Coupled Boson—Fermion Systems

Yukitoshi Motome

Nonlinear feedback effects due to dispersive bosons are studied in fermion-boson coupled systems by using an extension of dynamical mean-field theory. In the nonperturbative regime, strong coupling and nonadiabaticity reduce effective interaction between fermions as the width of boson dispersion increases. The boson field is completely softened at the crossover to this regime. The softening is significantly enhanced by the boson dispersion. We elucidate the controlling parameters of this nonperturbative regime where fluctuations of the dispersive bosons accelerate the delocalization of fermions.

B. Sørensen

V. A. Yatsun

Denis Lacroix A, Vazgen Valerikovich Sargsyan, G. G. Adamian +2 more

A method is proposed to describe Fermi or Bose systems coupled to one or several heat baths composed of fermions and/or bosons. The method, called the coupled equations of motion method, properly includes non-Markovian effects. The approach is exact in the full-coupling approximation when only bosonic particles are present in the system and baths. The approach provides an approximate treatment when fermions are present either in the system and/or in one or several environments. Our approach has the advantage of properly respecting the Pauli exclusion principle for fermions during the evolution. We illustrate the approach for the single fermionic or bosonic oscillator coupled to one or two heat baths assuming different types of quantum statistics (fermion or boson) for them. The cases of a Fermi system coupled to fermion or boson heat baths or a mixture of both are analyzed in detail. With the future goal of treating Fermi systems formed of an increasing number of two-level systems (qubits), we discuss possible simplifications that could be made in the equations of motion and their limits of validity in terms of the system-bath coupling or of the initial heat bath temperatures.

Douglas J. Scalapino, Robert L. Sugar

We have recently developed a formalism for carrying out Monte Carlo calculations on systems with coupled boson-fermion degrees of freedom. Here we present the results obtained using these techniques on some specific systems. First the quantum-mechanics problem of a single boson coordinate linearly coupled to a fermion is studied. The fermion Green's function and the boson coordinate displacement are calculated and compared with exact results. Extending this analysis to a one-dimensional array, a field theory in one space and one imaginary time dimension is investigated. The fermion and boson Green's functions are calculated and compared with analytic results obtained in the limit where the bare boson frequency is much larger or much smaller than the characteristic fermion energies.