Hanbury-Brown and Twiss exchange and non-equilibrium-induced correlations in disordered, four-terminal graphene-ribbon conductor

Z B Tan,P Lähteenmäki,L W Molenkamp,T Elo,F Duerr,K E Nagaev,P J Hakonen,A Puska,J Sarkar,C Gould

doi:10.1038/s41598-018-32777-5

Abstract

We have investigated current-current correlations in a cross-shaped conductor made of graphene. The mean free path of charge carriers is on the order of the ribbon width which leads to a hybrid conductor where there is diffusive transport in the device arms while the central connection region displays near ballistic transport. Our data on auto and cross correlations deviate from the predictions of Landauer-Büttiker theory, and agreement can be obtained only by taking into account contributions from non-thermal electron distributions at the inlets to the semiballistic center, in which the partition noise becomes strongly modified. The experimental results display distinct Hanbury – Brown and Twiss (HBT) exchange correlations, the strength of which is boosted by the non-equilibrium occupation-number fluctuations internal to this hybrid conductor. Our work demonstrates that variation in electron coherence along atomically-thin, two-dimensional conductors has significant implications on their noise and cross correlation properties.

Highlights

Disordered graphene is an extraordinary tunable system for studying electrical conduction ranging from nearly ballistic transport[1,2] to hopping conductivity[3,4,5,6,7,8]
We report and analyze experimental results on auto and cross correlations in a graphene nanoribbon cross where the mean free path mfp of charge carriers is on the order of the ribbon width
Our experiments reveal distinct Hanbury – Brown and Twiss (HBT) exchange correlations, the strength of which is boosted by the non-equilibrium occupation-number fluctuations internal to this hybrid conductor

Summary

Introduction

Disordered graphene is an extraordinary tunable system for studying electrical conduction ranging from nearly ballistic transport[1,2] to hopping conductivity[3,4,5,6,7,8]. These fluctuations take place if the average occupation numbers are different from zero and 1 and they account for the equilibrium thermal noise at a finite temperature This noise is proportional to the conductance of the system, and it is nonzero even for ballistic conductors, which lack any internal scattering. Arm, the distribution function to the scattering of particles inside the conductor, which partially reflects them back These fluctuations are called partition noise, and it may be observed even at zero temperature if there is a net current through the conductor. This noise is typical of systems with tunneling or diffusive transport where the incoming electrons are described by a Fermi distribution. The goal of our paper is not to test the exactness of individual noise models but rather to test whether the structure under investigation satisfies the assumptions of a particular model

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Conclusion