Non-equilibrium Hybridization Expansion Impurity-solver

Hartmut Hafermann, Philipp Werner, Emanuel Gull

Hiroshi Shinaoka, Emanuel Gull, Philipp Werner

Karyn Le Hur, Marco Schirò

C. Jung, A. Lieder, S. Brener +6 more

Abstract We present a generalization of the recently developed dual‐fermion approach introduced for correlated lattices to non‐equilibrium problems. In its local limit, the approach has been used to devise an efficient impurity solver, the superperturbation solver for the Anderson impurity model (AIM). Here we show that the general dual perturbation theory can be formulated on the Keldysh contour. Starting from a reference Hamiltonian system, in which the time‐dependent solution is found by exact diagonalization, we make a dual perturbation expansion in order to account for the relaxation effects from the fermionic bath. Simple test results for closed as well as open quantum systems in a fermionic bath are presented.

Hans Gerd Evertz, Martin Ganahl, Patrik Thunström +2 more

Eitan Eidelstein, Emanuel Gull, Guy Cohen

We present a numerically exact inchworm Monte Carlo method for equilibrium multiorbital quantum impurity problems with general interactions and hybridizations. We show that the method, originally developed to overcome the dynamical sign problem in certain real-time propagation problems, can also overcome the sign problem as a function of temperature for equilibrium quantum impurity models. This is shown in several cases where the current method of choice, the continuous-time hybridization expansion, fails due to the sign problem. Our method therefore enables simulations of impurity problems as they appear in embedding theories without further approximations, such as the truncation of the hybridization or interaction structure or a discretization of the impurity bath with a set of discrete energy levels, and eliminates a crucial bottleneck in the simulation of ab initio embedding problems.