Experimentelle Physik der kondensierten Materie
In dieser Vortragsreihe werden aktuelle Forschungsinhalte der Festkörperphysik behandelt. Die Vortragenden sind häufig internationale Gastwissenschaftler.
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| Datum | Sprecher | Titel |
|---|---|---|
| 09.01.2020 | Prof. Yishay Manassen (SPM Research Group, Ben Gurion University Beer Sheva, Israel) | Magnetic resonance on the single spin level |
| 29.11.2019 | Annika Kurzmann (Solid State Physics Laboratory, ETH Zürich, Schweiz) | Electrostatically-Defined Quantum Dots in Bilayer Graphene |
| 07.11.2019 | Prof. Sudeshna Chattopadhyay (Discipline of Physics, Indian Institute of Technology Indore) | Confined semiconductors and their composite systems: Effect of interface to control structure-properties and its applications |
| 02.05.2019 | Dr. Samer Houri (Physical Science Laboratory, NTT Basic Research Laboratories, Japan) | Acoustic Frequency Combs: Implementation and Prospects |
| 08.04.2019 | Dr. Knut Müller-Caspary (Ernst Ruska Centre, Forschungszentrum Jülich) | Strain, polarisation, atomic electric fields: Exploring the room at the bottom by momentum-resolved electron microscopy |
| 13.02.2019 | Dr. Xiaochun Huang (II. Physikalisches Institut B, RWTH Aachen) | Epitaxial growth and electronic structure modulation of tellurium films on graphene |
| 01.02.2019 | Dr. Antti Laitinen (Group of Pertti Hakonen, Aalto University School of Science, Finland) | Magnetization oscillations of graphene through mechanical resonance |
| 10.12.2018 | Dr. Bernd Kästner, PTB Berlin | Near-field IR Spectroscopy: Enhancing molecular sensitivity using compressed sensing and other techniques |
| 13.02.2018 | Alistair J. Brash (Department of Physics & Astronomy, Universität Sheffield, England) | Controlling the Dynamics of a Solid State Emitter with an Optical Nanocavity |
| 30.01.2018 | Dr. Adarsh Ganesan (Department of Engineering, University of Cambridge, UK) | Phononic Frequency Combs |
| 18.01.2018 | Prof. Stefan Linden, Physikalisches institut, Universität Bonn | Simulating condensed matter phenomena with plasmonic waveguide arrays |
| 18.12.2017 | Dr. Loic Huder (Institute for Nanoscience and Cryogenics, Université Grenoble Alpes) | Link between structural and electronic properties of moirés of graphene studied by scanning tunneling microscopy |
| 20.11.2017 | Dr. Alisa Javadi (Quantum-Photonics Group, Niels-Bohr-Institut, Kopenhagen, Dänemark) | Spin-photon interface and switching using a quantum dot in a nanobeam waveguide |
| 08.09.2017 | Dr. Alexander Paarmann, Fritz-Haber-Institut der Max-Planck Gesellschaft Berlin | Nonlinear Phonon Polariton Spectroscopy |
| 03.07.2017 | Prof. Evgeny Sherman (Department of Physical Chemistry, University of Basque Country UPV/EHU) | Anomalous velocity with spin-orbit coupling |
| 23.05.2017 | Dr. Christian R. Ast (Max-Planck-Institut für Festkörperforschung, Stuttgart) | Sensing the Quantum Limit in Scanning Tunneling Spectroscopy: From the Josephson Effect to Quantum Tunneling |
| 21.04.2017 | Dr. Alexey Chernikov (Institut für Experimentelle und Angewandte Physik, Universität Regensburg) | Coulomb engineering in 2D semiconductors |
| 26.01.2017 | Prof. Ferdinand Kuemmeth (Niels Bohr Institut, Universität Kopenhagen) | Nuclear notch filtering and long-distance spin exchange in GaAs quantum dots |
| 26.01.2017 | Dr. Kai Sotthewes (MESA+ Institute for Nanotechnology, Universität Twente, Niederlande) | The single-molecule switch and transistor |
| 06.12.2016 | Florian Libisch (Institut für Theoretische Physik, TU Wien) | Dynamically encircling an exceptional point for asymmetric mode switching |
| 14.11.2016 | Markus Ternes (Max-Planck-Institut für Festkörperforschung, Stuttgart) | Classical and quantum correlations in coupled spin systems and their influence on the tunnelling conductance |
| 19.10.2016 | Alexander Poshakinskiy (loffe Institute, St. Petersburg) | Spatiotemporal spin fluctuations in 2D electron gas |
| 18.10.2016 | Benoit Hackens (Institut de la Matière Condensée et des Nanosciences (IMCN), Université catholique de Louvain (UCL), Louvain-la-Neuve, Belgium | Imaging charge transport in quantum point contacts and graphene rings |
| 17.06.2016 | Yung-Woo Park (Department of Physics and Astronomy, Seoul National University, South Korea) | Magneto Resistance of Low Dimensional Conductors in High Electric Field |
| 12.05.2016 | Professor Norbert Koch von der Humboldt Universität in Berlin | Energy level tuning at inorganic/organic semiconductor heterojunctions |
| 28.04.2016 | Menno Veldhorst (Technische Universität Delft, Niederlande) | Universal quantum logic with silicon quantum dots |
| 21.04.2016 | Silvan Schmid (Department of Materials Science and Engineering, National University of Singapore) | Damping dilution in nanomechanical SiN resonators and their applications |
| 21.04.2016 | Dr. Sayanti Samaddar (Institut Néel, Universität Grenoble, Frankreich) | Charge Disorder and Screening in Graphene |
| 15.03.2016 | Benedikt Schwarz, Institute for Solid State Electronics, TU Wien, Vienna, Austria | Quantum cascade laser/detectors and plasmonics for mid-infrared integrated photonics |
| 01.02.2016 | Pol Forn-Diaz (Institute for Quantum Computing, University of Waterloo) | Strong light-matter interaction for superconducting artificial atoms |
| 17.12.2015 | Dr. Anna Isaeva, Bertold Rasche (Technische Universität Dresden) | Topological Insulators in Bismuth-Halide and Related Systems: Design, Synthesis, Optimisation and Properties |
| 17.12.2015 | Craig Prater, Ph.D. Chief Technology Officer Anasys Instruments | Nanoscale infrared spectroscopy and chemical imaging with AFM-IR |
| 16.12.2015 | Larissa Chizhova (Technische Universität Wien) | Graphene-laser interaction: nonlinear and magneto-optical responses |
| 29.09.2015 | Juliette Mangeney (Laboratoire Pierre Aigrain, Ecole Normale Supérieure, Paris) | Coherent THz radiation: from ultrafast spectroscopy to emission from graphene |
| 10.09.2015 | Priyamvada Bhaskar (Chalmers University of Technology, Göteborg, Schweden) | Magnetotransport in Topological Insulator |
| 28.07.2015 | Prof. Sergey Tarasenko (Ioffe Physical-Technical Institute, St. Petersburg, Russia) | Zitterbewegung in spin-orbit coupled systems |
| 27.05.2015 | Prof. Dr. Michel Pioro-Ladrière (Département de Physique, Université de Sherbrooke, Canada) | Goodies for the spin qubits toolbox |
| 24.04.2015 | Prof. Dr. Hiroshi Daimon (Nara Institute of Science and Technology, Japan) | Two-dimensional photoelectron spectroscopy for the analysis of atomic and electronic structure in detail using circularly and linearly polarized light |
| 04.03.2015 | Dr. Jörg Grenzer (Ion Beam Center, Helmholtz-Zentrum Dresden-Rossendorf) | X-ray Diffraction and Scattering from Nanostructures |
| 12.02.2015 | Stéphane Berciaud (Universität Straßburg) | Optical spectroscopy of suspended graphene and graphene-based hybrid systems |
| 06.02.2015 | Prof. Frank de Groot (Utrecht University, The Netherlands) | X-ray spectroscopy: XAS, RIXS and XMCD |
| 02.02.2015 | Dr. Oana Cojocaru-Mirédin (Max-Planck-Institut für Eisenforschung, Düsseldorf) | Advanced characterization of interfaces in photovoltaic materials using correlative microscopies |
| 29.01.2015 | Dr. Daniel Hernangómez Pérez (LPMMC, CNRS, Grenoble) | Microscopics of Disordered Quantum Hall Systems with Rashba Spin-Orbit Coupling |
| 15.01.2015 | Prof. Ivan K. Schuller (Physics Department of University of California San Diego, USA) | Hybrids: Materials and Physics |
| 05.12.2014 | Jose A. Garrido (Walter Schottky-Institut, TU München) | CVD-based graphene field-effect transistors and electrodes |
| 04.12.2014 | Dr. Florian Libisch (TU Wien) | Edge and substrate effects in graphene nanodevices |
| 27.11.2014 | Prof. Dr. Diederick Depla (Department of Solid States Sciences, Ghent University) | Magnetron sputter deposition of biaxial aligned thin films |
| 24.11.2014 | Dr. Vladimir Kaganer (Paul-Drude-Institut für Festkörperelektronik, Berlin) | X-ray diffraction studies of epitaxial films during their growth: molecular beam epitaxy in vivo |
| 06.10.2014 | Prof. Dr. Manfred Bayer (TU Dortmund) | Rydberg excitons in cuprous oxide |
Donnerstag, 9. Januar 2020, 11:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Prof. Yishay Manassen (SPM Research Group, Ben Gurion University Beer Sheva, Israel)
auf Einladung von Prof. Markus Morgenstern
Magnetic resonance on the single spin level
Single spin detection is one of the central challenges of nano science and technology. We have
developed an STM related technique for single spin magnetic resonance (ESR-STM). We
measure high frequency noise power densities in the STM tunneling current. When above a
single spin in an external magnetic field, it reveals peaks at the Larmor frequencies. This is done
at room temperature, and without spin polarized tunneling. In order to detect weak rf signals (of
the order of 1-3 pA) we use matching circuits, modulation techniques and sensitive detectors
(spectrum analyzers or rapid oscilloscopes).
ESR-STM measurements on different spin centers revealed g, hyperfine and zero field
splitting tensors up to a single spin levels. This reveals information on the local environment of
the single spin – which is not detectable microscopically. We use the hyperfine levels for doing
single spin double resonance measurements to detect the nuclear transitions. In addition we showthat the STM affects the molecular dynamics - making is anisotropic. In addition, the linewidth is linearly dependent on the bias voltage (at a fixed distance) and the exchange coupling with the tunneling electrons is estimated.
Current results have demonstrated the capability to detect a single spin hyperfine spectrum of
one Tempo molecule. When there is another molecule nearby, in the case that the molecules are
bound to give correlated motion, the dipolar interaction between the molecules, modifies the
spectrum and enables to calculate the distance between the two molecules. The zero field
splitting tensor is observed from the spectrum by easy spin software. Together with the order
parameter this gives the distance between the two spins. The distance is in agreement with the
STM image. We demonstrate that changing the tunneling conditions does not affect the observed
distance, although the spectrum looks different (broadens by the larger bias). We show that each
spectrum corresponds to a stable cluster which looks the same as the cluster seen in the image.
This opens the way of precise STM evaluation of the structure of soft disordered clusters of
molecules. It is actually a single molecule measurement of molecular docking. We also recall
that such dipolar interaction may be important in quantum information processing (CNOT gate)
and in the presence of exchange interaction – also for transport of the information.
Freitag, 29. November 2019, 11:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Annika Kurzmann (Solid State Physics Laboratory, ETH Zürich, Schweiz)
auf Einladung von Prof. Christoph Stampfer
Electrostatically-Defined Quantum Dots in Bilayer Graphene
Graphene is a promising candidate for future nano-electronic devices including building blocks for quantum information processing. Reasons are the expected long spin lifetimes and high carrier mobility. Recent improvements in fabrication technologies for graphene nanostructures, namely, the encapsulation between boron nitride, edge-contacting, graphite back-gates and the use of electrostatic gating of bilayer graphene, have leveraged the quality of quantum dots to such an extent, that few-electron or -hole quantum dots have been realized that are comparable to the best devices in gallium arsenide [1].
Here we confine charge carriers laterally by applying strong displacement fields forcing charge carriers to flow through a narrow channel (see inset in Fig. 1). In transport direction, charge carriers are confined by pn-junctions forming natural tunnel barriers, thus creating a p-type quantum dot coupled to n-type leads, or vice versa. In this ambipolar system, we can realize both single electron and single hole occupation of the respective quantum dots showing charging energies on the order of 5 meV (see Fig.1). In addition, we can use our design to form multi-dots [2,3].
We use finite bias spectroscopy to study and identify the single-particle and many-body ground- and excited states (marked with arrows in Fig. 1) of electrostatically-defined quantum dots in bilayer graphene trapping only one or two charge carriers [4]. While the properties of the material bear similarities to carbon nanotubes and silicon because of the two-fold valley and spin-degeneracies, the results of our experiments allow us to propose a remarkably clear level scheme for two-particle spectra, in which the spin- and valley-entanglement, as well as exchange interactions play a crucial role. With this level scheme at hand, future experiments can investigate spin- and valley-coherence and relaxation times, which are key parameters to be compared to other material systems.
Figure 1. Coulomb diamond measurement for a quantum dot that confines holes. Since the quantum dot can be completely depleted, we can label each diamond with the occupation number of the quantum dot. Inset: AFM image of the quantum dots. The white scale bar has a width of 100 nm.
[1] M. Eich, R. Pisoni, H. Overweg, A. Kurzmann, Y. Lee, P. Rickhaus, F. Herman, M. Sigrist, K. Watanabe, T. Taniguchi, T. Ihn and K. Ensslin, Phys. Rev. X 8, 031023 (2018).
[2] M. Eich, R. Pisoni, A. Pally, H. Overweg, A. Kurzmann, Y. Lee, P. Rickhaus, F. Herman, M. Sigrist, K. Watanabe, T. Taniguchi, K. Ensslin, and T. Ihn, Nano Lett. 18, 5042-5048 (2018).
[3] L. Banszerus, B. Froh, A. Epping, D. Neumaier, K. Watanabe, T. Taniguchi, and C. Stampfer, Nano Lett. 18, 4785-4790 (2018).
[4] A. Kurzmann, M. Eich, H. Overweg, M. Mangold, P. Rickhaus, R. Pisoni, Y. Lee, R. Garreis, C. Tong, K.Watanabe, T. Taniguchi, T.Ihn, and K. Ensslin, arXiv:1904.07185 [cond-mat.mes-hall] (201
Donnerstag, 7. November 2019, 11:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Prof. Sudeshna Chattopadhyay (Discipline of Physics, Indian Institute of Technology Indore)
auf Einladung von Prof. Dr. Uwe Klemradt
Confined semiconductors and their composite systems: Effect of interface to control structure-properties and its applications
Miniaturization of materials in nano-dimensions leads to the development of functional nanomaterials and induces a spectrum of novel applications which are generally different when compared to materials in micro-range. Semiconducting nanomaterials confined in different dimensions is a versatile platform to investigate and build functional devices, owing to rich physics occurring at the interface of the confined material. In this respect, surface and interfaces are the two significant parameters to maneuver the performance of semiconducting materials, particularly for hybrid structures and confined systems, in various reaction processes. Studies on effects of surface and interfaces for different semiconductors (e.g., direct band gap semiconductor, ZnO; indirect bandgap semiconductor SiC), indicating their great flexibility in structural tunability, bandgap engineering/optical and electronic properties, and broad application prospects in optoelectronic devices, electrochemical and photocatalytic efficiency, will be presented in this talk. The study demonstrates the identification and optimization of key parameter/s to provide the pathway to achieve the enhancement of material properties and its tuning related to a particular application: (a) A novel way of bandgap narrowing (up-to30% reduction) and tuning photocatalytic activity/efficiency (enhancement up to 1000%) in epitaxial graphene/silicon carbide (EG/SiC) yields high efficiency photocatalyst. Graphitization of SiC by high-temperature thermal decomposition method with different annealing time forms sets of EG/SiC composites having different quality of graphene layers. The Raman intensity ratio of the 2D band to the G band, I2D/IG, which represents a measure of quality and quantity of graphene and heterojunction interface layer between EG and SiC. I2D/IG plays a crucial role in tuning the bandgap and enhancement of photocatalytic activity of EG/SiC composites. (b) Significance of degree of confinement of soft-matter template (polymer, PT) to alter the interface of metal-oxide/polymer (ZnO/PT) hybrid system abruptly: Noticeable influence in growth of metal oxide thin film (ZnO), producing large variation in the defect levels, manifested in photoluminescence (PL) and absorption spectra, signifying importance in the research on opto-electronic applications of ZnO thin-films in the field of organic-electronics. (c) Significantly high electrochemical performance of atomic layer deposition (ALD) grown nanoscale ZnO film (< 50 nm) as light-weight, binder-free cathode for safer, low-cost, high performance rechargeable Al-ion battery.
Donnerstag, 2. Mai 2019, 11:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Dr. Samer Houri (Physical Science Laboratory, NTT Basic Research Laboratories, Japan)
auf Einladung von Prof. Dr. Christoph Stampfer
Acoustic Frequency Combs: Implementation and Prospects
Optical frequency combs are systems that generate a spectral output with equidistant frequency components, such systems have gained wide popularity in spectroscopy and timing applications. Such optical combs were first generated using mode-locked cavities, however, very interesting progress has been achieved recently using micro-resonators to produce Kerr combs. Acoustic (phononic) frequency combs, although underdeveloped compared to optical ones, are emerging as an equally interesting field of investigation, especially by leveraging nonlinearities in M/NEMS devices. In this presentation, I will briefly introduce optical frequency combs, and then move to discuss state of the art in acoustic frequency combs, their implementation, and future prospects, particularly the possibility to generate acoustic Kerr combs.
Montag, 8. April 2019, 16:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Dr. Knut Müller-Caspary (Ernst Ruska Centre, Forschungszentrum Jülich)
Strain, polarisation, atomic electric fields: Exploring the room at the bottom by momentum-resolved electron microscopy
By focusing electrons on probes with a diameter of 50 pm, aberration-corrected scanning transmission
electron microscopy (STEM) is now capable of probing subatomic details. With the recent advent of
ultrafast cameras, it is now possible to record millions diffraction patterns in a few minutes time while
scanning a focused electron probe over the specimen, being the birth of momentum-resolved STEM.
After a comprehensive introduction into the fundamentals of electron scattering and signal formation in
electron microscopy using basic physical concepts, it will be demonstrated how the wealth of
diffraction information can be interpreted so as to measure physical properties of materials directly. In
particular, recent showcases such as the high-precision strain mapping in field effect transistors, the
polarisation measurement in AlN/GaN nanodiscs, and the atomic-scale charge density mapping in 2D
materials will be presented. The talk closes with an outlook on future electron microscopy as a
multidimensional information channel with combined spatial, momentum, energy and time resolution.
Mittwoch, 13. Februar 2019, 10:00 Uhr, Seminarraum MBP2 117 (Physikzentrum)
Dr. Xiaochun Huang (II. Physikalisches Institut B, RWTH Aachen)
auf Einladung von Prof. Markus Morgenstern
Epitaxial growth and electronic structure modulation of tellurium films on graphene
In recent years, elementary two-dimensional (2D) materials, such as Graphene, Stanene, Bismuthene et al., have attracted tremendous interests. On one hand, these honeycomb materials are promising candidates of quantum spin Hall insulators, on the other hand, they are novel building blocks, which can be longitudinally stacked together to build various van der Waals heterostructures. Moreover, using these atomically thin materials, one can create in-plane p-n junctions which have great potential applications in next-generation devices, such as in-plane transistors.
This talk will focus on epitaxial growth and electronic structure modulation of tellurium films on graphene/6H-SiC(0001). I will show the fabrication of a seamless in-plane p–n junction with monolayer tellurium film.
Furthermore, I will show some transport measurements of the chiral anomaly in Weyl semimetal TaAs. Following these, I will briefly show my work on first-principles calculations of 3D topological insulators and the design of a hydrogenation chamber.
Freitag, 1. Februar 2019, 11:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Dr. Antti Laitinen (Group of Pertti Hakonen, Aalto University School of Science, Finland)
auf Einladung von Christoph Stampfer
Magnetization oscillations of graphene through mechanical resonance
Magnetization oscillations (i.e. the de Haas-van Alphen effect) have proven to be a useful tool for mapping Fermi surfaces of various interesting materials, such as unconventional superconductors. Graphene and 2D materials in general have also been investigated extensively theoretically as behavior different from bulk materials is expected. However, only few experimental studies about de Haas-van Alphen effect in graphene, such as Ref. [1], can be found. This is due to difficulties with fabricating good enough devices, but also because of competing quantum capacitance effects that can in fact overwhelm magnetization related phenomena, as demonstrated in Ref. [2].
I will discuss our recent measurements of mechanical resonance in monolayer graphene Corbino disks [3,4] as a function of perpendicular magnetic field strength. In these experiments, we have observed comparably small frequency shifts consistent with magnetization effects and the absence of quantum capacitance effects. I will discuss the competition of these two contributions in our system, as well as connections to the previous experiments.
[1] V. Singh, et al., Appl. Phys. Lett. 100, 233103 (2012).
[2] C. Chen, et al., Nature Phys. 12, 240 (2016).
[3] A. Laitinen, et al., Phys. Rev. B 97, 075113 (2018).
[4] M. Kumar, et al., Nature Commun. 9, 2776 (2018).
Montag, 10.12.18, 11:00-11:45 Uhr Physikzentrum, Modulbau 2, MBP2 015
Dr. Bernd Kästner
( PTB Berlin)
Near-field IR Spectroscopy: Enhancing molecular sensitivity using compressed sensing and other techniques
Near-field IR spectroscopy using the scattering-type SNOM allows not only sub-diffraction spatial resolution but also significantly higher molecular sensitivity than conventional far-field IR spectroscopy. However, some important applications, such as drug localization in biological cells, require even higher detection sensitivity. In this talk I will discuss recent strategies based on compressed sensing, plasmon interferometry, optimized radiation sources and detectors, as well as tailored tips for pushing the detection limit of surface molecules.
Dienstag, 13. Februar 2018, 10:30 Uhr, Seminarraum MBP2 116 (Physikzentrum)
Alistair J. Brash
(Department of Physics & Astronomy, Universität Sheffield, England)
Controlling the Dynamics of a Solid State Emitter with an Optical Nanocavity
The InAs/GaAs materials system presents a promising approach towards a fully-integrated platform for optical quantum technologies. In such a scheme, self-assembled quantum dots (SAQDs) emit single photons which are guided and manipulated by on-chip nano-optics before being detected by on-chip superconducting nanowire detectors. High performance single photon sources have been realised using SAQDs coupled to micropillar cavities. however these emit into free-space, orthogonal to the chip. By contrast, QDs coupled to nanophotonic structures such as waveguides for on-chip routing have often exhibited rather disappointing indistinguishabilities, attributed to charge noise from nearby etched surfaces.
In this seminar I will focus on our recent studies of an on-chip single photon source based on a SAQD in a waveguide-coupled H1 photonic crystal cavity. By developing a novel double π-pulse resonance fluorescence (DPRF) technique we are able to demonstrate a very short QD radiative lifetime of only 22.7 ps in the weak coupling regime. Using a variety of experimental techniques, we show that this enables operation as an on-chip, on-demand single photon source with high indistinguishability and repetition rate. In particular, we demonstrate that the short radiative lifetime renders pure dephasing of the emitter almost negligible and also acts to reduce the influence of spectral wandering on the photon indistinguishability on longer timescales. In addition, I will discuss the influence of the coupling between the QD-confined exciton and longitudinal acoustic (LA) phonons of the semiconductor lattice and present some initial results on how an optical nanocavity may be used to control these interactions.
Dienstag, 30. Januar 2018, 11:15 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Dr. Adarsh Ganesan (Department of Engineering, Universität Cambridge, England)
auf Einladung von Christoph Stampfer
Phononic Frequency Combs
The decades-old optical frequency comb has revolutionized optical frequency metrology. However, only very recently, the first experimental demonstration of its phononic analogue of such frequency combs (Ganesan et al. PRL 2017) came about in a micromechanical resonator device, confirming predictions made by numerical simulations of a Fermi-Pasta-Ulam system (Cao et al. PRL 2014). While the spectral features are similar, the physics of phononic frequency comb generation is conceivably different from that of its optical counterpart. My talk will not only highlight such differences but also will establish ‘why micromechanical resonator is an ideal experimental host for phononic frequency combs?’ The talk will also shed light on the possible engineering applications which are unique to phononic frequency combs.
Montag, 18. Dezember 2017, 14:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Dr. Loic Huder (Institute for Nanoscience and Cryogenics, Université Grenoble Alpes)
auf Einladung von Markus Morgenstern
Link between structural and electronic properties of moirés of graphene studied by scanning tunneling microscopy
The superimposition of two mismatched periodic patterns leads to the apparition of a moiré pattern. In condensed-matter physics, the moiré pattern is known to occur in Scanning Tunneling Microscopy (STM) images of two superimposed atomic layers with a twist angle and/or a lattice parameter difference between the two lattices. The moiré pattern depends highly on these mismatches and in addition acts as a superperiodic potential on the electrons. This leads to a strong link between the structure and the electronic properties of moiré systems, such as van der Waals heterostructures composed of different 2D materials.
The subject of this presentation is the study by low-temperature STM of this link in one of the simplest van der Waals structure: twisted graphene layers, i.e. the misorientated stacking of two graphene layers. Detailed analyses of STM images show that not only rotation but also strain between the layers are involved in the formation of the moiré pattern. This strain between the layers, called heterostrain, is shown to change more effectively the electronic local density of states (LDOS) than strain applied to both layers. The effect of this heterostrain on the LDOS was measured and well reproduced by theoretical calculations. The link between strain and electronic properties was also exploited by inducing deformations in the graphene layer with the STM tip to trigger changes in the LDOS. This effect appears to be modulated by the moiré periodicity and can lead to unexpected changes in STM/STS measurements. Finally, if time allows, I will present a novel approach to change the electronic properties of graphene by growing it on high-quality tantalum carbide. This process allows the formation of moirés of graphene in which superconductivity was induced by proximity effect.
18.01.2018 um 16.15 Uhr im Raum 28 B 110
Prof. Stefan Linden, Physikalisches institut, Universität Bonn
Titel: Simulating condensed matter phenomena with plasmonic waveguide arrays
hostet by Professor Taubner
Arrays of coupled waveguides can show interesting dynamics that resemble
quantum mechanical condensed matter phenomena . The basis for this is
the mathematical equivalence between the time-dependent Schrödinger
equation and the paraxial Helmholtz equation that describes the
propagation of light in such arrays. Hence, mapping the time-dependent
probability distribution of an electronic wave packet to the spatial
light intensity distribution in the waveguide arrays allows us to mimic
the quantum mechanical evolution in a coherent, yet classical wave
environment.
Here, we show that arrays of evanescently coupled dielectric-loaded
surface plasmon polariton waveguides (DLSPPWs) can be used to study the
dynamics of systems that can be described by a 1D single-particle
tight-binding Hamiltonian. In particular, we report on the observation
of topologically protected edge states and Anderson localization.
Montag, 20. November 2017, 14:00 Uhr, Seminarraum 28 A 301 (Physikzentrum)
Alisa Javadi (Quantum-Photonics Group, Niels-Bohr-Institut, Kopenhagen, Dänemark)
auf Einladung von Hendrik Bluhm
Spin-photon interface and switching using a quantum dot in a nanobeam waveguide
Single quantum emitters coupled to single optical modes can enable different blocks for optical quantum-information processing. In this talk, I will present our recent experimental results on interfacing the spin-state of an electron in a InAs/GaAs quantum dot to a nanobeam waveguide. I will show that we can achieve deterministic charging of a quantum dot with a single electron and that we can utilize the spin-state of the electron to control the transmission of the waveguide in a programmable manner. I will also touch on the prospects of using such an interface to build single-photon transistors and quantum gates between photons.
In the last part of the talk, I will give an overview of our recent effort towards realizing strong optical nonlinearities using a quantum dot embedded in a photonic-crystal waveguide. I will show that we can experimentally achieve a contrast of 30% in the transmission spectrum of our artificial-1D-atom along with nonlinearities at the single-photon level.
Dienstag, 23. Mai 2017, 12:00 Uhr, Hörsaal 28 B 110
Dr. Christian R. Ast (Max-Planck-Institut für Festkörperforschung, Stuttgart)
auf Einladung von Markus Morgenstern
Sensing the Quantum Limit in Scanning Tunneling Spectroscopy: From the Josephson Effect to Quantum Tunneling
The tunneling current in scanning tunneling spectroscopy (STS) is typically and often implicitly modeled by a continuous and homogeneous charge flow. If the charging energy of a single-charge quantum sufficiently exceeds the thermal energy, however, the granularity of the current emerges. In this quantum limit, the capacitance of the tunnel junction mediates an interaction of the tunneling electrons with the surrounding electromagnetic environment and becomes a source of noise itself, which cannot be neglected in STS. Using a scanning tunneling microscope operating at 15 mK, we show that we operate in this quantum limit, which determines the ultimate energy resolution in STS. A theoretical description requires quantum electrodynamics to be included resulting in the (E)-theory which describes the probability for a tunneling electron to exchange energy with the environment. I will discuss this effect using a superconducting aluminium tip and a superconducting aluminium sample, where it is most pronounced. These considerations will be most important also at higher temperatures for extremely sharp spectral features, such as superconducting gaps, sharp Kondo peaks or Yu-Shiba-Rusinov states.
Freitag, 21. April 2017, 10:30 Uhr, Raum 28 A 301
Dr. Alexey Chernikov (Institut für Experimentelle und Angewandte Physik, Universität Regensburg)
auf Einladung von Christoph Stampfer
Coulomb engineering in 2D semiconductors
Since the discovery of graphene, a single sheet of carbon atoms, research focused on two-dimensional (2D) van der Waals materials evolved rapidly due the availability of atomically thin, thermally stable crystals with intriguing physical properties. The 2D materials naturally inherit major traits associated with systems of reduced dimensionality: strongly enhanced interactions, efficient light-matter coupling, and sensitivity to the environment. In particular, the considerable strength of the Coulomb forces introduces a rich variety of many-body phenomena including significant renormalization of the bandgap and the emergence of tightly bound exciton quasi-particles.
In this talk, I will show how atomically-thin crystals offer an alternative approach to nanoscale bandgap engineering, based on the local tuning of the Coulomb interaction and the environmental sensitivity of 2D materials. I will demonstrate how careful tailoring of the surrounding dielectric environment allows us to tune the electronic bandgap of single layers of semiconducting transition-metal dichalcogenides by many 100’s of meV and present an in-plane dielectric heterostructure as an illustration. The unique advantages of the Coulomb engineering in 2D, including nanometer sensitivity and a high flexibility of resulting dielectric heterostructures, will be further discussed. Finally, I will give a brief outlook towards new pathways for manipulating and designing electronic bandgaps in the 2D plane.
03.07.2017, 11.00 Uhr, 28 A 301
Prof. Evgeny Sherman (Department of Physical Chemistry, University of the Basque Country UPV/EHU and IKERBASQUE, Basque Foundation for Science, Bilbao, Spain)
hosted by Prof. Markus Morgenstern
Anomalous velocity with spin-orbit coupling
Spin-orbit coupling is usually represented in condensed matter physics as a properly chosen symmetry-allowed combination of products of spin operators and the particle momentum components. By the general quantum mechanics rules, this interaction introduces a new spin-dependent component in the particle velocity, the so-called anomalous term. Here we present and discuss the general concept of this anomalous velocity and analyze several examples of its critical effects in the physics of the spin-orbit coupled condensed matter. These examples include: (1) Prohibition of collapse in spin-orbit coupled self-attractive Bose-Einstein condensates, (2) short-term spin dynamics in random one-dimensional systems, and (3) coherent spin dynamics in cold atomic gases described by the Fermi- or the Bose-statistics, in synthetic gauge fields.
Freitag, den 08.09.2017, 10.30 Uhr, Raum 28 B 110
Dr. Alexander Paarmann, Fritz-Haber-Institut der Max-Planck Gesellschaft Berlin
auf Einladung von Prof. T. Taubner
Nonlinear Phonon Polariton Spectroscopy
There has been much increasing interest recently in the mid-infrared spectral response of polar dielectric crystals due to a novel branch of photonics based on surface phonon polaritons (SPhPs) [1], which arise at the surfaces and interfaces of these materials due to the optical phonon resonances in their dielectric response. Using our in-house free-electron laser [2] as intense and tunable infrared light source, we have developed several new approaches of nonlinear solid state spectroscopy, focusing on phonon resonances in dielectrics. Specifically, mid-infrared second harmonic generation (SHG) spectroscopy is exemplified with a model study of SiC [3], and used to identify optical phonon hybridization in atomic-scale heterostructures. We also apply the technique to probe the optical field enhancement associated with resonantly excited SPhPs in sub-diffractional nanostructures [4], as well as prism-coupled propagating SPhPs using the Otto geometry [5]. The latter approach enables mapping of the full SPhP dispersion which, for instance, reveals a new phenomenon of tunable, strong coupling between ultrathin film polaritons and SPhPs, demonstrated for AlN films on a SiC substrate.
Donnerstag, 26. Januar 2017, 11:30 Uhr, Raum 28 A 301
Ferdinand Kuemmeth (Niels Bohr Institut, Universität Kopenhagen)
auf Einladung von Hendrik Bluhm
Nuclear notch filtering and long-distance spin exchange in GaAs quantum dots
Spin qubits based on semiconducting quantum dots are promising candidates for quantum computation, due to their potential for miniaturization, scalability and fault tolerance. In this talk I will present recent results on how to mitigate nuclear noise in GaAs spin qubits using nuclear notch filtering techniques, and on achieving coherent exchange operations between distant spins using a multielectron quantum dot.
In order to decouple a singlet-triplet qubit from nuclear spin fluctuations, we investigate Carr-Purcell-Meiboom-Gill (CPMG) sequences applied to GaAs double dots. At high magnetic fields we find that qubit dephasing is limited by narrow-band high-frequency noise arising from Larmor precession of 69Ga, 71Ga, 75As nuclear spins. By aligning the notches of the CPMG filter function with differences of the discrete nuclear Larmor frequencies we
demonstrate a qubit coherence time of 0.87 ms, i.e. more than five orders of magnitude longer than the duration of a π exchange gate in the same device.
Further, we use a singlet-triplet qubit implemented in a GaAs double dot to probe the exchange coupling between one of its dots and a nearby multielectron dot. We find that the spin-exchange energy can have opposite sign compared to exchange between singlyoccupied dots. This behavior occurs already at zero magnetic field, is robust to in-plane magnetic fields, and can be changed by applying out-of-plane magnetic fields or by changing the occupancy of the multielectron dot. By coupling a second singlet-triplet qubit to the multielectron dot, we map out different configurations useful for long-distance spin exchange, including superexchange, direct spin exchange, and on-site exchange mediated by the multielectron dot. Our results show a pathway to implementing fast, non-nearest
neighbor two-qubit gates in semiconducting spin qubits.
Donnerstag, 26. Januar 2017, 10:15 Uhr, Raum 28 A 301
Kai Sotthewes (MESA+ Institute for Nanotechnology, Universität Twente, Niederlande)
auf Einladung von Markus Morgenstern
The single-molecule switch and transistor
In order to design and realize single-molecule devices it is essential to have a good understanding of the properties of an individual molecule. For electronic applications, the most important property of a molecule is its conductance. Here we show how a single octanethiol molecule can be connected to macroscopic leads and how the transport properties of the molecule can be measured. Based on this knowledge we have realized two single-molecule devices: a molecular switch and a molecular transistor. The switch can be opened and closed at will by carefully adjusting the separation between the electrical contacts and the voltage drop across the contacts. This single-molecular switch operates in a broad temperature range from cryogenic temperatures all the way up to room temperature. Via mechanical gating, i.e., compressing or stretching of the octanethiol molecule, by varying the contact’s interspace, we are able to systematically adjust the conductance of the electrode-octanethiol-electrode junction. This two-terminal single-molecule transistor is very robust, but the amplification factor is rather limited.
Dienstag, 06.12.2016, 15:45 Uhr, Raum 28 A 301
Florian Libisch (Institut für Theoretische Physik, TU Wien)
auf Einladung von Markus Morgenstern
Dynamically encircling an exceptional point for asymmetric mode switching
Physical systems with loss or gain have resonant modes that decay or grow exponentially with time. Whenever two such modes coalesce both in their resonant frequency and their rate of decay or growth, an ‘exceptional point’ occurs, giving rise to fascinating phenomena that defy our physical intuition. Here we demonstrate dynamical encircling of an exceptional point through a two-mode waveguide with suitably designed boundaries and losses. We realize a corresponding waveguide structure that steers incoming waves around an exceptional point during the transmission process, inducing mode transitions that transform our device into a robust and asymmetric switch between different waveguide modes.
Montag, 14.11.2016, 11:00 Uhr, Raum 28 A 301
Markus Ternes (Max-Planck-Institut für Festkörperforschung, Stuttgart)
auf Einladung von Markus Morgenstern
Classical and quantum correlations in coupled spin systems and their influence on the tunnelling conductance
In recent years inelastic spin-flip spectroscopy using a low-temperature scanning tunneling microscope has been a very successful tool for studying not only individual spins but also complex coupled systems. When these systems interact with the electrons of the supporting electrodes correlated many-particle states can emerge, making them ideal prototypical quantum systems. In this presentation I will show how the controlled coupling of individual spin systems can lead not only to an energy shift of the eigenstates reminiscent of an externally applied field, but also to a bias asymmetry in the differential conductance. Using S = 1 and S = 1/2 model systems of CoHx on a h-BN/Rh(111) substrate in conjunction with model Hamiltonians which takes the coupling and correlation to the environment explicitly into account enables to precisely determine and control the emergence of correlations between the two subsystems on tip and sample.
Mittwoch, 19.10.2016, 14.00 Uhr, Raum 28 A 301
Alexander Poshakinskiy (loffe Institute, St. Petersburg)
auf Einladung von Dr. Bernd Beschoten, 2nd Institute of Physics A
Spatiotemporal spin fluctuations in 2D electron gas
Spin noise spectroscopy is the state-of-the-art tool for the study of spin dynamics in conditions close to thermal equilibrium and the measurement of spin relaxation times and the
frequencies of spin precession. In the talk we present a theory describing the thermal fluctuations of spin density in a two-dimensional electron gas. We show that temporal and
spatial correlations of spin fluctuations are coupled due to Brownian motion of electrons and spin-orbit interaction.
Dienstag, 18.10.2016, 11.15 Uhr, Raum 28 A 301
Benoit Hackens (Institut de la Matière Condensée et des Nanosciences (IMCN), Université catholique de Louvain (UCL), Louvain-la-Neuve, Belgium
auf Einladung von Professor Christoph Stampfer
Imaging charge transport in quantum point contacts and graphene rings
Quantum transport in nanodevices is usually probed thanks to measurements of the electrical resistance or conductance, which lack the spatial resolution necessary to probe local-scale electron behaviour inside the devices. Here, we will discuss real-space images of quantum transport phenomena inside two archetypal mesoscopic devices: quantum point contacts (QPCs) and quantum rings (QRs). The results were obtained using low temperature scanning gate microscopy (SGM), a technique based on mapping the electrical conductance of a device as an electrically-biased sharp metallic tip scans in its vicinity [1].
Freitag, 17.06.2016, 15:00 Uhr, Seminarraum 28 B 110
Yung-Woo Park (Dept. of Physics and Astronomy, Seoul National University, South Korea)
auf Einladung von Markus Morgenstern
Magneto Resistance of Low Dimensional Conductors in High Electric Field
The zero magneto resistance (ZMR) observed in polyacetylene nanofibers in high electric field is explained with the de-confined conduction of spinless charged solitons which is a 1-D topological insulator. The magneto resistance (MR) of 2-D MoS2 monolayer decreases becoming zero in high electric field. However, unlike the polyacetylene nanofibers, the reduction of the magneto resistance of MoS2 monolayer in high electric field is dominated by heating effect. The possibility of phase transformation between semiconducting (2H) and metal (1T) phases in MoS2 monolayer by annealing in high electric field is discussed in view of the Scanning Transmission Electron Microscope (STEM) studies of Yung-Chang Lin, et. al.
Donnerstag, 12.05.2016, 9.15 Uhr, Raum MBP1 026
Professor Norbert Koch (Humboldt Universität Berlin)
auf Einladung von Professor Matthias Wuttig
Energy level tuning at inorganic/organic semiconductor heterojunctions
The combination of inorganic and organic semiconductors provides for the potential to realizing high performance light emission or photovoltaic devices. While inorganic semiconductors feature high charge carrier mobility and support high excitation densities, organic semiconductors exhibit strong light-matter coupling and their energy spectrum can be tuned easily by molecular design. One of the key challenges to optimize inorganic/organic heterojunctions is controlling the interface electronic structure, which governs its functionality. To tune the "intrinsic" energy level alignment at the interface of a given material pair, e.g., to optimize energy transfer versus charge transfer, the work function of the inorganic semiconductor surface can be modified with appropriate monolayers of molecular donors or acceptors, so that the organic semiconductor levels are re-aligned accordingly. One mechanism that limits the range of level tuning is Fermi-level pinning at the frontier levels of the organic semiconductor. On the inorganic side, the energy and density of surface states turns out to play a crucial role for level tuning. Prototypical heterojunctions, comprising ZnO and GaN as inorganic component and intrinsic versus p-doped organic semiconductors, are discussed.
Donnerstag, 28.04.2016, 11:30 Uhr, Seminarraum 28 A 301
Menno Veldhorst (Technische Universität Delft, Niederlande)
auf Einladung von Hendrik Bluhm
Universal quantum logic with silicon quantum dots
Silicon, the material that defined our current information age, can be an excellent platform for large-scale quantum computation. In this talk, I will present our efforts in this direction. In the few-qubit regime, we have experimentally captured single electrons inside quantum dots, such that its corresponding spin states can be operated as qubits. These qubits can be individually controlled, operated with high fidelity, and coupled via the exchange interaction. In the many-qubit regime, we have designed a classical control architecture to operate an extendable two-dimensional qubit plane.
Donnerstag, 21.04.2016, 16:15 Uhr, Seminarraum 28 B 110
Silvan Schmid (Dept. of Materials Science and Engineering, National University of Singapore)
auf Einladung von Christoph Stampfer
Damping dilution in nanomechanical SiN resonators and their applications
Nanomechanical silicon nitride (SiN) resonator can have quality factors (Q) of several million. Such high Qs are facilitated by a tensile-stress-induced damping dilution mechanism. The high intrinsic tensile stress in SiN increases a resonator’s stored energy without significantly affecting the intrinsic energy losses. In essence, the tensile stress dilutes the effect of the intrinsic losses on Q. The analysis of stress-corrected intrinsic loss has exhibited a thickness dependence, suggesting ubiquitous surface loss as limiting damping mechanism in thin SiN resonators. High-Q nanomechanical SiN resonators have attracted a lot of attention in fundamental research, e.g. in quantum cavity optomechanics, and for sensing applications, e.g. for optoelectromechanical signal detection, photothermal microscopy and mass spectrometry of single nanoparticles, infrared spectroscopy of pictogram samples, and photothermal analysis of single nanoplasmonic antennas.
Donnerstag, 21.04.2016, 11:15 Uhr, Seminarraum 28 A 310
Sayanti Samaddar (Institut Néel, Universität Grenoble)
auf Einladung von Markus Morgenstern
Charge Disorder and Screening in Graphene
The charge carrier density in graphene on a dielectric substrate such as SiO2 displays inhomogeneities, the so-called charge puddles. Scanning probe studies have so far addressed the origin of such charge disorder and their role as scattering centers that generates quasiparticle interferences. However, the response of the charge puddles themselves to a change in carrier concentration has remained unanswered. Because of the linear dispersion relation in monolayer graphene, the puddles are predicted to grow near charge neutrality, a markedly distinct property from conventional two-dimensional electron gases. By performing scanning tunneling microscopy/spectroscopy on a mesoscopic graphene device, we directly observe the puddles' growth, both in spatial extent and in amplitude, as the Dirac point is approached. Self-consistent screening theory, together with the consideration of the impact of the STM tip as an electric gate, provides a unified description of both the macroscopic transport properties and the microscopically observed charge disorder.
Dienstag, 15.03.2016, 10:00 bis 12:00, MBP2 117
Benedikt Schwarz
Institute for Solid State Electronics, TU Wien, Vienna, Austria
Auf Einladung von Prof. Taubner
The increasing demand of rapid sensing and diagnosis in remote areas requires the development of compact and cost-effective mid-infrared sensing devices. I will present a monolithic integration approach for sensors, combining two major technologies: quantum cascade structures and surface plasmon polaritons [1].
A bi-functional quantum cascade laser/detector is used, where, by changing the applied bias, the device switches between laser and detector operation [2]. Recent results show, that bi-functional operation is not necessarily connected with a performance drawback. Once the layer structure has been grown on the substrate, different parts of the chip can be used for lasers and others for detectors.
The devices are connected via a dielectric-loaded surface plasmon polariton waveguide with a twofold function: it provides a high coupling efficiency and a strong interaction with the environment (e.g., a surrounding fluid). Our improved prototype sensor chip offers real-time monitoring of water in isopropanol with a 10ppm resolution over a large concentration range of 0-60%. An array of such laser/waveguide/detector units, each sensitive to another wavelength can be used as miniaturized spectrometer covering a range of 100-150cm-1.
Montag, 01.02.2016, 15.00 Uhr, Seminarraum 28 A 301
Pol Forn-Diaz (Institute for Quantum Computing, University of Waterloo)
auf Einladung von Lars Schreiber
Strong light-matter interaction for superconducting artificial atoms
The study of the interaction of radiation and matter has led to several fundamental discoveries as well as many important technologies. Over the last decades, great strides have been made in increasing the strength of this interaction at the single-photon level, leading to a constant exploration of new physics and applications. In recent years, a major achievement has been the demonstration of the so-called strong coupling regime, a key advancement enabling great progress in quantum information science. In this work, we demonstrate light-matter interaction at least an order of magnitude stronger than previously reported, reaching a new regime of ultrastrong coupling (USC). We achieve this using a superconducting artificial atom tunably coupled to the electromagnetic continuum of a one-dimensional waveguide. For the largest values of the coupling, the spontaneous emission rate of the atom is comparable to its transition frequency. In this USC regime, the conventional quantum description of the atom and light as distinct entities breaks down, and a new description in terms of hybrid states is required. Our results open the door to a wealth of new physics and new applications. Beyond light-matter interaction itself, the tunability of our system makes it promising as a model to study a number of important physical systems such as the well-known spin-boson and Kondo models.
Donnerstag, 17.12.2015, 11.00 Uhr, Seminarraum 28 A 301
Dr. Anna Isaeva, Bertold Rasche (Technische Universität Dresden)
auf Einladung von Prof. Morgenstern
Topological Insulators in Bismuth-Halide and Related Systems: Design, Synthesis, Optimisation and Properties
The present contribution reports some highlights of our quest for new topological insulators (TIs) in bismuth-rich halides performed at the Inorganic Chemistry Department of TU Dresden in tight cooperation with theoreticians and experimentalists from Germany (IFW Dresden, RWTH Aachen), Switzerland (EPFL) and Spain (DIPC).
Employing the concept of “confined metals” we have established rough guidelines toward the directed search of new topological insulators based on crystal-structure features. A two-dimensional TI fragment embedded as a low-dimensional structural fragment into the bulk structures of bismuth-rich metal-salt hybrids can account for weak or strong 3D TI properties of the entire compound. The salt-like part can be constructed from the polar iodide anions that favor for more defect-free, stoichiometric compounds as opposed to chalcogenides.
This approach has by now been exemplified by two weak 3D TIs (Bi14Rh3I9, Bi2TeI) and a strong 3D TI, β-Bi4I4, which electronic structure is in proximity of both the weak 3D TI phase and the trivial insulator phase. These bulk materials are built by two different 2D TI fragments: a decorated honeycomb intermetallic layer that resembles graphene and a bismuth bilayer, which is a building unit of the elemental bismuth structure.
Bi14Rh3I9 is so far the only known weak 3D TI, for which the occurrence of topological edge states was experimentally confirmed by scanning tunnelling microscopy. β-Bi4I4 is the only representative of the strong topological class Z2=1;(110) and features a highly anisotropic Dirac cone. A theoretical study on the Bi2TeI electronic structure predicts exotic topological surface states beyond the Z2-formalism.
Mittwoch, 16.12.2015, 14.00 Uhr, Seminarraum MBP2 116
Larisa Chizhova (Technische Universität Wien)
auf Einladung von Prof. Morgenstern
Graphene-laser interaction: nonlinear and magneto-optical responses
We theoretically study the excitation of graphene due to laser fields in the mid- and far-infrared spectral range. In the far-infrared region graphene responds nonlinearly to high-intensity THz radiation. We calculate high-harmonic spectra by solving the Dirac equation and the Schrödinger equation within a tight-binding approximation in time domain. The theoretical spectra agree very well with a recent experiment by Bowlan et.al. [PRB 89, 041408(R) (2014)]. We additionally highlight the influence of long- and short-range disorder on high-harmonic generation in graphene. In the mid-infrared region we investigate the magneto-optical response of graphene, which enables to probe substrate and many-body effects. Our tight-binding approach allows us to directly compare the magneto-optical response of pristine graphene with graphene aligned on hexagonal boron nitride giving rise to a periodic superlattice potential. Furthermore we include magneto-excitonic effects in our theoretical model resulting in a quantitative explanation of recently observed Landau-level dependent renormalizations of the Fermi velocity.
Donnerstag, 17.12.15 um 14.00 Uhr, 28 B 110
Nanoscale infrared spectroscopy and chemical imaging with AFM-IR
Craig Prater, Ph.D. Chief Technology Officer Anasys Instruments
This presentation will overview atomic force microscope based infrared spectroscopy (AFM-IR). The AFM-IR technique uses the tip of an AFM as a nanoscale detector of absorption of IR radiation. AFM-IR can be used to obtain IR absorption spectra and chemical imaging with resolution as fine as the AFM tip radius, >100X smaller than spatial resolution limits of conventional infrared spectroscopy. The AFM-IR technique can also achieve sensitivity down to the scale of single monolayers and biological membranes. The presentation will overview the underlying technology and complementary techniques for measuring nanoscale optical scattering, as well as nanomechanical and nanothermal analysis. The presentation will also highlight numerous applications of nanoscale spectroscopy and chemical imaging in physics, materials and life sciences. Applications include nanoscale chemical analysis of polymers, composites, semiconductors, 2D materials, plasmonics, biological cells, proteins, tissue and other areas.
Dienstag, 29.09.2015, 11.15 Uhr, Seminarraum 28 A 301
Juliette Mangeney (Laboratoire Pierre Aigrain, Ecole Normale Supérieure, Paris)
auf Einladung von Prof. Stampfer
Coherent THz radiation: from ultrafast spectroscopy to emission from graphene
The THz region of the electromagnetic spectrum is extremely interesting both for fundamental and applied research. In particular, THz time domain spectroscopy systems provide key insights into the ultrafast dynamics of various low energy phenomena. By exploiting ultra-short pulse lasers (<20 fs), ultrabroadband THz time-domain spectroscopy extending up to few tens of THz have been recently developed. In such ultrabroadband THz spectroscopy systems, strong diffraction limited focusing of THz radiation is essential for investigating single micrometer-scale objects as well as achieving intense ultrabroadband THz electric field. After a brief review of the basic principles of time domain spectroscopy system, I will focus on recent developments achieved in our lab to confine ultra-broadband THz radiation to the ultimate diffraction limit in THz spectroscopy systems.
In the second part of the talk, we will present a study of coherent THz emission from graphene excited by femtosecond optical pulses using THz time domain spectroscopy. I will discuss on the physical mechanism responsible for THz emission from graphene: the dynamical photon drag effect. This second-order nonlinear effect relies on the transfer of light momentum to the carriers by the ponderomotive electric and magnetic forces. Our investigation shows the potential of graphene as an emitter of ultra-broadband terahertz pulses at room temperature.
Donnerstag, 10. 09.2015, 10.00 Uhr, Seminarraum 28 A 301
Priyamvada Bhaskar (Chalmers University of Technology, Göteborg, Schweden)
auf Einladung von Prof. Morgenstern
Magnetotransport in Topological Insulator
Topological insulators (TI) provide an excellent platform to study topological quantum physics and exploring spintronic applications due to their gapless spin polarized helical metallic surface states. We explore novel semiconducting TI BiSbTeSe (BSTS) and report weak-anti localization and quantum oscillations. Further, spin signals originating from spin-momentum locking were observed up to room temperature. It was inferred that the observed transport mechanisms possibly originated from the surface states, although coupling to the bulk forming a single 2D channel cannot be ruled out. These studies provide a platform to pursue exotic physics and novel device applications predicted for TIs and its heterostructures with other 2D materials.
Dienstag, 28.07.2015, 11.15 Uhr, Seminarraum 28 A 301
Prof. Sergey Tarasenko (Ioffe Physical-Technical Institute, St. Petersburg, Russia)
auf Einladung von Dr. Beschoten
Coherent Zitterbewegung in spin-orbit coupled systems
We discuss a force-free trembling motion (Zitterbewegung) that may experience electrons in semiconductor structures. Such a phenomenon, first pointed out by Erwin Schrödinger for wave-packet solutions of the Dirac equation, is caused by the fact that the electron velocity operator does not commute with the Hamiltonian and, hence, the velocity is not a conserved observable in the presence of spin-orbit interaction. Here, we study the trembling motion of electrons emerging in the systems with k-linear Rashba/Dresselhaus spin-orbit coupling in an external magnetic field. It is shown that the alternating shift of electrons in real space due to Zitterbewegung gives rise to a net ac electric current oscillating at the Larmor frequency if electrons are spin polarized and coherently presses in the magnetic field. A fundamental relationship between the coherent Zitterbewegung and the spin-galvanic effect is established.
Mittwoch, 27.05.2015, 15:00 Uhr, Seminarraum 28 A 301
Prof. Dr. Michel Pioro-Ladrière (Département de Physique, Université de Sherbrooke, Canada)
auf Einladung von Dr. Schreiber
Goodies for the spin qubits toolbox
Spin qubits are more than ever serious contenders in the race to build the first quantum computer. The New Scientist magazine’s verdict on the topic is unequivocal: “Superconducting qubits might attract those who like to play it safe, but spin could just overtake it during the next decade. Everything else is for die-hard experimenters only.” In this talk, I will present some of the hardware being developed in my group for solid-state spin qubits, including (i) fast single spin rotations without cumbersome superconducting magnets, (ii) wait-as-much-as-you-like readout scheme, (iii) tunable radio frequency charge sensors, and (iv) truly scalable silicon quantum dots. Our approach is one way to deal with the obstacles on the road to the commercialization of spin-based quantum technologies.
Freitag, 24.04.2015, 14:30 Uhr, Seminarraum MBP2 117
Prof. Dr. Hiroshi Daimon (Nara Institute of Science and Technology, Japan)
auf Einladung von Prof. Morgenstern
Two-dimensional photoelectron spectroscopy for the analysis of atomic and electronic structure in detail using circularly and linearly polarized light
Two-dimensional photoelectron spectroscopy using circularly and linearly polarized light gives us fruitful information about materials. Two-dimensional photoelectron diffraction pattern from core level can be used as “stereo photograph of atomic arrangement” and “photoelectron holography” for the direct structure analysis of 3D atomic arrangement around specific atoms. Two-dimensional photoelectron angular distribution pattern from valence band using linearly polarized light can be used to analyze the atomic orbitals constituting the electronic energy bands. Their principles and recent results are shown in the seminar.
Mittwoch, 04.03.2015, 15:00 Uhr, Seminarraum MBP2 016
Dr. Jörg Grenzer (Ion Beam Center, Helmholtz-Zentrum Dresden-Rossendorf)
auf Einladung von Prof. Wuttig
X - ray Diffraction and Scattering from Nanostructures
Nowadays, the development of new materials is often associated with specific properties of functionalized nano structures. X-ray investigations are a very important tool to find the link between the functional (magnetism, luminescence) and the corresponding structural properties (size, orientation etc.) that are generating this function. This knowledge makes it possible to design new materials with specific properties.
This tutorial will show how modern X-ray scattering methods are used in material science. Beside standard X-ray diffraction techniques we will show that with up-to-date laboratory setups X-ray methods can be applied that were some years ago only possible using synchrotron radiation. The advantage and peculiarities of different geometries, 1- and 2- dimensional detectors will be discussed, e.g. they are very efficient for the measurement of large reciprocal space maps at medium resolution.
Different examples will be shown like the investigation of semiconductor nanostructures, of fluorescence up-converting nano particles potentially used in medical applications or grazing incidence diffraction measurements of thin magnetic metallic films.
Donnerstag,12.02.2015, 16:15 Uhr, Hörsaal 28 B 110
Stéphane Berciaud (Institut de Physique et Chimie des Matériaux de Strasbourg,
Université de Strasbourg)
auf Einladung von Prof. Stampfer
Optical spectroscopy of suspended graphene and graphene-based hybrid systems
Since 2004, graphene has risen as an outstanding system to investigate the interplay between photons, electrons and phonons in reduced dimensions. In addition, graphene is a promising candidate for realistic applications in electronic and optoelectronic nano-devices. In this seminar, we will focus on the two limiting cases of i) a suspended graphene monolayer and ii) a hybrid system composed of a single nano-emitter physisorbed on monolayer graphene.
First, we will show how the intrinsic properties of suspended graphene can be probed using inelastic light (or Raman) scattering. This technique provides quantitative information about the electronic structure, the position of the Fermi level (i.e. the doping level), as well as the influence of disorder, strain, temperature, etc.... We will introduce two original studies based on Raman spectroscopy: i) an all-optical determination of the mechanical properties (Grüneisen parameters and Young’s modulus) of a pressurized graphene blister and ii) the observation of (magneto-)Raman scattering by inter-Landau level electronic excitations in mono to penta-layer graphene.
The second part of this seminar will address the interaction between graphene and colloidal semiconductor nanostructures, another promising class of nano-materials for photonics and optoelectronics. We performed a detailed study of Förster-type resonant energy transfer (FRET) between individual CdSe-based (0-dimensional) nanocrystals and (two-dimensional) nanoplatelets deposited on (two-dimensional) graphene. Highly efficient energy transfer results in a shortening of the luminescence decay and subsequent luminescence quenching. The energy transfer rate is further monitored as a function of the distance between the nano-emitter and the graphene layer. Our work demonstrates the realization of prototype graphene-based molecular rulers and uncovers the influence of dimensionality on the energy transfer rate.
Freitag, 06. Februar 2015, 11:00 Uhr, Seminarraum MBP1 015
Prof. Frank de Groot (Department of Chemistry, Utrecht University, Netherlands)
auf Einladung von Prof. Güntherodt
X-ray spectroscopy: XAS, RIXS and XMCD
New developments in x-ray absorption (XAS), resonant inelastic x-ray scattering (RIXS) and x-ray magnetic circular dichroism (XMCD) will be discussed. First a brief introduction is given of x-ray absorption spectral shapes [1,2].
In 2p3d RIXS one scans through the 2p XAS edge and measures the optical excitation range. Experiments on cobalt oxides & nanoparticles are discussed, in particular the comparison with optical spectroscopy [3]. Related to the RIXS measurements is the analysis of Fluorescence yield (FY) detected x-ray absorption spectra (XAS), including the intrinsic deviations of FY-XAS spectral shape from the XAS spectrum [4]. In the last part of the talk I discuss the analysis of X-MCD spectra, including the accuracy of the spectral sum rules [5]. It is also possible to combine RIXS and MCD in RIXS-MCD experiments, both in the soft and hard x-ray range [6].
- Core Level Spectroscopy of Solids
Frank de Groot and Akio Kotani (Taylor & Francis CRC press, 2008) - Download the x-ray spectroscopy simulation software at http://www.anorg.chem.uu.nl/CTM4XAS/
- M. van Schooneveld et al. Angew. Chem. 52, 1170 (2012)
- F.M.F. de Groot, Nature Chemistry 4, 766 (2012)
- C. Piamonteze, P. Miedema and F.M.F. de Groot, Phys. Rev. B. 80, 184410 (2009)
- M. Sikora et al., Phys. Rev. Lett. 105, 037202 (2010)
New developments in x-ray absorption (XAS), resonant inelastic x-ray scattering (RIXS) and x-ray magnetic circular dichroism (XMCD) will be discussed. First a brief introduction is given of x-ray absorption spectral shapes [1,2].
Montag, 02. Februar 2015, 10:30 Uhr, Seminarraum MBP1 026
Dr. Oana Cojocaru-Mirédin (Max-Planck-Institut für Eisenforschung, Düsseldorf)
auf Einladung von Prof. Wuttig
Advanced characterization of interfaces in photovoltaic materials using correlative microscopies
Cu(In,Ga)Se2 (CIGS), Cu2ZnSnSe4 (CZTSe), and multicrystalline Si (mc-Si) solar cells possess a high efficiency1, despite the polycrystalline structure of the absorber layer. One of the major factors controlling the cell efficiency is the diffusion of the impurities during the fabrication process into the absorber layer and to the p-n junction2. However, the interaction between the defects and the impurities at the internal interfaces is not completely understood. This is due to a lack of information on the local chemical changes across the internal interfaces at the nanoscale.
As a step towards a better understanding of the impurity redistribution at the internal interfaces, we have developed novel approaches of preparing site-specific atom probe specimens using combined focused ion beam (FIB), (scanning) transmission electron microscopy ((S)TEM) and electron backscattered diffraction (EBSD). These approaches allow selected GBs in polycrystalline CIGS, CZTSe and mc-Si layers to be studied by atom probe tomography (APT).
Several examples of correlative EBSD-TEM-APT (see Figure 1) and STEM-APT (see Figure 2) studies will be presented in this work. Using APT, segregation of impurities at the GBs was directly observed. APT data of various types of GBs will be presented and discussed with respect to the possible effects on the cell efficiency.
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1 Empa [Internet]. Empa.ch: A new world record for solar cell efficiency, 2013. Available from: http://www.empa.ch/plugin/template/empa/3/131438/---/l=2 [cited 2013 January 18].
2 J. L. Shay, S. Wagner, H. M. Kasper, Appl. Phys. Lett. 27 (1975) 89, S. Yip and I. Shih, Proceedings of the 1st World Conference on Photovoltaic Energy Conversion (IEEE, Piscataway, 1994), p.210.
Donnerstag, 29.01.2015, 10:00 Uhr, Seminarraum 28 A 301
Dr. Daniel Hernangómez Pérez (Lab. de Physique et Modélisation des Mileux Condensés, CNRS, Grenoble)
auf Einladung von Prof. Morgenstern
Microscopics of Disordered Quantum Hall Systems with Rashba Spin-Orbit Coupling
We develop a coherent-state Green's function formalism to study disordered two-dimensional electron gases in the quantum Hall regime with the combined effect of random Rashba and Zeeman interactions. We then compute the energy spectrum and obtain a microscopic expression for the local density of states. This result is used to interpret scanning tunneling spectroscopy data through the study of the spatial dispersion and linewidth of the LDoS peaks. Next, we discuss charge / spin transport properties of these systems in the local equilibrium (hydrodynamic) regime. We prove that, in the semiclassical limit, the Hall conductance presents quantized Hall plateaus for any finite Rashba coupling. Finally, we show that the (semiclassical) spin Hall conductance at high magnetic fields is resonance-free, contrary to previous theoretical predictions.
Donnerstag,15.01.2015, 16:15 Uhr, Hörsaal 28 B 110
Prof. Ivan K. Schuller (Physics Department of University of California San Diego, USA)
auf Einladung von Prof. Güntherodt
Hybrids: Materials and Physics
Hybrid materials allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. In the area of magnetism, controlling the magnetic properties of ferromagnetic thin films without magnetic fields is an on- going challenge with multiple technological implications for low- energy consumption memory and logic devices. Interesting possibilities include ferromagnets in proximity to dissimilar materials such as antiferromagnets or oxides that undergo metal-insulator transitions. The proximity of ferromagnets to antiferromagnets has given rise to the extensively studied Exchange Bias. Our recent investigations in this field have addressed crucial issues regarding the importance of the antiferromagnetic and ferromagnetic bulk for the Exchange Bias and the unusual short time dynamics. In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization and dynamical properties such as the microwave response are clearly modified by the proximity effect and give rise to interesting perhaps useful properties.
Work supported by US-AFOSR and US-DOE
Freitag, 05.12.2014, 10:30 Uhr, Seminarraum 28 A 301
Jose A. Garrido (Walter Schottky-Institut, TU München)
auf Einladung von Prof. Stampfer
CVD-based graphene field-effect transistors and electrodes
This presentation will provide an overview on fundamentals and applications of solution-gated field-effect transistors and microelectrode arrays based on CVD-grown graphene. I will first introduce the science and technology of such electronic devices, both on rigid and flexible substrates, discussing the influence of intrinsic properties of CVD graphene (e.g. grain boundaries) and comparing their performance with other competing technologies. The presentation will also discuss on the functionalization of these devices aiming at the introduction of specific sensing mechanisms. Based on these developed technologies, I will report on designs aiming at the bidirectional communication with electrogenic cells as well as the detection of neurotransmitters.
Donnerstag, 04.12.2014, 16:15 Uhr, Hörsaal 28 B 110
Dr. Florian Libisch (Institut für Theoretische Physik, TU Wien)
auf Einladung von Prof. Morgenstern
Edge and substrate effects in graphene nanodevices
We investigate substrate and edge effects in graphene nanodevices of realistic size (up to 100 nm) using a tight-binding approach. We consider the periodic moire potential induced by placing graphene nanostructures on hexagonal boron nitride. In particular, we are interested in the ,Hofstadter butterfly emerging in the density of states when applying a perpendicular magnetic field. To assess the importance of edge effects, we calculate the eigenstates of finite-sized flakes with rough edges.
Our results compare well with current experimental results.
Donnerstag, 27.11.2014, 15:00 Uhr, Hörsaal 28 B 110
Prof. Dr. Diederick Depla (Department of Solid States Sciences, Ghent University)
auf Einladung von Prof. Wuttig
Magnetron sputter deposition of biaxial aligned films
The paper looks for a simple explanation for the growth mechanism of biaxial aligned thin films. A straightforward analytical model is proposed. The model starts from the gathered knowledge on the influence of the energy per arriving particle on the thin film texture and microstructure during magnetron sputter deposition. A Monte Carlo based particle trajectory code combined with the results from a published Molecular Dynamics simulation, enables to compare the ideas behind the analytical model with experiments. The influence of pressure, target-substrate distance on the in-plane alignment is studied for different materials.
The model fits for a wide range of materials, ranging from metals (Cr), over oxides (MgO, YSZ) to nitrides (InN,TiN). Further evidence for the proposed model is given by a detailed study on the growth of Mg(M)O (M=Al, Cr, Ti, Y and Zr) and biaxial aligned thin films. In this study, a second source is added to the set-up, a technique described as dual reactive magnetron sputtering. The addition of the second source increases the complexity of the growth, but by careful analysis, the obtained results showed to be consistent with the proposed model. The model can be further extended to YSZ, deposited now from two sources. The overall conclusion from this study is that the presented ideas have a solid foundation as they can be applied to different materials and experimental conditions.
Montag, 24.11.2014, 15:30 Uhr, Hörsaal 28 B 110
Dr. Vladimir Kaganer (Paul-Drude-Institut für Festkörperelektronik, Berlin):
X-ray diffraction studies of epitaxial films during their growth: molecular beam epitaxy in vivo
The crystal growth studies at the Paul Drude Institute beamline at the synchrotron BESSY II in Berlin will be overviewed, with the emphasison the theory of the growth processes and their x-ray diffraction analysis. The talk includes the results on layer-by-layer deposition and crystal surface recovery in the homoepitaxial growth of GaAs, kinetics of the Volmer-Weber growth of Fe3Si/GaAs, x-ray analysis of the registry of the lattices in the hetero epitaxial growth, the crystallization kinetics of the rare earth oxides.
Montag, 06.10.2014, 11:30 Uhr, Seminarraum MBP1 026
Prof. Dr. Manfred Bayer (Lehrstuhl für Experimentelle Physik 2, TU Dortmund)
Rydberg excitons in cuprous oxide
Cuprous oxide is the material in which excitons were discovered first by Evgenii Gross in 1952. The exciton levels can be well described by the hydrogen series. In the talk I will discuss the recent observation of highly excited excitons with principal quantum numbers up to n=25, corresponding to a giant extension in the μm-range. Similar to Rydberg atoms they show a huge interaction among each other leading to a Rydberg blockade effect. In magnetic field they allow one to enter the quantum chaos regime which for hydrogen atoms requires field strengths typical for white dwarf stars.