We consider a system of weakly interacting Fermions which satisfies all those requirements necessary for the derivation of Van Hove’s weak coupling master equation. We show how the Boltzmann equation can be derived with the help of a Factorisation Theorem which we then prove, in the limit of an infinite system. We then use this Factorisation Theorem to show that the electrical conductivity derived from the Kubo formula is equivalent to the conductivity derived from the Boltzmann equation, for the same system of Fermions. Finally, we illustrate how the theory must be modified for a weakly interacting system of electrons and phonons.
We consider a system of weakly interacting Fermions which satisfies all those requirements necessary for the derivation of Van Hove’s weak coupling master equation. We show how the Boltzmann equation can be derived with the help of a Factorisation Theorem which we then prove, in the limit of an infinite system. We then use this Factorisation Theorem to show that the electrical conductivity derived from the Kubo formula is equivalent to the conductivity derived from the Boltzmann equation, for the same system of Fermions. Finally, we illustrate how the theory must be modified for a weakly interacting system of electrons and phonons.
The many-body linear response expressions obtained in previous papers [J. Math. Phys. 19, 1345 (1978); 20, 2573 (1979)] are applied to systems of weakly interacting particles. General expressions for the susceptibility and conductivity in such systems are obtained. The diagonal parts depend on the scattering processes, for which we consider interactions with bosons with mass and electron-phonon interaction. For elastic collisions simple closed forms result. For general two-body collisions, the closed expressions are cumbersome, except when the current is due to collisional current through localized states, such as Landau orbits; in that event a generalized Adams-Holstein result is obtained. The nondiagonal electrical conductivity is shown to be of paramount importance for the quantum mechanical Hall effect. We also derive quantum mechanical Boltzmann equations, both for the diagonal occupancy operator 〈nζ〉t and for the nondiagonal operator 〈c+ζ′ cζ\〉t. The total Boltzmann equation is shown to be fully equivalent with the linear response results. Finally, in the last part we derive the Boltzmann equation for the Wigner function of inhomogeneous systems. In the classical limit this yields the usual Boltzmann transport equation. This equation has therefore been obtained by first principles from the von Neumann equation.
\nthe quantum calculation of the d.c. electrical conductivity for an electron system is reconsidered by using an improved form of the long-time solution of the density matrix. The starting time-evolution equation which is written as a linear system in the Liouville space of discrete quantum states, is first solved in terms of Cramer's determinants and next suitably adapted to tranport problems. The calculation of the statistical average of the many-body current density, in the presence of scattering potential, is carried out in the thermodynamic limit, using a characteristic property of collision matrices introduced earlier by van Hove as the so-called delta-singularity. In contrast to previous methods based on kinetic equations solved by perturbations, Kubo's theory, or Mori type memory functions, the resolution of an integral equation, initially included in Cramer's formulae, is no longer required. Among its major advantages, the method succeeds in overcoming the well known zero frequency divergent behaviour of the conductivity. It also yields an explicit expression of an effective relaxation time which is just constructed out of those diagonal transition sequences responsible for self-energy effects in van Hove's theory. This expression which only assumes the randomization of phases of initial states can be used to get tractable expansions in powers of the collision potential. Its range of validity is limited by the effect of collisions on equilibrium statistics, which is shown to be negligible within the Peierls criterion, hbar/taullmu, tau being the relaxation time and mu the Fermi energy. Finally, the comparison of the present approach with the elementary transport theory reveals that the relaxation time is close to the standard expression, even though the latter would be exactly recovered, in lowest order of collisions, only on disregarding divergent terms arising beyond second order.\n
To study transport properties, one needs to investigate the system of interest when coupled to biased external baths. This requires solving a master equation for this open quantum system. Obtaining this solution is very challenging, especially for large systems. This limits applications of the theories of open quantum systems, especially insofar as studies of transport in large quantum systems, of interest in condensed matter, is concerned. In this thesis, I propose three efficient methods to solve the Redfield equation --- an example of such a master equation. The first is an open-system Kubo formula, valid in the limit of weak bias. The second is a solution in terms of Green's functions, based on a BBGKY (Bogoliubov--Born--Green--Kirkwood--Yvon)-like hierarchy. In the third, the Redfield equation is mapped to a generalized Fokker-Planck equation using the coherent-state representation. All three methods, but especially the latter two, have much better efficiency than direct methods such as numerical integration of the Redfield equation via the Runge-Kutta method. For a central system with a d-dimensional Hilbert space, the direct methods have complexity of d³, while that of the latter two methods is on the order of order of polynomials of log d. The first method, besides converting the task of solving the Redfield equation to solving the much easier Schrödinger's equation, also provides an even more important conceptual lesson: the standard Kubo formula is not applicable to open systems. Besides these general methodologies, I also investigate transport properties of spin systems using the framework of the Redfield equation and with direct methods. Normal energy and spin transport is found for integrable but interacting systems. This conflicts with the well-known conjecture linking anomalous conductivity to integrability, and it also contradicts the relationship, suggested by some, between gapped systems (Jz > Jxy) and normal spin conductivity. I propose a new conjecture, linking anomalous transport to the existence of a mapping of the problem to one for non-interacting particles.
We consider a system perturbed by an external field and subject to dissipative processes. From the von Neumann equation for such a system in the weak coupling limit we derive an inhomogeneous master equation, i.e., a master equation with dissipative terms and streaming terms, using Zwanzig’s projection operator technique in Liouville space. From this equation the response function, as well as expressions for the generalized conductivity and susceptibility, is obtained. It is shown that for large times only the diagonal part of the density operator is required. The various expressions are found to be in complete harmony with previous results (Part I) obtained via the van Hove limit of the Kubo–Green linear response formulas. In order to account for the properties at quantum frequencies, the evolution of the nondiagonal part in the weak coupling limit is also established. The complete time dependent behavior of the dynamic variables in the van Hove limit is expressed by B (t) =exp[−(Λd−iℒ0) t] B, where Λd is the master operator and L0 the Liouville operator in the interaction picture. The cause of irreversibility is discussed. Finally, the inhomogeneous master equation is employed to obtain as first moment equation a Boltzmann equation with streaming terms, applicable to quantum systems.
The Kubo formula for the electrical conductivity is rewritten in terms of a sum of Drude-like contributions associated to the exact eigenstates of the interacting system, each characterized by its own frequency-dependent relaxation time. The structure of the novel and equivalent formulation, weighting the contribution from each eigenstate by its Boltzmann occupation factor, simplifies considerably the access to the static properties (dc conductivity) and resolves the long standing difficulties to recover the Boltzmann result for dc conductivity from the Kubo formula. It is shown that the Boltzmann result, containing the correct transport scattering time instead of the electron lifetime determined by the Green function, can be recovered in problems with elastic and inelastic scattering at the lowest order of interaction.