Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Montag
Höchst, Europhysics Letters 60 (2002) 615
[2] R. Manzke, R. Müller, C. Janowitz, M. Schneider, A. Krapf, H. Dwelk,
Phys. Rev. B 63 (2001) R 100504
TT 8.47 Mo 14:30 Poster A
Superconductivity and weak electron-phonon coupling in transition
metal diborides — •H. Rosner 1 , S.-L. Drechsler 2 , G.
Fuchs 2 , K.-H. Müller 2 , G. Behr 2 , H. Eschrig 2 , R. Heid 3 , E.
Forzani 4 , K. Winzer 4 , Y. Naidyuk 5 , and O. Otani 6 — 1 Max Planck
Institut für Chemische Physik fester Stoffe — 2 Institut für Festkörper und
Werkstoffforschung Dresden — 3 FZ Karlsruhe — 4 Universität Göttingen
— 5 NAS Kharkiv, Ukraine — 6 NIMS Tsukuba, Japan
After the surprising discovery of superconductivity at 40K in MgB2 an
intense search for other superconducting diborides was started. Several
contradicting reports about the superconductivity in TB2 (T = transition
metal) have been published so far, and the discussion is not settled
yet. Combining extensive electronic structure and model calculations
with various experiments (specific heat, resistivity, susceptibility,
de-Haas-van-Alphen oscillations, point contacts) we investigate the electronic
structure and the electron-phonon coupling λ in various TB2 with
focus on ZrB2. Based on our calculations, we provide a clear microscopic
picture for the rather weak electron-phonon coupling in these systems
and the absence of conventional superconductivity in agreement with
the empirically determined values from thermodynamic and transport
measurements. Open problems are briefly discussed.
TT 8.48 Mo 14:30 Poster A
”Supraleitende” und ”magnetische” Energiebänder bestimmter
Symmetrie als Voraussetzung für Supraleitung bzw. Magnetismus
— •Ekkehard Krüger — Kraussweg 5, 70597 Stuttgart
Beobachtungen an den Bandstrukturen der elementaren Metalle lassen
vermuten, dass zur Bildung stabiler Cooper-Paare sowie stabiler magnetischer
Strukturen gewisse Symmetriebedingungen im Leitungsband eines
Metalles erfüllt sein müssen, die sich im Rahmen der Gruppentheorie optimal
lokalisierter Wannier-Funktionen behandeln lassen. Zur Erklärung
der Beobachtungen wird ein nichtadiabatisches Modell atomartiger Leitungselektronen
vorgeschlagen.
TT 8.49 Mo 14:30 Poster A
Budd-Vannimenus theorem for superconductors — P. Lipavsk´y
1 , •K. Morawetz 2,3 , Jan Koláček 1 , J.J. Mares 1 , E.H.
Brandt 3 , and M. Schreiber 2 — 1 Institute of Physics, Academy of
Sciences, Cukrovarnická 10, 16258 Praha 6, Czech Republic — 2 Istitute
of Physics, Chemnitz University of Technology, 09107 Chemnitz,
Germany — 3 Max-Planck-Institute for the Physics of Complex Systems,
Nöthnitzer Str. 38, 01187 Dresden, Germany
The Budd-Vannimenus theorem is generalized to apply to superconductors
in the Meissner state. This identity links the surface value of the
electrostatic potential to the free energy in the bulk which allows one
to evaluate the observed potential without the explicit solution of the
charge profile at the surface.
[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
TT 8.50 Mo 14:30 Poster A
Pressure and Doping Dependence of the Super Exchange of
2D-Cuprates — •Helge Rosner 1 , Ulrike Nitzsche 2 , and Stefan-
Ludwig Drechsler 2 — 1 Max-Planck-Institute for Chemical Physics of
Solids, Noethnitzer Str. 40, D-01187 Dresden, Germany — 2 Leibniz Institute
for Solid State and Materials Research Dresden, P.O.B. 270016,
01171 Dresden, Germany
The dependence of superconducting Tc from the exchange integrals J
is investigated within many models devoted to HTSC. However, most of
these models ignore the dependence of the exchange integrals on both
the lattice parameters and the doping level because of lacking reliable
data. Here we present a systematic study for the two-dimensional model
compound CaCuO2. Based on LDA and LDA+U band structure calculations
and subsequently derived tight-binding models we calculate
the magnitude of the most relevant exchange integrals and their dependence
on doping and uniaxial pressure. We compare the results of
total energy calculations with those of various tight-binding models (oneband
and multi-band approaches) and the parameters obtained by microscopic
models analyzing the experimental data for the superconducting
HgBaCuO-family.
TT 8.51 Mo 14:30 Poster A
Crystal growth, perfection and selected physical properties
of RENi2B2C (RE=Y, Tb, Ho) — •Dmitri Souptel 1 , Günter
Behr 1 , Wolfgang Löser 1 , Günter Fuchs 1 , Konstantin
Nenkov 1 , and Andreas Kreyssig 2 — 1 IFW Dresden, Helmholtzstr.
20, 01171 Dresden — 2 IAPD, TU Dresden, 01062 Dresden
Single crystals of HoNi2B2C, YNi2B2C and TbNi2B2C intermetallic
compounds have been grown by a floating zone technique with optical
heating. Crystals are crack free, without traces of 2nd phases and single
crystalline in the volumes of 6 mm in diameter and 25-40 mm in length.
YNi2B2C shows superconducting transition temperature Tc=15.5 K
with the width ∆Tc=0.1-0.2 K and RRR=35-39. HoNi2B2C single crystals
exhibit a variety of physical properties. For the first time, macroscopic
single crystalline samples with zero-field re-entrant superconductivity
were prepared. Origins of variations of superconducting properties
have been related to composition, lattice parameters and local resistance
ratio measurements of crystal sections. The results corroborate the existence
of a finite homogeneity range of the HoNi2B2C intermetallic compound.
TT 8.52 Mo 14:30 Poster A
Tieftemperaturverhalten von CeNi2B2C — •Dmitrij Bogdanov,
Klaus Winzer und Eugenio Forzani — I. Physikalisches Institut,
Universität Göttingen, Tammannstr. 1, 37077 Göttingen
Die bisher vorliegenden Untersuchungen an CeNi2B2C lassen die Frage
nach dem Grundzustand des Systems bei tiefen Temperaturen offen.
Die Möglichkeit eines supraleitenden Grundzustandes bei T = 0,1 K [1]
konnte bisher nicht bestätigt werden. Für die Charakterisierung des Systems
wurden sowohl polykristalline Proben als auch Einkristalle hergestellt,
deren Phasenreinheit und Qualität durch Röntgendiffraktometrie
und Messungen des Restwiderstandsverhältnisses bestimmt wurden. Die
Temperatur- und Magnetfeldabhängigkeit des spezifischen Widerstandes
und der magnetischen Suszeptibilität wurden für Temperaturen bis 40
mK bestimmt. Mit einem SQUID-System wurden zusätzlich Messungen
des magnetischen Moments und der Suszeptibilität bis 1,8 K durchgeführt.
Der elektrische Widerstand zeigt im Temperaturbereich um 2
K einen stufenförmigen Verlauf, der auf eine geringe Aufspaltung des
Kristallfeld-Grundzustandes hindeutet. Bei tiefen Temperaturen wird ein
sehr steiler Anstieg der paramagnetischen Suszeptibilität mit einem Maximum
bei T = 100 mK beobachtet, das als Signatur einer magnetischen
Ordnung gedeutet wird. Messungen des spezifischen Widerstandes zeigen
keinerlei Anzeichen für das Auftreten von Supraleitung.
[1] M. El Massalami et al., Physica C 304 , 184 (1998)
TT 8.53 Mo 14:30 Poster A
The effect of noble-gas preplating on quench-condensed hydrogen
films — •Stefan Tibus, Jürgen Klier, and Paul Leiderer
— Universität Konstanz, Universitätsstraße 10, 78464 Konstanz
Triple-point dewetting of H2 on e.g. Au is a well-known phenomenon
and prevents the forming of a thick (i.e. more than a few monolayers)
solid equilibrium film on such surfaces. It was theoretically shown that
a modification of the substrate-adsorbate interaction by means of preplating
should lead to complete wetting and thus to a thick solid film at
saturated vapor pressure (provided that the substrate surface is ideally
smooth). Ne and Ar were found to be appropriate candidates as intermediate
layers, but investigations found no significant change in the film
thickness upon preplating, which was attributed to the roughness of the
substrate.
However, preliminary experiments with quench-condensed hydrogen
showed an effect of the preplating on the film’s structure. This is to be
investigated in more detail by means of a new experimental setup allowing
for both surface-plasmon microscopy (at a higher resolution than
before) as well as surface-plasmon spectroscopy.
TT 8.54 Mo 14:30 Poster A
Josephson coupling in Nb/Al 2 O3/Cu 1 Ni1−x/Nb tunnel junctions
— •Martin Weides 1 , Edward Goldobin 2 , Reinhold Kleiner 2 ,
and Hermann Kohlstedt 1 — 1 Institut für Festkörperforschung,
Forschungszentrum Jülich, D-52425 Jülich — 2 Physikalisches Institut-
Experimentalphysik II, Universität Tübingen, D-72076 Tübingen
The realization of qubits for quantum computation attracts considerable
interest. One of the approaches is to use π Josephson junctions (JJs)
which can be realized as hybrids of superconducting (S) tunnel junction