased to announce a seminar by leading expert Dr Riccardo Rossi (EPFL Lausanne) for the PQM discussion group on 11 Feb 2026, to share his latest progress in numerical simulations of magnetic systems:
Majorana Diagrammatics for Quantum Spin-1/2 Models
Frustrated quantum spin systems are believed to host exotic phases of matter such as spin liquids, but remain challenging to analyze with current theoretical techniques. The purpose of this talk is to introduce a diagrammatic framework for spin-1/2 systems based on an unconstrained “ancillary” mapping between spin and Majorana operators. This allows the use of standard tools of diagrammatic quantum many-body theory without requiring projections. We derive, in particular, the Feynman rules for the expansion around a color-preserving mean-field theory. We then present the numerical results obtained by computing the corrections up to second order for the Heisenberg model in one and two dimensions, showing that perturbative corrections are not only numerically important, but also qualitatively improve the results of mean-field theory. These results pave the way for the use of Majorana diagrammatic tools in theoretical and numerical studies of quantum spin systems.