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On the relation between current and partition function for impurity transport problems out of equilibrium

Sergei Skorik

Published Jul 29, 1997• 0 citations
OpenAlex

Abstract

We study the partition functions of quantum impurity problems in the domain of complex applied bias for its relation to the non-equilibrium current suggested by Fendley, Lesage and Saleur (cond-mat/9510055). The problem is reformulated as a certain generalization of the linear response theory that accomodates an additional complex variable. It is shown that the mentioned relation holds in a rather generic case in the linear response limit, or under certain condition out of equilibrium. This condition is trivially satisfied by the quadratic Hamiltonians and is rather restrictive for the interacting models. An example is given when the condition is violated.

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We study the partition functions of quantum impurity problems in the domain of complex applied bias for its relation to the non-equilibrium current suggested by Fendley, Lesage and Saleur (cond-mat/9510055). The problem is reformulated as a certain generalization of the linear response theory that accomodates an additional complex variable. It is shown that the mentioned relation holds in a rather generic case in the linear response limit, or under certain condition out of equilibrium. This condition is trivially satisfied by the quadratic Hamiltonians and is rather restrictive for the interacting models. An example is given when the condition is violated.
We study the partition functions of quantum impurity problems in the domain of complex applied bias for its relation to the nonequilibrium current suggested by Fendley, Lesage, and Saleur [J. Stat. Phys. 85, 211 (1996)]. The problem is reformulated as a certain generalization of the linear response theory that accommodates an additional complex variable. It is shown that the mentioned relation holds in a rather generic case in the linear response limit, or under certain condition out of equilibrium. This condition is trivially satisfied by the quadratic Hamiltonians and is rather restrictive for the interacting models. An example is given when the condition is violated.
Fendley, Ludwig and Saleur [Phys. Rev. B52 (1995) 8934] have obtained an expression for the non-equilibrium current through a constriction in the quantum Hall bar based on the Bethe ansatz technique and the Bolzmann equation. In this Comment we draw attention to a serious flaw in their derivation. We argue that their result is correct in the linear response limit but should be taken with a care out of equilibrium for finite bias. The reason is in the use of the equilibrium scattering matrix in the place of the transition probability amplitude out of equilibrium, the substitution which was not shown to be legimate.
Quantum dot devices may be tuned to Kondo models which have non-interacting limits such as the Toulouse limit. At sufficiently low temperatures the Toulouse limit can be used to make predictions for charge transport in the devices driven away from equilibrium, e.g. with an ac bias. We investigate non-equilibrium transport in a resonant level model with a single lead which may be mapped onto the Toulouse limit of the one-channel Kondo model. The main results are expressions for the charge current that are exact in the tunnelling couplings, while remaining in the linear response regime of the time-dependent voltage bias. We also show how the line of enquiry directly extends to the tunnelling junction studied by A.~Schiller and S.~Hershfield in which a Kondo impurity interacts separately with two leads.
The theoretical description of strongly correlated quantum systems out of equilibrium presents several challenges and a number of open questions persist. In this paper we focus on nonlinear electronic transport through an interacting quantum dot maintained at finite bias using a concept introduced by Hershfield [Phys. Rev. Lett. 70, 2134 (1993)] whereby one can express such nonequilibrium quantum impurity models in terms of the system's Lippmann-Schwinger operators. These scattering operators allow one to reformulate the nonequilibrium problem as an effective equilibrium problem associated with a modified Hamiltonian. In this paper we provide a pedagogical analysis of the core concepts of the effective equilibrium theory. First, we demonstrate the equivalence between observables computed using the Schwinger-Keldysh framework and the effective equilibrium approach, and relate the Green's functions in the two theoretical frameworks. Second, we expound some applications of this method in the context of interacting quantum impurity models. We introduce a novel framework to treat effects of interactions perturbatively while capturing the entire dependence on the bias voltage. For the sake of concreteness, we employ the Anderson model as a prototype for this scheme. Working at the particle-hole symmetric point, we investigate the fate of the Abrikosov-Suhl resonance as a function of bias voltage and magnetic field.
There exists a regime of overlap between the equilibrium and non-equilibrium behavior of a system, the non-equilibrium behavior of weakly perturbed states. When a system is perturbed ever so slightly, its response will be linear in the perturbation, say the current of the conduction electrons in a metal will be proportional to the strength of the applied electric field. This regime is called the linear response regime, and though the system is in a non-equilibrium state all its characteristics can be inferred from the properties of its equilibrium state. In the next chapter we shall go beyond the linear regime by showing how to obtain quantum kinetic equations. The kinetic-equation approach to transport is a general method, and allows in principle nonlinear effects to be considered. However, in many practical situations one is interested only in the linear response of a system to an external force. The linear response limit is a tremendous simplification in comparison with general non-equilibrium conditions, and is the subject matter of this chapter. In particular the linear response of the density and current of an electron gas are discussed. The symmetry properties of response functions, and the fluctuation–dissipation theorem are established. Lastly we demonstrate how correlation functions can be measured in scattering experiments, as illustrated by considering neutron scattering from matter. Needless to say, in measurements of (say) the current in a macroscopic body, far less information in the current correlation function is probed. Linear response In this section we consider the response of an arbitrary property of a system to a general perturbation.
By mapping steady-state nonequilibrium to an effective equilibrium, we formulate nonequilibrium problems within an equilibrium picture. The Hamiltonian in the open system is rewritten in terms of scattering states with appropriate boundary condition. We first study the analytic properties of many-body scattering states, impose the boundary-condition operator in a statistical operator and prove that this mapping is equivalent to the linear-response theory in the low-bias limit. In an example of infinite-UUU Anderson impurity model, we approximately solve the scattering state creation operators, based on which we derive the bias operator $\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{Y}$ to construct the nonequilibrium ensemble in the form of the Boltzmann factor ${e}^{\ensuremath{-}\ensuremath{\beta}(\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{H}\ensuremath{-}\stackrel{\ifmmode \hat{}\else \^{}\fi{}}{Y})}$. The resulting effective Hamiltonian is solved by noncrossing approximation. We obtain the $I\text{\ensuremath{-}}V$ features of Kondo anomaly conductance at zero bias, inelastic transport via the charge excitation on the quantum dot, and significant inelastic current background over a wide range of bias.