Low-dimensional (super)conductors at low temperature or high magnetic field are a fantastic playground for experimentalists. Our research activities aim at unveiling new quantum phenomena related to 2d superconductivity, quantum localization, and quantum Hall effect. We are currently focusing on:
- Quantum coherence and anyon physics in the quantum Hall effect in graphene
- Strongly disordered superconductors and their applications to quantum circuits
- Topological Josephson junctions
Our fields of expertise cover state-of-the-art nanofabrication, quantum transport of mesoscopic devices, and low-temperature/high field STM spectroscopy.
Highlights
Evidence for chiral supercurrent in quantum Hall Josephson junctions
In this work we have tackled the complex challenge of associating two emblematic quantum phenomena of condensed matter: the quantum Hall effect and superconductivity. We demonstrated the existence of a chiral Josephson supercurrent with unprecedented properties flowing between two superconducting electrodes, connected by a graphene nanoribbon in the quantum Hall regime. This supercurrent is transported along the quantum Hall edge channels of the graphene, forming a chiral loop whose rotation direction depends on the sign of the magnetic field.
H. Vignaud, D. Perconte, W. Yang, B. Kousar, E. Wagner, F. Gay, K. Watanabe, T. Taniguchi, H. Courtois, Z. han, H. Sellier, B. Sacépé, Nature, Nov. 29th (2023)
Absence of edge reconstruction for quantum Hall edge channels in graphene devices
This study report on the first scanning tunneling spectroscopy of quantum Hall edge channels in graphene under a perpendicular magnetic field. Our work reveals that these edge channels are confined to a few magnetic lengths at crystal edges, confirming them as ideal 1D chiral channels without electrostatic reconstruction. The findings carry implications for electron and heat transport experiments in graphene and other 2D crystalline materials.
A. CoissardScience Advances 9, eadf7220(2023)
Imaging tunable quantum Hall broken-symmetry orders in graphene
When electrons populate a flat band their kinetic energy becomes negligible, forcing them to organize in exotic many-body states to minimize their Coulomb energy. The zeroth Landau level of graphene under a magnetic field is a particularly interesting strongly interacting flat band because interelectron interactions are predicted to induce a rich variety of broken-symmetry states with distinct topological and lattice-scale orders, which we evidence in this work.
A. Coissard, D. Wander, H. Vignaud, A. G. Grushin, C. Repellin, K. Watanabe, T. Taniguchi, F. Gay, C. B. Winkelmann, H. Courtois, H. Sellier, B. Sacépé, Nature 605, 51 (2022)
A tunable Fabry-Pérot quantum Hall interferometer in graphene
Electron interferometry with quantum Hall (QH) edge channels in semiconductor heterostructures can probe and harness the exchange statistics of anyonic excitations. In this work we show that high-mobility monolayer graphene constitutes an alternative material system, not affected by charging effects, for performing Fabry–Pérot QH interferometry in the integer QH regime.
C. Déprez, L. Veyrat, H. Vignaud, G. Nayak, K. Watanabe, T. Taniguchi, F. Gay, H. Sellier and B. Sacépé, Nature Nanotechnology 16, 555 (2021)
Review Article: Quantum breakdown of superconductivity in low-dimensional materials
This Review Article summarizes recent progress in understanding of the 
various pathways that lead to the destruction of superconductivity by the interplay of disorder, localization and interactions, including experiments in low dimensional materials and application in superconducting quantum devices.
B. Sacépé, M. Feigel’man, T. Klapwijk, Nature Physics 7, 734 (2020)
Helical quantum Hall phase in graphene on SrTiO3
We unveiled a new interaction-induced topological phase in the zeroth Landau level of graphene. This phase is insulating in the bulk and exhibits a pair of spin-filtered, helical edge channels. It emerges under moderate perpendicular magnetic field when graphene is placed in proximity to a high-k dielectric substrate. In this work we demonstrated a remarkably robust quantum spin Hall effect that withstands up to 110 K over micron-long distances which opens a new avenue for spintronics and topological superconductivity.
L. Veyrat, C. Deprez, A. Coissard, X. Li, F. Gay, K. Watanabe, T. Taniguchi, Vitto Han, B. Piot, H. Sellier, B. Sacépé, Science 367, 781 (2020)
See also CNRS press release here.
When Joule poisons Majorana in topological Josephson junctions
The search for experimental evidence of Majorana modes is an area of intense research in condensed matter and quantum physics and uncovering clear evidence is complicated. We investigate the impact of Joule heating which can influence the analysis of experimental features related to Majorana bound states in topological Josephson junctions.
K. Le Calvez, L. Veyrat, F. Gay, P. Plaindoux, C. Winkelmann, H. Courtois, B. Sacépé, Communications Physics, 2:4 (2019)
Low-temperature anomaly in disordered superconductors near Bc2 as a vortex-glass property
Pair-breaking quantum phase transition in superconducting nanowires
Transport measurements performed on MoGe superconducting nanowires reveal a quantitative agreement with quantum critical behaviour driven by a pair-breaking mechanism.
Hyunjeong Kim, Frédéric Gay, Adrian Del Maestro, Benjamin Sacépé, Andrey Rogachev. Nature Physics 14, 912-917 (2018)
Pinching off electron edge-channels in graphene
High mobility graphene van der Waals heterostructures equipped with split-gate electrodes enable to control the transmission of integer and fractional quantum Hall edge channels in a gate tunable fashion, demonstrating Quantum Point Contact operation in the quantum Hall regime of graphene.
Anna Jordan, Frédéric Gay, Kenji Watanabe, Takashi Taniguchi, Zheng Han, Vincent Bouchiat, Hermann Sellier, Benjamin Sacépé, Nature Communications 8:14983 (2017)