DQMP Forum - Charge-transfer insulation and superconductivity in twisted bilayer graphene & Microfocus laser-ARPES on encapsulated mono-, bi- and few-layer 1T'-WTe2

11.12.2018 13:00 – 14:00

Charge-transfer insulation and superconductivity in twisted bilayer graphene
Louk Rademaker (group of Prof. Abanin)

Last March an unexpected discovery was made in so-called 'twisted' bilayer graphene. This system, which
consists of two misaligned graphene layers with a small relative twist angle, displays both Mott insulatorlike
behavior and superconductivity up to 1.7 Kelvin. In this talk I will present my thoughts on the relevant
mechanism that can explain these experiments. For that, I will show that the real-space structure of the
orbitals in the Moiré patterns form 'ring' and 'center' structures, and a charge-transfer from center to ring
orbitals lies at the root of the observed correlation effects.


Microfocus laser-ARPES on encapsulated mono-, bi- and few-layer 1T'-WTe2
Irène Cucchi (group of Prof. Baumberger)

In the latest years, remarkable physics has shown to arise in WTe2 when exfoliated down to its ultrathin
2D forms. In 2017, the quantum Spin Hall state with protected edge states has been discovered in the
monolayer. Later, in 2018, it has been found that this topological material becomes superconducting when
electrostatically doped. With the unexpected discovery of ferroelectricity in the bi- and tri-layer, the
interest in this crystal expanded even further. Investigating the band structure of such systems is
paramount to rationalize all these phenomena. Therefore, we probed mono-, bi- and few-layer WTe2 with
angle-resolved photoelectron spectroscopy (ARPES). This proved challenging because of technical
difficulties: the high-quality ultrathin crystals produced by micromechanical exfoliation are small in
lateral size (a few micrometers) and WTe2 degrades when exposed to air (it cannot be cleaned by simple
annealing in vacuum). Here, I will present our advances to overcome such hindering. First, the micro-
ARPES setup in our lab has a high spatial resolution without paying the price of a lowered energy
resolution. Second, encapsulating the WTe2 flake with monolayer graphene proved effective in protecting
it while still permitting ARPES experiments on the covered sample. This study paves the way for
electronic structure measurements on previously inaccessible ultrathin 2D materials.

Forum Committee: C. Lichtensteiger, N. Ubrig, A. Tamai (6.12.2018)

Lieu

Bâtiment: Ecole de Physique

Auditoire Stückelberg

entrée libre