Walter Schottky Institute
Center for Nanotechnology and Nanomaterials

Welcome to the Holleitner group
Group leader: Prof. Dr. Alexander Holleitner


[Holleitner 1]
A typical day in our researchers' life

We investigate novel types of optoelectronic systems that consist of atomically thin semiconductors, heterostacks, to topological quantum materials. We focus on the following main research themes.



Ultrafast electronics
We apply a unique on-chip THz-time domain spectroscopy to nanoscale materials for measuring electronic currents with a time-resolution as fast as ~500fs. We can access non-equilibrium currents from femtosecond photoemission processes, the ballistic electron transport up to thermoelectric processes in corresponding nano-circuits. The scheme goes back to David Auston in the 1980s, which has been very successfully applied to THz-spectroscopy e.g. of semiconductors. Since about 10 years, we have been refining it to measure the non-equilibrium charge and thermal currents in several nanoscale materials, such as individual carbon nanotubes, graphene, nano-diamonds with NVcenters, 2D materials, semiconductor nanowires, topological insulators and nanoscale tunnel junctions.
Present collaboration partners: Reinhard Kienberger (TUM) and Martin Wöhrle (TUM).




Nanoscale optoelectronics
Nanoscale materials form a unique platform to study optoelectronic processes in geometries smaller or equivalent to intrinsic physical lengthscales. This allows to explore the physical fundaments of optoelectronic dynamics but also to enhance the efficiency of photovoltaic and thermoelectric processes. Recently, we revisited the photocurrent dynamics in the prototypical 2D material MoS2, where we found that drastic changes of the band gap and exciton binding energies and ultrafast non-radiative relaxation processes after a photo-excitation dominate the optoelectronic response in such 2D materials. Moreover, a laser-annealing process, an effect naturally occurring in standard photocurrent experiments, enhances the contact morphologies.
Present collaboration partners: Jon Finley, Michael Kaniber, Franz Kreupl, Ursula Wurstbauer (all TUM).




Topological materials
Topological materials have intriguing electron and spin properties that stem from the topology of the underlying electron states. We are particularly fascinated by the non-equilibrium spin and charge transport in such materials. Our approach exploits a local optical excitation scheme e.g. with helical light and an electronic read-out. Recently, we explored ultrafast helical photo-galvanic surface currents and discovered a spin-Hall photoconductance at the facets of 3D topological insulators at room temperature.
Present collaboration partners: Marko Burghard & Klaus Kern (both MPI Stuttgart), Sergey Ganichev (U. Regensburg), Yongqing Li (IOP, Bejing)

[Holleitner 4]

Key papers:
Kastl et al., Nature Comm. 6, 6617 (2015)
Seifert et al., Nature Comm. 9, 331 (2018)



Two-dimensional materials are atomistically thin, but still crystalline materials with very good electronic and optoelectronic properties. We follow two main directions in this field. We aim at fabricating nanoscopic circuits by a helium-ion microscope lithography with functionalities down to the single defect level, which are attractive as quantum emitters but also as catalytic active sites. Moreover, we stack different 2D materials, such as MoSe2 and WSe2, and explore the many-body properties of so-called indirect excitons, i.e. bosons formed by electrons and holes in different layers.
Present collaboration partners: Jon Finley (TUM), Michael Kaniber (TUM), Michael Knap (TUM), Richard Schmidt (MPQ).

[Holleitner 5]

Key papers:
Miller et al., Nano Letters 17, 5229 (2017)
Klein et al., 2D materials 5, 011007 (2018)



We explore the transport and confinement of long-living spatially indirect dipolar excitons in a semiconductor double quantum well, aiming for a better understanding of many-body phenomena in dipolar excitonic ensembles, in which the attractive forces between electrons in one quantum well and holes in the adjacent one cause excitonic binding and are complemented by in-plane repulsive forces of the thus formed dipoles. Depending on confinement, excitonic densities, and temperature, these interactions are expected to possibly result in phase transitions ranging from Wigner crystallization of such dipolar excitons via Bose-Einstein condensation in fully confined systems to a Mott transition into an electron-hole plasma at highest densities. Nanofabricated electrostatic traps allow us to confine even single dipolar excitons.
Present collaboration partners: Jorg Kotthaus (LMU Munich), Aron Pinczuk (Columbia U.), Mike Manfra (Purdue), Andreas Wieck (U. Bochum), W. Wegscheider (Zurich), Ursula Wurstbauer (TUM).

[Holleitner 6]

Key papers:
Schinner et al., PRL 110, 127403 (2013)
Dietl et al., PRB 95, 085312 (2017)



Nanofabrication

[Holleitner 7]


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.


Publications:
A list of our group’s publications can be found here.

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) and the Deutsche Forschungsgemeinschaft (DFG) for funding in several projects. In particular, we are grateful for the support of the Nanosystems Initiative Munich (NIM).








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