Inducing superconducting correlation in quantum Hall edge states

Abstract

The quantum Hall (QH) effect supports a set of chiral edge states at the boundary of a 2-dimensional electron gas (2DEG) system. A superconductor (SC) contacting these states can induce correlations of the quasi-particles in the dissipationless 1D chiral QH edge states. If the superconducting electrode is narrower than the superconducting coherence length, the incoming electron is correlated to the outgoing hole along the chiral edge state by the Andreev process 1-3 across the SC electrode. In order to realise this crossed Andreev conversion (CAC) 4-7 , it is necessary to fabricate highly transparent and nanometer-scale superconducting junctions to the QH system. Here we report the observation of CAC in a graphene QH system contacted with a nanostructured NbN superconducting electrode. The chemical potential of the edge states across the SC electrode exhibits a sign reversal, providing direct evidence of CAC. This hybrid SC/QH system is a novel route to create isolated non-Abelian anyonic zero modes, in resonance with the chiral QH edge 7-12 .Inducing superconducting correlations via the proximity effect into a 2DEG in the QH regime has been a long standing challenge and has attracted renewed attentions due to its promise for realising non-Abealian zero modes [13][14][15] . Unlike conventional conductors, the 2DEG can exhibit an insulating incompressible bulk electronic state under perpendicular quantizing magnetic fields. In this QH regime, the conduction of electric charge occurs only along the edges via 1D chiral edge states, which the SC can make contacts to. In order to realise the hybrid system of QH and SC, the upper critical field of the SC needs to be high enough such that Cooper pairs in the SC are correlated mostly to the quasiparticles in well-developed 1D QH edge states. The experimental realisation of such hybrid systems often encounters challenges in semiconductor 2DEGs due to the formation of large Schottky barriers at the SC/semiconductor interfaces 16 . Graphene is a compelling candidate for the SC/QH platform, since the zero-band gap of graphene ensures Ohmic contacts for most metals, including SCs with high upper critical fields. Highly transparent SC/graphene interfaces have been demonstrated with strong superconducting proximity interactions and Josephson couplings 15,[17][18][19][20][21][22][23] . In addition, high mobility hBN-encapsulated graphene channels exhibit integer and fractional QH effects 24,25 at much lower magnetic field than the upper critical field of a few select SCs.

Graphene-Based Josephson-Junction Single-Photon Detector

“…The doped silicon substrate serves as a back-gate electrode. The superconducting terminals consist of 5-nm-thick titanium and 60-nm-thick niobium nitride (NbN) deposited after reactive ion etch and electron beam deposition of 5-nm titanium to form the etched one-dimensional contact [48]. The distance between the superconducting electrodes L is about 200 nm, forming a proximitized JJ with monolayer graphene as the weak link.…”

Fibonacci Topological Superconductor

“…Another promising venue is graphene, which has a four-fold degenerate zeroth Landau level. Coexistence of SC with the quantum Hall effect in these systems appears feasible [48,49].…”

Two-terminal transport along a proximity-induced superconducting quantum Hall edge

“…This is because quantum Hall effect requires a strong magnetic field, which is abhorred by superconductors. Nevertheless, a stable proximity effect in the quantum Hall regime has been achieved recently [5][6][7][8][9][10]. The key element of this success is the ability to manufacture a hybrid structure using either superconducting materials with high critical fields or two-dimensional electron gas that exhibits quantum Hall effects in lower magnetic fields.…”

“…In contrast, in Refs. [6][7][8][9][10], graphene was used as the two-dimensional electron gas that exhibits quantum Hall effects in lower magnetic fields.This experimental breakthrough offers an opportunity to test the predictions of earlier theoretical works such as the tunneling current from a superconducting point contact into a quantum Hall liquid [11] and the critical current [12] along with the upstream information transfer [13] in a superconductor-normal metal-superconductor (SNS) junction, where the normal metal is in the quantum Hall regime. Furthermore, if one can extend the stable proximity effect into the fractional quantum Hall regime, one might be able to create novel excitations with nontrivial braiding statistics [14][15][16][17].…”