Direct evidence for flat bands in twisted bilayer graphene from nano-ARPES

Transport experiments in twisted bilayer graphene revealed multiple superconducting domes separated by correlated insulating states. These properties are generally associated with strongly correlated states in a flat mini-band of the hexagonal moir\'e superlattice as it was predicted by band structure calculations. Evidence for such a flat band comes from local tunneling spectroscopy and electronic compressibility measurements, reporting two or more sharp peaks in the density of states that may be associated with closely spaced van Hove singularities. Direct momentum resolved measurements proved difficult though. Here, we combine different imaging techniques and angle resolved photoemission with simultaneous real and momentum space resolution (nano-ARPES) to directly map the band dispersion in twisted bilayer graphene devices near charge neutrality. Our experiments reveal large areas with homogeneous twist angle that support a flat band with spectral weight that is highly localized in momentum space. The flat band is separated from the dispersive Dirac bands which show multiple moir\'e hybridization gaps. These data establish the salient features of the twisted bilayer graphene band structure.Comment: Submitted to Nature Materials. Nat. Phys. (2020

On the nature of ionic liquid gating of Nd₂CuO₄ thin films

Recently, ionic liquid gating has been used to modulate the charge carrier properties of metal oxides. The mechanism behind it, however, is still a matter of debate. In this paper, we report experiments on doped and undoped Nd₂CuO₄. We find major resistance drops of the bilayer coupled to observations of the presence of a considerable Faradeic component in the gate current and of the appearance of charge transfer peaks in the cyclic voltammetry data. This leads us to propose a mechanism of gating based on irreversible electrochemical reactions, likely due to trace amounts of contaminations present in the ionic liquid. This work is therefore in line with previous reports confirming the presence of irreversible electrochemistry in ionic liquid gated electron- doped cuprates

On the Formation of a Conducting Surface Channel by Ionic-Liquid Gating of an Insulator

Ionic-liquid gating has become a popular tool for tuning the charge carrier densities of complex oxides. Among these, the band insulator SrTiO3 is one of the most extensively studied materials. While experiments have succeeded in inducing (super)conductivity, the process by which ionic-liquid gating turns this insulator into a conductor is still under scrutiny. Recent experiments have suggested an electrochemical rather than electrostatic origin of the induced charge carriers. Here, experiments probing the time evolution of conduction of SrTiO3 near the glass transition temperature of the ionic liquid are reported. By cooling down to temperatures near the glass transition of the ionic liquid, the process develops slowly and can be seen to evolve in time. The experiments reveal a process characterized by waiting times that can be as long as several minutes preceding a sudden appearance of conduction. For the conditions applied in our experiments, a consistent interpretation in terms of an electrostatic mechanism for the formation of a conducting path at the surface of SrTiO3 is found. The mechanism by which the conducting surface channel develops relies on a nearly homogeneous lowering of the surface potential until the conduction band edge of SrTiO3 reaches the Fermi level of the electrodes

TANGO Project Mini Symposium 2020 - Presentations

Presentation of all work packages at the TANGO project mini symposium in 2020, includes milestone achievements and preliminary/advanced results adjusted for public consumption