Exfoliated van der Waals crystals can be stacked to form a large variety of heterostructures displaying intriguing new electronic properties. One such system is magic angle twisted bilayer graphene, an assembly of two graphene sheets with a small misalignment of 1.1° between them. Using synchrotron-based nano-ARPES, we recently achieved the first direct measurements of the flat moiré bands in this system, which are at the heart of the correlated phenomena (superconductivity, insulating states) observed in transport (Nature Physics (2020)).
To study air-sensitive 2D materials we developed a technique for encapsulating exfoliated 2D materials withgraphene flakes. This allowed us to perform first electronic structure measurements of the quantum spin Hall state in exfoliated monolayer Td-WTe2 using our home-built µ-ARPES system (Nano Letters 19, 554 (2019)). Building on the success of this experiment, we are presently employing the same method to investigate other topical 2D materials, such as the direct gap semiconductor black phosphorus (BP) and the topological semimetal Td-MoTe2.
Correlated electron systems
Ruthenium based 4d transition metal oxides show a wide range of non-trivial phases including low-dimensional Fermi liquids, spin-triplet superconductivity, Mott insulating and orbitally ordered states. The extremely high purity of ruthenate single crystals opens a unique window to the correlated electron problem: It allows one to study an interacting fluid in a nearly perfect low-dimensional lattice, free of the complications arising from doping induced disorder. Recently we have demonstrated for the first time the enhancement of spin-orbit interaction by electron correlations in the unconventional superconductor Sr2RuO4 (Phys. Rev. X 9 021048 (2019)). In addition our study has provided one the most stringent tests to date of the fundamental locality assumption of dynamical mean field theory.
Oxide thin films and oxide surfaces
The creation and control of novel electronic phases at the interfaces or surfaces of transition metal oxides lies at the heart of the emergent field of oxide electronics. We grow oxide thin films in-situ using sputter deposition (in Geneva) and PLD and MBE (at SLS/PSI) to study their electronic structure by ARPES without any exposure to air. Current interests include the electronic structure of SrRuO3 and SrMoO3 and in particular the evolution of their correlated metallic states as the film thickness is reduced down to a single unit cell. We also study 2D electron gases created by the chemical modification of the bare surfaces of the insulating oxides SrTiO3, KTaO3 and TiO2, a field co-founded by us [Nat. Mat. 10, 114 (2011)].