Effect of the electromagnetic environment on current fluctuations in driven tunnel junctions

TL;DR

This paper investigates how the electromagnetic environment influences current fluctuations in driven tunnel junctions, revealing a complex spectral function with contributions from Johnson-Nyquist noise, shot noise, and their cross-correlations.

Contribution

It provides an exact treatment of driven electromagnetic modes and derives a spectral function for current fluctuations, extending understanding of noise in ac-driven tunnel junctions.

Findings

Spectral function includes Johnson-Nyquist, shot noise, and cross-correlation contributions.

Results applicable to both dc and ac driven junctions via a photo-assisted tunneling relation.

Specific analysis for Ohmic environments and resonator-driven junctions.

Abstract

We examine current fluctuations in tunnel junctions driven by a superposition of a constant and a sinusoidal voltage source. In standard setups the external voltage is applied to the tunneling element via an impedance providing an electromagnetic environment of the junction. The modes of this environment are excited by the time-dependent voltage and are the source of Johnson-Nyquist noise. We determine the autocorrelation function of the current flowing in the leads of the junction in the weak tunneling limit up to terms of second order in the tunneling Hamiltonian. The driven modes of the electromagnetic environment are treated exactly by means of a unitary transformation introduced recently. Particular emphasis is placed on the spectral function of the current fluctuations. The spectrum is found to comprise three contributions: a term arising from the Johnson-Nyquist noise of the environmental impedance, a part due to the shot noise of the tunneling element and a third contribution which comes from the cross-correlation between fluctuations caused by the electromagnetic environment and fluctuations of the tunneling current. All three parts of the spectral function occur already for devices under dc bias. The spectral function of ac driven tunneling elements can be determined from the result for a dc bias by means of a photo-assisted tunneling relation of the Tien-Gordon type. Specific results are given for an Ohmic environment and for a junction driven through a resonator.