The chromium chalcogen halides (CrXH) are an emerging family of 2D semiconductors with a direct bandgap, highly anisotropic structural and electronic properties, and robust magnetic order with ordering temperatures up to room temperature. We seek to understand and engineer interactions between the magnetic order in CrXH compounds and the excitons they host. The unique magnetic and optical properties of CrXH materials present unique opportunities to develop spintronic devices with magnetic and optical controllability.

Superlattices are artificial, periodic potentials which are used to confined particles and quasiparticles such as atoms, electrons, or excitons. If the trapped quasiparticles are allowed to interact, the resulting many-body state can capture essential behaviors of canonical quantum many-body theories (e.g. the Hubbard model). Such artificial quasiparticle lattices are often called analogue quantum simulators. A bit like a quantum wind tunnel, the quantum simulator serves as an engineerable scale model of a real physical system (electrons or other particles in a real crystalline solid) which allows us to probe complicated many-body physics with a high level of control. Such many-body physics are often hard to describe or predict theoretically, so our quantum simulators provide a route to benchmark leading theories and discover new phases of matter beyond current theoretical predictions.

We generate optically active defects in atomically thin 2D materials on a spatial scale of below 10 nm, such that scalable opto/electronics based on individual atomic states seems at reach. We use a helium-ion microscope (HIM) to generate individual defects, e.g. in semiconducting MoS₂. Our single-defect technology is principally applicable to the wide range of 2D materials with more than a thousand different materials and is compatible with standard cleanroom manufacturing steps. This combination makes it possible to realize first devices based on single defects, such as gate-switchable single photon emitters and photodetectors or even solar cells based on individual atomic defects. At the same time, fundamental processes, such as the coherent single electron tunneling dynamics and many-body interactions of localized states in a Fermion boson mixture are experimentally accessible.

Collaborators: Jonathan Finley (TUM), Kai Müller (TUM), Nicolas Leitherer-Stenger (DTU), Sivan Refaely-Abramson (Weizmann), Alex Weber-Bargioni (Berkeley)

Increasing the interaction strength between quasi-particles in solid-state materials can cause strong correlations, collective phenomena and the transition to macroscopic quantum phases, such as a Bose-Einstein condensate. Heterostructures made of semiconducting 2D materials, such as MoSe₂ and WSe₂, are ideal systems to realize interacting exciton ensembles, because they provide large exciton binding energies, long photoluminescence lifetimes, as well as a permanent exciton dipole. The latter allows the manipulation of the exciton ensembles, e.g. via electric fields. Recently, we have reported on several signatures regarding photoluminescence intensity, linewidth, as well as spatial and temporal coherences in accordance with the predicted degeneracy of an exciton ensemble at low temperature. The ongoing studies want to explore further predicted phases, such as an excitonic superfluidity and possible condensation phenomena in momentum space.

Collaborators: Ursula Wurstbauer (University of Münster), Andreas Knorr (TU Berlin).

While 2D semiconductors offer unique electronic and optical properties, realization of their full potential requires scalable growth and controlled integration into functional assemblies. In our research, we aim to create defined 2D/3D heterostructures using chemical vapor deposition (CVD) of transition metal dichalcogenides (TMDs) onto engineered substrates. These heterostructures include thin film junctions and laterally patterned substrates that enable precise spatial control of the 2D material and its electronic environment. In addition, we take advantage of low-temperature atomic layer deposition (ALD) processes to precisely tune interaction lengths and strengths between TMDs and their surroundings. A major emphasis of this work on 2D semiconductor assemblies is devoted to elucidating interfacial mechanisms that govern charge transfer pathways, energetic alignment, and structural and chemical interactions, providing a basis for future applications spanning from advanced optoelectronics to photocatalytic energy conversion.