Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
YBCO-Filme der Dimension 10mm x 42mm x 300nm. Unterhalb des
Films befand sich eine supraleitende Pfannkuchenspule an welche RF-
Pulse angelegt wurden. War der an der Probe angelegte Bias-Strom ausreichend
hoch, konnte damit der Übergang in die Normalleitung ausgelöst
werden. Mittels des magnetooptischen Verfahrens war es möglich
die Flussdichteverteilung in dem YBCO-Streifen vor, während und nach
dem Schaltvorgang zu beobachten und somit Rückschlüsse auf die jeweiligen
Stromverteilungen zu ziehen. Als Ergebnis werden magnetooptische
Aufnahmen und ein theoretisches Modell des RF-induzierten Schaltvorgangs
vorgestellt.
TT 8.32 Mo 14:30 Poster A
Magnetic Properties of YBa2Cu3O7 Ring Structures —
•K. Schindler 1 , M. Ziese 1 , P. Esquinazi 1 , H. Hochmuth 2 ,
M. Lorenz 2 , E. Salamatin 3 , and K. Zimmer 3 — 1 Division of
Superconductivity and Magnetism, University of Leipzig, 04103 Leipzig.
— 2 Semiconductor Physics Group, University of Leipzig, 04103 Leipzig.
— 3 IOM, Institute for Surface Modification, 04318 Leipzig.
YBa2Cu3O7 (YBCO) films were grown by pulsed laser deposition on
CeO2-buffered r-plane sapphire substrates. Film thickness was 260 nm;
the films were covered with a gold layer of 220 nm. From a single 3 inch
wafer various ring structures were fabricated using photolithographical
techniques and subsequent etching with either an Excimer laser or an
ultrafast Ti:sapphire laser, respectively. The magnetic properties of the
YBCO ring structures were investigated by ac-susceptibility and SQUID
magnetometry. In order to quantify and assess the material damage introduced
by the different etching techniques, the critical current density
of 12 single rings with 2 mm outer diameter and ring widths between 15
and 110 µm was determined by magnetic field dependent ac-susceptibility
measurements. The complex susceptibility of the rings at least up to the
13th harmonic could be accurately modelled within a critical state model.
The values of the critical current density extracted did not show a systematic
dependence on the patterning technique. Furthermore, structures
with 1024 rings of outer diameter 20 µm and ring width 1 and 2 µm were
investigated. FC-, ZFC- and remanent magnetization measurements were
used to determine the volume of shielded material. The data will be further
discussed within a critical state model.
TT 8.33 Mo 14:30 Poster A
Frequency dependence up to microwave frequencies of the abplane
conductivity peak near Tc of YBa2Cu2O7−δ — •D. Görlitz
and J. Kötzler — Institut f. Angewandte Physik, Univ. Hamburg, D-
20355 Hamburg
The linear suceptibility χ ′ − iχ ′′ of d=50-600 nm thin films has been
measured between 30mHz and 22GHz in ac-fields perpendicular to the
CuO2-planes. For frequencies up to 800 kHz mutual induction techniques
were employed, while between 0.45 and 22 GHz the complex resonance
frequency, ˜ fr = fr + i∆f, of helical resonators and of cylindrical cavities
was determined. The complex shifts δ ˜ fr = ˜ fr(T) − ˜ fr(0) are proportional
to the dynamic susceptibility χ(ω), and DC-susceptibilities were
employed to calibrate δfr. The ab-plane conductivity σ(ω) = σ ′ − iσ ′′
is determined from χ(ω) by means of an exact inversion routine [1].
The loss component σ ′ shows a peak at a temperature Tω near Tc, and
the peak temperatures Tω exhibit an Arrhenius behaviour. Using the inverse
kinetic inductivity determined from σ ′′ ,(µ0Lk(T)) −1 = ωσ ′′ (T, ω),
the Arrhenius barriers are related to the nucleation energy of vortex
cores threading the film. These barriers are found to disappear at T0,
which is distinctly above the true critical temperature Tc, also extracted
from L −1
k (T)[2]. At the largest frequency, 21.3 GHz, the σ ′ -peak becomes
strongly suppressed since Tω approaches T0.
[1] J. Kötzler et al., Phys.Rev. B 50, 3384 (1994)
[2] J. Kötzler et al., Phys.Rev.Lett. 87, 127005 (2001) and 89,149704
(2002)
TT 8.34 Mo 14:30 Poster A
Evidence for a percolation-driven transition to coherent surface
superconductivity — •Lars von Sawilski, Sara Casalbuoni, and
Jürgen Kötzler — Institut für Angewandte Physik und Zentrum
für Mikrostrukturforschung, Universität Hamburg, Jungiusstrasse 11, D-
20355 Hamburg, Germany
Above the upper critical field Hc2 we have investigated the field dependences
of the surface conductance, G ′ − iG ′′ , and the critical current
Jc of different treated Niobium cylinders: as grown, chemically and electrolytically
polished. The low frequency limits of G ′ (H) and G ′′ (H) and
of Jc(H) display power-law singularities, defining a transition to coherent
surface superconductivity at H c c3 > Hc2. The critical exponents as well as
the dynamical scaling of G ′ − iG ′′ studied between 0.1 Hz and 1 MHz are
consistent with predictions for a two-dimensional percolation transition.
Relating H c c3 to the conventional onset field for surface superconductivity
Hc3, we find H c c3 /Hc3 = 0.81. Surprisingly, this ratio turns out to
be independent of significant variations of Hc3 due to differently treated
surfaces. This points to a new universal relation between the nucleation
and the onset of long-range coherent surface superconductivity.
TT 8.35 Mo 14:30 Poster A
Influence of Fermi surface topology on the quasiparticle spectrum
in the vortex state — •Siegfried Graser, Thomas Dahm,
and Nils Schopohl — Institut für Theoretische Physik, Universität
Tübingen, Auf der Morgenstelle 14, D-72076 Tübingen
We study the influence of Fermi surface topology on the quasiparticle
density of states in the vortex state of type II superconductors. We
observe that the field dependence and the shape of the momentum and
spatially averaged density of states is affected significantly by the topology
of the Fermi surface. We show that this behavior can be understood
in terms of characteristic Fermi surface functions and that an important
role is played by the number of points on the Fermi surface at which the
Fermi velocity is directed parallel to the magnetic field. A critical comparison
is made with a broadened BCS type density of states, that has been
used frequently in analysis of tunneling data. We suggest a new formula
as a replacement for the broadened BCS model for the special case of a
cylindrical Fermi surface. We apply our results to the two gap superconductor
MgB2 and show that in this particular case the field dependence
of the partial densities of states of the two gaps behaves very differently
due to the different topologies of the corresponding Fermi surfaces, in
qualitative agreement with recent tunneling experiments.
TT 8.36 Mo 14:30 Poster A
Local Quasiparticle Density of States in unconventional Type-
II Superconductors — •Christian Iniotakis and Nils Schopohl
— Institut für Theoretische Physik, Universität Tübingen, Auf der Morgenstelle
14, D-72076 Tübingen
Our aim is to calculate the quasiclassical propagator in unconventional
(d-wave) superconductors in the vortex state. We assume a clean and
homogeneous superconductor. Near the phase transition at Hc2 the spacial
variation of the gap function is given by the well-known Abrikosov
solution. Concentrating on the quadratic and hexagonal vortex lattice
we present calculations of the local quasiparticle density of states. Our
results have been achieved by using the Riccati-parametrization of the
quasiclassical Eilenberger theory. Additionally, basic symmetry relations
of the vortex lattice have been taken into account.
TT 8.37 Mo 14:30 Poster A
Mirage phenomena in quantum corrals of s-wave superconductors
— •Markus Schmid and Arno P. Kampf — Theortical
Physics III, Center for Electronic Correlations and Magnetism, Institute
of Physics, University of Augsburg, 86135 Augsburg
We investigate the local pairing amplitude and the local density of
states (LDOS) for an s-wave superconductor in an elliptic quantum corral.
Using a T-matrix analysis we explore the spatial structure of the
LDOS in the presence of a single non-magnetic impurity and observe a
variety of quantum mirage phenomena. In particular, we discuss mirage
effects for localized impurity bound states and analyze the interference
patterns for the scattering processes from two impurities in the quantum
corral.
TT 8.38 Mo 14:30 Poster A
Extraction of the nonlinear response function from intermodulation
distortion experiments and application to
high-temperature superconductor resonators — •Roland
Hott, Alexander Zaitsev, and Rudolf Schneider —
Forschungszentrum Karlsruhe, Institut für Festkörperphysik, P.O.B.
3640, D-76021 Karlsruhe, Germany
We present a general scheme how to extract the response function
Z(H) = �
m ZmHm for a nonlinear reaction of a quantity
E(H) = Z(H)H on an input H from two-tone intermodulation distortion
(IMD) experiments using a time-dependent excitation H(t) =
h cos(ω0t) cos(δ0t). We applied this method with success to hightemperature
superconductor (HTS) rf resonators to identify here the
mechanism of nonlinear signal distortions. Comparison of the experimental
3rd-order IMD I3(pcirc) measured as function of rf power pcirc