Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
[1] R. Egger, W. Häusler, C.H. Mak and H. Grabert, Phys. Rev. Lett 82,
3320 (1998)
[2] B. Reusch, R. Egger, Europhysics Letters, 64, 84 (2003)
[3] M. Governale, Phys. Rev. Lett 89, 206802 (2002)
TT 17.10 Di 14:30 Poster A
Conductance Measurements on Ferromagnetic Breakjunctions
— •C. Bacca , R. Arnold, and E. Scheer — Department of Physics,
University of Konstanz
We investigate lithographically fabricated breakjunctions of ferromagnetic
(Co,Ni) nanobridges with or without superconducting leads (Al).
With the help of a three-point bending mechanism, the bridges can be
opened to a single-atom contact, broken to a vacuum-tunnel contact and
closed again repeatedly at low temperatures. As reported for nonmagnetic
materials [1] we observe steps in the conductance that are due to
atomic rearrangements in the contact region. From the opening and closing
curves with and without magnetic field we calculate conductance
histograms in order to determine the preferred conductance values. We
do not find indications of maxima at half-integer conductance quanta
e 2 /h. The latter is often assumed to give indications for the existence of
spin-polarized conductance channels.
We observe very high magnetoconductance effects up to 150% for
single-atom or 500% tunnel contacts in magnetic fields up to 5 T. However
the details of the magnetoconductance curves are not very reproducible.
[1] J.M. Krans et al. Nature 375, 767 (1995)
TT 17.11 Di 14:30 Poster A
Aharonov-Bohm-Oszillationen in mesoskopischen metallischen
Ringen: Einfluss magnetischer Verunreinigungen
und eines inhomogenen Magnetfelds — R. Häussler 1 , H. v.
Löhneysen 1,2 , C. Paschke 1 , H. B. Weber 3 und E. Scheer 4 —
1 Physikalisches Institut, Universität Karlsruhe, 76128 Karlsruhe —
2 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, 76344
Karlsruhe — 3 Forschungszentrum Karlsruhe, Institut für Nanotechnologie,
76344 Karlsruhe — 4 Fachbereich Physik, Universität Konstanz,
78457 Konstanz
Wir berichten über Oszillationen des Magnetleitwerts als Funktion eines
äußeren Magnetfelds (Aharonov-Bohm-Oszillationen) in Cu-Ringen
mit Durchmesser von etwa 1µm und Stegbreite von etwa 80nm. Eine
mit 640pm Mn dotierte Probe zeigt eine klare Signatur von h/e-
Oszillationen, die jedoch einer Leitwertänderung von nur etwa ∆G =
2 · 10 −4 e 2 /h entsprechen gegenüber ∆G = 5 · 10 −2 e 2 /h für reine Cu-
Proben ähnlicher Geometrie. Aufbringen eines ferromagnetischen Kerns
aus Dy (Durchmesser 1µm) direkt oberhalb des Cu-Rings (durch eine
30nm dicke SiO2- Schicht von diesem separiert) erzeugt ein inhomogenes
radiales Magnetfeld in der Ebene des Cu-Rings. Wir diskutieren
die Möglichkeit einer dadurch hervorgerufenen Berry-Phase in den
Aharonov-Bohm-Oszillationen.
TT 17.12 Di 14:30 Poster A
Transmission Coefficients for Oscillating Barriers — •Mario Fischer,
Frank Grossmann und Thomas Kunert — Techn. Univ.
Dresden, Inst. f. Theor. Physik, 01062 Dresden
Hamiltonians with localized oscillating potentials serve as model systems
for break junctions and lead-molecule-lead systems in laser fields.
Because for this class of Hamiltonians energy is not conserved and absorption
resp. emission of photons occurs, the definition of transmission
coefficients is not straightforward. For those Hamiltonians a new numerical
approach to define and calculate transmission coefficients is presented.
By perfoming a limited amount of wave packet propagations the
transmission coefficients for all interesting energies and photon exchange
numbers can be obtained. Using this approach basic phenoma as the rectification
of current through molecules [ 1 ] can be examined.
[ 1 ] J. Lehmann, S. Kohler, P. Hänggi, J. Chem. Phys. 118, 3283 (2002)
TT 17.13 Di 14:30 Poster A
Einfluss von Laserlicht auf den Stromtransport zwischen zwei
Metallspitzen — •Daniel Schmidt, Paul Leiderer und Elke
Scheer — Universität Konstanz, Universitätsstraße 10, 78457 Konstanz
Wir untersuchen die Auswirkungen von Laserlicht verschiedener Wellenlängen
auf den elektronischen Transport in mechanisch kontrollierten
Bruchkontakten. Dabei soll der Einfluss der Polarisation des Lichts
berücksichtigt, sowie Effekte der Gleichrichtung untersucht werden.
Auf Grund ihrer Abmessungen sind die Metallspitzen des Bruchkontakts
auch optische Antennen. Diese Eigenschaft besser zu verstehen, ist ein
weiteres Ziel unseres Experiments.
TT 17.14 Di 14:30 Poster A
Conductance properties of single-molecule junctions under
pulsed laser illumination — •J. U. Würfel 1 , S. Dantscher 2 , C.
Kennerknecht 2 , W. Pfeiffer 2 , S. Schramm 2 , and H. B. Weber 1
— 1 Forschungszentrum Karlsruhe, Institut für Nanotechnologie, PO-Box
3640, D-76021 Karlsruhe — 2 Physikalisches Institut EP1, Universität
Würzburg, Am Hubland, D-97074 Würzburg
We investigate electron transport properties of single-molecule junctions,
formed by the mechanically-controlled break-junction technique.
Recent experiments demonstrated that we are able to contact single
molecules [1]. Here, we report on measurements carried out under illumination
with a femtosecond TiSa laser system. We observe a photoinduced
enlargement of the conductance. As possible mechanism a thermally
induced diminishment of the contact distance is discussed and contrasted
to a picture of photo-excited electrons in the leads. When diodelike
molecules are used, different photo-induced conductance changes for
positive and negative bias voltage are observed.
[1] Phys. Rev. Lett., 88, 176804 (2002)
TT 17.15 Di 14:30 Poster A
Conductance fluctuations in nonequilibrium — •Thomas Ludwig
1,2 , Alexander Mirlin 1,2 , and Yaroslav Blanter 3 — 1 Institut
für Nanotechnologie, Forschungszentrum Karlsruhe, 76021 Karlsruhe,
Germany — 2 Institut für Theorie der Kondensierten Materie, Universität
Karlsruhe, 76128 Karlsruhe, Germany — 3 Department of NanoScience,
Delft University of Technology, Lorentzweg 1, 2628 CJ Delft, The Netherlands
We investigate the amplitude of mesoscopic fluctuations of the differential
conductance of a metallic wire at arbitrary bias voltage. We identify
the parameter regime where inelastic scattering is important. In the
noninteracting case we show quantitative results for T = 0, in the interacting
case we qualitatively identify the asymptotic behaviour. We show
that, due to inelastic scattering, conductance fluctuations cannot grow
with increasing voltage in a way that would lead to negative differential
resistance.
TT 17.16 Di 14:30 Poster A
Spin Hall Effect in paramagnetic Metal — •Christian Debuschewitz,
Sergio O. Valenzuela, and Michael Tinkham —
Department of Physics Gordon McKay Laboratory, Harvard University
When a current is circulating in a paramagnetic metal, electrons with
spin up and spin down are predicted to separate due to skew scattering
[1]. The generated spin imbalance results in a spin Hall voltage which
can be measured by attaching a ferromagnet, perependicular to the current
on one side of a gold wire [2]. By changing the magnetization of the
ferromagnet, we observed a voltage difference of ≈ 100 nV.
[1] J. E. Hirsch PRL 83, 1834 (1999)
[2] S. Zhang PRL 85, 393 (2000)
TT 17.17 Di 14:30 Poster A
Quantum holonomies with Josephson-junction devices
— •Mateusz Cholascinski — Institut für Theoretische
Festkörperphysik, Universität Karlsruhe, D-76128 Karlsruhe, Germany
— Nonlinear Optics Division, Adam Mickiewicz University, 61614
Poznan, Poland
We examine properties of a Josephson-junction system composed of
two coupled Cooper-pair boxes (charge qubits) as a candidate for observation
of quantum holonomies. We construct a universal set of transformations
in a two-fold degenerate ground state, and discuss the effects of
noise in the system. The cheme can be applied to any system of two coupled
qubits (independently of physical realization) provided the number
of tunable parameters is sufficiently large.
TT 17.18 Di 14:30 Poster A
Compensation of telegraph noise decoherence by means of
bang-bang control — •Henryk Gutmann 1 , William Kaminsky 2 ,
Seth Lloyd 3 , and Frank Wilhelm 1 — 1 Sektion Physik and CeNS,
LMU, 80333 München — 2 Department of Physics, MIT, Cambridge,
Mass. 02139 — 3 Department of Mechanical Engineering, MIT, Cambridge,
Mass. 02139
With growing success in isolating solid-state qubits from external noise
sources the origins of decoherence inherent to the material start to play a