Propagation of superconducting coherence via chiral quantum-Hall edge channels

Geon-Hyoung Park,Minsoo Kim,Kenji Watanabe,Takashi Taniguchi,Hu-Jong Lee

doi:10.1038/s41598-017-11209-w

Geon-Hyoung Park, Minsoo Kim + Show 3 more

Open Access

https://doi.org/10.1038/s41598-017-11209-w

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Abstract

Recently, there has been significant interest in superconducting coherence via chiral quantum-Hall (QH) edge channels at an interface between a two-dimensional normal conductor and a superconductor (N–S) in a strong transverse magnetic field. In the field range where the superconductivity and the QH state coexist, the coherent confinement of electron- and hole-like quasiparticles by the interplay of Andreev reflection and the QH effect leads to the formation of Andreev edge states (AES) along the N–S interface. Here, we report the electrical conductance characteristics via the AES formed in graphene–superconductor hybrid systems in a three-terminal configuration. This measurement configuration, involving the QH edge states outside a graphene–S interface, allows the detection of the longitudinal and QH conductance separately, excluding the bulk contribution. Convincing evidence for the superconducting coherence and its propagation via the chiral QH edge channels is provided by the conductance enhancement on both the upstream and the downstream sides of the superconducting electrode as well as in bias spectroscopy results below the superconducting critical temperature. Propagation of superconducting coherence via QH edge states was more evident as more edge channels participate in the Andreev process for high filling factors with reduced valley-mixing scattering.

Highlights

An electron incident from a normal conductor (N) to a superconductor (S) undergoes retroreflection as a hole at the normal conductor and a superconductor (N–S) interface, which is the Andreev reflection (AR)[1]
The conductance plateaus are clearly enhanced from the expected values denoted by dashed lines, with stronger conductance deviation as more edge states participated in the AR
Non-ideal transparency of the graphene–S interface (ΔGU ~30% at B = 0 T in bilayer graphene (BLG)) and valley-degenerate edge states (n ≥ 2) partly breaks the Andreev pairs in the two opposite edge states, which reduces the enhancement of QH conductance from the expected doubled QH conductance

Summary

Introduction

An electron incident from a normal conductor (N) to a superconductor (S) undergoes retroreflection as a hole at the N–S interface, which is the Andreev reflection (AR)[1]. The coexistence of the QH edge states and the superconductivity can be realised as one reaches the QH regime in an N layer at sufficiently low magnetic field to allow the survival of the superconductivity in an S electrode This requires the N layer to have high carrier mobility with a mean free path longer than the magnetic length (lB) and a good superconducting proximity contact as well at the 2DEG–S interface. Inset shows a magnified view near zero bias for T = 0.16, 0.36, 0.6, and 0.8 K. good edge contact between a graphene layer and an Nb superconducting electrode with a high critical field (Hc2 ~ 3.5 T) allows the superconducting proximity effect in the QH regime for a magnetic field above ~1 T. Unlike the AES in this study, their result of the negative resistance in the downstream edge states, which represents the hole current, was caused by the nonlocal coherence between electrons and holes

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Results

Conclusion