Twisted bilayer graphene dances with light


Artistic visualization of interband collective excitations in twisted bilayer graphene. Credit: ICFO / Matteo Ceccanti

An international team of researchers reports in Physics of nature on how light and electrons move together in material when illuminated by infrared light.

When two coats of graphene are placed on top of each other and twisted between them at a very small angle, a “moiré pattern” is formed and the physical properties of the system are found to change drastically. In particular, near the “magic” angle of 1 degree, electrons slow down considerably, promoting interactions between electrons. Such interactions give rise to a new type of superconductivity and insulating phases in twisted bilayer graphene.

Along with many other fascinating properties discovered over the past three years, this material has proven to exhibit extremely rich physical phenomena, but most importantly, it has proven to be an easily controllable quantum material. Now, even though this carbon-based material exhibited these incredibly diverse states, the interaction between twisted bilayer graphene and light has been shown to have theoretically fascinating results, but no experiment has so far. now been able to clearly show how this interaction works.

In a recent work published in Physics of nature, ICFO researchers Niels Hesp, Iacopo Torre, David Barcons-Ruiz and Hanan Herzig Sheinfux, entrusted by Professor ICREA to ICFO Frank Koppens, in collaboration with the research groups of Professor Pablo Jarillo-Herrero (MIT), Prof. Marco Polini (University of Pisa), Prof. Efthimios Kaxiras (Harvard), Prof. Dmitri Efetov (ICFO) and NIMS (Japan), have discovered that twisted bilayer graphene can be used to guide and control light to the nanometer to climb. This is possible thanks to the interaction between light and the collective movement of electrons in the material.

By exploiting the properties of plasmons, in which electrons and light move together as a single coherent wave, scientists were able to observe that the plasmons propagate in the material, while being highly confined in the material, up to l nanoscale. Moreover, by observing the unusual collective optical phenomena occurring in the material, they were able to understand the type of particular properties of electrons. This observation of the propagation of light, confined to the nanoscale, can be used as a platform for the optical detection of gases and biomolecules.

To obtain the results of this discovery, the team used a near-field microscope, which probes optical properties with a spatial resolution of 20 nanometers, a resolution that exceeds the diffraction limit. In short, the scientists took two layers of graphene, layered them on top of each other, twisting them near the magic angle, and then, at room temperature, illuminated the material with infrared light on a nano-sized spot. They saw that the plasmons behave very differently from the usual plasmons, for example in metals or graphene, and this deviation is related to the particular movement of electrons within the moiré superlattice of bilayer graphene.

This work lays the foundation for nano-optical studies on the exotic phases of twisted bilayer graphene at low temperature. In particular, he demonstrates that twisted bilayer graphene is a remarkable nanophotonic material, especially since it serves as an intrinsic host (no external voltage is required) of collective excitations.

Reference: “Observation of interband collective excitations in twisted bilayer graphene” by Niels CH Hesp, Iacopo Torre, Daniel Rodan-Legrain, Pietro Novelli, Yuan Cao, Stephen Carr, Shiang Fang, Petr Stepanov, David Barcons-Ruiz, Hanan Herzig Sheinfux, Kenji Watanabe, Takashi Taniguchi, Dmitri K. Efetov, Efthimios Kaxiras, Pablo Jarillo-Herrero, Marco Polini and Frank HL Koppens, September 27, 2021, Physics of nature.
DOI: 10.1038 / s41567-021-01327-8

This research was partially supported by the European Research Council, the European Graphene Flagship, the Government of Catalonia, the Fundació Cellex and the Severo Ochoa Excellence Program of the Spanish government.

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