Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
Fachsitzungen
– Haupt-, Fach-, Kurzvorträge und Posterbeiträge –
TT 1 Supraleitung: Herstellung und Charakterisierung
Zeit: Montag 09:30–11:00 Raum: H20
TT 1.1 Mo 09:30 H20
Molecular beam epitaxy of electron doped high-temperature
superconductors — •Yoshiharu Krockenberger 1 , Michio
Naito 1 , Akio Tsukada 1 , Hideki Yamamoto 1 , Lambert Alff 2 ,
and Rudolf Gross 2 — 1 NTT Basic Research Laboratories, NTT Corporation,
3-1 Morinosato-Wakamiya, Atsugi-shi, Kanagawa 243-0198,
Japan — 2 Walter-Meissner-Institut, 85748 Garching, Germany
The field of high temperature superconductors has been investigated
intensively. Among the studied materials, electron doped high temperature
superconductors are a minority class and only two families are
known: The T ′ and infinite-layer compounds. The superconducting transition
temperatures (Tc) are moderate even for optimum doping: 30K for
T ′ -compounds. Furthermore, sample preparation is very difficult. Nevertheless,
n-type cuprates are of high interest and essential from the basic
physics point of view, especially as to whether or not electron-hole symmetry
holds in the high-Tc superconductors. As reported recently by Alff
et al.[1] tunneling spectroscopy on thin film electron doped cuprates pinpointed
detailed features of the pseudogap behavior. Therefore, we have
investigated the growth and especially the reduction conditions (removal
of apical oxygen) of T ′ -RE2−xCexCuO4+δ thin films for a wide doping
variation.
[1] L. Alff et al., Nature 422, 698 (2003)
TT 1.2 Mo 09:45 H20
Thickness dependence of superconducting properties of ultrathin
Nb and NbN films — •Konstantin Il’in 1 , Michael Siegel 1 ,
Alexei Semenov 2 , Andreas Engel 2 , Heinz-Wilhelm Hübers 2 ,
Eugen Hollmann 3 , Gregory Gol’tsman 4 , and Boris Voronov 4
— 1 Institut für Mikro- und Nanoelektronische Systeme, Universität Karlsruhe,
Karlsruhe — 2 DLR Institut für Planetenforschung, Berlin —
3 Forschungszentrum Jülich GmbH, Jülich — 4 MSPU, Moscow, Russia
We studied systematically superconducting properties of Nb and NbN
films widely used in modern detector technology. The films with a thickness
from 3.5 to 100 nm were deposited by reactive magnetron sputtering.
The Ginzburg-Landau coherence length and the electron diffusion constant
have been obtained from the temperature dependence of the second
critical magnetic field, HC2. We have found that diffusivity in ultra-thin
films reduces with thickness. The non-linear temperature dependence of
HC2 of NbN films at temperatures close to TC was attributed to the
granular structure of the films. At low temperatures, where the normal
core of a magnetic vortex is smaller than the grain size, HC2 depends
linear on temperature. The grain size of 40-80 nm was estimated from
the crossover temperature, where the dependence of HC2 on temperature
changes from linear to non-linear behaviour. We found that grain size and
its dependence on film thickness is determined by sputtering conditions.
TT 1.3 Mo 10:00 H20
Superconducting thin films of the heavy fermion compound
UNi2Al3
— •M. Jourdan, A. Zakharov, M. Foerster, and H. Adrian —
Institut für Physik, Johannes Gutenberg Universität, 55099 Mainz
Thin films of the presumably unconventional superconductor UNi2Al3
were prepared to obtain dedicated samples for novel experiments on the
superconducting order parameter (e.g.tunnelling spectroscopy). UNi2Al3
was deposited by coevaporation of the elementary components in an MBE
system. The preparation on various substrates was studied and epitaxial
growth in (100)-direction was observed. With optimized deposition
parameters thin films showing the typical properties known from bulk
samples were obtained. The best films up to now show a superconducting
transition with Tc ≃ 0.75K. Measurements of the angular dependence
of the upper critical field Bc2(θ) will be presented.
TT 1.4 Mo 10:15 H20
Transition metal, boride and oxide doped Fe/MgB2 tapes and
wires — •Bing Liu, S. Schlachter, and W. Goldacker —
Forschungszentrum Karlsruhe ITP 76021 Karlsruhe,P.O. Box 3640
Transition metal, boride and oxide doped mono- and multifilamentary
Fe/MgB2 tapes and wires have been prepared by the powder-intube
technique at ambient pressure from different precursors. For each
composition, the heating process has been optimized to the full extent
according to DTA measurements. We determined Tc and the transport
critical current density in magnetic fields up to 9T and investigated the
microstructure with high-resolution SEM and quantitative EDX analysis.
Compared with undoped samples, Jc and Hirr of doped samples have
been enhanced significantly in high fields by effective pinning centers and
improved grain connection due to sintering aid effect from the additives,
with the highest Jc value over 5000A/cm2 at 4.2K/9T and insignificant
Tc suppression. However, the improvement of thermal and mechanical
stability of Fe/MgB2 tapes and wires remains a challenge for the future.
TT 1.5 Mo 10:30 H20
Mechanisms of current improvement at grain boundaries in
High-Tc superconductors: Magneto-optic and electron-optic results
— •Christian Jooss 1 , Karsten Guth 1 , Volker Born 1 , Eva
Brinkmeier 1 , Harald Jarzina 1 , Wilko Westhäuser 1 , Marvin
Schofield 2 , Marco Beleggia 2 , and Yimei Zhu 2 — 1 Institut für Materialphysik,
Universität Göttingen, Tammannstr. 1, 37077 Göttingen —
2 Brookhaven National Laboratory, Upton, NY 11973 USA
Grain boundaries in high-temperature superconductors represent the
major factor limiting high current applications. We apply a full quantitative
magneto-optical imaging technique to the investigation of local
transport properties of low angle grain boundaries in YBaCuO and Cadoped
YBaCuO thin films. In order to analyse microscopic mechanisms of
current suppression, the local transport measurements grain boundaries
are combined with the determination of electrostatic potentials by electron
holography. By developing quantitative phase retrieval procedures
for electron holography, we obtained for the first time direct evidence of
a negative potential at the grain boundary dislocations which is significantly
reduced in its spatial extend as well as in its value in Ca-doped
samples. Our results nicely explain the observed enhancement of the intergranular
current densities by Ca-doping and shed light onto the puzzling
electronic properties of high-temperature superconducting oxides at
grain boundaries. We find evidence for the presence of a new insulating
electronic phase at the grain boundary in the strongly overdoped region.
This work is partially supported by the Deutsche Forschungsgemeinschaft
(DFG).
TT 1.6 Mo 10:45 H20
Transport properties of Ca doped superconducting epitaxial
YBCO films — •Jürgen Halbritter 1 , Ermile Gaganidze 1 , Holger
Hochmuth 2 , and Michael Lorenz 2 — 1 FZK, IMF I, Postfach
3640, 76021 Karlsruhe, Germany — 2 Universität Leipzig, Fakultät für
Physik und Geowissenschaften, Linnéstr. 5, D-04103 Leipzig, Germany
O doping enhancing the CuO plane density of states nm is well
known to improve normal conducting and superconducting dc, ac and
rf transport properties of YBCO by orders of magnitude. By Ca doping
Y1−xCaxBa2Cu3O7−δ enhancing nm similarely transport improvements
by factors below 2 to 10 have been found for bicrystal junctions and single
crystals, only. To study the effect of Ca doping on epitaxial PLD films
on CeO2/Al2O3 we performed systematic transport studies for nominal
x = 0.1, 0.15 and 0.2. The resulting superconducting rf properties, and
therewith the mean values of the weak link critical current density jcJ,
degraded with Ca content confirming trends found by dc measurements
of PLD films on STO and on LAO and by rf measurements of doped
films on Al2O3 and on LAO. The normal conducting weak link resistance