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• Single-atom circuits
• Topological materials and electronics
• Excitonic many-body states including Bose-Einstein condensates
• Femtosecond on-chip electronics
• Nanofabrication

Single-atom circuits

We recently demonstrated how to 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. Until now, this goal was reserved only to materials and methods in the ultra-high vacuum. 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.
Present collaboration partners: Jonathan Finley (TUM), Frank Jahnke (Bremen), Kai Müller (TUM), Alex Weber-Bargioni (Berkeley).

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Topological materials and electronics

Topological surface states are very promising for future opto/electronic circuits, since computing schemes can be envisaged where the information carrying surface state is protected by topology. Van der Waals materials and their heterostructure are an ideal platform to engineer and explore topological states. On the one hand, we can control and break the relevant symmetries of the Hamiltonian at will by interfacing different van der Waals materials with different symmetries. On the other hand, we can directly address the symmetry of the electron-Bloch states in the van der Waals crystal by external electric field 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. Monolayers of Weyl semi-metals, such as WTe₂, also have a superconducting phase far from charge neutrality. As a result, exotic phenomena such as Higgs and Majorana modes seem to be possible in the near future in such topological atomistic materials and field-effect heterostructures.
Present collaboration partners: Marko Burghard (MPI Stuttgart), Yongqing Li (IOP Bejing).

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Excitonic Many-Body States including Bose-Einstein Condensates

Increasing the interaction strength between quasi-particles in solid-state materials can cause strong correlations, collective phenomena and the transition to large-scale quantum phases, for example to a Bose-Einstein condensate. Heterostructures made of semiconducting 2D materials, such as MoSe₂ and WSe₂, are ideal systems to realize a condensation of excitons. The latter are Coulomb-bound electron hole pairs. The heterostructures enable large exciton ­binding energies,­ long photoluminescence lifetimes, as well as a permanent exciton dipole, which allows the manipulation of the exciton ensembles, e.g. via electric fields. In one of our current studies, we observe several signatures regarding photoluminescence intensity, linewidth, temporal coherence and excitonic propagation ­dynamics in accordance with the predicted condensation temperature of an excitonic Bose-Einstein condensation at about 10 Kelvin. This allows us to investigate quantum mechanical many-body physics in nano-structured­ circuits. In particular, the Berezinskii-Kosterlitz-Thouless transition to a superfluidic excitonic phase at very low temperatures (<1 K) and also the Mott transition to a degenerate electron-hole-fermi gas at high excitonic densities appear experimentally accessible. With our advanced nanolithography, e.g. using HIM, we design such coherent multi-particle circuits and investigate them for nanophotonic applications.
Present collaboration partners: Ursula Wurstbauer (University of Münster), Andreas Knorr (TU Berlin).

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Towards femtosecond on-chip electronics

The vision of this research topic is to generate electric current pulses with a duration of only a few femtoseconds for an on-chip signal conversion at the interface between electronics and optics, the so-called THz-gap. For signal generation we use amongst others, photo-emission processes, such as multiphoton absorption and strong field tunnel processes in plasmonic nanocontacts; but also femtosecond processes within solid state materials. The signal propagation occurs via electromagnetic THz modes in coplanar strip conductors. For the on-chip THz-detection, we are working on an electrical detection on the 100 fs scale and faster. Currently we reach 350 fs by using the relaxation dynamics in amorphous silicon photo-switches. Our group manufactures all circuits in our own laboratory, and we use phase-stable femtosecond lasers to drive the on-chip THz circuits coherently. Intriguingly, atomically thin 2D materials can be integrated into the THz circuits without much effort, such that the 2D materials can act as functional THz modulators. In the same way, the electron and heat dynamics in the 2D materials can be investigated on a femto- to picosecond timescale.
Present collaboration partner: Reinhard Kienberger (TUM).

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Nanofabrication

We are heading the Center for Nanotechnologies and Nanomaterials (ZNN), which is a shared nanofabrication facility of the Walter Schottky Institute of TUM. Students, researchers, and scholars from the greater scientific Munich area have access to state-of-the-art nanolithography and nanoanalytic instruments for building nanoscale electronic, optoelectronic, and photonic circuits. The methodologies include electron beam-, focused-ion-beam-, and helium-ion-beam lithography.

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Open positions

We are always looking for highly motivated students. Interested applicants should contact Prof. Holleitner for an informal discussion on the opportunities available.

Acknowledgement

For all above research directions, we thank the European Research Council (ERC), the International Graduate School for Science and Engineering of TUM (IGSSE), the Bavaria California Technology Center (BaCaTec) and the Deutsche Forschungsgemeinschaft (DFG) for funding in several projects. In particular, we are grateful for the support of the excellence clusters Munich Center for Quantum Science & Technology (MCQST) and e-conversion.

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