Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
The preparation, manipulation and detection of single spin states are
the essential prerequisites for spin based quantum computation. The coupling
of a single C13 spin (I=1/2) to the ground state triplet spin (S=1)
of a single nitrogen-vacancy defect center (NV-center)in diamond forms a
single 2-qubit system. The joint approach of single molecule spectroscopy
and optically detected magnetic resonance (ODMR) allows to detect optically
individual NV-centers and to manipulate the spin subsystem of
the defect.
Coherent oscillations of the triplet spin in a single NV-Center have
been detected. Further pulsed ODMR sequences, derived from optical
detected electron nuclear double resonance experiments, allows the coherent
control of the coupled spin states. As a first quantum computation
operation the conditional rotation gate (CROT) has been implemented.
The poster presents the measurements and discusses the limitations due
to coherence decay.
TT 17.27 Di 14:30 Poster A
Towards an Autonomous Quantum Thermodyamic Machine —
•Friedemann Tonner und Günter Mahler — Institut für Theoretische
Physik 1, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart
On a fundamental level thermodynamic machines necessarily consist of
quantum mechanical parts. Here we address quantum mechanical analogs
to each functional part of a thermodynamic machine. Thus we construct
time-independent Hamiltonians for the gas system, the work variable and
the control system responsible for driving the thermodynamic process,
normally controlled from the outside. Baths are dealt with in Markov
approximation.
We are interested not in quantum mechanical modeling of existing
(classical) machines, but rather in exploring the small system (quantum)
limit: how small can a machine be made to still exhibit machine functions?
TT 17.28 Di 14:30 Poster A
Schrödinger-dynamics of a spin-chain in contact with a quantum
environment — •Markus Henrich, Michael Hartmann, Mathias
Michel, and Günter Mahler — Institut für Theoretische Physik
1, Universität Stuttgart, Pfaffenwaldring 57, D-70550 Stuttgart
We study the exact Schrödinger-dynamics of a spin-chain with nearestneighbour
interaction coupled to a finite quantum environment. The relaxation
behavior of the spin-chain is analyzed for two coupling topologies;
1. all spins are directly coupled to the environment, 2. only one
edge of the spin-chain is coupled to the environment. Various coupling
strengths between the spins as well as various initial states are considered.
We then analyze the conditions under which a thermodynamical
description should become appropriate for the entire spin-system or even
for the induvidual spins.
TT 17.29 Di 14:30 Poster A
Viscous Transport in Dirty Fermi Superfluids — •Dietrich
Einzel 1 and Jeevak M. Parpia 2 — 1 Walther-Meissner-Institut, Bayerische
Akademie der Wissenschaften, 85748 Garching — 2 Dept. of
Physics, Cornell University, Ithaca, NY 14853, USA
The transport theory of clean Fermi superfluids like 3 He–A,B is generalized
to include strong elastic scattering from impurities represented by
a system of low density silica arogel strands. The dynamics of the aerogel
system is taken into account within a collision–drag model. In particular,
we calculate for the first time the viscosity of superfluid 3 He–B in the
presence of impurity scattering both in the Born and the unitary limit.
The shear viscosity dominates the attenuation of transverse sound, the
general dispersion relation of which is also provided by our theory. The
observability of the shear viscosity is discussed in context with recent
torsional oscillator experiments.
TT 17.30 Di 14:30 Poster A
Pressure Driven 3 He Flow Through Submicron Sized Apertures
— •S.V. Pereverzev and G. Eska — Physikalisches Institut, Universität
Bayreuth
The flow of liquid 3 He through two different weak link structures was
investigated at temperatures of T< 1 mK to 1 K by measuring the characteristic
time τ which was needed for pressure equilibration between
two volumes connected by the weak links. For the flow through a regular
structure of 65x65 holes of 150 nm dia. in a 50 nm thick SiN membrane
we observed the expected T-dependence and the transition from Knudsen
to viscous flow behavior. To the contrary, no T-dependence was found for
normal fluid 3 He-flow through 10 5 holes of 15 nm diameter, statistically
distributed over a 14 nm thick nitrocellulose film. Below the superfluid
transition the relaxation rate (1/τ ∼flow-channel conductance) increased
by orders of magnitude for both types of weak links. Our data are in qualitative
agreement with experiments by other groups. We present a model
which describes our normal flow results quantitatively for the 150 nm
aperture, but which does not explain the flow through the 15 nm holes.
TT 17.31 Di 14:30 Poster A
Light propagation and random lasing — •Andreas Lubatsch
and Hans Kroha — Physikalisches Institut, Universitaet Bonn
We present caculations and numerical results for light propagation in
disordered media. We consider vector waves passing through a system
of randomly distributed Mie - scatterers. The scaterers themselves can
consist of optical active media. We present as well results for such systems,
which are known as random lasers. In order to find the results,
we calculated the necessary transport properties including e.g. the exact
Ward Identity and the diffusion constant.
TT 17.32 Di 14:30 Poster A
Channel analysis of electrical conductance through a Zn
single-atom contact — •Michael Häfner, Jan Heurich,
Juan-Carlos Cuevas und Gerd Schön — Institut für Theoretische
Festkörperphysik, Universität Karlsruhe, 76128 Karlsruhe
Today it is possible to manipulate atomic-sized structures in a controlled
way. Special attention is payed to the electronic transport properties
of single-atom contacts. For noble and s-p like metals the conduction is
carried by the s-band (one channel) and the s- and p-bands (up to four
channels) respectively. Scheer et al. have recently measured the conductance
of Zn single-atom contacts. As Zn is intermediate between noble
and s-p like metals it constitutes an excellent testbed for the detailed
study of the interplay between electronic configuration and conduction
properties in this regime.
In this work we present a theoretical analysis of the experiment using
tight binding transport calculations. This approach allows us to show
how the conduction channels arise from the atomic orbitals and elucidate
which specific properties of the individual orbitals determine their
contribution to the current. For the case of Zn single-atom contacts we
find a typical total transmission of ∼ 0.6 in good agreement with the
experiment. In contrast to the noble or s-p like metals, the current is
predominantly carried by two channels which can be related to a combination
of the s and p valence bands of Zn.
TT 18 Supraleitung: Tunneln, Josephson-Kontakte, SQUIDs
Zeit: Mittwoch 14:30–16:30 Raum: H20
TT 18.1 Mi 14:30 H20
Electron tunneling into superconducting MgB2 — •J. Geerk, R.
Schneider, A. Zaitsev, R. Heid, and G. Linker — Forschungszentrum
Karlsruhe, Institut für Festkörperphysik
We have prepared tunnel junctions of the sandwich-type on superconducting
thin films of MgB2. The films were prepared by physical vapor
deposition from an evaporation source for magnesium and a sputtering
source for boron with Tc values near 33 K. The tunnel junctions showed
an energy gap between 2.5 and 3.0 meV and phonon structures in the
reduced density of states with a strength of about 0.8%. The inversion of
the tunnel data using standard single-band Eliashberg equations revealed
an Eliashberg function with 3 distinct peaks at 40, 60 and 90 meV. It can
be deduced that the boron-like phonons (60 and 90 meV) couple about 3
times stronger than the magnesium like phonons (40 meV). The strong
peak due to the E2g-mode near 70 meV which is predicted by several
theoretical calculations could not be detected in the tunnel data.