Walter Schottky Institute
Center for Nanotechnology and Nanomaterials

Topological Electronics and Materials - Research
Group leader: Dr. Christoph Kastl


Control of symmetry and topology in van der Waals heterostructures

Van der Waals materials and their heterostructure are an ideal platform to engineer and explore topological states. We can control and break the relevant symmetries of the Hamiltonian at will by interfacing different van der Waals materials with different symmetries. Furthermore, we can directly address the symmetry of the electron-Bloch states in the van der Waals crystal by external electric fields in atomic field effect structures. In turn, a wide range of non-trivial, i.e. topological quantum phenomena can be explored, such as the orbital Edelstein effect, Berry plasmons or even the interplay of plasmons and fringe states in quantum spin Hall-systems. Currently, we explore spin-orbit and magnetic proximity interactions in graphene-based vdW heterostructures of 2D materials on a fundamental level. A major focus are the opto-spintronic and magneto-electronic properties of the heterostructures and their control via charge tuning, layer number, quality of the material interfaces, as well as the crystallographic alignment of the monolayers.

We use a set of complementary experimental techniques to interrogate the interfacial spin texture, including (ultrafast) photocurrent and Kerr microscopy as well as magnetotransport spectroscopy. For heterostructure device fabrication we employ stacking methods in controlled, inert atmosphere and standard lithography techniques. The goal of this project is to achieve a full electronic control of the band topolgy in these heterostructures.

Berry-curvature optoelectronics in topological materials

In so called Weyl semimetals, the energy bands have the topology of massless chiral fermions and they carry a quantized monopole of Berry curvature. The Berry-curvature effectively acts as a magnetic field in momentum space, and it introduces a quantum mechanical correction to the charge carrier velocity, which gives rise to new (opto)electronic phenomena, such as the non-linear anomalous Hall effect, photocurrent generation in topological metals, as well as topological Kerr effects. In our group, we investigate the dynamics of electronic systems with large Berry curvature by ultrafast optoelectronic methods. Since Berry curvature driven effects are enhanced near small or vanishing energy gaps, our goal is to develop optoelectronic THz spectroscopy to resonantly probe the Berry curvature dynamics. Currently, we focus on van der Waals Weyl semimetals, such as WTe2 or MoTe2, wich can be integrated in micro- and nanoscale optoelectronic Thz circuits.

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