Research Interests

My research is uses quantum transport and polarisation-resolved photocurrent measurements to probe effects of strong electronic interactions and topology in moiré quantum materials. Graphene is a material of choice in this field; combinations with other two-dimensional materials or graphene layers enables a plethora of electronic properties.

General Reading:

Moiré quantum materials

The discovery of two-dimensional (2D) materials has brought together the possibility to stack together crystalline layers of different parent crystals. Unlike traditional growth techniques, material selection is free from constraints imposed by lattice mismatch, bringing together a virtually limitless field for material creation. This has brought together new electronic and photonic phases with tuneable spin-orbit coupling, tuneable bandgaps and exciton amplitudes, or energy shifts and scattering rates. More recently, the development of rotational stages has enabled to precisely control the twist angle between two or more layers. In that case, the twist angle controls the size of the band-folded Brillouin zone and possible inter-layer hybridisation, therefore enabling control of interaction strength in a single material. So far, all known electronic phases (superconductors, ferromagnets, Wigner crystals, stripe phases, multiferroic orders, bosonic exciton crystals, fractional Chern insulators, etc.) have been discovered in Moiré materials, making them a material of choice for condensed matter physics.

Notably, I have introduced a new family of quasiparticles emerging in moiré materials: the Brown-Zak fermions, occurring at rational fractions of the magnetic flux per moiré unit cell. I have also used superlattice reconstruction in marginally-twisted bilayer graphene to create a new one-dimensional system, unique in its ability to support proximity superconductivity. In addition, it is possible to use mid-infrared spectroscopy to resolve moiré minibands.

Recent articles on the topic:
  1. Infrared Spectroscopy for Diagnosing Superlattice Minibands In Magic-Angle Twisted Bilayer Graphene, Nano Lett. (2024). [arXiv:2404.05716] [DOI]
  2. One-Dimensional Proximity Superconductivity in the Quantum Hall Regime, Nature 628, 741 (2024). [arXiv:2402.14451] [DOI] [PDF]
  3. Long-Range Ballistic Transport of Brown-Zak Fermions in Graphene Superlattices, Nature Communications 11, 5756 (2020). [arXiv:2006.15040] [DOI] [PDF] [SI]

Strong electron interactions

The simplified picture of isolated electrons, often presented in textbooks, breaks down in the presence of interactions between electrons, giving rise to rich and complex behaviours. Specifically, collective dynamics can emerge in quantum materials, and are notoriously difficult to predict theoretically and understand experimentally. To address these challenges, I have developed novel techniques to tune down the magnitude of Coulomb interactions in a system, based on van der Waals heterostructures. The possibility to induce screening at sub-nanometer distances provides a powerful new tuning knob to explore many-body physics. We are now able to measure the effects of electron-electron interactions while varying their strength.

For example, we have identified new correlated phases in materials such as rhombohedral graphite, which exhibits phase transitions and hysteresis reminiscent of multiferroic systems and heavy metal compounds, or in twisted monolayer-bilayer graphene, where electrostatically tuneable van Hove singularities and Mott insulating states emerge at integer filling of the moiré minibands. In typical moiré superconductors, screening allowed us to understand the electronic origin of unconventional superconductivity. Remarkably, even in seemingly simple systems, like monolayer graphene (which ultra-relativistic spectrum was known since 2004), new phenomena such as an electron-hole plasma have been uncovered at low carier densities, where screening becomes ineffective.

Recent articles on the topic:
  1. Coulomb Screening of Superconductivity in Magic-Angle Twisted Bilayer Graphene, (2024). [arXiv:2412.01577] [DOI]
  2. Giant Magnetoresistance of Dirac Plasma in High-Mobility Graphene, Nature 616, 270 (2023). [arXiv:2302.06863] [DOI]
  3. Electronic Phase Separation in Multilayer Rhombohedral Graphite, Nature 584, 210 (2020). [arXiv:1911.04565] [DOI]

Topology

In crystalline materials, Bloch bands characterise the behaviour of the Bloch electronic states’s evolution as a function of momentum. Recently, the quantum geometry has emerged as a new parameter, linking quantum geometrical quantities such as the quantum metric and the Berry curvature, to observable quantities. The quantum metric describes the distance between two Bloch states in momentum state, while the Berry curvature tracks the geometric phase acquired by an electron as it evolves in a small loop in momentum space. These quantum geometrical properties are responsible for the optoelectronic properties of a crystal. Because of large unit cells of moiré quantum materials, their effect is enhanced. It is for example possible to probe large injection or shift photocurrents, where symmetry rules forbid one or another. Another possibility is to probe the bulk photogalvanic effect, which consists in the generation of rectified DC current from an oscillating electric field. Such photocurrents are determined by the geometrical properties of the wave function.

In the THz range, we have notably discovered that twisted bilayer graphene hosts inversion symmetry-breaking states, untdetectable through usual transport measurements, but with sharp changes in the polarisation axes caused by interactions.

Recent articles on the topic:
  1. Terahertz Photocurrent Probe of Quantum Geometry and Interactions In Magic-Angle Twisted Bilayer Graphene, Nature Materials 1034 (2025). [arXiv:2406.16532] [DOI]