University of Bremen: Graph-like materials: University involved in SPP

Graphene now belongs to a large family of two-dimensional (2d) materials which, due to their versatile properties, have led to remarkable research activity worldwide. These include the priority program SPP2244 of the German Research Foundation with the title “2D materials – the physics of van der Waals [hetero] structures (2DMP)”, which is coordinated by the TU Dresden. “The program connects many of the leading university research groups in Germany and has a total funding volume of around 7 million euros,” says Dr. Christopher Gies from the Institute for Theoretical Physics at the University of Bremen. “The University of Bremen is represented in SPP2244 with three projects and a funding volume of around 650,000 euros.

According to Gies, the fact that the University of Bremen is so sustainably present in the ambitious research program is also due to the high level of expertise that has been developed over the past few years as part of the Bremen graduate school “Quantum Mechanical Materials Modeling – QM 3 “. For four and a half years, the graduate school has been working intensively on the development of models for the computer-aided simulation of novel quantum materials.

Focus on heterostructures of 2d materials
In the DFG priority program, the focus is on heterostructures of 2d materials. Due to the weak bond between the individual layers in the crystal, atomically thin layers can be easily detached and stacked on top of one another in a variety of combinations. This modular system enables material design on the quantum level: Researchers can set and manipulate electronic and optical properties and thus create materials for new types of applications and basic research.

Applications in optics and research into strongly correlated electron states are of great interest to the Bremen researchers. In the first case, it is about materials that can emit light in a very specific color, but are extremely sensitive to their direct environment (for example the presence of certain substances in the air) and react to tension. “These properties can be used, for example, for LEDs and microlasers that are integrated on chips, or for innovative optical sensors,” explains the physicist Gies. Strong correlations between electrons play an important role in research into superconductivity, a special form of electrical conductivity in which electricity can be transported without any loss.

What are the Bremen projects about?
In the project of Dr. Christopher Gies focuses on the interaction of light with electrons in heterostructures of semiconducting 2d materials. If you stack individual atomic layers of so-called transition metal dichalcogenides (these are, for example, materials such as MoSe 2 or WSe 2 ), a so-called type II heterostructure is created in which electrons excited by light are separated and distributed in the two layers. Due to the strong interaction, the charge carriers are still bound. They thus form a unit that researchers refer to as interlayer excitons (IX).

Because the components of the IX are spatially separated from one another, they are particularly long-lived and enable research into correlation effects such as Bose-Einstein condensation and super radiancy. Embedding the heterostructure in optical cavities should further facilitate the creation of these special forms of matter. Optical cavities are known from lasers, for example. They serve to contain light and are supposed to intensify the interaction of the excitons with light to such an extent that new quasiparticles are created – i.e. particles that combine the properties of both species, the IX and light quanta.

Complex quantum phenomena in nanomaterials
Dr. Alexander Steinhoff focuses on heterostructures made from semiconducting transition metal dichalcogenides. Due to the extremely strong particle-particle interaction and the controllable formation of special spatial structures (so-called moiré grids), these offer a unique platform for studies of complex quantum phenomena in nanomaterials. The aim of the research project is to use these remarkable properties to investigate gases from quantum mechanical particles. The focus will be on previously unexplored many-particle states, which are based on the conventional image of so-called interlayer excitons (bound states of positive and negative particles,

Understand and control the interplay between interactions
The project carried out by Professor Tim Wehling investigates how the collective quantum mechanical behavior of electrons in 2d materials consisting of moiré lattices can be controlled. In particular the family of semiconducting TMDCs – such as MoS 2 , WS 2 , MoS 2 , WSe 2- forms a promising material platform here. In corresponding [hetero] structures, a complex and novel interplay of electron-electron and electron-phonon interactions with minibands, strong spin-orbit coupling and multivalley effects is expected. Depending on the material combination, the twist angle and the doping, strong electron correlations should be realizable, which could drive these systems into Mott-isolating, superconducting, magnetic, excitonic or other (quasi) ordered states with interwoven degrees of spin, valley and band degrees of freedom. “Our goal is to understand the interaction of the above-mentioned degrees of freedom and interactions,” says Tim Wehling.

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