Phase Coherent Transport of Charges in Graphene Quantum Billiard

TL;DR

This paper demonstrates that graphene acts as a phase-coherent quantum billiard with ballistic transport, exhibiting conductance oscillations, proximity effects, and potential for novel superconducting states.

Contribution

It provides experimental evidence of phase-coherent transport and quantum interference in graphene, and explores proximity effects with superconducting electrodes, revealing new phenomena.

Findings

Minimum conductivity approaches theoretical value in wide, short graphene strips

Quantum interference causes conductance oscillations at low temperatures

Proximity effects include enhanced conductance and dips indicating novel superconductivity

Abstract

We experimentally investigate electrical transport properties of graphene, which is a two dimensional (2D) conductor with relativistic energy dispersion relation. By investigating single- and bi-layer graphene devices with different aspect ratios, we confirm experimentally that the minimum conductivity in wide and short graphene strips approaches the theoretical value of 4e^2/\pi\h. At low temperatures, quantum interference of multiply-reflected waves of electrons and holes in graphene give rise to periodic conductance oscillations with bias and gate voltages. Thus graphene acts as a quantum billiard, a 2D ballistic, phase coherent electron system with long phase coherence length that exceeds 5 microns. Additional features in differential conductance emerge when graphene is coupled to superconducting electrodes. We observe proximity-induced enhanced conductance at low bias, and conductance dips at energy scales far above the superconducting gap of the electrodes. The latter provides preliminary evidence for a novel superconducting material that consists of graphene coated with metallic atoms.