Consequences of Charge Fractionalization in a Graphene Quantum Hall Point Contact

This is the second of two talks describing tunneling measurements in a dual-graphite gated, hBN encapsulated, graphene quantum point contact. In this segment I will discuss the physical consequences of a ν=1/3 to ν=1 quantum Hall point heterojunction beyond the universal weak coupling limit. This system can be directly mapped onto the two branches of a g = 2 Luttinger liquid scattering at an impurity and can be solved non-perturbatively for all coupling strengths [1]. While this model is not expected to hold universally, we find substantial agreement with the predicted tunneling characteristics for a large range of bias, temperature, and tunnel couplings. Most surprisingly, as the bias, temperature, or coupling becomes large we find that the tunneling conductance exceeds 1/3 e²/h and saturates to nearly 1/2 e²/h. This excess conductance is accompanied by a negative reflected voltage measured downstream of the QPC. We interpret this behavior as arising from correlated transmission across the QPC, analogous to crossed Andreev reflection in superconductors [2]. In this scenario, single electron charges are transferred across the junction when two incident e/3 quasiparticles are accompanied by a retro-reflected -e/3 quasi-hole. We exploit this ‘Andreev’ like process to make a (nearly) dissipationless DC voltage step-up transformer which yields a gain of 1.46 and a 97% power efficiency. Our measurements imply that the heterojunction can be made nearly adiabatic, presenting a significant advance in the state-of-the-art for quantum Hall mesoscopics.



[1] Claudio de C. Chamon, Eduardo Fradkin, Distinct universal conductances in tunneling to quantum Hall states: The role of contacts, Phys Rev. B, 56, vol 4., 15 July, 1997

[2] Nancy P. Sandler, Claudio de C. Chamon, Eduardo Fradkin, Andreev reflection in the fractional quantum Hall effect, Phys Rev. B, 57, 15 May, 1998