May 2020
Abstracts of the QSIT Lunch Seminar, Thursday, May 7, 2020
Scheduled Zoom meeting - NEW TIME: starting @ 14:00
The upper branch broadening in ultrastrongly coupled THz Landau polaritons
Shima Rajabali – Quantum Optoelectronics Group (Faist group), ETH Zurich
In this work, we present a study on the coupling between the Landau level transitions in a two dimensional electron gas and a resonant mode of our terahertz metasurfaces. A higher coupling rate is always demanded for a higher performance and study the exotic physics in higher coupled systems such as virtual photons in the ground state of these system. To increase the coupling rate, the capacitive gap of the resonator is reduced systematically to the enhance vacuum field fluctuations in a smaller volume mode. However, a broadening in the upper polariton mode is observed when we reduce the gap size further. One possible explanation for such broadening is the coupling of the polaritonic mode to the surface plasmon polariton mode at the interface of the two-dimensional electron gas and the substrate due to a large wave vector introduced to our system by the narrow gap size which can be retrieved by confining the electrons to small areas to prevent the surface plasmon polariton excitation and propagation.
Moiré band engineering and correlations in graphene-based materials
Tobias Wolf – Quantum Condensed and Coherent Systems (Blatter group), ETH Zurich
Twisted honeycomb lattices provide a versatile platform to engineer metamaterials with novel emergent properties. In these structures, the resulting geometric moiré superlattice rearranges the electronic spectrum into minibands in each original valley. Paradigmatic realizations are graphene on hexa-boron nitride (G/hBN) and twisted bilayer graphene (TBG). The latter has attracted much attention because of flat minibands at fine-tuned twist angles, such that interactions introduce strong correlations (magnetism, superconductivity, etc). In this talk, we will first identify a flat-band regime in which interactions lead to Stoner magnetism that can be controlled electrically through interlayer bias. We will then discuss a different regime in which proximity to ferromagnetic insulators (e.g. CrI3) can induce flat minibands with valley interactions, leading to spontaneous valley spiral ordering.