Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
with a ferromagnetic inter-layer (F) on one side of the tunnel barrier (I),
i.e. having SIFS structure. The magnetic film thickness determines the
Josephson coupling and sets the phase coherence to either 0 or π.
SIFS multilayers consisting of Nb(140 nm)/Al (4) −
Al2O3 (1)/CuNi (0 − 30)/Nb (40) were fabricated. The process
parameters were derived from Josephson junctions with critical currents
of about 0.1 − 1 kA cm 2 . The junction areas are ranged from 20.000 µm 2
down to 10 µm 2 .
The first transport measurements of the junctions showed a monotonic
decrease of IcRn for increasing FM-thickness. The generation of a semifluxon
(cf. 1) pinned at the discontinuity of the magnetic layer thickness
will be discussed. Since the semifluxon is a degenerated two state systems,
it can be used as a qubit when the JJ is in the quantum limit.
[E. Goldobin et al., PRB 66, 100508 (2002)]
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TT 9 Supraleitung: Eigenschaften, elektronische Struktur, Ordnungsparameter
Zeit: Dienstag 09:30–13:00 Raum: H20
Hauptvortrag TT 9.1 Di 09:30 H20
Wavefunction Imaging Studies of Cuprate Superconductivity —
•Séamus Davis 1 , K. McElroy 1 , E. W. Hudson 2 , J. E. Hoffman 3 ,
D.-H. Lee 3 , H. Eisaki 4 , and S. Uchida 5 — 1 Department of Physics,
LASSP, Cornell University, Ithaca, NY — 2 MIT — 3 UC Berkeley —
4 AIST-Tsukuba — 5 Tokyo University
High temperature superconductivity in the cuprates emerges when the
localized electrons of a Mott-insulator become itinerant due to carrierdoping.
Since cuprate superconductivity develops from atomically localized
electrons and exhibits nanoscale disorder, simultaneous information
on electronic structure in both the real-space and momentum-space is
required. I will describe a combination of novel scanning tunneling microscopy
(STM) techniques which achieves these apparently contradictory
aims.
The first technique, atomic-resolution spectroscopic mapping, allows
imaging of interactions between quasiparticle wavefunctions and the realspace
environment at the atomic-scale.
A second technique, Fourier-transform scanning tunneling spectroscopy
(FT-STS), is used to study interference patterns of the
delocalized wavelike electronic states. For optimally doped samples,
analysis of these patterns as due to quasiparticle interference , yields the
Fermi surface and the d-wave superconducting energy gap, in excellent
agreement with ARPES.
Finally I will describe FTSTS experiments designed to detect and identify
the electronic ground state in other regions of the cuprate phase diagram
including studies of the vortex core and of strongly underdoped
samples.
TT 9.2 Di 10:00 H20
The LDA band structure of Bi2Sr2CaCu2O8 revisited. — •Denis
Mertz, Helmut Eschrig, and Klauss Koepernik — IFW Dresden,
P.O.B. 27 0016, D-01171 Dresden, Germany
We study the effects of structural optimizations on the electronic properties
of Bi2Sr2CaCu2O8 high-Tc superconductor by means of the LDA
approach to the density functional theory. The LDA calculations are done
using the FPLO method [1]. Oxygen and Bismuth position optimizations
in a √ 2 × √ 2 supercell with A2aa symmetry yield important changes in
the band structure near the Fermi energy. The effects of buckling in the
CuO planes are also considered.
[1] K. Koepernik and H. Eschrig, Phys. Rev. B 59, 1743 (1999)
TT 9.3 Di 10:15 H20
Anomalous enhancement of the coupling strength upon underdoping
in Pb-Bi2212. — •Sergey Borisenko, Alexander Kordyuk,
Andreas Koitzsch, Martin Knupfer, and Joerg Fink —
Institute for Solid State Research, IFW-Dresden
Angle-resolved photoemission spectroscopy is used to study the mass
renormalization of the charge carriers in the high-Tc superconductor
(Pb,Bi)2Sr2CaCu2O8+δ (Pb-Bi2212) in the vicinity of the (π, 0)-point in
the superconducting and the normal states. Using matrix element effects
at different photon energies and due to a high momentum and energy
resolution the bonding and the antibonding bands could be separated in
the whole dopant range. A huge coupling to a bosonic collective mode
is observed below Tc for both bands, in particular, for the underdoped
case. Above Tc , a weaker coupling to a continuous spectrum of modes
is detected.
TT 9.4 Di 10:30 H20
Electronic structure of doped Bi-cuprates — •R.-S. Unger, M.
Schneider, T. Stemmler, L. Lasogga, H. Dwelk, A. Krapf,
C. Janowitz und R. Manzke — Institut für Physik, Humboldt-
Universität zu Berlin, Newtonstr.15, 12489 Berlin
We present angle-resolved photoemission data from La-doped n=1
and Y-doped n=2 Bi2Sr2−y(Lay)Ca1−n−x(Yx)CunO2n+4+δ single crystals.
The crystal quality is proved by Laue and LEED, the superconducting
properties by susceptibility measurements. So we have the
opportunity to investigate the electronic structure of a high-Tc superconductor
and its parent compound over the entire hole doping range.
We discuss our measured spectra series of the metallic/superconducting
and insulating state of the Bi-cuprate single crystals along high symmetric
directions of the Brillouin zone. This work was supported by the
BMBF project no. MA2371/1-3
TT 9.5 Di 10:45 H20
The bulge in the basal CuO2 area of cuprate superconductors
— •Jürgen Röhler — Universität zu Köln, D-50937 Köln, Germany
Hole doping of the superconducting cuprates not only shrinks their
basal area B (square of the basal lattice parameter) – to be expected
if electrons are removed from the antibonding Cu3d x 2 +y 2O2pxy states –,
but creates also a strong bulge around optimum doping, nopt ≃ 0.16.
The bulge starts growing in the lightly underdoped, and collapses in the
overdoped regime near critical doping, ncrit ≃ 0.19 [1]. We show that
the observed expansion of the basal lattice around nopt and its collapse
at ncrit results from the Aufbau of the strongly correlated many electron
state in the CuO2 sheets with “self-protecting” singlets (SPS) [2].
In them not only the Cu sites are protected against occupation with more
than one hole, but also the oxygen cages of the Zhang-Rice (ZR) singlets.
This additional non-double occupant constraint favours the formation of
non magnetic (S= 0), bond centered molecular pairs. The paired SPS exchange
two oxygen holes across antiferromagnetic chains of 4 Cu spins,
thereby flipping them, and create a delocalized singlet of oxygen holes.
Most closest packing of paired SPS occurs at 1/6 = 0.166 ≃ nopt in a
tweed-like mesoscopic pattern. At higher hole densities, n ≥ 0.2 ≃ ncrit,
the non-double-occupant constraint for the oxygen cage collapses. We
show that the exchange of two oxygen holes in a paired SPS is connected
with the local half-breathing (LO) mode of planar oxygen vibrations
found up to nopt, and that it expands the basal CuO2 area as observed.
[1] J. Röhler, Physica C (2003), cond-mat/0304628.
[2] J. Röhler, J. Supercond. (2003), cond-mat/0307310.
TT 9.6 Di 11:00 H20
Electronic structure and aspects of unconventional superconductivity
in NaxCoO2 . yH2O — •S.-L. Drechsler 1 , H. Rosner
2 , G. Fuchs 1 , A. Wälte 1 , G. Krabbes 1 , K.-H. Müller 1 , H.
Eschrig 1 , and A. Handstein 1 — 1 IFW-Dresden, D-01171, P.O. Box
270116, Dresden, Germany — 2 MPI-CPfS Dresden, Germany
We examine the electronic structure of NaxCoO2 . yH2O within the local
density approximation. The parametrization of the band which forms
the largest hole-Fermi surface centered at Γ shows significant deviations
from what is frequently assumed in recent sophisticated theoretical studies.
In particular, the commonly used nearest neighbor approaches in the
framework of single band pictures are found to be unrealistic. The special
role of H2O in screening the disorder in the charge reservoir is briefly
discussed and compared with the case of Y1−xCaxCu3O6+δ. We report
strucural, magnetic susceptibility, and specific heat measurements of the
unhydrated nonsuperconducting compounds. The results are compared
with our findings for the superconducting perovskite MgCNi3 which is
close to a ferromagnetic instability.