Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Dienstag
TT 9.7 Di 11:15 H20
Spectroscopic investigations of the electronic structure of
NaxCoO2 — •Thomas Kroll 1 , Martin Knupfer 1 , Thomas
Pichler 1 , Bernd Büchner 1 , Chengtian Lin 2 und Bernhard
Keimer 2 — 1 Leibniz Institute for Solid State and Materials Research
Dresden, 01171 Dresden, Germany — 2 Max Planck Institute for Solid
State Research, 70569 Stuttgart, Germany
The discovery of superconductivity in Na0.35CoO2 · 1.3H2O by Takada
et al. at a Tc of about 5K initiated an increasing interest in those
compounds. This superconducting compound consists of two-dimensional
CoO2 layers separated by a thick insulating layer of Na + ions and H2O.
Based on the analogy to the known high Tc cuprates, it has been speculated
that there is a similar underlying mechanism. In this contribution we
present measurements of the electronic structure (using methods such as
Electron Energy Loss Spectroscopy (EELS) and X-ray Absorption Spectroscopy
(XAS)) of the mother compound NaxCoO2 which may provide
an important contribution to understand the physics of cobalt superconductors.
11:30 Pause
TT 9.8 Di 11:45 H20
Phonons and electron-phonon coupling in nickel borocarbides
— •R. Heid, W. Reichardt, and K.-P. Bohnen — Institut für
Festkörperphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe
Superconductivity in nickel borocarbides exhibits various unusual
properties [1], which raises the question if electron-phonon coupling
is the main source of pairing in this class of materials. This subject
can be elucidated by modern first principles methods based on
density-functional theory which provide a detailed microscopic picture
of the electron-phonon interaction. We present calculations of the
bandstructure, the phonon dispersion, and the electron-phonon coupling
for superconducting YNi2B2C (Tc=15.5K) and compare them with
results for the non-superconducting reference compounds LaNi2B2C and
Y(Ni.75Co.25)2B2C. The calculations reproduce a pronounced phonon
anomaly in the vicinity of the wave vector (0.6,0,0) in a low frequency
branch of ∆4 symmetry that has been observed experimentally both in
YNi2B2C and LuNi2B2C [2]. We find that the magnitude of the anomaly
and of the electron-phonon coupling depend very sensitively on the
chosen structural parameters (volume, c/a ratio). Another pronounced
anomaly is predicted in an optic branch of the same symmetry with
predominant B and C elongations. Although the low frequency phonon
spectrum is strongly dominated by the vibrations of the heavy metal ions
we find that B and C contribute more than 50% to the electron-phonon
coupling constant λ.
[1] K. Izawa et al., Phys. Rev. Lett. 89, 137006 (2002)
[2] C. Stassis et al., Physica C 317-318, 127 (1999)
TT 9.9 Di 12:00 H20
Strong electron–phonon coupling in the superconducting
perovskite MgCNi3 — •Andreas Wälte 1 , G. Fuchs 1 , K.-H.
Müller 1 , A. Handstein 1 , K. Nenkov 1 , V.N. Narozhnyi 1 , H.
Rosner 2 , S.-L. Drechsler 1 , S.V. Shulga 1 , and L. Schultz 1 —
1 Institut für Festkörper- und Werkstoffforschung Dresden, Helmholtzstr.
20, D-01171 Dresden — 2 MPI CPFS Dresden, Nöthnitzer Str. 40,
D-01187 Dresden
The superconducting state of MgCxNi3, a recently discovered perovskite
superconductor [1], was characterized by measurements of the
specific heat in a temperature range between 2 and 300 K and the resistivity.
Analyzing the upper critical field Hc2(0) = 11 T and the London
penetration depth in terms of impurity scattering rates, evidence
for strong electron–phonon coupling was found. From the characteristic
phonon frequency ωln ≈ 140 K derived from the specific heat data, a
superconducting transition temperature of Tc ≈ 15 K is expected. The
reduced experimental value of Tc ≈ 7 K implies considerable pair break-
ing contribution, which is ascribed to ferromagnetic spin fluctuations.
Our results are discussed in comparison with related borocarbide superconductors.
[1] T. He et al. Superconductivity in the non-oxide perovskite MgCNi3.
Nature, Vol. 411, May 2001.
TT 9.10 Di 12:15 H20
Renormalization of the electron-phonon coupling in the oneband
Hubbard model — •Roland Zeyher and Erik Koch —
Max-Planck-Institut für Festkörperforschung, Stuttgart
We investigate the effect of electronic correlations on the coupling of
electrons to Holstein phonons in the one-band Hubbard model. We calculate
the static electron-phonon vertex within linear response of Kotliar-
Ruckenstein slave-bosons in the paramagnetic saddle-point approximation.
Within this approach the on-site Coulomb interaction U strongly
suppresses the coupling to Holstein phonons at low temperatures. Moreover
the vertex function does not show particularly strong forward scattering.
Going to larger temperatures kT ∼ t we find that after an initial
decrease with U, the electron-phonon coupling starts to increase with U.
We show that this behavior is related to an unusual reentrant behavior
from a phase separated to a paramagnetic state upon decreasing the
temperature.
TT 9.11 Di 12:30 H20
Analytisches Zwei-Flüssigkeitsmodell für unkonventionelle Supraleiter
— •Dietrich Einzel — Walther-Meissner-Institut, Bayerische
Akademie der Wissenschaften, 85748 Garching
Dieser Beitrag untersucht die Möglichkeit, einfache analytische Ausdrücke
für thermodynamische und Responsefunktionen im Rahmen einer
Theorie schwach koppelnder unkonventioneller Supraleiter bei allen
Temperaturen (0 ≤ T ≤ Tc) zu finden. Es werden sehr präzise Interpolationsformeln
abgeleitet, insbesondere für Entropie, Wärmekapazität,
Spinsuszeptibilität und normalfluide Dichte eines Gases aus thermischen
Anregungen (Bogoliubov-Quasiteilchen), sowie für die superfluide Dichte,
welche die Magnetfeld-Eindringtiefe, und damit den Response des Kondensats
bestimmt. Die Resultate können auf alle bekannten unkonventionellen
Supraleiter wie auch auf Fermi–Supraflüssigkeiten wie 3 He–A
und –B angewendet werden und etablieren somit eine analytische Zwei–
Flüssigkeits–Beschreibung für diese Systeme.
TT 9.12 Di 12:45 H20
Bernoulli potential in type-I and weak type-II superconductors
— •K. Morawetz 1,2 , P. Lipavsk´y 3 , Jan Koláček 3 , J.J. Mares 3 ,
E.H. Brandt 2 , and M. Schreiber 1 — 1 Istitute of Physics, Chemnitz
University of Technology, 09107 Chemnitz, Germany — 2 Max-
Planck-Institute for the Physics of Complex Systems, Nöthnitzer Str. 38,
01187 Dresden, Germany — 3 Institute of Physics, Academy of Sciences,
Cukrovarnická 10, 16258 Praha 6, Czech Republic
The electrostatic potential close to the surface of superconductors in
the Meissner state is discussed. We show that beside the Bernoulli potential,
the quasiparticle screening, and the thermodynamic contribution
due to Rickayzen, there is a non-local contribution which is large for
both type-I and weak type-II superconductors [1]. A generalization of
the Budd-Vannimenus theorem is found which allows one to evaluate the
observed potential without the explicit solution of the charge profile at
the surface [2]. The electrostatic potential above the Abrikosov vortex
lattice is evaluated numerically. We propose an experimental measurement
by NMR [3] to access this field which can yield informations about
material parameters.
[1] P. Lipavsk´y, K. Morawetz, J. Koláček, J. J. Mareˇs, E. H. Brandt, M.
Schreiber, Phys. Rev. B in press (2003), cond-mat/0311166
[2] P. Lipavsk´y, K. Morawetz, J. Koláček, J. J. Mareˇs, E. H. Brandt, M.
Schreiber, Phys. Rev. B submitted, cond-mat/0311167
[3] P. Lipavsk´y, J. Kolacek, K. Morawetz, E. H. Brandt, Phys. Rev. B 66
(2002) 134525