Electron beam lithography system in our clean room which allows to write structures with nanoscale dimensions. [Open image in a new tab for higher resolution]

Honeycomb pattern, defined by electron beam lithography and etched into the ferromagnetic semiconductor GaMnAs with 6 nm wide semiconductor bars, shows the ultimate potential of top down patterning techniques. Such ultra small structures are used to probe interference phenomena of charge carriers in ferromagnetic semiconductors. [Open image in a new tab for higher resolution]

By employing lithographic techniques like electron beam lithography, dry etching and lift-off techniques we pattern semiconductor, metal, and graphene layers on the nanoscale. All the technology is done in clean room facilities. We are interested in the modified electronic and magnetic properties of nanoscale devices. Typical transport measurements, e.g. measurements of the electrical resistance, are carried out at ultra low temperatures and in high magnetic fields. Recently our research focuses on employing both, electric charge and electron spin for a modified form of electronics, called spintronics.

An actual topic in this field is spin injection into a semiconductor with transistor-like geometry. In contrast to conventional transistor structures the channel resistance of a 'spin-transistor' is expected to depend on the electrons' spin orientation, i.e. on the magnetization direction of the ferromagnetic source and drain contacts.

Our research group is part of the Collaborative Research Center SFB 689 on "Spin phenomena in reduced dimensions" funded by the German Science Foundation (DFG) and of the DFG Research Training Group GRK 1570 on "Electronic Properties of Carbon based Nanostructures".

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/weiss/.

Top 5 publications:

• S. D. Ganichev, V. V. Bel’kov, S. A. Tarasenko, S. N. Danilov, S. Giglberger, C. Hoffmann, E. L. Ivchenko, D. Weiss, C. Gerl, D. Schuh, W. Wegscheider, W. Prettl,
  Zero-bias spin separation
  Nature Physics 2, 609 (2006)
• K. Wagner, D. Neumaier, M. Reinwald, W. Wegscheider, D. Weiss,
  Dephasing in (Ga,Mn)As nanowires and rings
  Phys. Rev. Lett. 97, 056803 (2006)
• J. Moser, A. Matos-Abiague, D. Schuh, W. Wegscheider, J. Fabian, D. Weiss,
  Tunneling Anisotropic Magnetoresistance and Spin-Orbit Coupling in Fe/GaAs/Au Tunnel Junctions
  Phys. Rev. Lett. 99, 056601 (2007)
• D. Neumaier, K. Wagner, S. Geißler, U. Wurstbauer, J. Sadowski, W. Wegscheider, D. Weiss,
  Weak Localization in Ferromagnetic (Ga;Mn)As Nanostructures
  Phys. Rev. Lett. 99, 116803 (2007)
• D. Neumaier, M. Schlapps, U. Wurstbauer, J. Sadowski, M. Reinwald, W. Wegscheider, D. Weiss,
  Electron-electron interaction in one- and two-dimensional ferromagnetic (Ga,Mn)As
  Phys. Rev. B 77, 041306 (2008)

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Linear conductance of a benzene interference single electron transistor as a function of the gate voltage for the two different configurations depicted on the right hand side. A systematic conductance suppression is observed when the molecule is connected in the meta- rather than in the para-configuration and a transition from a non-degenerate groundstate (A_1g or B_2g symmetry) to a degenerate groundstate (E_2u or E_2g) occurs. The gate voltage is expressed in units of b/e where b is the hopping energy and e is the electron charge. Moreover the conductance is given in units of e2G/k_BT where G is the bare tunnelling rate, T is the temperature and k_B is the Boltzmann constant. Adapted from Begemann et al. Phys. Rev. B 77, 201406(R) (2008). [Open image in a new tab for higher resolution]

Left: Dissipative time evolution of the population difference P(t) of the two states of a quantum bit (qubit) coupled to a linear oscillator (time is in units of the inverse qubit frequency Δ_0). The entanglement between oscillator and the qubit is visible in the Fourier transform F(ω), which displays a pronounced "Rabi splitting" of the order of the qubit-oscillator coupling. Adapted from Hausinger and Grifoni, New. J. Phys. 10, 115015 (2008), Nesi et al., New J. Phys. 9, 317 (2007).
Right: Experimental realization in a superconducting circuit interrupted by Josephson junctions, adapted from Chiorescu et al. Nature 431, 159 (2004). The oscillator is in this case the dc-SQUID used to measure the state of the qubit. [Open image in a new tab for higher resolution]

Main focus of our group is on QUANTUM TRANSPORT in low dimensional systems, as e.g. quantum dots, single-molecule bridges or quantum wires. In particular, we investigate how the current-voltage characteristics of these systems is affected due to the presence of strong electron-electron interactions, coupling to mechanical degrees of freedom or orbital symmetries. Moreover we investigate novel forms of information transfer, e.g., upon using the spin rather than the charge degree of freedom of an electron, as a quantum logic element.

We also focus on the influence of a quantum environment on the non-equilibrium properties of small quantum systems. On the one hand the entanglement of the small system with the degrees of freedom of the environment yields a loss of quantum coherence of the small quantum system. We aim at understanding the origin of damping and its optimization in real devices. We study e.g. decoherence in solid state nanocircuits used for quantum computation. We also study model systems, called Brownian motors, where noise does not constitute a nuisance. In contrast, it can be used to drive particles along pre-assigned directions.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/grifoni/ .

Top 5 publications:

• S. Smirnov, D. Bercioux, M. Grifoni, K. Richter,
  Quantum dissipative Rashba spin ratches
  Phys. Rev. Lett. 100, 230601 (2008)
• G. Begemann, D. Darau, A. Donarini, M. Grifoni,
  Symmetry fingerprints of a benzene single-electron transistor
  Phys. Rev. B 77, 201406(R) (2008)
• S. Koller, L. Mayrhofer, M. Grifoni,
  Spin transport across carbon nanotube quantum dots
  New Journ. Phys. 9, 348 (2007)
• F. Nesi, M. Grifoni, E. Paladino,
  Dynamics of a qubit coupled to a broadened harmonic mode at finite detuning
  New Journ. Phys., 9, 316 (2007)
• H. W. Ch. Postma, T. Teepen, Z. Yao, M. Grifoni, C. Dekker,
  Carbon nanotube single-electron transistors at room temperature
  Science , 293, 76 (2001)

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Split-coil magnet cryostat and spectrometer [Open image in a new tab for higher resolution]

Time-resolved Faraday rotation is a pump-probe technique to observe spin dynamics in semiconductor heterostructures with sub-picosecond time resolution. A circularly-polarized pump pulse generates spin-polarized electron-hole pairs. The resulting spin polarization can be detected with a delayed probe pulse via the Faraday effect. [Open image in a new tab for higher resolution]

Our research field is optical spectroscopy on semiconductor quantum structures at low temperatures and in high magnetic fields. Specifically, we are focussing on the spin properties of free carriers - electrons or holes. The semiconductor quantum structures are, e.g., GaAs-AlGaAs heterostructures, which host high-mobility two-dimensional electron systems (2DES) or two-dimensional hole systems (2DHS). Furthermore, we are interested in novel magnetic semiconductor heterostructures, consisting, e.g., of a combination of magnetic (GaMnAs) and nonmagnetic (e.g., InGaAs-GaAs) layered semiconductors. Those systems are prototyps of spin-injection structures for a possible future semiconductor spintronics.

For the investigation of the spin dynamics of free carriers we are employing ultrafast experiments - e.g., time-resolved Faraday rotation, time-resolved Kerr rotation or time-resolved photoluminescence. By means of Raman spectroscopy (inelastic light scattering) we are studying the elementary excitations of the crystal lattices (phonons) or of the charge carrier systems (plasmons, spin-density waves, spinflip excitations) in the nanostructures. Here, we are planning to do time-resolved Raman experiments to explore the dynamics of the excitations. Recently we have extended the range of investigated materials to graphene (single layer of graphite) and to one-dimensional nanostructures, like Carbon nanotubes or GaAs nanorods.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/schueller/index-e.phtml.

Top 5 publications:

• M. Kugler, T. Andlauer, T. Korn, A. Wagner, S. Fehringer, R. Schulz, M. Kubova, C. Gerl, D. Schuh, W. Wegscheider, P. Vogl, C. Schüller,
  Gate control of low-temperature spin dynamics in two-dimensional hole systems
  Phys. Rev. B 80, 035325 (2009)
• V. V. Belkov, P. Olbrich, S. A. Tarasenko, D. Schuh, W. Wegscheider, T. Korn, C. Schüller, D. Weiss, W. Prettl, S. D. Ganichev,
  Symmetry and spin dephasing in (110)-grown quantum wells
  Phys. Rev. Lett. 100, 176806 (2008)
• D. Stich, J. Zhou, T. Korn, R. Schulz, D. Schuh, W. Wegscheider, M. W. Wu, C. Schüller,
  Effect of Initial Spin Polarization on Spin Dephasing and the Electron g Factor in a High-Mobility Two-Dimensional Electron System
  Phys. Rev. Lett. 98, 176401 (2007)
• D. Stich, J. H. Jiang, T. Korn, R. Schulz, D. Schuh, W. Wegscheider, M. W. Wu, C. Schüller,
  Detection of large magneto-anisotropy of electron spin desphasing in a high mobility two-dimensional electron system in a [001] GaAs/AlGaAs quantum well
  Phys. Rev. B 76, 073309 (2007)
• D. Stich, J. Zhou, T. Korn, R. Schulz, D. Schuh, W. Wegscheider, M. W. Wu, C. Schüller,
  Dependence of spin dephasing on initial spin polarization in a high-mobility two-dimensional electron system
  Phys. Rev. B 76, 205301 (2007)

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Atomic structure of Fe/GaAs layers. We investigate the spin-orbit effects in such structures from first principles to find the magnetic, optical, and transport anisotropies [Open image in a new tab for higher resolution]

Scheme of a ferromagnetic resonant tunneling diode. The interplay of charging, electronic transport, as well as ferromagnetic ordering in the quantum well, leads to the appearance of ac current under nominally dc bias voltages. [Open image in a new tab for higher resolution]

Our group explores fundamental and applied aspects of the field of spintronics, investigating the properties of the electron spin in specific solid state environments. The goal is to find ways the electron spin influences the electronic characteristics of solid-state structures, as well as to invent electric control mechanisms to manipulate the spin states. We employ a variety of analytical and numerical tools, such as effective Hamiltonian descriptions, many-body techniques, realistic device simulations, and electronic structure modeling from tight-binding schemes to state-of-the-art first-principles calculations.

The broad spectrum of the investigated themes and material systems comprises the spin dynamics of electrons confined in semiconductor quantum dots, spin effects in graphene, the spin physics of nonmagnetic and ferromagnetic semiconductors, ferromagnet/semiconductor interfaces, and simulation and design of spintronic devices.

We are funded by the Collaborative Research Center SFB 689 on "Spin phenomena in reduced dimensions" of the German Science Foundation (DFG), by the DFG Research Training Group GRK 1570 on "Electronic Properties of Carbon based Nanostructures", and by the DFG Priority Program SPP1285 "Semiconductor Spintronics".

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/fabian/.

Top 5 publications:

• I. Zutic, J. Fabian, S. Das Sarma,
  Spintronics: fundamentals and applications
  Rev. Mod. Phys. 76, 323 (2004)
• I. Zutic, J. Fabian, S. C. Erwin ,
  Spin injection and detection in silicon
  Phys. Rev. Lett. 97, 026602 (2006)
• P. Stano, J. Fabian,
  Phonon-induced spin relaxation in laterally coupled quantum dots
  Phys. Rev. Lett. 96, 186602 (2006)
• J. Fabian, A. Matos-Abiague, C. Ertler, P. Stano, I. Zutic,
  Semiconductor spintronics
  Acta Physica Slovaca 57, 565-907 (2007)
• C. Ertler, J. Fabian,
  Self-sustained magnetoelectric oscillations in magnetic resonant tunneling structures
  Phys. Rev. Lett. 101, 077202 (2008)

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The image shows individual copper atoms on a copper surface that were arranged one after another by atomic manipulation to form the logo of the University of Regensburg. Some color was added for better recognition. [Open image in a new tab for higher resolution]

The text contains further information about this image. [Open image in a new tab for higher resolution]

The objective of our team is to study the physical and chemical properties of single adsorbates and adsorbate structures on the atomic length scale. The technique of choice is low-temperature scanning probe microscopy, primarily scanning tunneling microscopy (STM). The strength of the scanning probe techniques lies in their ability to analyze the structural environment of one particular defect or adsorbate on the atomic length scale and to probe the local properties of this individual, well-characterized structure.

In the last years we were mainly interested in STM experiments on individual adsorbates on ultrathin insulating films. Ultrathin insulating films on metal substrates facilitate the use of the scanning tunneling microscope to study the electronic properties of single atoms and molecules, which are electronically decoupled from the metallic substrate. Most importantly, many physical and chemical properties are not only quantitatively but also qualitatively different on an insulating surface from those on a (semi-)conducting surface. Hence, it becomes of supreme importance to the scope of science, as it advances to the atomic length scale, to include insulating materials. New experimental possibilities which are to be examined comprise (meta-)stable charging processes of individual adsorbates and STM-induced chemistry of single molecules on insulators. Furthermore, these investigations shall open new research avenues in molecular electronics, as they combine the following two elements: the electronic decoupling of an adsorbate provided by the insulator and the ability of STM to analyze the structural environment of an adsorbed molecule.

Our running scanning tunneling microscope works at a temperature of 5K under ultra-high vacuum conditions. A second machine is presently being built up that includes the possibility to measure forces on the atomic length scale. In the following years, a third apparatus is planned which will allow measurements in strong magnetic fields to additionally study magnetic properties.

The group was established in March 2007 by the hiring of Jascha Repp on a Lichtenberg-professorship, funded by the Volkswagenstiftung.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/repp/.

Top 5 publications:

• P. Liljeroth, J. Repp, G. Meyer,
  Current-Induced Hydrogen Tautomerization and Conductance Switching of Naphthalocyanine Molecules
  Science 317, 1203 (2007)
• F. E. Olsson, S. Paavilainen, M. Persson, J. Repp, G. Meyer,
  Multiple Charge States of Ag Atoms on Ultrathin NaCl Films
  Physical Review Letters 98, 176803 (2007)
• J. Repp, G. Meyer, S. Paavilainen, F. E. Olsson, M. Persson,
  Imaging Bond Formation Between a Gold Atom and Pentacene on an Insulating Surface
  Science 312, 1196 (2006)
• J. Repp, G. Meyer, S. M. Stojkovic, A. Gourdon, C. Joachim,
  Molecules on insulating films: Scanning tunneling microscopy imaging of individual molecular orbitals
  Physical Review Letters 94, 026803 (2005)
• J. Repp, G. Meyer, F. Olsson, M. Persson,
  Controlling the charge state of individual gold adatoms
  Science 305, 493 (2004)

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Time resolved scanning Kerr microscope
[Open image in a new tab for higher resolution]

Sample design for the detection of the Spin wave Doppler effect in a (Ga,Mn)As stripe shown in blue.
[Open image in a new tab for higher resolution]

Our experimental effort is focussed on magnetization dynamics and magneto-transport in nanosized magnetic elements. Using time resolved magneto-optic techniques we investigate the time dependent response of nanomagnets to different external stimuli. The magnetic systems may be stimulated by ultra short laser pulses, short magnetic field pulses and by microwaves. In recent experiments we have addressed the generation and detection of pure spin currents in all metallic nanostructures.

As an experimental tool we mostly use a state of the art time resolved scanning Kerr microscope with sub-picosecond temporal resolution and a spatial resolution of 250 nm. Recently this instrument has been extended for use at low temperatures and a current topic of research is the detection of the spin wave Doppler effect in ferromagnetic (Ga,Mn)As nanostripes.

Our research group is part of the Collaborative Research Center SFB 689 on "Spin phenomena in reduced dimensions" funded by the German Science Foundation (DFG).

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/back/.

Top 5 publications:

• A. Vansteenkiste, K. W. Chou, M. Weigand, M. Curcic, V. Sackmann, H. Stoll, T. Tyliszczak, G. Woltersdorf, C. H. Back, G. Schütz, B. Van Waeyenberge,
  X-ray imaging of the dynamic magnetic vortex core deformation
  Nature Physics 5, 332 (2009)
• I. Radu, G. Woltersdorf, M. Kiessling, A. Melnikov, U. Bovensiepen, J.-U. Thiele, C. H. Back,
  Laser-Induced Magnetization Dynamics of Lanthanide-Doped Permalloy Thin Films
  Phys. Rev. Lett. 102, 117201 (2009)
• F. Maccherozzi, M. Sperl, G. Panaccione, J. Minar, S. Polesya, H. Ebert, U. Wurstbauer, M. Hochstrasser, G. Rossi, G. Woltersdorf, W. Wegscheider, C. H. Back,
  Evidence for a Magnetic Proximity Effect up to Room Temperature at Fe/(Ga,Mn)As Interfaces
  Phys. Rev. Lett 101, 267201 (2008)
• G. Woltersdorf, O. Mosendz, B. Heinrich, C. H. Back,
  Magnetization dynamics due to pure spin currents in magnetic double layers
  Phys. Rev. Lett. 99, 246603 (2007)
• P. Kotissek, M. Bailleul, M. Sperl, A. Spitzer, D. Schuh, W. Wegscheider, C. H. Back, G. Bayreuther,,
  Cross-sectional imaging of spin injection into a semiconductor
  Nature Physics 3, 872 (2007)

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Overlook of the experimental hall with terahertz laser systems. [Open image in a new tab for higher resolution]

Observation of the Seebeck ratchet effect. The effect is measured in semiconductor heterostructures with a one-dimensional lateral potential excited by terahertz radiation. The photocurrent generation is based on the combined action of a spatially periodic in-plane potential and a spatially modulated light, which gives rise to a modulation of the local temperature. Besides a polarization independent current due to the Seebeck ratchet effect, we observe a photon helicity dependent response and propose a microscopic mechanism to interpret the experimental findings. (a) Sample design. (b) Experimental geometry. (c) Electron micrograph. The graph in the lower panel shows the photocurrent as a function of the radiation helicity given by the angle phi measured in a structured sample at three angles of incidence theta. The ellipses on top illustrate the polarization for various angles phi. [Open image in a new tab for higher resolution]

The main focus of our group is on terahertz and infrared physics of low dimensional systems. The scientific aims involve fundamental research in the topical fields of semiconductor spintronics, optical and opto-electronic properties of graphene, quantum ratchet effects in semiconductor nanostructures, terahertz (THz) radiation induced tunnelling phenomena, nonlinear optics, electron gas heating as well as the application of terahertz radiation in material science, development of THz sources, detectors, components and techniques.

Our research group is part of the Collaborative Research Center SFB 689 on "Spin phenomena in reduced dimensions" and Priority Program SPP 1285 funded by the German Science Foundation (DFG) as well as of the Regensburg Terahertz Center (TERZ).

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/ganichev/.

Top 5 publications:

• S. D. Ganichev, W. Prettl,
  Intense Terahertz Excitation of Semiconductors
  Oxford University Press 2006 [book]
• S. D. Ganichev, S. A. Tarasenko, V. V. Bel'kov, P. Olbrich, W. Eder, D. R. Yakovlev, V. Kolkovsky, W. Zaleszczyk, G. Karczewski, T. Wojtowicz, D. Weiss,
  Spin currents in diluted magnetic semiconductors
  Phys. Rev. Lett. 102, 156602 (2009)
• P. Olbrich, E.L. Ivchenko, T. Feil, R. Ravash, S.D. Danilov, J. Allerdings, D. Weiss, S. D. Ganichev,
  Ratchet effects induced by terahertz radiation in heterostructures with a lateral periodic potential
  Phys. Rev. Lett. 103, 090603 (2009)
• V. V. Bel'kov, P. Olbrich, S. A. Tarasenko, D. Schuh, W. Wegscheider, T. Korn, C. Schüller, D. Weiss, W. Prettl, S. D. Ganichev,
  Symmetry and spin dephasing in (110)-grown quantum wells
  Phys. Rev. Lett. 100, 176806 (2008)
• S. D. Ganichev, V. V. Bel'kov, S. A. Tarasenko, S. N. Danilov, S. Giglberger, Ch. Hoffmann, E. L. Ivchenko, D. Weiss, W. Wegscheider, Ch. Gerl, D. Schuh, J. Stahl, J. De Boeck, G. Borghs, W. Prettl,
  Zero-bias spin separation
  Nature Physics (London) 2, 609 (2006)

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[Fig. 1] Spin injection from a graphene nanoribbon (left part) into a graphene flake, a monoatomic graphite layer (right). Blue and red parts mark the charge carrier densities of spin-up and spin-down electrons (taken from reference 3). [Open image in a new tab for higher resolution]

[Fig. 2] Quantum mechanically computed charge density of interacting electrons in a quantum dot. [Open image in a new tab for higher resolution]

The main research area of our group is theoretical condensed matter physics. More specifically, our research ranges from molecular electronics to spin electronics, from mesoscopic physics of electronic and atomic systems to the topic of chaos in quantum systems. Transport phenomena at mesoscopic and nano-scales (such as the conductance in nanostructures and single molecules, spin-dependent transport) are in the foreground and provide the mutual theme and methodical framework. Quantum coherence together with the influences of reduced dimensions in nanosystems, as well as with disorder-, spin- and interaction-effects lead to a multitude of interesting quantum phenomena.

Our research group is part of the Collaborative Research Center SFB 689 on "Spin phenomena in reduced dimensions", of the DFG Research Training Group GRK 1570 on "Electronic Properties of Carbon based Nanostructures", of the DFG Priority Porgramm 1243 on "Quantum transport at Molecular Scales", of the German-Japanese Research Unit "Topological Electronics" and of the DFG Research Unit FOR 760 on "Scattering Systems with Complex Dynamics".

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/richter/richter/ .

Top 5 publications:

• V. Krueckl and K. Richter,
  Switching Spin and Charge between Edge States in Topological Insulator Constrictions
  Physical Review Letters 107, 086803 (2011)
• M. Scheid, M. Kohda, Y. Kunihashi, K. Richter, J. Nitta,
  All-electrical detection of the relative strength of Rashba and Dresselhaus spin-orbit interaction in quantum wires
  Physical Review Letters 101, 266401 (2008)
• M. Wimmer, I. Adagideli, S. Berber, D. Tomanek, K. Richter,
  Spin Transport in Rough Graphene Nanoribbons [→ please see also figure 1]
  Physical Review Letters 100, 177207 (2008)
• G. Cuniberti, G. Fagas, K. Richter [eds.],
  Introducing Molecular Electronics
  Springer, Berlin, 2005 [book]
• D. Frustaglia, K. Richter,,
  Spin interference effects in ring conductors subject to Rashba coupling
  Physical Review B 69, 235310 (2004)

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Room temperature UHV STM/AFM system with pump electronics (left), vacuum chamber (middle) and control unit (right). [Open image in a new tab for higher resolution]

Left: FM-AFM image (raw data) and profile of the Si(111) unit cell. Right: Enlargement of a single adatom image showing subatomic features. Adapted from Giessibl et.al, Science 289, 422-425 (2000).[Open image in a new tab for higher resolution]

[ Please move the mouse over here for a German translation]

We study solid surfaces and nanostructures on the atomic scale. Our key tool is the atomic force microscope and its predecessor, the scanning tunnelling microscope. Electronic, magnetic and mechanical properties of matter are probed on the length scale from picometers to micrometers. The group is a member of the DFG Research Training Group GRK 1570 "Electronic Properties of Carbon based Nanostructures" and of the Collaborative Research Center SFB 689 "Spin phenomena in reduced dimensions". Within GRK 1570, we study electronic and mechanical properties of graphene, few layer graphene and highly oriented pyrolytic graphite. In SFB 689 we investigate the possibility of spin-resolved force microscopy as well as magnetic force microscopy. Furthermore, we are pushing the fundamental resolution limits of scanning probe microscopy with the development of novel force sensors.

External collaborators include the IBM Research Laboratories in San Jose (USA) and Zurich (Switzerland) and the surface science division of the Czech Academy of Sciences in Prag (Czech Republic).

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/giessibl/neu/index_e.phtml.

Top 5 publications:

• L. Gross, F. Mohn, P. Liljeroth, J. Repp, F. J. Giessibl, G. Meyer,
  Measuring the Charge State of an Adatom with Noncontact Atomic Force Microscopy
  Science 324, 1428-1431 (2009)
• M. Ternes, C. P. Lutz, C. F. Hirjibehedin, F. J. Giessibl, A. J. Heinrich,
  The Force Needed to Move an Atom on a Surface
  Science 319, 1066 (2008)
• S. Hembacher, F. J. Giessibl, J. Mannhart,
  Force Microscopy with Light Atom Probes
  Science 305, 380-383 (2004)
• F. J. Giessibl,
  Advances in Atomic Force Microscopy
  Reviews of Modern Physics 75 (3), 949-983 (2003)
• F. J. Giessibl, S. Hembacher, H. Bielefeldt, J. Mannhart,
  Subatomic Features on the Silicon(111)-(7×7) Surface Observed by Atomic Force Microscopy
  Science 289, 422-425 (2000)

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Left: Scanning electron micrograph of a carbon nanotube with two superconducting Nb contacts attached. Right: Current-voltage characteristics of the Josephson junction with the nanotube as ‘weak link’.
[Open image in a new tab for higher resolution]

A freely suspended nanotube between two metallic contacts. The gate electrode below changes both the charge state of the tube and its mechanical tension.
[Open image in a new tab for higher resolution]

Carbon nanotubes present a fascinating physical system, combining semiconductor physics, the properties of a defect-free macromolecule, and extreme mechanical stiffness. We are interested in three particular aspects: Nanotubes that are contacted with superconducting metals can carry a proximity-induced supercurrent and/or act as a weak link in a device structure. The interplay of the localized electronic structure of the nanotube and the physics of superconductivity is an active field of research. Here, niobium as contact material promises larger critical currents and more robust superconductivity with respect to temperature and magnetic field compared to previous processes. A complementary approach is given by using ferromagnetic contacts, and thereby probing the spin transport properties of electron systems trapped inside the macromolecules.

Last but not least, nanotubes also excel in their mechanical properties. Here, both fully quantized behaviour and classical resonator mechanics have been observed in different vibration modes. Longitudinal vibrations, with a high spring constant and thereby excitation energy, lead to quantized, phonon-like excitations, and can be observed in the current through the nanotube via the Franck-Condon effect. The bending or transversal mode (analogous to the motion of a guitar or piano string) has been used as classical resonator, although its resonance frequency is still very high compared to other types of nanomechanical devices. Current work aims e. g. towards observing the transition from a classical to a quantum mechanical oscillator in one and the same mechanical system – detecting true quantum-"mechanics".

There are two more main areas of research of our group, namely Superconductor (S) — Normal Conductor (N) — Superconductor (S) Structures and Electronic Interferometry which are discussed in more detail on our homepage (link below).

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/strunk/.

Top 5 publications:

• VV. M. Vinokur, T. I. Baturina, M. V. Fistul, A. Yu. Mironov, M. R. Baklanov, C. Strunk,
  Superinsulator and Quantum Synchronization
  Nature 452, 613 (2008)
• B. Stojetz, C. Miko, L. Forró, C. Strunk,
  Effect of Band Structure on Quantum Interference in Multiwall Carbon Nanotubes
  Phys. Rev. Lett., 94, 186802 (2005)
• A. Bauer, J. Bentner, M. Aprili, M. L. Della Rocca, M. Reinwald, W. Wegscheider, C. Strunk,
  Spontaneous supercurrent induced by ferromagnetic p-junctions
  Phys. Rev. Lett. 92, 217001 (2004)
• A. Bachtold, C. Strunk, J.-P. Salvetat, T. Nussbaumer, L. Forró, C. Schönenberger,
  Aharonov-Bohm oscillations in carbon nanotubes
  Nature 397, 673-675 (1999)
• M. Henny, S. Oberholzer, C. Strunk, T. Heinzel, K. Ensslin, M. Holland, C. Schönenberger,,
  The fermionic Hanbury-Brown Twiss experiment
  Science 284, 296-298 (1999)

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Electron holographic image of a triangular magnetic particle with superimposed magnetic flux lines inside and outside the specimen. The emerging stray field is clearly visible. [Open image in a new tab for higher resolution]

a): TEM cross section image of a GaN/InGaN/GaN quantum well. b): Element distribution map of Indium content (color coded) with atomic resolution. [Open image in a new tab for higher resolution]

We use transmission electron microscopes (TEMs), operated at 300 kV to investigate and charac-terize materials on the nanometer scale. The preparation of the specimens is generally achieved by mechanical thinning and ion polishing, until a thickness well below 100 nm is reached. This is sufficiently thin for high energy electrons to penetrate the specimen. When transmitted through the specimen, the electrons interact with the intrinsic physical properties of the specimen, i.e. they may loose characteristic amounts of energy, they may be deflected or absorbed. From these interactions, we learn important details about the specimens properties, such as atomic structure, local composition on an atomic scale, and even magnetic properties.

The areas of interest in this group can be split in two major topics:

Imaging and quantitatively measuring the properties of magnetic materials

There are many attempts to use magnetic materials for new technologies, such as the spin transistor or magnetic memory devices other than the hard disks known from computers. These devices are built on the basis of small, sub-µm sized magetic particles. As magnetism is a phenomenon, where many atoms have to interact, it is obvious that for very small particles, where less atoms are left for interaction, the magnetic properties may change. For future devices and applications, it is vital to be able to characterize these new properties in utmost detail. This is part of our efforts in SFB 689: Spin phenomena in reduced dimensions.

Characterizing the structure and composition of "quantum wells" in LEDs or laser diodes

Future displays will use three lasers in red, green and blue to project information onto screens, even in bright daylight. However, green lasers which can easily be integrated in a projecting device are still not available. In order to build these lasers one needs so-called quantum well structures, i.e. InGaN layer of only a few atoms in thickness between GaN layers. The efficiency of the light production is determined by the structure, the chemical homogeneity of the InGaN layer and also by piezoelectric fields in the material. We use electron microscopy and elemental analysis techniques on a nanometer scale to characterize such devices during their development.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/zweck/.

Top 5 publications:

• P. Schattschneider, M. Stöger-Pollach, S. Rubino, M. Sperl, Ch. Hurm, J. Zweck, J. Rusz,
  Detection of magnetic circular dichroism on the two-nanometer scale
  Physical Review B, 78, 104413 (2008)
• Ch. Dietrich, R. Hertel, M. Huber, D. Weiss, R. Schäfer, J. Zweck,
  Influence of perpendicular magnetic fields on the domain structure of permalloy microstructures grown on thin membranes
  Physical Review B — Condensed Matter and Materials Physics 77 (17): 174427 (2008)
• U. Wurstbauer, M. Sperl, M. Soda, D. Neumaier, D. Schuh, G. Bayreuther, J. Zweck, W. Wegscheider,
  Ferromagnetic GaMnAs grown on (110) faced GaAs
  Applied Physics Letters 92 (10): 102506 (2008)
• M. Heumann, T. Uhlig, J. Zweck,
  True single domain and configuration-assisted switching of submicron permalloy dots observed by electron holography
  Physical Review Letters 94 (7): 077202/1-4 (2005)
• T. Uhlig, M. Rahm, C. Dietrich, R. Hollinger, M. Heumann, D. Weiss, J. Zweck,
  Shifting and pinning of a magnetic vortex core in a Permalloy dot by a magnetic field
  Physical Review Letters 95 (23): 237205/1-4 (2005)

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Snapshot of an excited Landau-level wavepacket on a graphene flake in the presence of disorder. Additional movies can be viewed at http://www.quantumdynamics.de/graphene . [Open image in a new tab for higher resolution]

Snapshot showing the scattering of a wavepacket in a Hall cross. The red wavepackets acts as a receiver and records the time-dependent flux from the blue wavepacket into the left arm. We reconstruct the scattering matrix from the time-dependent flux information for a large energy window. [Open image in a new tab for higher resolution]

Our group develops a time-dependent approach to electronic transport based on the propagation of electronic wave packets. Wave packets are versatile tools which allow us to predict and analyze the electronic pathways in devices with gates and in the presence of disorder potentials. Additionally, we explore new ways to describe open, current-carrying systems, where no exact quantum-mechanical results exist. The time-dependent approach enables us to calculate conductivities over the large energy ranges needed for matching theory and experiments. Furthermore we study time-dependent excitations which are precisely controllable by strong laser fields and are used to engineer specific electronic states.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/kramer/.

Top 5 publications:

• V. Krueckl, T. Kramer,
  Revivals of quantum wave-packets in graphene
  New Journal of Physics 11, 093010 (2009)
• T. Kramer, E. Heller, R. Parrott,
  An efficient and accurate method to obtain the energy-dependent Green function for general potentials
  J. Phys.: Conference Series 99, 012010 (2008)
• K. Aidala, R. Parrott, T. Kramer, R. Westervelt, E. Heller, M. Hanson, A. Gossard,
  Imaging Magnetic Focusing of Coherent Electron Waves
  Nature Physics 3, 464-468 (2007)
• C. Bracher, M. Kleber, T. Kramer,
  New mathematical tools for quantum technology
  Mathematics of Quantum Computation and Technology [book: Chapman & Hill/CRC (2007); edited by G. Chen, L. Kauffman, S. Lomonaco]
• C. Bracher, M. Kleber, T. Kramer,
  Electron propagation in crossed magnetic and electric fields
  J. Opt. B: Quantum Semiclass. Opt. 6, 21-27, (2004)

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FemtosekundenZeitlupenkamera. Ein Lichtimpuls löst ultraschnelle Dynamik aus, z.B. die Vibration eines Kristallgitters. Ein zweiter Impuls macht eine Momentaufnahme. Schnapschüssen zu verschiedenen Verzögerungszeiten werden zu einem Zeitlupenfilm zusammengesetzt.

Blick auf eine hochintensive Femtosekunden-Lichtquelle an unserem vormailgen Standort (Universität Konstanz). Eine ähnliche Quelle der neuesten Generation entsteht derzeit in Regensburg.

Ein Laserblitz erzeugt freie Elektronen (blaue Kugeln) in einem Halbleiter-Lichtresonator und erzeugt eine Elektron-Photon-Mischung.

Viele Eigenschaften der uns umgebenden Materie werden durch Be­­we­gungen ihrer ele­men­taren Bausteine ver­ur­sacht. Das Haupt­in­­te­r­­esse unserer 2010 ein­ge­richteten Grup­pe be­steht darin, diese Quan­­ten­­physik zeit­auf­gelöst zu beob­ach­ten und zu kon­trol­lie­ren. Eine echte He­raus­forderung, denn die rele­vante Zeitskala ist durch die Femto­se­kun­de (1 fs = 10‑15 s) gegeben, den millions­ten Teil einer Milliardstel Se­kunde. Wir ent­wic­­keln Zeit­lu­pen­ka­me­ras1 auf Basis spe­ziel­ler Ultrakurz­puls-Laser2, die eine Rei­he von Re­kor­den hal­ten. Neben kürzesten Impuls­dauern zählen hierzu auch die weltweit höchsten Spitzenintensitäten im Terahertz-Fre­quenz­band3, die selbst Groß­­­for­schungs­­ein­rich­tungen über­treffen. Mit die­ser Tech­nologie sind wir für einen direk­ten Zu­gang zur ultraschnellen Quan­tenwelt moder­ner Nano­strukturen gerüstet:
Intensive Terahertz-Im­pul­se erlauben uns etwa, den Spin von Elektronen auf der Re­kord­zeitskala einer einzigen Licht­schwingung zu manipulieren4 – ein Lichtblick für die Fest­kör­per­physik ebenso wie für Hoch­ge­schwin­dig­keits-Datenspeicher der Zukunft.

Mit Halbleiter-Nanostrukturen lässt sich Licht speichern. Elektronen, die von einem Laser­impuls im Halbleiter erzeugt werden, können mit den einge­schlos­senen Pho­to­nen zu Licht-Materie-Mischteilchen verschmel­zen – und zwar schneller als eine einzelne Licht­schwin­gung5. Nach aktuellsten Theorien treten dabei neuartige nicht­adiaba­ti­sche Quan­ten­phänomene auf, ähnlich der pos­tu­lier­ten Hawking-Strahlung schwar­zer Löcher. Diese neue Quantenwelt des Vaku­ums zu er­for­schen ist ein weiteres unserer zahl­rei­chen Pro­jek­te, die wir in lokalen und inter­na­tio­na­len Ko­ope­rationen vorantreiben.

Ultraschnelle Quanten­physik beschreibt ein rasch wach­sen­des interdisziplinäres For­schungs­gebiet, das zusammen mit der nötigen inno­vativen Photonik gleichermaßen relevant für die Grund­lagen­­forschung und An­wen­dun­gen ist. Wir stehen am Anfang eines fas­zi­nie­ren­den Weges, der täglich neue wis­sen­schaft­li­che Überraschungen bereithält.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/huber/home.html.

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Impressions from the fluorescence microscopy lab with broadly-tunable laser excitation.[Open image in a new tab for higher resolution]

Light-harvesting action spectroscopy of single tetrapod-shaped semiconductor nanoparticles. Three different cases of band alignment at the CdSe/CdS interface can be identified in the plots of emission energy versus excitation energy.[Open image in a new tab for higher resolution]

Blinking of single molecule emission under the fluorescence microscope
[Movie file - click to download]

Light-harvesting action spectroscopy of single tetrapod-shaped semiconductor nanoparticles. Three different cases of band alignment at the CdSe/CdS interface can be identified in the plots of emission energy versus excitation energy.

We aim to use optical techniques such as time-resolved luminescence and fluorescence microscopy to correlate the physical structure of an object with its electronic properties. Examples of materials studied include organic semiconductors, in particular pi-conjugated polymer molecules, semiconductor colloidal nanocrystals and metallic nanoparticles. These materials can all be interpreted as individual nanostructures, where the physical shape and arrangement in space directly impact on electronic characteristics and the underlying light-matter coupling.

The approach of the group bridges current thrusts in mesoscopic physics, in particular in quantum dot spectroscopy, with open questions in biological, chemical and polymer physics. The formation, migration and dissipation of single charges and excitons is tracked optically in individual nanostructures. Ultimately, we aim to develop a toolbox to design new functional materials from the bottom up.

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/lupton.

Top 5 publications:

• N. J. Borys, M. J. Walter, J. Huang, D. V. Talapin, J. M. Lupton,
  The role of particle morphology in interfacial energy transfer in CdSe/CdS heterostructure nanocrystals
  Science 330, 1371 (2010)
• D. R. McCamey, K. J. van Schooten, W. J. Baker, S.-Y. Lee, S.-Y. Paik, J. M. Lupton, C. Boehme,
  Hyperfine-field-mediated spin beating in electrostatically-bound charge carrier pairs in organic light-emitting diodes
  Phys. Rev. Lett. 104, 017601 (2010)
• J. M. Lupton,
  Single molecule spectroscopy for plastic electronics: materials analysis from the bottom up
  Adv. Mater. 22, 1689 (2010)
• M. J. Walter, J. M. Lupton,
  Unraveling the inhomogeneously broadened absorption spectrum of conjugated polymers by single-molecule light-harvesting action spectroscopy
  Phys. Rev. Lett. 103, 167401 (2009)
• M. Reufer, M. J. Walter, P. G. Lagoudakis, A. B. Hummel, J. S. Kolb, H. G. Roskos, U. Scherf, and J. M. Lupton,
  Spin-conserving carrier recombination in conjugated polymers
  Nature Mat. 4, 340 (2005)

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We perform computer simulations of nanostructures interacting with light. The goal is to understand how the absorbed energy is released in luminescence, converted into heat and atomic motion or transported from one place to another. To this end, equations equivalent to the time dependent Schrödinger equation are solved numerically for large numbers of electrons without empirical parameters. Applications of the theory include for example novel forms of solar cells, organic LED, or fluorescent nanocrystals.

Our research is funded by the DFG Priority Program SPP 1243 "Quantum Transport" and the DFG Research Training Group GRK 1570 on "Electronic Properties of Carbon based Nanostructures.

Left: Schematic design of a dye-sensitized solar cell. Absorption of sun light by a dye molecule leads to the excitation of an electron that is transferred to neighboring Titanium oxide nanoparticles and reaches finally the upper electrode. The electron vacancy left in the molecule is neutralized by the electrolyte, which itself is regenerated at the lower electrode. In this way the electronic circuit is closed and an usable electromotive force is generated.
Right: Results of a quantum mechanical simulation. Shown is the energtically most favourable conformation of a Cyanidine dye on a Titanium oxide nanowire. [Open image in a new tab for higher resolution]

Excited state molecular dynamics simulation of C₆₀ subjected to a laser pulse of 100 fs duration with 1.55 eV carrier frequency and a fluence of 1.3 J/cm². -- Don't worry, we love football!
[Movie file - click to download]

Homepage

The homepage of our research group can be found at http://www.physik.uni-regensburg.de/forschung/niehaus/index.html.

Top 5 publications:

• Schulze, G., Franke, K., Gagliardi, A., Romano, G., Lin, C., Da Rosa, A., Niehaus, T., Frauenheim, T., Di Carlo, A., Pecchia, A. und Pascual, J.,
  Resonant Electron Heating and Molecular Phonon Cooling in Single C(₆₀) Junctions
  Phys. Rev. Lett. 100, 136801 (2008)
• Unusual size dependence of the optical emission gap in small hydrogenated silicon nanoparticles,
  Wang, X., Zhang, R., Lee, S., Niehaus, T. und Frauenheim, T.
  Appl. Phys. Lett. 90, 123116 (2007)
• Simdyankin, S., Niehaus, T., Natarajan, G., Frauenheim, T. und Elliott, S.,
  A new type of charged defects in amorphous chalcogenids
  Phys. Rev. Lett. 94, 086401 (2005)
• Niehaus, T., Suhai, S., Della Sala, F., Lugli, P., Elstner, M., Seifert, G. und Frauenheim, T.,
  Tight-binding approach to time-dependent density-functional response theory
  Phys. Rev. B 63, 085108 (2001)
• Torralva, B., Niehaus, T., Allen, R., Elstner, M., Frauenheim, T. und Suhai, S.,
  Response of C₆₀ and C_n to ultrashort laser pulses
  Phys. Rev. B 64, 153105 (2001)

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