Events & News at the Walter Schottky Institute
6 Aug 2019
Site-selective positioning of quantum emitters
Spatially highly localized luminescent defects in solid-state materials constitute potential rudimentary building blocks for prospective quantum hardware. In a recent publication in Nature Communications the physicists Julian Klein, Prof. Jonathan Finley and Prof. Alexander Holleitner reported on a deterministic and site-selective route to incorporate single optically active defects in the atomically thin semiconductor MoS2. 
Previous circuits on chips rely on electrons as the information carriers. In the future, photons which transmit information at the speed of light will be able to take on this task in optical circuits. Quantum light sources, which are then connected with quantum fiber optic cables and detectors are needed as basic building blocks for such new chips. The coupling of quantum light sources to optical elements is crucially dependent on the positioning accuracy. Conventional three-dimensional materials, like silicon or diamond, which host quantum emitters suffer from spatial inaccuracies in the creation of defects. For such materials the emitters are typically embedded several nm to tens of nm from the crystal surface. In two-dimensional materials, like MoS2, the emitter is naturally restricted to an about 1 nm thick atomic layer.
For harnessing the quantum properties of defects in solids they have to be positioned site-selectively with nanometer precision in all spatial dimensions and also need to emit light at the same frequency. In this study, the researchers used a sub-nanometer focused helium ion beam in the new helium ion microscope at the Walter Schottky Institute’s Center for Nanotechnology and Nanomaterials to locally irradiate a monolayer of MoS2. Subsequent to irradiation the MoS2 is then encapsulated in the high quality insulator hBN, another material out of the exciting 2D family. The treatment results in spatially localized luminescent defects in the atomically thin semiconducting monolayer. This is the first time light sources can be generated in such a deterministic and controlled way by simultaneously maintaining nanometer accuracy. The irradiation creates defects, predominantly Mo and S vacancies, in the material that then serve as exciton traps under optical excitation. In collaboration with the theorists Michael Lorke and Matthias Florian from the University of Bremen and Richard Schmidt and Michael Knap from the TUM, a model has been developed to describe the energy states observed in the experiment. In future work, more complex light source patterns, like 2D lattices of trapped excitons are envisioned as model platform to realize Bose-Hubbard physics. Moreover, coupling to photonic elements like waveguides and cavities can now be realized with unprecedented accuracy and the high sensitivity of defects renders them for potential use as quantum sensors on the nanoscale.
 Site-selectively generated photon emitters in monolayer MoS2 via local helium ion irradiation. Nature Communications, 10, 2755 (2019).
J. Klein, M. Lorke, M. Florian, F. Sigger, L. Sigl, S. Rey, J. Wierzbowski, J. Cerne, K. Müller, E. Mitterreiter, P. Zimmermann, T. Taniguchi, K. Watanabe , U. Wurstbauer, M. Kaniber, M. Knap, R. Schmidt, J.J. Finley & A.W. Holleitner
Image: Christoph Hohmann / MCQST
17 Apr 2019
Topological quantum conductance
Crystals with symmetry-protected topological
order, such as topological insulators, promise coherent spin and charge transport
phenomena even in the presence of disorder, which
renders these topological phases promising candidates for quantum information
applications. In Physical review Letters, the
physicists Paul Seifert, Prof. Alexander Holleitner and Dr. Christoph Kastl now demonstrated
a novel optoelectronic scheme to read-out the quantum conductance of
topological surface states. 
classification of solid-state materials by the mathematical concept of topology
revolutionized condensed matter physics in the past decade, as recognized by
the Nobel prize in 2016, and lead to the discovery of a plethora of previously
unknown topological materials and quantum phases. Topological materials exhibit
surface states with unusual physical properties. For example, in so called topological
insulators (e.g. Bi2Se3) no electric current can flow through the
crystal, but the crystal surfaces conduct current almost perfectly. Intriguingly,
due to the non-trivial topology of the crystal, surface electrons moving in opposite
directions have exactly opposite spin, a property called helicity, and an
electrical current is always accompanied by a spin current in the same
direction. Unlike in other materials, the surface states are stabilized against
material imperfections or external influences by the topology of the crystal.
Therefore, these helical surface states are of interest for realizing robust spin-based
quantum information application, potentially up to room temperature.
the surface state
topological insulator materials into electronic circuits, it is important to be
able to read-out the surface conductance independent of a residual bulk conductance.
In previous studies, the researchers already successfully used so called scanning
photocurrent microscopy to image the effect of disorder on the surface conductance
of topological insulators. [2,3] In the current study, where they imaged the
current distribution in a field effect transistor made from the topological
insulator Bi2Se3, they made the surprising discovery that,
at the edges of the device, electrons showed a well-defined conductance which precisely
matched the conductance quantum e²/h. This finding directly showed that the
detected current was carried by a single, spin-helical surface mode, since in topologically
trivial materials two spin-degenerate modes would add up to 2e²/h.
their findings in the framework of the so-called Shockley-Ramo theory, which has
traditionally been applied to describe the signal formation
process in particle detectors. In simple words, this theory states that, in a
detector device, a moving charge creates a quasi-instantaneous macroscopic
detector current parallel to the field lines in the detector. Therefore, the
require a coherent transport of charge carriers all the way to the
in contrast to the conventional Landauer-Büttiker theory. Such optoelectronic schemes may provide a generalizable platform for
studying surface state conductance in further topological
quantum materials such
as Weyl-semimetals and quantum spin-Hall insulators.
 Quantized Conductance in Topological Insulators Revealed by
the Shockley-Ramo Theorem. Phys. Rev. Lett. 122, 146804 (2019).
P. Seifert, M. Kundinger, G. Shi, X. He, K. Wu, Y. Li, A.W. Holleitner, C.
Potential Fluctuations in Topological Insulator BiSbTe3 Films Visualized by
Photocurrent Spectroscopy. 2D Materials 2 024012 (2015).
C. Kastl, P. Seifert, X. He, K. Wu, Y. Li, A. Holleitner
 Local photocurrent generation in thin films of the
topological insulator Bi2Se3. Applied Physics Letters101, 25
18 Feb 2019
Valleytronics at room temperature
Essential in the development of new ultrathin information carriers is
the stabilization of the excited state, which carries the information.
The NIM physicists Prof Ursula Wurstbauer, Prof Alexander Holleitner and
Prof Alexander Högele now found a tunable method, even at room
temperature. This enables applications in so called “valleytronics”.
Important feature of semiconductor materials are their energetic
properties, especially with regard to an application in optoelectronics
and information technologies. Light is used to optically generate
the information within the semiconductor crystal. Irradiation with
visible light excites a so called exciton, a mobile excited state that
propagates like a wave. So far, in practice such excited, information
carrying states of the exciton, also called valley polarization, were
stable at low temperatures (100 K, liquid nitrogen) only. The NIM physicists Professor Ursula Wurstbauer, Professor Alexander Holleitner and Professor Alexander Högele
now clarified the underlying mechanism resulting in the depolarization
at increasing temperatures. In parallel, they developed a tunable method
to suppress that process and stabilize the valley polarization at
elevated temperatures (220 K) and even at room temperature. In Nature Communications, they present impressive results with an increased degree of valley polarization of 20 % and more.
Efficient single layers
Stabilization of excited states
Molybdenum disulfide (MoS2)
is a member of the emerging material class of two-dimensional
transition metal dichalcogenides. Its properties include stability in
the presence of oxygen, water and diluted acids as well as a high
melting temperature. All making the semiconductor an ideal candidate in
the development of new information technology devices. Additionally, even single-layered MoS2
is very sensitive to light-dependent processes. So called valley
degrees of freedom, specific energy levels within the lattice structure,
allow for valley-specific selective optical excitation and local
increase of the charge density. Stabilization of the valley polarization
is prerequisite for lossless devices and their commercial
Theoretically, the valley
polarization should be stable after the excitation with light at room
temperature, in the practice this was not the case. The physicists now
could elucidate the nearly-breakdown of the polarization of excitons in
semiconductor crystals is due to the coupling of excitons to specific
lattice oscillations, so called phonons. “One could picture the
depolarization process, especially at elevated temperatures, as the
coupling of around themselves rotating excitons and phonons,” Ursula
Wurstbauer explains, “and this movement is propagating like a wave front
on a water.” The coupling of the excitons to longitudinal optical
phonons is called Fröhlich exciton-phonon interaction and enhances the
depolarization. “Doping of the semiconductor with electrons can
longlastingly suppress that coupling,” Wurstbauer describes the basis of
the new strategy to stabilize the valley polarization.“Mechanistically, the added electrons shield the electrical field
induced by lattice oscillations. Hence, the coupling of excitons is
suppressed,” the physicist further explains, “Comparing the system to a
water again, we fill the lake with electrons and thereby smooth the
oscillations of the electric field. The scattering of a valley-polarized
exciton on phonons after excitation with light is suppressed.” The
gained basic understanding of the depolarization mechanism enabled the
scientists to develop a strategy to stabilize the valley polarization.
Doping of the semiconductor with electrons makes the depolarization
process tunable. Another extra of their method is the reduction of
scattering due to lattice imperfections and disorder.
Tuning the Fröhlich exciton-phonon scattering in monolayer MoS2. Miller
B, Lindlau J, Bommert M, Neumann A, Yamaguchi H, Holleitner A, Högele
A, Wurstbauer U. Nature Communications 10, 807 (2019), DOI: 10.1038/s41467-019-08764-3
30 Jan 2019
Fundamental work on dislocation-mediated effects on thermal transport published in Nature Materials
Extended defects such as dislocations are common in technologically important III-V semiconductors, and affect heat dissipation, for example, in nitride-based high-power electronic devices. For decades, dislocations were predicted to induce anisotropic heat transport depending on their direction, however, experimental observation has been still lacking.
In a well-concerted collaboration between teams at the National University of Singapore (NUS), Oak Ridge National Laboratory (ORNL, USA) and WSI-TUM, highly oriented threading dislocation (TD) arrays were designed in few-µm thick, epitaxial InN (indium nitride) films for which the strong thermal transport anisotropy was measured for the first time using time-domain thermoreflectance. The results published in B. Sun, et al, Nature Materials 18, 136 (2019) show that the cross-plane thermal conductivity is almost 10-fold higher than the in-plane thermal conductivity for state-of-the-art TD densities of ~1010/cm2. With enhanced understanding of dislocation-phonon interactions obtained here, this work fuels the developments for directed heat dissipation in the thermal management of diverse device applications.
15 Jan 2019
Best Poster Award at MRS Fall Meeting for Thomas Stettner and Daniel Ruhstorfer
Thomas Stettner and Daniel Ruhstorfer, both PhD students at the Walter Schottky Institute share a Best Poster Award that was recently awarded at the MRS Fall Meeting 2018 held by the Materials Research Society in Boston, USA (Nov. 25-30, 2018). The MRS Fall Meeting is one of the largest international conferences for materials research with over 50 symposia and ca. 6000 attendees annually.
The awardees received their prize for their contribution entitled “GaAs-AlGaAs core-shell nanowire lasers on silicon” which was presented in the Symposium “Nanowires and Related 1D Nanostructures – New Opportunities and Grand Challenges”. Thomas Stettner and Daniel Ruhstorfer are both students supervised by PD Dr. Gregor Koblmüller at the Semiconductor Quantum Nanosystems Chair of Prof. Jonathan Finley at WSI.
23 Nov 2018
The institute mourns for Prof. Markus-Christian Amann. He passed away unexpectedly on 23. Nov. 2018.
Markus-Christian Amann passed away unexpectedly on 23.11.2018 at the age of 67
years. Until April 2018 he was Full Professor for Semiconductor Technology at
TUM and Director of the Walter Schottky Institut.
completing his studies at the Faculty for Electronic and Computer Engineering of
TUM, Markus-Christian Amann joined the research laboratories of Siemens AG in
Munich in 1981 where he remained until 1994, rising to lead the department for Semiconductor
Research & Optoelectronics. Subsequently, he accepted a call to a Chair for
Technical Electronics at the University of Kassel where he taught and
researched until 1997. In 1997 he received a call to the TUM, where he held the
chair for Semiconductor Technology at the Walter Schottky Institut until his
retirement in April 2018.
Amann was a very well-known and highly-respected figure in the international
scientific community. He performed
groundbreaking work in the field of III-V semiconductor optoelectronic devices,
including vertical cavity surface emitting lasers - key components for high
bandwidth optical data communication systems and optical metrology and sensing
systems. His work is documented by more than 500 scientific publications,
patents and conference proceedings that also formed the intellectual basis for
the successful launch of several start-up companies.
Amann received many prizes and accolades for his scientific work. Specific examples
include the Karl-Heinz-Beckurts prize in 2007, from the Foundation bearing the
same name and a nomination to be Fellow of the IEEE Society in 2009. Beyond his
research, he made major contributions to scientific administration being
referee for many leading international scientific journals, as a committee
member for the Germany Science Foundation, as vice Dean and Dean of the faculty
for Electronic and Computer Engineering and as Managing Director of the Walter
31 Oct 2018
A new hybrid device for renewable electricity and fuels from sunlight
Our group has developed and demonstrated a new device that allows for simultaneous production of electricity and chemical fuels from sunlight. The new device, called a hybrid photoelectrochemical and photovoltaic (HPEV) cell, overcomes central challenges in the area of artificial photosynthesis, as described in our recent paper in Nature Materials:
G. Segev, J.W. Beeman, J.B. Greenblatt, & I.D. Sharp, Nature Materials (2018).
A press release describing this exciting advancement can be found here.
27 Sep 2018
WSI celebrates successes in Excellence Initiative !
Walter Schottky Institute scientists were thrilled with the great news on 27.09.18 that Technical University of Munich was again successful in the extremely competitive Excellence Initiative organised by the Federal and Regional Governments. From Jan 2019 onwards, research groups from WSI led by Profs. Brandt, Finley, Holleitner, Sharp and Stutzmann will be strongly involved in two DFG Clusters of Excellence for the next seven years. After a two-step selection procedure lasting over 18 months, a high-level international committee approved two clusters for funding with participation of WSI groups - e-Conversion and MCQST (Munich Center for Quantum Science and Technology)
e-CONVERSION (Finley, Holleitner, Sharp, Stutzmann)
The e-conversion Cluster of Excellence explores ways to deliver a stable, efficient and sustainable supply of energy by combining nanoscience with energy sciences. This cluster focuses on the energy conversion processes of different technologies – from photovoltaics through (photo-)electrocatalysis to battery technologies. To date, inadequate control of these processes in nanomaterials and at relevant interfaces has led to resistance, recombination losses or overvoltage, all of which compromise the efficiency of power generation. e-conversion will study the basic mechanisms of energy conversion with a time resolution in the femtosecond range. The findings will enable scientists to design and synthesize energy materials with atom-scale precision. The cluster will build an electron microscopy center in order to characterize the materials. Alongside WSI scientists, e-conversion will involve many TUM and LMU groups, the Max Planck Institutes for Chemical Energy Conversion (Mülheim/Ruhr) and for Solid State Research (Stuttgart).
MUNICH CENTER FOR QUANTUM SCIENCE AND TECHNOLOGY (Brandt, Finley, Holleitner)
Breakthroughs in quantum mechanics have inspired everyday technologies such as microchips, computers and lasers. Quantum mechanics describes the physical properties of the smallest particles, and work in this area revolutionized the world of science in the 20th century. A technological leap forward is currently taking place known as “Quantum 2.0”. It is based on the use of superposition and entanglement of quantum states. The number of potential applications is huge, with ultra-high-performance quantum computers and secure quantum communication systems being just two examples. The objective of the Munich Center for Quantum Science and Technology is to further the scientific understanding of quantum mechanics phenomena and thus advance basic components, materials and concepts for quantum technologies. Interdisciplinary research extends from the analysis of entanglement in multiparticle systems to quantum chemistry, astronomy and precision metrology. Alongside WSI Scientists, MCQST will involve many TUM and LMU groups working besides scientists from the Max Planck Institute of Quantum Optics, the Walther Meißner Institute of the Bavarian Academy of Sciences and Humanities and the Deutsches Museum.
27 Jun 2018
Closing the gap: On the road to Terahertz electronics
Asymmetric plasmonic antennas deliver femtosecond pulses for fast optoelectronics
A team headed by the TUM physicists Alexander Holleitner and Reinhard Kienberger has succeeded for the first time in generating ultrashort electric pulses on a chip using metal antennas only a few nanometers in size, then running the signals a few millimeters above the surface and reading them in again a controlled manner. The technology enables the development of new, powerful terahertz components.
Classical electronics allows frequencies up to around 100 gigahertz. Optoelectronics uses electromagnetic phenomena starting at 10 terahertz. This range in between is referred to as the terahertz gap, since components for signal generation, conversion and detection have been extremely difficult to implement.
The TUM physicists Alexander Holleitner and Reinhard Kienberger succeeded in generating electric pulses in the frequency range up to 10 terahertz using tiny, so-called plasmonic antennas and run them over a chip. Researchers call antennas plasmonic if, because of their shape, they amplify the light intensity at the metal surfaces.
The shape of the antennas is important. They are asymmetrical: One side of the nanometer-sized metal structures is more pointed than the other. When a lens-focused laser pulse excites the antennas, they emit more electrons on their pointed side than on the opposite flat ones. An electric current flows between the contacts – but only as long as the antennas are excited with the laser light.
"In photoemission, the light pulse causes electrons to be emitted from the metal into the vacuum," explains Christoph Karnetzky, lead author of the Nature work. "All the lighting effects are stronger on the sharp side, including the photoemission that we use to generate a small amount of current."
Ultrashort terahertz signals
The light pulses lasted only a few femtoseconds. Correspondingly short were the electrical pulses in the antennas. Technically, the structure is particularly interesting because the nano-antennas can be integrated into terahertz circuits a mere several millimeters across.
In this way, a femtosecond laser pulse with a frequency of 200 terahertz could generate an ultra-short terahertz signal with a frequency of up to 10 terahertz in the circuits on the chip, according to Karnetzky.
The researchers used sapphire as the chip material because it cannot be stimulated optically and, thus, causes no interference. With an eye on future applications, they used 1.5-micron wavelength lasers deployed in traditional internet fiber-optic cables.
Holleitner and his colleagues made yet another amazing discovery: Both the electrical and the terahertz pulses were non-linearly dependent on the excitation power of the laser used. This indicates that the photoemission in the antennas is triggered by the absorption of multiple photons per light pulse.
"Such fast, nonlinear on-chip pulses did not exist hitherto," says Alexander Holleitner. Utilizing this effect he hopes to discover even faster tunnel emission effects in the antennas and to use them for chip applications.
Pulses of femtosecond length from the pump laser (left) generate on-chip electric pulses in the terahertz frequency range. With the right laser, the information is read out again. (Image: Christoph Hohmann / NIM, Holleitner / TUM)
Towards femtosecond on-chip electronics based on plasmonic hot electron nano-emitters.
C. Karnetzky, P. Zimmermann, C. Trummer, C. Duque-Sierra, M. Wörle, R. Kienberger, A. Holleitner; Nature Communications June 25, 9, 2471 (2018).
The experiments were funded by the European Research Council (ERC) as part of the "NanoREAL" project and the DFG Cluster of Excellence "Nanosystems Initiative Munich" (NIM).
Prof. Dr. Alexander Holleitner
Walter Schottky Institute / Department of Physics
Center for Nanotechnology and Nanomaterials
Technical University of Munich
Am Coulombwall 4a, 85748 Garching, Germany
Tel: +49 89 289 11575
Prof. Dr. Reinhard Kienberger
Technical University of Munich
James-Franck-Str. 1, 85748 Garching, Germany
Tel: +49 89 289 12840
13 Feb 2018
Understanding life cycles of charge carriers in functional photoelectrodes
The next generation of solar energy conversion systems requires the discovery of semiconductors with properties tailored to their desired function. In this pursuit of such new materials, detailed understanding of optoelectronic properties, driving forces for charge separation and extraction, and loss mechanisms that limit device performance is essential for achieving high efficiencies. However, few experimental methods are available for direct characterization of these fundamental processes in artificial photosystems that are designed to convert sunlight into fuels. To overcome this gap, researchers are developing new methods for operando characterization of photoelectrochemical systems comprising illuminated and electrified semiconductors in aqueous environments that are incompatible with many traditional spectroscopic tools.
Reporting in the journal Energy and Environmental Science, an international team of scientists working at the Walter Schottky Institute and the Lawrence Berkeley National Laboratory, led by Ian Sharp, have developed a new approach for identification and quantification of photocarrier transport and efficiency loss mechanisms in operating photoelectrodes with nanometer depth resolution. In their work, they applied this method to a newly identified and thus poorly understood semiconductor, copper vanadate, which possesses an ideal bandgap for photoelectrochemical energy conversion. In doing so, they provided key insights into charge transport and loss mechanisms that are ubiquitous in a broad range of recently identified transition metal oxide materials. Such details regarding photocurrent generation and recombination at operating semiconductor/electrolyte junctions not only provide guidance to future material discovery efforts, but also motivate strategies for nanostructuring photoelectrodes, engineering interfaces, and integrating catalysts. More broadly, this work represents a significant leap forward in the ability to understand the key aspects of functional photosystems by probing the nanoscale life cycles of charge carriers in advanced materials interacting with complex environments.
Quantification of the loss mechanisms in emerging water splitting photoanodes through empirical extraction of the spatial charge collection efficiency
G. Segev, C.-M- Jiang, J.K. Cooper, J. Eichhorn, F.M. Toma & I.D. Sharp, Energy Environ. Sci. (2018) doi: 10.1039/C7EE03486E.
17 Jan 2018
ERC Consolidator Grant for Gregor Koblmüller
A Consolidator Grant was awarded by the European Research Council (ERC) in its latest funding round to PD Dr. Gregor Koblmueller (WSI & Physics Department, TU Munich). Gregor Koblmueller received the award for his project QUANtIC which focuses on novel "Quantum Nanowire Integrated Photonic Circuits". Consolidator Grants are among the largest and most prestigious single-PI awards within the European research community and are given to outstanding researchers who have demonstrated independent and highly creative research. The award is worth nearly 2 Mio € and allows the recipient to conduct cutting-edge research on the proposed topic over the next five years.
Gregor Koblmueller (picture) will use the Consolidator Grant to create and explore new links between semiconductor nanowires with precisely tailored quantum electronic properties and nanoscale integrated photonic circuits. The vision is to enable thereby highly deterministic and site-selectively integrated nanoscale coherent light sources, such as efficient nanolasers and single photon emitters, direcly on photonic and quantum photonic hardware. These integrated light sources are expected to provide novel grounds for future applications in the fields of on-chip light processing, quantum communication, as well as lab-on-chip sensing.
Gregor Koblmueller has been active at WSI for many years where he is leading the Semiconductor Quantum Nanomaterials group. More information on his research activities can be found at www.wsi.tum.de/koblmueller
16 Jan 2018
Light-steering of spin-polarized currents in topological insulators
Topological insulators are a fascinating group of materials. A spin-polarization occurs, as soon as an electric current flows in the material. WSI scientist Prof Dr Alexander Holleitner and his cooperation partners measured this now for the first time optically at room temperature. In particular, they succeeded to steer spin-polarized currents towards the edges by a circularly polarized light beam and to read-out the electron spin-polarization at the facets of the circuits.
About ten years ago, scientists discovered a group of materials called "topological insulators" with unusual electronic properties. The interior acts as an insulator, but the surface conducts electricity better than average. The group of the NIM physicist Professor Alexander Holleitner (weblink: www.nanoptronics.de) has succeeded to guide electrons with opposite magnetization, in short spin-polarization, towards the opposite edges of a topological insulator.
Key feature is that no external magnetic field is needed to generate this phenomenon. The opposite spin-polarization rather derives from an effect called spin-orbit-coupling. The direct coupling between the electron’s spin and the direction of the electron motion allows its manipulation. The physicists found this effect to be reversible. By inducing a certain magnetization with polarized light, they can control the electric current at the sample’s edges. Their results are presented in the latest issue of Nature Communications (Weblink: https://www.nature.com/articles/s41467-017-02671-1).
The best-known representatives of three-dimensional topological insulators are heavy metal alloys, such as bismuth selenide or bismuth telluride. Scientists assign the exceptional electronic properties to be a phenomenon of quantum physics: the so-called spin-Hall-effect. One observes that all electrons moving in the surface layers have a well-defined spin. In doing so, they differ "topologically" from electrons inside the materials. The direction of the surface currents is directly linked to the electron spin. In such spin-orbit materials, an electron with positive spin always flows in the opposite direction compared to an electron with negative spin.
Holleitner and colleagues now made the stunning discovery that this also holds for the material’s interior, if it is electrically conducting. When a current flows through the topological insulator, electrons with opposite spin move in opposite directions and accumulate at the topological edges of the material. The imbalance in the spin distribution results in a magnetization of the surface states.
Spin-polarized currents at the facets
In conventional conductors, electric currents are always carried by electrons with an arbitrary spin-orientation. In topological insulators, however, the direct coupling between the electron’s spin and the direction of movement allows a particular control of the electrons without the necessity of a sophisticated magnetic field or magnetic materials.
"Such control of the electronic spin is the basic requirement for the realization of so-called spin-based electronics.", explains first author Paul Seifert, who designed and carried out the experiments. The scientists hope that this technology will be applied in the development of more powerful computers or the secure encryption of data.
Measurements with polarized light
Very small electric currents and their magnetization can be directly detected with polarized light. In the actual experiment, they contact a topological insulator between two electrodes and excite the material with a circularly polarized laser. By choosing the correct polarization, they can induce a magnetization in the material, as electrons with different spin can be excited selectively.
Through a circuit, the scientists are able to track how a spin-polarized current at the edges of the topological insulator changes when they change the polarization of the light. In addition, the scientists observed the local magnetization of the topological insulator to change the polarization of the reflected light. Thus, they were able to directly detect the magnetization or spin polarization generated by the current flow.
The experiments are funded by the Deutsche Forschungsgemeinschaft within DFG Projects 3324/8-1 of the SPP 1666 “topological insulator“ and the excellence cluster “Nanosystems Initiative Munich“ (NIM). The co-authors Dr. K. Vaklinova, Prof. K. Kern and Dr. M. Burghard work at the Max Planck Institute for Solid State Research in Stuttgart. Co-author Sergey Ganichev works at the Terahertz Center of the University of Regensburg.
Spin Hall photoconductance in a three-dimensional topological insulator at room temperature. Paul Seifert, Kristina Vaklinova, Sergey Ganichev, Klaus Kern, Marko Burghard und Alexander W. Holleitner. Nature Communications
Prof Dr Alexander Holleitner
Walter Schottky Institute and Physics-Department
Center for Nanotechnology and Nanomaterials
Technische Universität München
Am Coulombwall 4a
Web: Holleitner Group
Helicity-dependent edge conductance.
18 Oct 2017
Quantum sensing of GHz frequency signals
Quantum sensors can detect signals at much higher frequencies than previously thought. This is a finding that our quantum sensing group reports in Nature Communications this week. A novel sensing scheme based on this insight could lead to a new generation of quantum devices, such as detectors for single microwave photons.
Nature Communications 8, 964 (2017)
01 Sept 2017
WSI Welcomes Prof. Ian Sharp
We extend a warm welcome to our new colleague, Prof. Ian Sharp, who joined the Walter Schottky Institute on September 1st, 2017. Prof. Sharp, who holds the Chair for Experimental Semiconductor Physics at the Technical University of Munich, pursues research into functional materials and interfaces for renewable energy conversion. His interests include development of artificial photosystems that convert sunlight into chemical fuels, synthesis and characterization of new semiconductors and nanosystems, and investigation of physical and chemical mechanisms of energy conversion.
Before coming to the Technical University of Munich, Prof. Sharp was a Staff Scientist at the Lawrence Berkeley National Laboratory. While there, he served as Thrust Lead at the Joint Center for Artificial Photosynthesis and, in 2016, was recipient of the prestigious U.S. Department of Energy Early Career Award. From 2007 to 2011, Dr. Sharp was a post-doctoral fellow in the group of Prof. Stutzmann, initially as an Alexander von Humboldt Fellow and later as a Carl von Linde Junior Fellow of the TUM Institute for Advanced Study.
It is a pleasure to welcome Ian Sharp back to the Walter Schottky Institute!
10 Aug 2017
Best Poster Awards for Ganpath Veerabathran and Alexander Andrejew at iNOW 2017
Ganpath Veerabathran and Alexander Andrejew, both doctoral candidates from Prof. Amann's group (E26) at the WSI, received Best Poster Awards at the International Nano-Optoelectronics Workshop (iNOW) in Tianjin, Qian’an & Chengde, China (2017). Ganpath Veerabathran's poster titled "GaSb-based vertical-cavity surface-emitting lasers at 4 μm using type-II quantum wells" was awarded the 1st place out of 42 posters. The prize consists of a certificate and carries a value of 400 USD. Alexander Andrejew's poster titled “Electrically pumped mid-infrared vertical-cavity surface-emitting lasers emitting at 3 μm” was selected for the ‘Honorary mention’ award.
27 Jun 2017
Best Poster Award at Nanowire Week for Jochen Bissinger
Jochen Bissinger, PhD student at the Walter Schottky Institut was awarded with the Best Poster Award at Nanowire Week 2017 in Lund Sweden (29th May - 2nd June, 2017). The Nanowire Week (combined 10th Nanowire Growth Workshop and 9th Nanowires Workshop) is nowadays one of the most significant international conferences for semiconductor nanowire-related research with over 300 participants annually.
Selected from 180 poster presentations Jochen Bissinger received the Best Poster prize for his contribution entitled "Simulation of monolithically integrated Ga(Al)As-InGaAs core-multishell nanowire lasers on silicon waveguides". Jochen Bissinger is a member of the WSI Nanowire Group and is co-supervised by Dr. Gregor Koblmüller, Dr. Michael Kaniber, and Prof. Jonathan Finley. His PhD thesis work is performed in collaboration with the Electrical Engineering Department at TUM and is funded by the International Graduate School of Science and Engineering (TUM-IGSSE).
15 Mar 2017
Dr. Kai Müller admitted to the “Junges Kolleg” of the Bavarian Academy of Sciences
Recently, Dr. Kai Müller a subgroup leader at the chair E24 (Prof. Finley) was admitted to the “Junges Kolleg” of the Bavarian Academy of Sciences and Humanities. The “Junges Kolleg” was found in 2010 in order to support promising junior scientist and foster interdisciplinary research. It consists of 20 young scientists, carrying our research across all disciplines and all universities / research institutions in Bavaria. This year, 5 researchers out of 60 highly qualified applicants were admitted. More information can be found here:
Press release from BAdW
Homepage of the “Junges Kolleg”
27 Feb 2017
Two-photon pulses from a single two-level system
Sources of non-classical states of light are key components needed for future quantum photonic technologies such as intrinsically secure communication, distributed quantum information processing and precision metrology. By far the most commonly non-classical light sources investigated to date can generate individual quanta of light - single photons. A well-established technique to generate single photons on-demand involves using short laser pulses to resonantly excite an individual two-level quantum system, such as an atom or semiconductor quantum dot. A team of scientists lead by Dr. Kai Müller (E24 / Prof. J. Finley) have now discovered that resonantly driven two-level systems can also act as sources of two-photon pulses when excited by laser pulses having specially tailored properties. The work has been performed in the framework of a collaboration between the WSI and the group of TUM-IAS Hans Fischer senior fellow Prof. J. Vuckovic at Stanford University and is also supported financially by the BMBF and the DFG via the Nanosystems Initiative Munich. The paper is published in Nature Physics and can be viewed at:
Emission of two-photon pulses from a quantum two-level system
Kevin A. Fischer, Lukas Hanschke, Jakob Wierzbowski, Tobias Simmet, Constantin Dory, Jonathan J. Finley, Jelena Vuckovic and Kai Müller
Nature Physics (2017) doi:10.1038/nphys4052
05 Aug 2016
Best Poster Award at iNow 2016 for Hannes Schmeiduch
The contribution "Selective Area Epitaxy and Growth on Patterned Surfaces of Indium Phosphide using LP-MOVPE for MIR-QCL" presented by Hannes Schmeiduch at International Nano-Optoelectronics Workshop 2016 (iNow) in München and Würzburg, has been award the 3rd "Best Poster Award" out of 38 posters. The research team consists of researchers from Chair E26 at the Walter Schottky Insitut/TU München (H. Schmeiduch, F. Demmerle, S. Saller, S. Sprengel, W. Oberhausen, R. Meyer, and M.-C. Amann). We especially thank Mr. Demmerle for his encouragement.
27 Jul 2016
Best Poster Award for "Few-QD nanolaser" at PECS-XII
The contribution "A few-emitter solid-state multi-exciton laser" presented by Michael Kaniber at "The 12th International Symposium on Photonic and Electromagnetic Crystal Structures (PECS-XII)" in York, UK, has been selected out of 40 posters for a runner-up poster award. The research team consists of researchers from Chair E24 at the Walter Schottky Insitut/TU München (S. Lichtmannecker, T. Reichert, M. Blauth, Dr. M. Kaniber, and Prof. J. J. Finley) and from the Solid-state Theory-group at Universität Bremen (Dr. M. Florian, Dr. C. Gies, and Prof. F. Jahnke). We thank the whole team for their efforts and great work! Congratulations!
13 Jul 2016
Best Poster Award at HeFIB for Julian Klein
Julian Klein, Ph.D student at the Walter Schottky Institut, has very recently been awarded with the 1st price for his Poster contribution at the HeFib, "1st International Conference on Helium Ion Microscopy and Emerging Focused Ion Beam Technologies", in Luxembourg (June 8-10). His poster entitled "Optical properties of 2D materials exposed to helium ions" combines novel methods of nanostructuring performed by a helium ion microscope applied to semiconducting atomically thin 2D materials. Julian Klein is supervised by Dr. Michael Kaniber, Dr. Ursula Wurstbauer, Prof. Jonathan Finley and Prof. Alexander Holleitner in the Integrated Quantum Photonics Group at the Walter Schottky Institut. He investigates optical properties of atomically thin 2D materials in combination with plasmonic nanostructures.
04 Jul 2016
CSW 2016 Best Paper Award for Bernhard Loitsch
Bernhard Loitsch, PhD student at the Walter Schottky Institut won the Best Student Paper Award at
the international Compound Semiconductor Week 2016 (CSW2016) held in Toyama (Japan). CSW2016 is
a joint venue for the 43rd International Symposium on Compound Semiconductors (ISCS) and the
28th International Conference on Indium Phosphide and Related Materials (IPRM) and a premier
forum for science, technology and applications in all areas of compound semiconductors.
He gave an oral presentation entitled “Quantum Confinement Phenomena in Ultrathin
GaAs Nanowires”. Based on an evaluation of the quality of abstracts and oral
presentations by a panel of international experts, only three papers were selected from a
large number of eligible contributions.
Bernhard Loitsch is a student in the Nanowire
Group led by Dr. Gregor Koblmüller at the Semiconductor Quantum Nanosystems Chair
(Prof. Finley) and investigates growth, structure-property correlations, and advanced optical
properties in III-V semiconductor nanowire systems in his Ph.D. thesis work.
14 Jun 2016
Poster Award at 10th IGSSE Forum in Raitenhaslach
The “Nanowire lasers”-team (IGSSE Project 9.08) working at WSI-TUM (Chair Prof. J. J. Finley) and TUM-EE (Chair Prof. P. Lugli) were recently awarded the Best Poster Award at the 10th IGSSE Forum held in Raitenhaslach (June 1-4). The PhD students Thomas Stettner, Jochen Bissinger, Armin Regler and the Project Team Leader Dr. Michael Kaniber won the 3rd prize with their contribution entitled “Nanowire lasers for information technologies and sensing”. This collaborative IGSSE Project brings together students from the Physics Department/Walter Schottky Institut as well as from the Faculty of Electrical and Computer Engineering and explores the potential of III-V semiconductor nanowires as coherent light sources for applications in future optical on-chip and interconnects communication.
21 Mar 2016
IBM Ph.D. Fellowship for Bernhard Loitsch
Bernhard Loitsch, Ph.D. student at the Walter Schottky Institut, has received the prestigious IBM Ph.D Fellowship award. This is the second successful nomination after an initial award in 2014, which was followed by an ongoing scientific collaboration and mentorship with Dr. Heike Riel, IBM Fellow and Manager of the Nanoscale Electronics group at IBM Research – Zurich. The IBM Ph.D. Fellowship Awards Program is an intensely competitive worldwide program, which honors exceptional Ph.D. students with a $20.000 stipend for one academic year. Bernhard Loitsch is a Ph.D. student supervised by Dr. Gregor Koblmüller and Prof. Jonathan Finley in the Quantum Nanomaterials Group at the Walter Schottky Institut. He investigates the epitaxial growth of GaAs-based nanowire heterostructures as well as their optical and electrical properties.
07 Jan 2016
Arnold Sommerfeld Prize 2015 for Gregor Koblmueller!
Our congratulations to Gregor Koblmueller on being awarded the Arnold Sommerfeld Prize 2015 by the Bavarian Academy of Sciences for his leading work on the realization of Semiconductor nanowire heterostructures and their use for next generation electronic and photonic devices ! Gregor Koblmueller is one of the leading material scientists worldwide and has been active in WSI for many years working on the growth of such nanomaterials and the investigation of their fundamental properties. More information on the research topics for which he has received his prize can be found on the research pages of the nanowire subgroup of E24. The prize was presented by the President of the Bavarian Academy, Prof. Dr. Karl-Heinz Hoffmann as part of the annual general meeting of the academy in December 2015 (see photo). Join us in congratulating Gregor on this great recognition of the leading work performed by his group, his students and colleagues and wishing all a great start into 2016 !
c/o BAdW, Foto: A. Heddergott
09 Nov 2015
Two Student Awards at Nanowire Growth/Nanowires-2015 Workshop
Two PhD students from WSI-TUM, Martin Hetzl and Julian Treu, were recently awarded with Best Poster Paper Awards at the international Nanowire Growth Workshop (NWG) and Nanowires-2015 Workshop in Barcelona, Spain (October 26-30). Martin Hetzl received a 1st prize Best Poster Award for his contribution entitled “Growth and electrical transport properties of GaN nanowire/diamond heterojunctions". Julian Treu was awarded 2nd prize Best Poster Award for his presentation on “Widely tunable InGaAs nanowire heterostructures and devices”.
Martin Hetzl and Julian Treu are both Ph.D. students supervised by Prof. Martin Stutzmann and Dr. Gregor Koblmüller (Prof. Finley group) and are investigating growth, structure-property correlations, and advanced optical and electrical properties in III-V and nitride-based semiconductor nanowire systems in their Ph.D. thesis work.
18 May 2015
EMRS Graduate Student Award for Julian Treu
Julian Treu, PhD student at the Walter Schottky Institut and TUM Physics Department was awarded with the EMRS Graduate Student Award at the 32nd European Materials Research Symposium in Lille, France. The EMRS Meeting is one of the largest conferences in materials science worldwide with over 2000 participants annually.
Based on an evaluation by international experts, his oral presentation entitled “Surface passivation and confinement in lattice-matched InGaAs-InAlAs core-shell nanowires was selected as the award winning contribution in the Symposium I “Semiconductor Nanostructures towards Electronic & Optoelectronic Device Applications”. Julian Treu is a Ph.D. student supervised by Dr. Gregor Koblmüller in the group of E24 and is investigating III-V semiconductor nanowires for photonic and light harvesting applications. His excellent contribution was recently also published in J. Treu, et al., Nano Letters 15, 3533 (2015).
27 Mar 2015
Best Paper Student Awards in Nanowire Research
Two PhD students from WSI-TUM, Julian Treu and Benedikt Mayer, were recently awarded with Best Paper Student Awards. Julian Treu received the best student award at the 18th European Molecular Beam Epitaxy Workshop (Euro-MBE) in Canazei, Italy (March 15-18) for his oral presentation entitled “Growth and optical properties of composition-tuned InGaAs-based core-shell nanowire arrays”. In addition, Benedikt Mayer was awarded at the 582. WE Heraeus Seminar on “III-V Nanowire Photonics” in Bad Honnef, Germany (March 22-25) for his presentation on “Monolithically integrated GaAs-AlGaAs core-shell nanowire lasers on Silicon”.
Julian Treu and Benedikt Mayer are both students in the Nanowire Group led by Dr. Gregor Koblmüller at the Semiconductor Quantum Nanosystems Chair (Prof. Finley) and are investigating growth, structure-property correlations, and advanced optical properties in III-V semiconductor nanowire systems in their Ph.D. thesis work.
26 Mar 2015
Optoelectronic quantum transport on a topological surface
Topological insulators are an exceptional group of materials. Their interior acts as an insulator, but the surface conducts electricity extraordinarily well. The group of Alexander Holleitner could measure this now for the first time directly, with extremely high temporal resolution. In addition, they succeeded to influence the direction of the surface currents with a polarized laser beam.
Artistic sketch of a polarized laser exciting surface currents in the topological insulator Bi2Se3, which is contacted by two gold electrodes. (c) nature.com and Cristoph Hohmann (NIM).
Original publication: C. Kastl, C. Karnetzky, H. Karl, A.W. Holleitner "Ultrafast helicity control of surface currents in topological insulators with near-unity fidelity" Nature Comm. 6, 6617 (2015).
02 Dec 2014
Graphene layer reads optical information from nanodiamonds electronically
In a recent publication in Nature Nanotechnology, we demonstrate that the spin of nitrogen-vacancy centers in diamond can be electronically read-out using a graphene layer on a picosecond time-scale. Nitrogen-vacancy centers in diamonds could be used to construct vital components for quantum computers. But hitherto it has been impossible to read optically written information from such systems electronically. The work was led by the group of Alexander Holleitner in collaboration with Frank Koppens (ICFO, Barcelona).
Image: Christoph Hohmann / NIM
Link to press release
Original publication: A. Brenneis, L. Gaudreau, M. Seifert, H. Karl, M.S. Brandt, H. Huebl, J.A. Garrido, F.H.L. Koppens, and A.W. Holleitner "Ultrafast electronic read-out of diamond NV centres coupled to graphene" Nature Nanotechnology 10, 135 (2015).
22 Nov 2014
Nanoday at the Deutsches Museum !
„Nano – what does that mean exactly? How is it able to work on that tiny scale? What is the use of the research results?" You will get answers to these and many other questions at first hand by our scientists.
At the information booths you can do a lot of nano-experiments yourself and in the stage program professors will explain their cutting edge research projects. The program is completed by the comedian Georg Eggers who presents science with a twinkle in his eye.
This year on Saturday 22nd November 10:00-17:00 you will have the chance to experience the world of nanoscience by visiting the NanoDay 2014 at the Deutsches Museum in Munich - Entrance to the exhibit is entirely free !
click here for more information!
11 Oct 2014
Tag der offenen Tür am Walter Schottky Institut (WSI) und Zentrum für Nanotechnologie und Nanomaterialien (ZNN)
Programm des Walter Schottky Instituts: Programm
Das komplette Programm gibt es hier: forschung-garching.de
05 Sep 2014
New research group on diamond quantum sensors
A newly established Emmy-Noether research group has joined the WSI on the 1st of September 2014. Its research will focus on quantum sensors based on color centers in diamond and their application in life science, in particular nuclear magnetic resonance (NMR) spectroscopy of single biomolecules.The new group is led by Friedemann Reinhard, formerly a senior scientist in the lab of Prof. Dr. Jörg Wrachtrup at the University of Stuttgart. He has been at the forefront of research on diamond quantum sensors for several years, demonstrating among other results the first detection of NMR signals from a 5nm small sample volume.
Friedemann and his group will join department E24 (Prof. Jonathan Finley). Funding for the group is provided by the Emmy-Noether program of the Deutsche Forschungsgesellgeschaft (DFG), a support program providing outstanding young scientists with an independent research group. All of us at the Walter Schottky Institut wish the new group a highly successful start and look forward to fruitful collaborations over the coming years !
17 Jul 2014
The art of photon "bundling"...
In a recent publication in Nature Photonics led by WSI alumnus Fabrice Laussy in collaboration with Jonathan Finley's group, we published a paper in which a quantum optical light source can emit optical energy strictly via "bundles" of curious N-photon quanta, where N can be any integer. Remarkably, what appears to be the simplest possible configuration in cavity quantum electrodynamics - a single-mode optical cavity containing a two-level system like a quantum dot - is capable of exhibiting a broad range of interesting, and even counter-intuitive behaviours when pumped with an external laser. Fabrice has written an excellent article on the art of photon buldling on his blog.
02 Jul 2014
Hocheffiziente nichtlineare Metamaterialien für die Laser-Technik
Trotz aller Fortschritte gibt es noch immer nicht für alle gewünschten Frequenzen geeignete Laser-Systeme. Manche dieser Frequenzen kann man mit Frequenzverdopplern erzeugen, die nichtlineare optische Eigenschaften nutzen. Wissenschaftler der Technischen Universität München (TUM) und der University of Texas (Austin, USA) haben nun einen optischen Baustein entwickelt, dessen nur 400 Nanometer dicke Schicht, 100-mal dünner als ein menschliches Haar, verschiedenste Frequenzen verdoppeln kann und eine Million mal effizienter ist als traditionelle Materialien mit nichtlinearen optischen Eigenschaften.Weiter zur TUM Pressemeldung...
11 Mar 2014
IBM Ph.D. Fellowship for Bernhard Loitsch
Bernhard Loitsch, Ph.D student at the Walter Schottky Institut, has received the prestigious IBM Ph.D Fellowship award. This award is an intensely competitive worldwide program, which recognizes the great potential of individual Ph.D. students in their very early career as well as the quality of the research institution in focus areas of interest to IBM. Dr. Heike Riel, IBM Fellow and Manager of the Nanoscale Electronics group at IBM Research – Zurich, will act as his mentor throughout this fellowship. The award aims to strengthen collaborations between IBM and the core research group of the awardee at WSI-TUM. It covers a $20.000 stipend for one academic year and can be renewed yearly. Bernhard Loitsch is a Ph.D. student supervised by Dr. Gregor Koblmüller and Prof. Jonathan Finley in the Quantum Nanomaterials Group of E24. He investigates the epitaxial growth of GaAs-based nanowire heterostructures as well as their optical and electrical properties.
22 Jan 2014
Official opening of E24 optics laboratories in the Walter Schottky Institut after renovation works!
Recently, it was our great pleasure to welcome Dr. U. Kirste from the Bayerische Staatsministerium für Wissenschaft, Forschung und Kunst (Bavarian Science Ministry) during a visit to the group E24 of the Walter Schottky Institut. It was a great opportunity for Prof. Finley and colleagues to discuss current and future research activities. Moreover, after many months in which the first group of laboratories were renovated and prepared for new experiments, we could officially declare the optical laboratories "open" and drink a toast to our future successes ! The hard work building up the laser spectroscopy experiments has now begun, students and staff working tirelessly over the Christmas vacations - check back later in spring 2014 to see how we are getting along!