Layered quantum systems | Dr. Felix Lüpke

Assembly of van der Waals heterostructures

We assemble heterostructures from exfoliated flakes of layered materials using novel polymer-based fabrication techniques, allowing the manipulation of flakes with micron precision, while maintaining atomically clean surfaces and interfaces.

Van der Waals (vdW) materials have a layered crystal structure consisting of atomically thin sheets with strong chemical bonds inside the plane, but weakly coupled to neighboring layers. The resulting two-dimensional nature of the sheets often gives rise to unique properties, especially when a single layer is isolated, making vdW materials not only interesting for fundamental research, but also applications. Since the discovery of graphene, many other vdW materials with a wide range of properties have been synthesized − from insulating to superconducting and magnetic. Due to the weak interlayer coupling, we can isolate atomically thin layers of vdW materials and manipulate them in a controlled way. We continuously advance our heterostructure fabrication techniques and have recently developed a ‘dry-transfer flip method’ which allows a complete in-situ preparation of heterostructures from air-sensitive materials while maintaining atomically clean surfaces. Using this technique, we are able to combine materials with different properties to engineer heterostructure properties via proximity effects, e.g., to realize topological superconductivity¹.

Furthermore, we investigate how a controlled mismatch of the atomic lattices in two or more layers, e.g., via rotational misalignment, gives rise to moiré effects which can alter the electronic properties of the materials altogether^{2 3} (see also our work on twisted graphene interfaces).

Topological states

Topological edge and surface states have unique electronic properties which make them promising candidates for applications, e.g. in quantum computing. Using scanning probe techniques, we study the local properties of topological boundary states in layered materials.

Topological materials have in common an inverted band structure that results in topologically protected boundary states at interfaces bordering to a topologically trivial material, including vacuum. The material class of Bi₂Te₃ is a prototypical three-dimensional topological insulator, with two-dimensional Dirac cone-like surface states, which we have studied extensively in the past^{4 5 6}.

When induced with magnetism, in form of MnBi₂Te₄, we are further able to study the interplay of magnetism and topology^{7 8 9}. By incorporating layered topological insulators into heterostructures, we are able to engineer topological superconductors¹⁰, which can host so-called Majorana states and are proposed to have applications in topological quantum computing.

Proximity effects

By exploiting proximity effects between materials with different properties, van der Waals heterostructures allow the systematic engineering of quantum states. In our research, we study the local strength of proximity effects as a function of, e.g., material thickness, temperature, and magnetic field in a scanning probe setting.

The possibility to isolate individual materials into thin two-dimensional sheets and combine them with other layered materials with vastly different electronic properties in a Lego-like fashion allows the realization of physical properties which are otherwise unattainable. The physics that allows the engineering of properties in heterostructures is based on proximity effects which stem from an overlap of the wave functions between neighboring layers. At the same time, the properties of the individual layers remain mostly intact due to the relative weak coupling between the vdW layers. We identify novel materials¹¹ to include into heterostructures and characterize the resulting proximity effects as function of parameters such as material thicknesses, interlayer distance, temperature and magnetic field^{12 13}.

Our work on the superconducting proximity effects as a means to realize novel topological superconducting phases is supported by the DFG Priority Programme 2244.

Moiré lattices

A moiré lattice is a periodic interference pattern that occurs when two or more regular patterns overlap, creating a new pattern with a larger unit cell. This phenomenon is commonly observed when two lattice structures, such as layers of two dimensional crystals, with different lattice periodicities or a rotational misalignment, are combined. Moiré superlattices result in new electronic properties that can be manipulated and tuned, making them a versatile platform for the enginnering of quantum states.

We study the realization of flat electronic bands in graphene and transition metal dichalcogenide heterostructures^{14 15} as a path towards novel electronic states and superconducting phases.

Our work on tuning superconductivity in moiré lattices is supported by the DFG through the Emmy Noether Programme.

Functionalized scanning probes

In our labs, we implement and develop novel types of scanning probe sensors, such as scanning quantum dot sensors and nanowire based probe sensors. Our goal is to the extend the capabilities of our scanning probe microscopes to provide information about local electric fields, spin sensitivity and Josephson tunneling, to achieve a better understanding of our studied samples systems.

Our work on the developement of nanowire probe sensors is supported by the DFG through the Emmy Noether Programme.