Archive

Blocking transport resonances via Kondo many-body entanglement in quantum dots

Michael Niklas, Sergey Smirnov, Davide Mantelli, Magdalena Marganska, Ngoc-Viet Nguyen, Wolfgang Wernsdorfer, Jean-Pierre Cleuziou, Milena Grifoni

Nature Communications 7, 12442 (2016)

We study the Kondo effect in carbon nanotubes. If the coupling between the nanotube and the leads is weak a single electron can become trapped inside the tube due to Coulomb repulsion and its spin acts as a scattering region. If the coupling becomes stronger the scattered electrons form a bound state, the Kondo cloud, which results in a zero bias conductance peak. Interestingly not the real electron spin is involved in this process, but a pseudospin degree of freedom.

(01.03.2017)
Secondary electron interference from trigonal warping in clean carbon nanotubes

A. Dirnaichner, M. del Valle, K. J. G. Götz, F. J. Schupp, N. Paradiso, M. Grifoni,
Ch. Strunk, and A. K. Hüttel
Physical Review Letters 117, 166804 (2016)

http://dx.doi.org/10.1103/PhysRevLett.117.166804

Imagine a graphene "sheet" of carbon atoms rolled into a tube - and you get a carbon
nanotube. Carbon nanotubes come in many variants, which influence strongly their
electronic properties. They have different diameter, but also different "chiral
angle", describing how the pattern of the carbon atoms twists around the tube axis.
In our work, we show how to extract information on the nanotube structure from
measurements of its conductance. At low temperature, electrons travel ballistically
through a nanotube and are only scattered at its ends. For the quantum-mechanical
electron wavefunction, metallic nanotubes act then analogous to an optical Fabry-
Perot interferometer, i.e., a cavity with two semitransparent mirrors at either end,
where a wave is partially reflected. Interference patterns are obtained by tuning the
wavelength of the electrons; the current through the nanotube oscillates as a
function of an applied gate voltage. The twisted graphene lattice then causes a
distinct slow current modulation, which, as we show, allows a direct estimation of
the chiral angle. This is an important step towards solving a highly nontrivial
problem, namely identifying the precise molecular structure of a nanotube from
electronic measurements alone.

(22.11.2016)

Direct determination of spin-orbit interaction coefficients and realization of the persistent spin helix symmetry
Nature Nanotechnology  (2014), doi:10.1038/nnano.2014.128

A. Sasaki, S. Nonaka, Y. Kunihashi, M. Kohda, T. Bauernfeind, T. Dollinger, K. Richter und J. Nitta

Link to publication:
http://www.nature.com/doifinder/10.1038/nnano.2014.128

The spin–orbit interaction plays a crucial role in diverse fields of condensed matter. In III–V zinc-blende semiconductor heterostructures, two types of spin–orbit interaction—Rashba and Dresselhaus—act on the electron spin as effective magnetic fields. They are characterized by coefficients α and β, respectively. When α is equal to β, the so-called persistent spin helix symmetry is realized. In this condition, invariance with respect to spin rotations is achieved even in the presence of the spin–orbit interaction, implying strongly enhanced spin lifetimes for spatially periodic spin modes. Existing methods to evaluate α/β require fitting analyses that often include ambiguity in the parameters used.
In a recent publication of Klaus Richter's theory group with experimental physicists from Tohoku University in Sendai (Japan), a simple all-electrical and fitting parameter-free technique has been demonstrated to determine α/β and to deduce the absolute values of α and β. The method is based on the detection of the effective magnetic field direction and the strength induced by the two spin–orbit interactions. Moreover,  the persistent spin helix symmetry has been demonstrated by gate tuning.


Electron microscope images of the wire structures (left) and schematic illustration of measurement configuration

Press release (In German):
http://www.uni-regensburg.de/pressearchiv/pressemitteilung/410193.html

(18.07.2014)

Dissertation Prize 2014 of the Condensed Matter Division awarded to Dr. Bernhard Endres

The Dissertation Prize 2014 of the Condensed Matter Division (SKM) was awarded to Dr. Bernhard Endres during the DPG spring meeting in Dresden (30.03 – 4.4.2014) in recognition of his research in the field of semiconductor spintronics, in particular on spin injection into Galliumarsenide. Efficient spin injection is crucial for the development of spin-based transistors with the promise of lower power consumption compared to conventional devices. In his doctoral thesis carried out within the SFB 689 Bernhard Endres has found a novel method of spin injection into semiconductors, the so-called spin solar cell 1).

1) B. Endres, M. Ciorga, M. Schmid, M. Utz, D. Bougeard, D. Weiss, G. Bayreuther, C.H. Back,
Demonstration of the spin solar cell and spin photodiode effect, Nature Communications 4, 2068 (2013)

Klaus Richter, Bernhard Endres, Johanna Stachel und Ludwig Schultz (v.l.)
© DPG / Sauer

(25.04.2014)
Erasmus Intensive Program Spintronics and Applications

1st Erasmus Intensive Program Spintronics and Applications (SPEA)

22 / 7 / 2013 - 2 / 8 / 2013

Department of Electronics, TEI of Crete, Chania, Crete, Greece

http://spea.chania.teicrete.gr

 

(17.09.2013)

Bulk electronic structure of the dilute magnetic semiconductor Ga1-xMnxAs through hard X-ray angle-resolved photoemission

A. X. Gray, J. Minar, S. Ueda, P. P. Stone, Y. Yamashita, J. Fujii, J. Braun, L. Plucinski, C. M. Schneider, G. Pannaccione, H. Ebert, O. D. Dubon, K. Kobayashi, and C. S. Fadley
Nature Materials 11, 957-962 (2012)

In a recent publication Jan Minar, Jürgen Braun and Hubert Ebert as part of a multi-institutional collaboration of researchers report on a new technique to probe bulk electronic structure called HARPES (Hard x-ray Angle-Resolved Photo Emission Spectroscopy). They used this technique to investigate the prototypical dilute magnetic semiconductor Ga1-xMnxAs and as reference undoped GaAs. The authors compared the experimental data with their theory based on the coherent potential approximation and fully relativistic one-step-model photoemission calculations including matrix-element effects. Direct observation of Mn-induced states between the GaAs valence band maximum and the Fermi level, centered about 400 meV binding energy, as well as changes throughout the whole range of the valence band, indicates that ferromagnetism in Ga1-xMnxAs must be considered to arise from both p-d and double exchange, leading to a coherent picture for the electronic structure of this interesting and challenging material.

(05.11.2012)

Spin Transistor Action from Hidden Onsager Reciprocity

İ. Adagideli, V. Lutsker, M. Scheid, Ph. Jacquod, and K. Richter

Phys. Rev. Lett. 108, 236601 (2012)

In a recent publication of the group of Klaus Richter together with two external groups (Inanc Adagideli and Philippe Jacquod)  new implications for spin transport in systems with spatially inhomogeneous spin-orbit coupling have been found. Using a local gauge transformation the authors show how spin conductance in such systems is related to the magneto conductance of spinless fermions in a fictitious orbital magnetic field. They  further show how the resulting spin conductance  vanishes in a two-terminal setup, and how it is turned on by opening additional transport terminals, thus allowing one to switch the generated spin current on or off.


(22.06.2012)
Awarding of the honorary doctorate by the faculty of physics of the University of Regensburg to Stuart Parkin on December 20, 2011

Dr. Parkin (*1955) is one the founders of Spintronics and Nano-Magnetism. He studied in Cambridge at the Cavendish Laboratory and received his PhD degree in 1980. After a short stay in France he moved to the IBM research centre in San Jose, now IBM Almaden in 1982. His original field of research was superconductivity, in particular high temperature superconductor. In the mid 1980’s he has changed his research interest to thin film magnetism. He is considered as one of the discoverers of the oscillatory exchange interaction, which was – in combination with the GMR effect – a necessary prerequisite for so-called „ spin engineering “. In the last years he was active in the field of the tunnel-magneto-resistance and the so-called „Racetrack Memory”. Recently he has rediscovered the field of oxide materials and in particular oxidic interfaces.

For his work in the area of the GMR effect he has received the American Physical Society International New Materials Prize in 1994 and the European Physical Society Hewlett-Packard Europhysics Prize in 1997. In the year 2009 he received the “International Union of Pure and Applied Physics Magnetism Award“ (IUPAP) as well as the die Louis Neel Medal. Parkin is IBM Fellow as well as elected member of the Royal Society, the American Physical Society, the Institute of Physics (London), the Institute of Electrical and Electronics Engineers, and the American Association for the Advancement of Science. He is an author and/or Co-author of approx. 350 articles and holds approx. 50 patents.

Altogether 22 groups of the physics department are involved in the SFB initiative „ spin phenomena in reduced dimensions “ (DFG-SFB 689), among them 14 professors. This initiative is centred on the topics of Magneto-Electronics or Spin-Electronics. Stuart Parkin is one the founders of this research area. Together with the Nobel Laureates Dr. Albert Fert and Dr. Peter Grünberg he contributed substantially to the understanding and technological implementation of the Giant Magneto-Resistance effect. In particular his contributions to spin engineering to the understanding of oscillatory exchange coupling is acknowledged world wide. Stuart Parkin was considerably involved in the exploitation of the GMR and TMR effects for sensing applications. Only 10 years after the discovery of GMR, sensors based on this principle were introduced into recording heads by IBM.

Due to his scientific excellence and the proximity of the work of Mr. Parkin to one of the most important research areas of the faculty for physics, Stuart Parkin was awarded with the honorary doctor of the faculty for physics of the University of Regensburg on 20 December 2011.

 

 

(18.01.2012)
Thin Fe film boosts Curie temperature of (Ga,Mn)As

"Proximity Induced Enhancement of the Curie Temperature in Hybrid Spin Injection Devices"

C. Song, M. Sperl, M. Utz, M. Ciorga, G. Woltersdorf, D. Schuh, D. Bougeard, C. Back and D. Weiss

Physical Review Letters 107, 56601 (2011)

In a recent publication the groups of Christian Back, Dominique Bougeard and Dieter Weiss demonstrated the enhancement of the Curie temperature (TC) of (Ga,Mn)As layers by placing a thin film of iron on top of the (Ga,Mn)As sheet. Depending on the thickness of the (Ga,Mn)As film TC can be increased by up to 100%. This opens the way to applications at room temperature if thin (Ga,Mn)As layers with sufficiently high TC can be grown.

Normalized enhancement of T <sub>c for differently thick (Ga,Mn)As films

Fig.: Normalized enhancement of Tc for differently thick (Ga,Mn)As films

(14.09.2011)
"Geometric Correlations and Breakdown of Mesoscopic Universality in Spin Transport"

I. Adagideli, Ph. Jacquod, M. Scheid, M. Duckheim, D. Loss and K. Richter

Phys. Rev. Lett.105,246807 (2010)

Geometry-dependent spin currents in mesoscopic conductors

In disordered or ballistic low-dimensional, spin-orbit based conductors exhibiting ergodic charge carrier dynamics, no net spin conductance has been expected, at least according to random-matrix theory. In a recent work, where a group of the SFB has been involved, it could be demonstrated, both analytically and numerically, that the spin current is indeed finite in such cavities and that its direction is determined by the direction of the average current flow, i.e. depending on the orientation of the leads.

(15.12.2010)
"Fast Domain Wall Propagation under an Optimal Field Pulse in Magnetic Nanowires"

Z. Z. Sun and J. Schliemann

Phys. Rev. Lett. 104, 037206 (2010)

A theoretical group of the Regensburg Physics Department has investigated field-driven domain wall (DW) propagation in magnetic nanowires. The study was carried out within in framework of the Landau-Lifshitz-Gilbert equation and led to a new concept to speed up the DW motion in a uniaxial magnetic nanowire by using an optimal space-dependent field pulse synchronized with the DW propagation. Depending on the damping parameter, the DW velocity can be increased by about two orders of magnitude compared to the standard case of a static uniform field.

(15.12.2010)
Extension of SFB 689/2 is approved.
Die Verlängerung des SFB 689/2 ist bewilligt.

DFG fördert Physik an der Uni Regensburg mit rund 10 Millionen Euro

Die Physik auf der Nanometerskala ist seit mehreren Jahren das zentrale Thema der Regensburger Festkörperphysiker und stellt mit den größten Forschungsschwerpunkt der Universität Regensburg dar. Nun hat die Deutsche Forschungsgemeinschaft (DFG) den Sonderforschungsbereich 689 an der Regensburger Physikfakultät um vier Jahre verlängert. Dabei mussten sich die Regensburger, deren wissenschaftliches Programm und bisherige Erfolge von den Gutachtern als hervorragend bewertet wurden, in einem höchst kompetitiven Verfahren gegen andere Forschergruppen durchsetzen. Damit können die Physiker weiter auf höchstem internationalem Niveau auf dem Gebiet der "Spinphänomene in reduzierten Dimensionen", so der Titel des SFB, arbeiten. Dieses aktuelle Forschungsgebiet umfasst die zusätzliche Nutzung des Spins, also des magnetischen Momentes eines Elektrons, für die Elektronik auf kleinsten Längenskalen. Diese Nano-Spintronik kann einerseits zu neuartigen Datenspeichern führen, ist aber andererseits auch ein wichtiges Forschungsfeld für die Quanteninformationsverarbeitung, bei dem quantenmechanische und nicht-klassische Regeln die Rechenoperationen bestimmen.

Mit den von der DFG zur Verfügung gestellten Mitteln können für die nächsten vier Jahre über 30 zusätzliche Doktoranden und Postdoktoranden finanziert werden. Viele der bislang im Sonderforschungsbereich ausgebildeten Absolventen sind zwischenzeitlich bei High-Tech Firmen in der Region tätig und geben so wichtige Impulse für die heimische Wirtschaft. Bei der personellen Struktur setzt der SFB stark auf Nachwuchswissenschaftler/innen. Neben Wissenschaftlerstellen werden aber auch Investitionsmittel zur Verfügung gestellt mit denen modernste Geräte beschafft werden können. Insgesamt sind 23 Wissenschaftler in den Sonderforschungsbereich integriert, Sprecher des SFB ist Professor Dieter Weiss vom Institut für Experimentelle und Angewandte Physik.

(10.03.2010)
"Ratchet Effects Induced by Terahertz Radiation in Heterostructures with a Lateral Periodic Potential"

P. Olbrich, E. L. Ivchenko, R. Ravash, T. Feil, S. D. Danilov, J. Allerdings, D. Weiss, D. Schuh, W. Wegscheider and S. D. Ganichev

Phys. Rev. Lett. 103, 090603 (2009)

(18.11.2009)
"All-Electrical Detection of the Relative Strength of Rashba and Dresselhaus Spin-Orbit Interaction in Quantum Wires"

M. Scheid, M. Kohda, Y. Kunihashi, K. Richter, J. Nitta

Phys. Rev. Lett. 101, 266401 (2008)

We propose a new method to determine the relative strength of Rashba and Dresselhaus spin-orbit interaction from transport measurements without the needo f fitting parameters.

(17.08.2009)
"Evidence for a Magnetic Proximity Effect up to Room Temperature at Fe/Ga(Mn,As) Interfaces"

F. Maccherozzi, M. Sperl, G. Panaccione, J. Minár, S. Polesya, H. Ebert, U. Wurstbauer, M. Hochstrasser, G. Rossi, G. Woltersdorf, W. Wegscheider, C. H. Back

Phys. Rev. Lett 101, 267201 (2008)

Physics 1, 43 (2008)

In collaboration with a research group at the synchrotron Elettra in Trieste research groups within the SFB 689 have succeeded in convincing the diluted ferromagnetic (Ga,Mn)As to become ferromagnetic even at room temperature. This was realized by utilizing the proximity polarization effect of a thin ferromagnetic Fe overlayer. Element specific hysteresis curves have been measured using XMCD at the APE beamline in Trieste. Ab-initio calculations as well as Monte Carlo Simulatione support the ex-perimental findings. The article has been highlighted in "Viewpoint in Physics".

(05.05.2009)
Review of the workshop

"Spin Phenomena in Reduced Dimensions"

(12.01.2009)
A member of the SFB 689, Dr. Christian Ertler, was awarded the "AIV-Massimo Sancrotti Award for young scientists" at the 22nd General Conference of the Condensed Matter Divison of the European Physical Society, for the best publication.
(12.01.2009)
"Conductivity of graphene: How to distinguish between samples with short and long range scatterers"

Maxim Trushin, John Schliemann

Europhys. Lett. 83, 17001 (2008)

Research Highlight in Nature Physics

(02.12.2008)
"Self-sustained Magnetoelectric Oscillations in Magnetic Resonant Tunneling Structures"

Christian Ertler and Jaroslav Fabian

Phys. Rev. Lett. 101, 077202 (2008)

The dynamic interplay of transport, electrostatic, and magnetic effects in the resonant tunneling through ferromagnetic quantum wells is theoretically investigated. It is shown that the carrier-mediated magnetic order in the ferromagnetic region not only induces, but also takes part in intrinsic, robust, and sustainable high-frequency current oscillations over a large window of nominally steady bias voltages. This phenomenon could spawn a new class of quantum electronic devices based on ferromagnetic semiconductors.

(19.09.2008)
"Symmetry and spin dephasing in (110)-grown quantum wells"

V.V. Bel’kov, P. Olbrich, S.A. Tarasenko, D. Schuh, W. Wegscheider, T. Korn, Ch. Schüller, D. Weiss, W. Prettl, S.D. Ganichev

Phys. Rev. Lett. 100, 176806 (2008)

Symmetry and spin dephasing in (110)-grown GaAs quantum wells (QWs) are investigated applying magnetic field induced photogalvanic effect and time-resolved Kerr rotation. We show that magnetic field induced photogalvanic effect provides a tool to probe the symmetry of (110)-grown quantum wells. The photocurrent is only observed for asymmetric structures but vanishes for symmetric QWs. Applying Kerr rotation we prove that in the latter case the spin relaxation time is maximal. Therefore, these structures set the upper limit of spin dephasing in GaAs QWs. We also demonstrate that structure inversion asymmetry can be controllably tuned to zero by variation of δ-doping layer positions.

(20.08.2008)
"Spin Transport in Rough Graphene Nanoribbons: Universal Fluctuations and Spin Injection"

M. Wimmer, I. Adagideli, S. Berber, D. Tomanek, K. Richter

Phys. Rev. Lett. 100, 177207 (2008)

Abstract:
We investigate spin conductance in zigzag graphene nanoribbons and propose a spin injection method based only on graphene. Combining density functional theory with tight-binding transport calculations, we find that nanoribbons with asymmetrically shaped edges show a non-zero spin conductance and can be used for spin injection. Furthermore, nanoribbons with rough edges exhibit mesoscopic spin conductance fluctuations with a universal value of rmsGs = 0.4 e/4π

(03.07.08)
A member of the SFB 689 is invited to present his results at the March-Meeting 2008 of the American Physical Society (APS) in New Orleans.

I. Adagideli, Boundaries Between Current Carrying Semiconductors and Metallic Contacts

Abstract:
An important goal of semiconductor spintronics is the generation of spin accumulations in non-magnetic semiconductors by applying charge currents. I will talk about two alternative methods of spin injection without using ferromagnetic leads using spin-orbit interaction: the spin Hall effect and current induced spin accumulation. First, I will focus on bulk semiconductor systems and discuss how to extract and detect current-induced spins. To this end, I will discuss the boundary conditions satisfied by spin accumulations near diffusive contacts and show how the effective boundary conditions depend on the width of the contacts. Surprisingly, narrow (i.e. width smaller than the spin relaxation length) diffusive contacts are more effective in extracting current-induced spins than wide contacts. I will demonstrate the general concepts by solving the transport equations for the diffuse Rashba system near a Hall contact as well as by numerical quantum transport simulations. Next I will focus on mesoscopic size systems and discuss the fluctuations of the spin current at ballistic contacts. Finally, I will conclude by discussing other systems that feature spin Hall effect such as graphene nanobridges.

(03.04.2008)
"Microwave assisted switching of single domain NiFe elements"

Georg Woltersdorf und Christian H. Back

Phys. Rev. Lett. 99, 227207 (2007)

A group of our collaborative research center succeeded to demonstrate micro-wave assisted switching of single domain magnetic elements. The magnetization dynamics was measured using time and spatially resolved magneto-optic Kerr effect (MOKE). It was shown that in 2 nm thick hexagonal NiFe elements with a lateral size of 1 micron the ferromagnetic resonance allows to trigger magnetic reversal in significantly reduced magnetic fields.

(12.12.2007)
"Weak localization in ferromagnetic (Ga,Mn)As nanostructures "

D. Neumaier, K. Wagner, S. Geißler, U. Wurstbauer, J. Sadowski, W. Wegscheider and D. Weiss

Phys. Rev. Lett. 99, 116803 (2007)

A group of our collaborative research center succeeded in observing weak antilocalization in a ferromagnetic material. The transport experiments were carried out in the ferromagnetic semiconductor material (Ga,Mn)As. To extract the quantum mechanical conductivity correction the resistance across twenty-five, 7.5 μm long wires with 40 nm x 40 nm cross section, connected in parallel, was measured at millikelvin temperatures. From the data we extract a phase coherence length between 100 nm and 200 nm and a spin orbit length of ~ 100 nm at 20 mK.

(05.11.2007)
"Spin-orbit interaction in symmetric wells and cycloidal orbits without magnetic fields"

E.S. Bernardes, J. Schliemann, M. Lee, J. C. Egeus and D. Loss

Phys. Rev. Lett. 99, 076603 (2007)

Spin-orbit coupling in quantum wells with more than one occupied subband. A theoretical collaboration involving our department's Collaborative Research Centre has most recently investigated spin-orbit coupling in quantum wells with more than one occupied subband. Here one finds a "Rashba-like" contribution even for symmetric potentials, and, among other effects, a finite spin Hall conductivity is predicted.

(20.09.2007)
"Tunneling Anisotropic Magnetoresistance and Spin-Orbit Coupling in Fe/GaAs/Au Tunnel Junctions"

J. Moser, A. Matos-Abiague, D. Schuh, W. Wegscheider, J. Fabian and D. Weiss

Phys. Rev. Lett. 99, 056601 (2007)

A group of our collaborative research center succeeded in observing an anisotropic tunnelling magnetoresistance (TAMR) in a tunnelling device involving the conventional ferromagnet iron. Within the Fe/GaAs/Au sandwich, the iron was grown epitaxially onto the 8 nm thin GaAs barrier. The observed resistance across the stack depends not only on the direction of the magnetization in the iron film but can be swapped by applying a bias voltage between the Fe and Au contact. The figure below shows the bias dependent switching of the anisotropic resistance.

(14.09.2007)
"Effect of initial spin polarization on spin dephasing and electron g factor in a high-mobility two-dimensional electron system"

D. Stich, J. Zhou, T. Korn, R. Schulz, D. Schuh, W. Wegscheider, M. W. Wu and C. Schüller

Phys. Rev. Lett. 98, 176401 (2007)

A group of our collaborative research center has reported a strong increase of the spin dephasing time with increasing initial degree of spin polarization in a high-mobility two-dimensional electron system. The left panel of the figure below shows time-resolved experiments, which demonstrate the increase of spin dephasing time. At small polarizations and low temperatures, coherent spin oscillations due to spin-orbit interaction could be observed (right panel). The manuscript is accepted for publication in Physical Review Letters.

(13.07.2007)
Two members of the SFB 689 were invited to present their results at the March-Meeting 2007 of the American Physical Society (APS) in Denver.

J. Fabian gave an invited talk on "Theory of phonon-induced spin relaxation in coupled lateral quantum dots",

D. Weiss on "Dephasing in (Ga,Mn)As Nanowires & Rings"

(02.04.2007)
Group photo Workshop Frauenchiemsee

Lectures 2006 (intern)

(29.03.2007)
"Zero-bias spin separation"

S.D. Ganichev, V.V. Bel'kov, S.A. Tarasenko, S.N. Danilov, S. Giglberger, C. Hoffmann, E.L. Ivchenko, D. Weiss, W. Wegscheider, C. Gerl, D. Schuh, J. Stahl, J.de Boeck, G. Borghs and W. Prettl

Nature Physics 2, 609 (2006)

A team of our SFB 689 reports in Nature Physics the observation of a novel effect causing spin separation of charge carriers in a terahertz field. The effect offers the possibility of generating pure spin currents.

Fig.: Principle of spin-separation: Spin dependent scattering of charge carriers under terahertz irradiation causes separation of spin-up and spin-down charge carriers.

(15.02.2007)
Lecture Prof. Ganichev Int. Spintronics Workshop, Singapore 2006

"Spin-galvanic effect and spin orientation by current in non-magnetic semiconductors"

First observation of phase-coherent phenomena in the ferromagnetic semiconductor (Ga,Mn)As

"Dephasing in (Ga,Mn)As nanowires and rings"

K. Wagner, M. Reinwald, W. Wegscheider and D. Weiss

Phys. Rev. Lett. 97, 056803 (2006)

Konrad Wagner reports the first observation of universal conductance fluctuations and Aharonov-Bohm oscillations (Ga,Mn)As nanostructures in the August 4 issue of Physical Review Letters. From an analysis of the conductance fluctuations he extracted a phase coherence length of lΦ ~ 100 nm at 20 mK as well as a lΦ ~ T -1/2 temperature dependence.

Figure: Magnetoresistance of a (Ga,Mn)As ring with a diameter of 400 nm and a ring width of 40 nm. The inset displays a top view of the ring. The temperature was varied from 1 K (bottom trace) to 30 mK (top trace) and the current through the device was set to 100 pA. To demonstrate the reproducibility of the resistance oscillations observed below 200 mK the 30 mK trace is shown for an up-(blue line) and down-sweep (dashed black line) of the B-field.

The smallest football pitch of Germany

Konrad Wagner (LS Weiss) took part in the contest "The smallest football pitch of Germany" announced by Spiegel Online. Wagner’s 700 nm x 500 nm pitch can be seen above and in SPIEGEL Online (together with the fields from the "competition"). The technique used here, electron beam lithography and dry etching, was developed to study phase coherent effects in GaMnAs nanostructures, a topic within our SFB.

For more info see:
http://www.spiegel.de/wissenschaft/mensch/0,1518,424268,00.html

Prof. Zweck, Plenary lecture Spring Meeting 2006

"Imaging and magnetic characterization of individual nanostructures in a transmission electron microscope (TEM)"