Research area 1 Superconductivity and superconductors

Reports from Research areas
Research area 1
Co-ordinators:
Prof. H. Eschrig
Prof. J. Fink
Prof. L. Schultz
Superconductivity and superconductors
The basic research in the area of high-
T c superconductors was focused on
theoretical and experimental electronic
structure studies and on investigations
of the influence of the grain boundary
network on the critical currents in these
materials. In the field of the transition
metal borocarbides, most of the activities
were related to the coexistence of
magnetism and superconductivity. The
analytical tools and the possibilities to
prepare new compounds, single crystals
and thin films of interesting new
superconductors have been improved.
Technology development were continued
successfully in the field of high-T c
superconductors. In particular, using
the Rolling Assisted Biaxially Textured
Substrated (RABITS) method superconducting
tapes with exceptional high
current densities and increased
mechanical strength could be achieved
on new substrated developed in the
IFW Dresden. Using the powder in tube
technique the overall critical current
density of high-T c wires for cables and
transformer could be improved. Finally,
by chemical replacement or introduction
of impurities in superconducting
permanent magnet materials new
record values for magnetic fields could
be obtained. The use of these materials
for friction-free superconducting
bearings, for the development of a
pump for liquid gases and the construction
of a reluctance motor has
been pushed forward.
Torsten Fahr,
Claus Fischer,
Volker Haas,
Wolfgang Häßler,
Bernhard Holzapfel,
Christian Rodig,
Margitta Schubert,
Hans-Peter Trinks
Fig.:
Critical current density j c
of a 55 filament tape in
dependence on
temperature and O 2
partial pressure of the
second annealing step
Funded by:
BMBF/Siemens AG
Superconducting Bi 2 Sr 2 Ca 2 Cu 3 O x /Ag-tapes
For improving the application possibilities of Bi 2 Sr 2 Ca 2 Cu 3 O x /Ag-tapes in cables or
transformers it is necessary to improve the overall critical current density and to
prepare special tapes with low AC-losses.
For enhancing of the overall critical current j e density the filling factor of the tapes
was increased from 25% to 34% and the thermo-mechanical treatment (TMT)
process was optimized. Due to the great number of parameters of the TMTprocess
(at least temperature, oxygen partial pressure and dwell time of three different
treatment steps if the temperature ramps and the
parameters of the intermediate rolling procedures are
kept constant) the application of the method of design of
experiments (DOS) can be very useful. The number of
experiments can be reduced and the method gives
additional informations about the process , especially the
statistical significance of the process parameters and
informations about interactions between different parameters.
A response surface describing the dependence
of the critical current density on the process parameters
can be calculated (Fig.). As the result of these investigations
j e could be enhanced to 10 kAcm -2 (I c =90A).
On twisted tapes with a twist pitch of 10mm without additional
ceramic barriers a reduced AC-loss of 0,6 mW/Am
at a j e of 5 kAcm -2 was measured.
In situ neutron and x-ray diffraction experiments on real
tapes were carried out to investigate the reaction formation of the 2223 phase and
the changes in the secondary phase assemblage under real reaction conditions.
A phase formation mechanism with heterogeneous nucleation and growth of 2223
and 2212 template grains beginning at the Ag/superconductor interface is proposed.
Precursor powders with different phase compositions have been prepared to investigate
the influence of the precursor phase composition on the critical current.
Cooperation: Siemens AG Erlangen, Vacuumschmelze Hanau, FZ Karlsruhe,
Universität Erlangen, Nexans Superconductors GmbH Huerth, Universität Graz
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Reports from Research areas
Sergey V. Borisenko,
Alexander A. Kordyuk,
Sibylle Legner,
Konstantin Nenkov,
Mark S. Golden,
Jörg Fink
Fig.: Fermi surface maps of
Pb-Bi2212 compounds with
different hole doping levels:
underdoped (UD), as
grown (AG) and overdoped
(OD). Critical temperatures
are also indicated.
Funded by:
BMBF, DFG
Holger Bitterlich,
Günter Behr,
Stephan-Ludwig
Drechsler,
Jörg Fink,
Günter Fuchs,
Gerald Graw,
Wolfgang Löser,
Konstantin Nenkov,
Ludwig Schultz
Fig.
Upper critical fields
H c2
ll
and H c2
ll
vs.
temperature T of a
Tb 0.2 Y 0.8 Ni 2 B 2 C single
crystal for different
orientations parallel to
the a-axis and parallel to
the c-axis, respectively.
Inset: Magnetization vs.
external field plots for
two different orientations
M(H ll ) and M(H ll )
at T = 4.5 K.
Funded by:
DFG
Doping dependence of the Fermi surface
in Bi-based superconducting cuprates
We have continued our systematic studies of the Fermi surface (FS) of
Pb 0.44 Bi 1.56 Sr 2 CaCu 2 O 8+d by studying the evolution of the FS topology as a function
of the hole doping level range, covering from underdoped (Tc=76 K) to overdoped
(Tc=69K) samples. Utilizing the high energy and momentum resolution offered by
our angle-scanning photoemission set-up, we recorded maps of the momentum
distribution of the Fermi level photemission intensity, which give direct image of
the FS (see Fig.).
A visual inspection of the data already reveals that the size of the rounded barrels
(representing the hole-like FS) increases on going from the underdoped to the
overdoped regime. A more quantitative analysis is currently underway to determine
the charge carrier concentration directly from the ARPES results. A further
result is that the FS topology tends towards stronger nesting as the doping increases
– i.e. the form changes from more circular to a squarer shape (with rounded
corners). We stress however, that for all doping levels concerned, the FS remains
hole-like, indicating that even for the strongest overdoped crystals the flat bands
located near to the [p,0] point are still below the chemical potential. As regards the
lifetime of the states involved, an analysis of the width of the E F momentum distribution
curves crossing the FS shows that the lifetime varies along the FS following
sin 2 (2f), where f is the angle with respect to the nodal direction measured
with [p,p] as the origin. This would fit into a hot-spot / cold-spot scenario describing
the k-dependence of the interactions in the system. Remarkably, however, the
f dependence of the lifetime is independent of doping, which would not be expected
from this picture. Finally, we note that relative intensity of the shadow FS to the
main FS is found to be maximal at optimal doping, decreasing on going either to
the under- or overdoped side, which suggests that the origin of this feature is
important for our understanding the mechanism of the high temperature superconductivity.
Cooperation: Institut de Physique Applique, EPFL, Lausanne, Switzerland,
DP/IGA, EPFL, Lausanne, Switzerland, Univ. Birmingham, MPI für Festkörperforschung
Stuttgart
Crystal growth and intrinsic properties
of Rare earth-Transition metal intermetallic compounds
Bulk single crystals of Y 1-x Tb x Ni 2 B 2 C borocarbides have been grown by the floating
zone method with inductive and optical heating, respectively, in order to study the
interplay of paramagnetic Tb-ions with superconducting properties. Substitution of
Y- by Tb-ions not only reduces the upper critical field H c2 , but an anisotropy of H c2
along the c-axis and in the plane perpendicular to the c-axis was induced. Moreover,
the magnitude of the anisotropy can be tuned by changing the fraction x of
Tb-ions. The maximum anisotropy was reached at x = 0.2 (Fig. 1). The anisotropy
H c2
ll[001]
> H c2
ll[100]
is induced by the in-plane magnetic moments of Tb-ions, which is
emphasized by the magnetization measurements. The behaviour is explained by
a simplified theoretical model, which is primarily based on the interaction of conduction
electrons with magnetic moments of the rare earth ions.
For YNi 1-x Cu x BC compounds the effect of alloy composition
on phase relations and superconducting
properties was revealed. The peritectic solidification
mode could be proven. A maximum Cu solubility
of x Å 0.42 of YNi 1-x Cu x BC compounds was
inferred. The superconducting transition temperature
increases with both the Cu and B content up to
a maximum T C = 9.1 K.
For the Tb 2 PdSi 3 intermetallic compound the interaction
between magnetic moments of Tb-ions with conduction electrons leads to
a considerable anisotropy of the negative giant magnetoresistance (GMR) at low
temperatures. An unusual anisotropy of the magnetocaloric effect was found for
the first time for bulk Tb 2 PdSi 3 single crystals prepared.
Cooperation: INPG-ENSPG Grenoble, Univ. Frankfurt, Tata Institute of Fundamental
Research India, TU Clausthal, TU Dresden, Univ. of Minsk
54

Reports from Research areas
Karl-Hartmut Müller,
Günter Fuchs,
Axel Handstein,
Stefan-Ludwig
Drechsler,
Helge Rosner,
Sergej Shulga,
Jens Freudenberger,
Konstantin Nenkov,
Kerstin Häse,
Stuart Wimbush,
Bernhard Holzapfel,
Holger Bitterlich,
Wolfgang Löser,
Günter Behr,
Ludwig Schultz
Fig.: Several quantities
for Y x Lu 1-x Ni 2 B 2 C
compounds determined
by resisitivity (left panel)
and specific heat
measurements (right
panel) vs. Y concentration:
Superconducting
transition temperature T c ,
the upper critical field
parameters a and H c2 *
(according to
H c2 (T)=H c2 *[1-T/T c ] 1+a ), the
residual resisitivity ratio
RRR, the parameter ( of
the relation g(H)/g N µ
[H/H c2 (0)] 1-b and the Sommerfeld
parameter g N .
Funded by:
DFG (SFB 463)
Magnetism and superconductivity of borocarbides
The effect of substitutional disorder-induced local lattice distortions on the superconducting
properties of nonmagnetic Y x Lu 1-x Ni 2 B 2 C compounds was studied by
resistivity and specific heat measurements. Substitutional disorder was found to
reduce several relevant quantities as the superconducting transition temperature
T c , the upper critical field H c2 (0) at T=0, a characteristic positive curvature of H c2 (T)
observed for these compounds, the Sommerfeld parameter g N in the normal state
as well as the curvature of g (H) with g as the electronic specific heat coefficient in
the mixed state. These quantities have their highest values for the pure compounds
and show a minimum for the highest degree of substitutional disorder at
x ~ 0.5 without reaching the case of dirty limit where the curvatures of g(H) and of
H c2 (T) at T c would disappear (i.e. b=0 and a=0). Starting from a two-band model
description of H c2 (T) within the clean limit, a correlation between H c2 (0) and the
unusual g(H) dependence was established and related to the impurity scattering
rate. Also epitaxial YNi 2 B 2 C and HoNi 2 B 2 C thin films were prepared for the first time
by UHV laser deposition. Epitaxial c-axis oriented films were produced at high
deposition temperatures, whereas a-axis oriented YNi 2 B 2 C films could be prepared
by a low deposition temperature. The
availibilty of epitaxial thin films of nonmagnetic
borocarbides enables
phase-sensitive tunneling experiments
which could help to clarify the
symmetry of the superconducting
order parameter in borocarbides. Furthermore,
the effect of paramagnetic
Tb-ions on H c2 (T) of Tb x Y 1-x Ni 2 B 2 C
bulk single crystals was studied. With
increasing Tb content x both T c and
H c2 are reduced. Different from YNi 2 B 2 C a reversed anisotropy H c2 || (001) > H c2 ||
(100) was observed resulting from the in-plane magnetic moments of uncoupled
Tb-ions. The magnitude of this anisotropy was found to rise up to a Tb concentration
of x = 0.2. The reduction of the anisotropy for higher Tb concentrations can
be explained by magnetic ordering of Tb-ions.
Cooperation: TU Dresden, MPG cPfS, ILL Grenoble
Stefan-Ludwig
Drechsler,
Helge Rosner,
Ingo Opahle,
Sergey Shulga,
Helmut Eschrig
Fig.1
Fermi surface and Fermi
velocity (in atomic units)
distribution of a band
composed mainly by Ni
3d xy and Ni 3d z2 states
for the Y(NiB) 2 C
superconductor.
Fig.2
Fermi surface and Fermi
velocity (in atomic units)
distribution of a band
composed mainly by
Ni 3d xy and Ni 3d z2 states
for the La(NiB) 2 C
nonsuperconductor.
Electronic structure and superconductivity
of transition metal borocarbides
The role of the electronic structure in the mechanism of superconductivity and
selected thermodynamic properties has been investigated in the frame of modern
density functional theory as well as multi-band Eliashberg theory. The bandstructure,
density of states, the distribution of Fermi velocities v F , the orbital character,
and nesting properties of rather complex Fermi surfaces of both superconducting
as well as nonsuperconducting members of the R(TB) 2 C
family with R=Y, Lu, Sc, Th, La, Ho and T=Ni, Co, Pt, Pd,
Re, Ru have been studied in detail. The great complexity of
the first glance bandstructure can be resolved into relatively
separate subsystems providing a well-defined basis for a
systematic study of interesting many-body problems in near
future. R(TB) 2 C-Superconductors are characterized by a
broad distribution of Fermi velocities containing both very
slow and very fast electrons as well as the presence of
nested regions which play a decisive also in the phonon
softening as well as in special incommensurate magnetic
structures for R(TB)2C compounds with magnetic
rare earth ions R. These features are absent in the nonsuperconducting
members of the R(TB) 2 C family. Candidates
for relatively isolated bands have been figured out
which might alter conventional properties such as the shape
of the upper critical field as well as the field dependence of
the electronic specific heat in the mixed state.
55

Reports from Research areas
Carla Vogt,
Wolfgang Gruner,
Rolf Kucharkowski,
Alexei Plotnikov,
Günter Behr
Fig. 1
a) Microscopic view of
the laser ablation trace
on YNi 2.02 B 2.00 C system;
length 20 mm, width
60 µm, ablation rate
20 µm/s.
b) ICP-MS signal of the
ablated material. In
heterogenic samples the
composition of phases
could be calculated from
isotopic ratios (shown
here 11 B/ 58 Ni) of all
elements.
Funded by:
DFG
Stoichiometry, purity and homogeneity of superconducting materials
Precise analysis was performed for Ca-Mg-Cu-O, Y(RE)-Ni-B-C, Bi-Pb-Sr-Ca-Cu-
O and some other material systems by ICP-OES and carrier gas hot extraction
methods for accurate oxygen determinations. In ICP-OES measurements optimised
multicomponent determination was realised by multi-line measurements
complemented by specially adapted data evaluation routines for precursors and
multi-filament super conductors. For a typical Bi-Pb-Sr-Ca-Cu-O 2223-phase precursor
with lead in the oxidation state 4 the cationic stoichiometry was determined
with optimum accuracy, e.g. Bi 1.734±0.006 Pb 0.3864±0.0003 Sr 1.935±0.006 Ca 1.74±0.02 Cu 3.000±0.003 .
The development of a special method for precise oxygen determination in oxides
was the prerequisite to determine the total composition of Ca-Mg-Cu-O compounds
experimentally for the first time. Thus, the exact stoichiometry of such complex
composed cuprates, like Ca 2.42±0.05 Mg 0.446±0.012 Cu 6.979±0.022 O 10.000±0.027 , could be
determined without any assumptions.
Laser ablation ICP-MS has been optimised
for the fast control of materials
homogeneity in cm-regions (Fig.1)
with lateral resolution down to 20 µm.
This makes the identification of phases
with dimensions below 200 µm
possible. The quantification of YNiBC systems was successful within 2 % RSD
using binary, ternary and quaternary synthetical standards.
Gernot Krabbes,
Günter Fuchs,
Stefan Gruß,
Gudrun Stöver,
Peter Verges,
Roland Hayn,
Larisa Shlyk,
Karl-Hartmut Müller,
Ludwig Schultz,
Jörg Fink
Fig. top:
Bulk cylinders, tiles and
rings
Fig. right:
Contours of the trapped
magnetic field of a
YBCO multiseeded ring
(diameter 80 mm)
Funded by:
BMBF
High performance HTSC bulk materials
High performance bulk materials are one of the preconditions for a new type of permanent
magnets and its application in newly designed electromotors, frictionless
bearings and physical technologies. Remanent induction („trapped field“) and
appearing forces between the superconductor and an applied field depend on both
the critical current density j c and diameter of the pseudo single crystalline material.
With respect to applications materials of appropriate size and shape are needed.
The properties of YBa 2 Cu 3 O 7 based melt grown material (YBCO) have been
remarkably altered by chemical modifications. Substitutions by cations of differing
valence, i.e. Y 3+ by Ca 2+ , influence the hole density in (CuO 2 ) x -planes, thus affecting
the critical temperature T c . On the other hand, substitution of in plane Cu 2+
especially by Zn 2+ was found to result in remarkably improved critical current densities
in applied fields of more than 2 T already at 77 K. An explanation can be
based on local magnetic moments which appear if Cu 2+ (S = 1/2) is substituted by
cations of differing spins S (Ni 2+ , S = 1; Zn 2+ , S = 0), thus causing alternative pinning
centres. An optimum dopand level was observed since the appearing
moments decrease T c . The trapped field on the top surface of a YBCO cylinder at
77 K achieves 1.2 T. The trapped field increases to nearly 2 T, if additional columnar
defects have been generated by neutron irradiation (co-operation H. Weber,
Vienna).
YBCO materials grown by the modified
melt crystallisation process (MMCP,
developed in IFW) distinguish by a
remarkable mechanical strength. Thus it
was possible to generate a mini-magnet
from YBCO : Zn material – reinforced by
an applied ring bandage from stainless
steel – with a trapped field of 11.2 T
already at T = 47 K. This is the highest
field ever achieved in bulk material at T >
40 K.
A multiseeding technique was developed to produce large tiles or rings consisting
of uniformly and well aligned and coherently bounded single grains. At 77 K,
the intergrain j c amounts to 10 % of the intragrain critical current density. Size and
shape are appropriate to its use in bearings and rotors of electrical motors.
Cooperation: Oswald Elektromotoren GmbH Miltenberg, Solvay Barium Strontium
GmbH, Bad Hönningen, IPHT Jena, Atominstitut der Österreichischen Universitäten
Wien, ZFW Göttingen, Univ. Stuttgart, MAI Moskau
56