Experimental Condensed Matter Physics

 

The dates of the experimental condensed matter physics seminar are also announced via mailing lists. Anonye interested can join the lists here. Note that many people will automatically receive the announcement via institute lists.

 
Datum Sprecher Title
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 Institute for Solid State Research, Stuttgart) Sensing the Quantum Limit in Scanning Tunneling Spectroscopy: From the Josephson Effect to Quantum Tunneling
21/04/2017 Dr. Alexey Chernikov (Department of Physics, University of Regensburg) Coulomb engineering in 2D semiconductors
26/01/2017 Prof. Ferdinand Kuemmeth (Niels Bohr Institute, University of Copenhagen) Nuclear notch filtering and long-distance spin exchange in GaAs quantum dots
26/01/2017 Dr. Kai Sotthewes (MESA+ Institute for Nanotechnology, University of Twente, Netherlands) The single-molecule switch and transistor
06/12/2016 Florian Libisch (Institute for Theoretical Physics, Vienna University of Technology) Dynamically encircling an exceptional point for asymmetric mode switching
14/11/2016 Markus Ternes (Max Planck Institute for Solid State Research, 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 (Humboldt Universität in Berlin) Energy level tuning at inorganic/organic semiconductor heterojunctions
28/04/2016 Menno Veldhorst (Technical University of Delft, Netherlands) 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, University of Grenoble, France) 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 Bertold Rasche (Technical University 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 (Technical University Vienna, Austria) 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, Sweden) 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 (Department of Physics, University of Sherbrooke, Canada) Goodies for the spin qubits toolbox
24/04/2015 Prof. Dr. Hiroshi Daimon (Nara Institute of Science and Technology, Japan)
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 (University of 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 Institute, University of 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
 

Tuesday, 23rd May 2017, 12:00am, Room 28 B 110

Dr. Christian R. Ast  (Max Planck Institute for Solid State Research, Stuttgart)

hosted by 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.

 

Friday, 21st April 2017, 10:30am, Room 28 A 301

Dr. Alexey Chernikov (Department of Physics, University of Regensburg)

hosted by 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, 11.00h, 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.

 

 

Friday, 08.09.2017, 10.30h, Room 28 B 110

Dr. Alexander Paarmann, Fritz-Haber-Institut der Max-Planck Gesellschaft Berlin
hosted by 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.  

 

Thursday, 26th January 2017, 11:30am, Room 28 A 301

Ferdinand Kuemmeth (Niels Bohr Institute, University of Copenhagen)

hosted by 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.

 

Thursday, 26th January 2017, 10:15am, Room 28 A 301

Kai Sotthewes (MESA+ Institute for Nanotechnology, University of Twente, Netherlands)

hosted by 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.

 

Tuesday, 6th December 2016, 3:45p.m., Room 28 A 301

Florian Libisch (Institute for Theoretical Physics, Vienna University of Technology)

hosted by 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.

 

Monday, 18th of November 2016, 11:00am, Room 28 A 301

Markus Ternes (Max-Planck-Institute for Solid State Research, Stuttgart)

hosted by 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.

 

Wednesday, October 19, 2016, 2:00pm

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].

 

Friday, 17th of June 2016, 3:00pm, Room 28 B 110

Yung-Woo Park (Dept. of Physics and Astronomy, Seoul National University, South Korea)

hosted by 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.

 

Thursday, 12 May 2016, 9.15h, room MBP1 026

Professor Norbert Koch (Humboldt Universität Berlin)

hosted by Professor 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.

 

Thursday, 28th of April 2016,11:30am, Room 28 A 301

Menno Veldhorst (Technical University of Delft, Netherlands)

hosted by 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.

 

Thursday, 21st of April 2016, 4:15pm, Room 28 B 110

Silvan Schmid (Dept. of Materials Science and Engineering, National University of Singapore)

hosted by 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.

 

Thursday, 21st of April 2016, 11:15am, Room 28 A 301

Sayanti Samaddar (Institut Néel, University of Grenoble)

hosted by 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.

 

Tuesday, 15.03.2016, 3:00pm, Room MBP2 117

Benedikt Schwarz
Institute for Solid State Electronics, TU Wien, Vienna, Austria

hosted by 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.

 

Monday, 01.02.2016, 3:00pm, Room 28 A 301

Pol Forn-Diaz (Institute for Quantum Computing, University of Waterloo)

hosted by 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.

 

Thursday, 17.12.2015, 11:00am, Room 28 A 301

Bertold Rasche (Technical University Dresden)

hosted by 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.

 

Wednesday, 16.12.2015, 2.00pm, Room MBP2 116

Larissa Chizhova (Technical University Vienna)

hosted by 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.

 

Thursday, December 17, 2015, 2:00pm, 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.

 

 

Tuesday, 29.09.2015, 11:15am, Room 28 A 301

Juliette Mangeney (Laboratoire Pierre Aigrain, Ecole Normale Supérieure, Paris)

hosted by 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.

 

Thursday, 10.09.2015, 10:00am, Room 28 A 301

Priyamvada Bhaskar (Chalmers University of Technology, Göteborg, Schweden)

hosted by 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.

 

Tuesday, 28.07.2015, 11:15am, Room 28 A 301

Prof. Sergey Tarasenko (Ioffe Physical-Technical Institute, St. Petersburg, Russia)

hosted by 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.

 

Wednesday, 27.05.2015, 3pm, Room 28 A 301

Prof. Dr. Michel Pioro-Ladrière (Department of Physics, University of Sherbrooke, Canada)

hosted by 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.

 

Friday, 24.04.2015, 02:30 pm, Room MBP2 117

Prof. Dr. Hiroshi Daimon (Nara Institute of Science and Technology, Japan)

hosted by 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.

 

Wednesday, 04.03.2015, 03:00 pm, Room MBP2 016

Dr. Jörg Grenzer (Ion Beam Center, Helmholtz-Zentrum Dresden-Rossendorf)

hosted by 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.

 

Thursday, 12.02.2015, 04:15 pm, Room 28 B 110

Stéphane Berciaud (Institut de Physique et Chimie des Matériaux de Strasbourg

Université de Strasbourg)

hosted by 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)

hosted by 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].

  1. Core Level Spectroscopy of Solids
    Frank de Groot and Akio Kotani (Taylor & Francis CRC press, 2008)
  2. Download the x-ray spectroscopy simulation software at  http://www.anorg.chem.uu.nl/CTM4XAS/
  3. M. van Schooneveld et al.  Angew. Chem. 52, 1170 (2012)
  4. F.M.F. de Groot, Nature Chemistry 4, 766 (2012)
  5. C. Piamonteze, P. Miedema and F.M.F. de Groot, Phys. Rev. B. 80, 184410 (2009)
  6. 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].

     

 

Monday, 02.02.2015, 10:30 am, Room MBP1 026

Dr. Oana Cojocaru-Mirédin (Max-Planck-Institut für Eisenforschung, Düsseldorf)

hosted by 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.

________________________________________________

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.  

 

Thursday, 29.01.2015, 10:00 am, Room 28 A 301

Dr. Daniel Hernangómez Pérez (Lab. de Physique et Modélisation des Mileux Condensés, CNRS, Grenoble)

hosted by 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.

 

Thursday, 15.01.2015, 04:15 pm, Room 28 B 110

Prof. Ivan K. Schuller (Physics Department of University of California San Diego, USA)

hosted by 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 [5-7] 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

 

Friday, 05.12.2014, 10:30 pm, Room 28 A 301

Jose A. Garrido (Walter Schottky-Institut, TU München)

hosted by 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.

 

Thursday, 04.12.2014, 04:15 pm, Room 28 B 110

Dr. Florian Libisch (Institut für Theoretische Physik, TU Wien)

hosted by 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.

 

Thursday, 27.11.2014, 03:00 pm, Room 28 B 110

Prof. Dr. Diederick Depla (Department of Solid States Sciences, Ghent University)

hosted by 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.

 

 

Monday, 24.11.2014, 03:30 pm, Room 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.

r. Vladimir Kaganer (Paul-Drude-Institut für Festkörperelektronik, Berlin):

X-ray diffraction studies of epitaxial films during their growth: molecular beam epitaxy in vivo

 

 

Monday, 06.10.2014, 11:30 am, Room 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.