Current noise and Keldysh vertex function of an Anderson impurity in the Fermi liquid regime

TL;DR

This paper develops a microscopic Fermi-liquid theory for nonequilibrium transport and noise in Anderson impurities under bias, incorporating Keldysh formalism and vertex functions to describe current conservation and quasiparticle interactions.

Contribution

It provides a comprehensive derivation of the Keldysh vertex functions and noise formulas for Anderson impurities at finite bias, extending Fermi-liquid theory to nonequilibrium conditions.

Findings

Derived the formula for current noise up to order (eV)^3 at zero temperature.

Calculated Keldysh vertex functions including linear order terms in bias, temperature, and frequency.

Showed that Ward identities and collision integrals ensure current conservation in the nonequilibrium Fermi-liquid regime.

Abstract

We present a complete microscopic Fermi-liquid description for next-to-leading order transport through an Anderson impurity under a finite bias voltage $V$. It is applicable to multilevel quantum dots without particle-hole or time-reversal symmetry, and is constructed based on the nonequilibrium Keldysh formalism, taking into account the current conservation between electrons in the impurity levels and the conduction bands. Specifically, we derive the formula for the current noise generated in the steady flow up to terms of order $(eV)^3$ at zero temperature $T=0$. To this end, we calculate the Keldysh vertex functions $\Gamma_{\sigma\sigma';\sigma'\sigma}^{ \nu_1\nu_2;\nu_3\nu_4} (\omega,\omega'; \omega',\omega)$, which depend on branches $\nu_1, \nu_2, \nu_3$ and $\nu_4$ of the time-loop contour and on spin degrees of freedom $\sigma$ and $\sigma'$, up to linear-order terms with respect to $eV$, $T$, and frequencies $\omega$ and $\omega'$. The coefficients of these linear-order terms are determined by a set of the parameters, defined with respect to the equilibrium ground state: the phase shift, static susceptibilities, and nonlinear three-body susceptibilities of the impurity electrons. The low-energy expressions of the vertex components are shown to satisfy the Ward identities with the Keldysh Green's functions expanded up to terms of order $\omega^2$, $(eV)^2$, and $T^2$. We also find that the imaginary part of the Ward identities can be described in terms of the $eV$-dependent collision integrals for a single-quasiparticle excitation and that for a single quasiparticle-quasihole pair excitation. These collision integrals ensure the current conservation of the next-to-leading order Fermi-liquid transport due to the quasiparticles with a finite damping rate.