Research Highlights

Professor Hu-Jong Lee’s Realization of Dream Come True Supercurrent Junction on Graphene

2015-07-29 312

A research team consisting of Dr. Gil-Ho Lee, Prof. Hu-Jong Lee, Sol Kim, and Prof. Seung-Hoon Jhi of Department of Physics, reported realization of “short and ballistic” Josephson junctions based on a monolayer graphene sheet. The paper was published in the February issue of Nature Communications.
Josephson junction, a nanometer-scale hybrid superconducting device, consists of two superconducting (S) electrodes weakly coupled by a normal-conducting (N) insert. It supports a resistance-free junction supercurrent along with quantum interference effects and has been utilized for realizing diverse quantum devices including quantum bits, field-effect supercurrent transistors, quantum electron pumps, etc. However, these efforts have often been hindered by non-ideal S-N contact characteristics, attributed to the short electronic mean free path (l) and/or a short superconducting coherence length (x), compared with the junction channel length (L).
The team realized a vertical graphene Josephson junction (vGJJ) by attaching two superconducting electrodes on top and bottom of a monolayer graphene sheet and, for the first time, confirmed the short (L<x) and ballistic (L<l) Josephson coupling reaching the theoretically predicted limit. The atomically thin single-crystalline graphene layer serves as an ultimately short conducting channel, with highly transparent interfaces with superconductors. The vGJJ is expected to enable studies on exotic, but highly elusive to date, quantum phenomena arising from strong Josephson coupling.  
The vertical hybridization scheme adopted in the study is not limited to superconducting electrodes and graphene, but is readily applicable to a variety of electrodes (e.g., ferromagnets) and exotic cleavable materials, such as topological insulators, layered cuprate and iron-pnictide superconductors, or various transition-metal dichalcogenides.
Professor Lee stresses that the scheme will open the pathway to a wide range of research opportunities for the fundamental sciences manifested at the atomic-scale interfaces of different materials, as well as the applications for highly coherent and scalable superconducting hybrid quantum devices.
This work was supported by National Research Foundation of Korea (NRF) through the SRC Center for Topological Matter (Grant No. 2011-0030788) and the GRF Center for Advanced Soft Electronics (Grant No. 2011-0031640).