Speaker
Description
We show how the stability conditions for a system of interacting fermions can be also expressed in terms of local two-particle correlators, instead of conventional derivatives of thermodynamic potentials. By inspecting the spectral representation of the generalized local charge susceptibility and its lowest negative eigenvalues, we first illustrate the applicability of this stability conditions for the phase-transitions of a multi-orbital model for strongly correlated electrons. As a second step, we investigate the intrinsic relation linking the thermodynamic instabilities to the local generalized susceptibilities on a more fundamental level. In particular, we show that these quantities possess intrinsic non-Hermitian matrix properties, solely due to Fermi-Dirac statistics and imaginary time-ordering, which enable the occurrence of exceptional points, i.e. singularities where two eigenvalues and eigenvectors coalesce. By means of both analytic and numerical approaches, we demonstrate that exceptional points promote the correlation-induced thermodynamic instabilities to a topologically stable phenomenon.
