Models of non-interacting fermions coupled to auxilliary classical degrees of freedom are relevant to the understanding of a wide variety of problems in many body physics, e.g. the description of manganites, diluted magnetic semiconductors or strongly interacting electrons on lattices. Monte Carlo sampling over the classical fields is a powerful, yet notoriously challenging, method for this class of problems -it requires the solution of the fermion problem for each classical field configuration. Conventional Monte Carlo methods minimally utilize the information content of these solutions by extracting single temperature properties. We present a flat-histogram Monte Carlo algorithm that simulates a novel statistical ensemble which allows to acquire the full thermodynamic information, i.e. the partition function at all temperatures, of sampled classical configurations.
Introduction -Bilinear Hamiltonians of lattice fermions coupled to classical degrees of freedom (continuous or discrete) are ubiquituous in contemporary manybody physics. These often arise as suitable approximations in the description of systems where many different degrees of freedom contrive to yield complex and interesting physics. In these cases, some subsystem is treated classically as e.g. localized spins in double-exchange models [1][2][3], models of Mn-doped (III, IV) semiconductors [4], the Ising t-J model [5], adiabatic phonons in polaron models [6][7][8][9], or one species of fermions in the Falicov-Kimball model [9][10][11]. Exactly solvable models can also take this form, such as the seminal honeycomb lattice Kitaev model [12] where interacting spins are mapped onto Majorana fermions coupled to static gauge fields. More generally, auxiliary field methods, e.g. based on the Hubbard-Stratonovich transformation, allow to decouple fermion-fermion or fermion-boson interactions -the fields are then treated classically in conjunction with the application of the Suzuki-Trotter decomposition [13,14] (see [15] for a recent approximate scheme with static fields).