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Plenarvorträge - DPG-Tagungen

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Tiefe Temperaturen Dienstag<br />

TT 9.7 Di 11:15 H20<br />

Spectroscopic investigations of the electronic structure of<br />

NaxCoO2 — •Thomas Kroll 1 , Martin Knupfer 1 , Thomas<br />

Pichler 1 , Bernd Büchner 1 , Chengtian Lin 2 und Bernhard<br />

Keimer 2 — 1 Leibniz Institute for Solid State and Materials Research<br />

Dresden, 01171 Dresden, Germany — 2 Max Planck Institute for Solid<br />

State Research, 70569 Stuttgart, Germany<br />

The discovery of superconductivity in Na0.35CoO2 · 1.3H2O by Takada<br />

et al. at a Tc of about 5K initiated an increasing interest in those<br />

compounds. This superconducting compound consists of two-dimensional<br />

CoO2 layers separated by a thick insulating layer of Na + ions and H2O.<br />

Based on the analogy to the known high Tc cuprates, it has been speculated<br />

that there is a similar underlying mechanism. In this contribution we<br />

present measurements of the electronic structure (using methods such as<br />

Electron Energy Loss Spectroscopy (EELS) and X-ray Absorption Spectroscopy<br />

(XAS)) of the mother compound NaxCoO2 which may provide<br />

an important contribution to understand the physics of cobalt superconductors.<br />

11:30 Pause<br />

TT 9.8 Di 11:45 H20<br />

Phonons and electron-phonon coupling in nickel borocarbides<br />

— •R. Heid, W. Reichardt, and K.-P. Bohnen — Institut für<br />

Festkörperphysik, Forschungszentrum Karlsruhe, D-76021 Karlsruhe<br />

Superconductivity in nickel borocarbides exhibits various unusual<br />

properties [1], which raises the question if electron-phonon coupling<br />

is the main source of pairing in this class of materials. This subject<br />

can be elucidated by modern first principles methods based on<br />

density-functional theory which provide a detailed microscopic picture<br />

of the electron-phonon interaction. We present calculations of the<br />

bandstructure, the phonon dispersion, and the electron-phonon coupling<br />

for superconducting YNi2B2C (Tc=15.5K) and compare them with<br />

results for the non-superconducting reference compounds LaNi2B2C and<br />

Y(Ni.75Co.25)2B2C. The calculations reproduce a pronounced phonon<br />

anomaly in the vicinity of the wave vector (0.6,0,0) in a low frequency<br />

branch of ∆4 symmetry that has been observed experimentally both in<br />

YNi2B2C and LuNi2B2C [2]. We find that the magnitude of the anomaly<br />

and of the electron-phonon coupling depend very sensitively on the<br />

chosen structural parameters (volume, c/a ratio). Another pronounced<br />

anomaly is predicted in an optic branch of the same symmetry with<br />

predominant B and C elongations. Although the low frequency phonon<br />

spectrum is strongly dominated by the vibrations of the heavy metal ions<br />

we find that B and C contribute more than 50% to the electron-phonon<br />

coupling constant λ.<br />

[1] K. Izawa et al., Phys. Rev. Lett. 89, 137006 (2002)<br />

[2] C. Stassis et al., Physica C 317-318, 127 (1999)<br />

TT 9.9 Di 12:00 H20<br />

Strong electron–phonon coupling in the superconducting<br />

perovskite MgCNi3 — •Andreas Wälte 1 , G. Fuchs 1 , K.-H.<br />

Müller 1 , A. Handstein 1 , K. Nenkov 1 , V.N. Narozhnyi 1 , H.<br />

Rosner 2 , S.-L. Drechsler 1 , S.V. Shulga 1 , and L. Schultz 1 —<br />

1 Institut für Festkörper- und Werkstoffforschung Dresden, Helmholtzstr.<br />

20, D-01171 Dresden — 2 MPI CPFS Dresden, Nöthnitzer Str. 40,<br />

D-01187 Dresden<br />

The superconducting state of MgCxNi3, a recently discovered perovskite<br />

superconductor [1], was characterized by measurements of the<br />

specific heat in a temperature range between 2 and 300 K and the resistivity.<br />

Analyzing the upper critical field Hc2(0) = 11 T and the London<br />

penetration depth in terms of impurity scattering rates, evidence<br />

for strong electron–phonon coupling was found. From the characteristic<br />

phonon frequency ωln ≈ 140 K derived from the specific heat data, a<br />

superconducting transition temperature of Tc ≈ 15 K is expected. The<br />

reduced experimental value of Tc ≈ 7 K implies considerable pair break-<br />

ing contribution, which is ascribed to ferromagnetic spin fluctuations.<br />

Our results are discussed in comparison with related borocarbide superconductors.<br />

[1] T. He et al. Superconductivity in the non-oxide perovskite MgCNi3.<br />

Nature, Vol. 411, May 2001.<br />

TT 9.10 Di 12:15 H20<br />

Renormalization of the electron-phonon coupling in the oneband<br />

Hubbard model — •Roland Zeyher and Erik Koch —<br />

Max-Planck-Institut für Festkörperforschung, Stuttgart<br />

We investigate the effect of electronic correlations on the coupling of<br />

electrons to Holstein phonons in the one-band Hubbard model. We calculate<br />

the static electron-phonon vertex within linear response of Kotliar-<br />

Ruckenstein slave-bosons in the paramagnetic saddle-point approximation.<br />

Within this approach the on-site Coulomb interaction U strongly<br />

suppresses the coupling to Holstein phonons at low temperatures. Moreover<br />

the vertex function does not show particularly strong forward scattering.<br />

Going to larger temperatures kT ∼ t we find that after an initial<br />

decrease with U, the electron-phonon coupling starts to increase with U.<br />

We show that this behavior is related to an unusual reentrant behavior<br />

from a phase separated to a paramagnetic state upon decreasing the<br />

temperature.<br />

TT 9.11 Di 12:30 H20<br />

Analytisches Zwei-Flüssigkeitsmodell für unkonventionelle Supraleiter<br />

— •Dietrich Einzel — Walther-Meissner-Institut, Bayerische<br />

Akademie der Wissenschaften, 85748 Garching<br />

Dieser Beitrag untersucht die Möglichkeit, einfache analytische Ausdrücke<br />

für thermodynamische und Responsefunktionen im Rahmen einer<br />

Theorie schwach koppelnder unkonventioneller Supraleiter bei allen<br />

Temperaturen (0 ≤ T ≤ Tc) zu finden. Es werden sehr präzise Interpolationsformeln<br />

abgeleitet, insbesondere für Entropie, Wärmekapazität,<br />

Spinsuszeptibilität und normalfluide Dichte eines Gases aus thermischen<br />

Anregungen (Bogoliubov-Quasiteilchen), sowie für die superfluide Dichte,<br />

welche die Magnetfeld-Eindringtiefe, und damit den Response des Kondensats<br />

bestimmt. Die Resultate können auf alle bekannten unkonventionellen<br />

Supraleiter wie auch auf Fermi–Supraflüssigkeiten wie 3 He–A<br />

und –B angewendet werden und etablieren somit eine analytische Zwei–<br />

Flüssigkeits–Beschreibung für diese Systeme.<br />

TT 9.12 Di 12:45 H20<br />

Bernoulli potential in type-I and weak type-II superconductors<br />

— •K. Morawetz 1,2 , P. Lipavsk´y 3 , Jan Koláček 3 , J.J. Mares 3 ,<br />

E.H. Brandt 2 , and M. Schreiber 1 — 1 Istitute of Physics, Chemnitz<br />

University of Technology, 09107 Chemnitz, Germany — 2 Max-<br />

Planck-Institute for the Physics of Complex Systems, Nöthnitzer Str. 38,<br />

01187 Dresden, Germany — 3 Institute of Physics, Academy of Sciences,<br />

Cukrovarnická 10, 16258 Praha 6, Czech Republic<br />

The electrostatic potential close to the surface of superconductors in<br />

the Meissner state is discussed. We show that beside the Bernoulli potential,<br />

the quasiparticle screening, and the thermodynamic contribution<br />

due to Rickayzen, there is a non-local contribution which is large for<br />

both type-I and weak type-II superconductors [1]. A generalization of<br />

the Budd-Vannimenus theorem is found which allows one to evaluate the<br />

observed potential without the explicit solution of the charge profile at<br />

the surface [2]. The electrostatic potential above the Abrikosov vortex<br />

lattice is evaluated numerically. We propose an experimental measurement<br />

by NMR [3] to access this field which can yield informations about<br />

material parameters.<br />

[1] P. Lipavsk´y, K. Morawetz, J. Koláček, J. J. Mareˇs, E. H. Brandt, M.<br />

Schreiber, Phys. Rev. B in press (2003), cond-mat/0311166<br />

[2] P. Lipavsk´y, K. Morawetz, J. Koláček, J. J. Mareˇs, E. H. Brandt, M.<br />

Schreiber, Phys. Rev. B submitted, cond-mat/0311167<br />

[3] P. Lipavsk´y, J. Kolacek, K. Morawetz, E. H. Brandt, Phys. Rev. B 66<br />

(2002) 134525

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