Plenarvorträge - DPG-Tagungen

Tiefe Temperaturen Mittwoch
nahe 0,4 GPa verschwindet.
Wir präsentieren hier Ergebnisse aus Messungen der magnetischen
Suszeptibilität und spezifischen Wärme unter Druck. Danach sind noch
schwache Kennzeichen für einen magnetischen Ordnungsübergang jenseits
der 0,4 GPa zu finden, die nahe 0,5 GPa definitiv verschwinden.
Dabei bildet sich sukzessive ein Verlauf von χ heraus, der mit abnehmender
Temperatur ungefähr logarithmisch ansteigt.
TT 24.29 Mi 14:30 Poster A
Specific heat of PrOs4Sb12 at high magnetic fields — •K.
Grube 1 , S. Drobnik 2 , C. Pfleiderer 2 , A. Faisst 2 , R. Vollmer 2 ,
H. v. Löhneysen 1,2 , E.D. Bauer 3 , and M.B. Maple 3 —
1 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021
Karlsuhe — 2 Physikalisches Institut, Universität Karlsruhe, D-76128
Karlsuhe — 3 Department of Physics and Institute of Pure and
Applied Physical Sciences, University of California San Diego, La Jolla,
California 92093-0319
PrOs4Sb12 is the first Pr based heavy-fermion superconductor. Detailed
measurements of the low temperature specific heat [1] have helped
identify the superconductivity in PrOs4Sb12 as a strong contender for
quadrupolar pairing, i.e., superconductivity that is neither electronphonon
nor magnetically mediated. Here we focus on a related issue,
the formation of an ordered phase at low temperatures and high magnetic
fields, that appears to be connected with the Zeeman splitting of
the crystal electric field ground state. We have revisited this high field
phase in a detailed study of the specific heat up to 14T at temperatures
down to 0.4K. The possible nature of the high-field phase is discussed in
the context of the crystal field ground state and the evidence for several
superconducting phases.
[1] R. Vollmer et al., Phys. Rev. Lett. 90 (2003) 057001
.
TT 24.30 Mi 14:30 Poster A
Thermische Ausdehnung von CeCu6−xAux — •K. Grube 1 , P.
Estrela 2 , A. de Visser 2 , O. Stockert 3 und H. v. Löhneysen 1,4 —
1 Forschungszentrum Karlsruhe, Institut für Festkörperphysik, D-76021
Karlsruhe — 2 Van der Waals-Zeeman Institute, Universiteit van Amsterdam,
1018XE Amsterdam — 3 Max-Planck-Institut für chemische Physik
fester Stoffe, D-01187 Dresden — 4 Physikalisches Institut, Universität
Karlsruhe, D-76128 Karlsruhe
CeCu6−xAux ordnet oberhalb von x > 0.1 antiferromagnetisch.
Während der magnetische Ordnungsvektor sich drastisch zwischen x =
0.4 und 0.5 ändert, steigt die Néeltemperatur zwischen x = 0.1 und 1.0
linear an. Am Übergang zur magnetischen Ordnung zeigen die linearen
thermischen Ausdehnungskoeffizeinten für x = 0.3 positive Sprünge in a
und b Richtung und einen negativen Sprung in c Richtung. Für x = 0.5
wechseln die Sprünge entlang der a und b Achse ihr Vorzeichen. Dieses
Verhalten scheint die drastische Änderung des Ordnungsvektors widerzuspiegeln.
Mit Hilfe der elastischen Konstanten lassen sich die resultierenden
uniaxialen Druckabhängigkeiten der Néeltemperatur in Dehnungsabhängigkeiten
umrechnen und mit der Dotierungs- und Druckabhängigkeit
der magnetischen Struktur vergleichen.
TT 24.31 Mi 14:30 Poster A
Ferromagnetic to paramagnetic quantum phase transition of
single crystal CeSi1.81 at high pressure — •S. Drotziger 1 , C.
Pfleiderer 1 , M. Uhlarz 1 , H. v. Löhneysen 1,2 , D. Souptel 3 , W.
Löser 3 , and G. Behr 3 — 1 Physikalisches Institut, Universität Karlsruhe,
D-76128 Karlsuhe — 2 Forschungszentrum Karlsruhe, Institut für
Festkörperphysik, D-76021 Karlsuhe — 3 Institut of Solid State and Materials
Research Dresden, Postfach 270116, D-01171 Dresden
Silicon deficient CeSi2−x crystallizes over a large range of compositions
in the orthorhombic α-GdSi2 structure. Below a critical Si content,
easy-plane ferromagnetism is observed. Being one of the rare examples
of a ferromagnetic Ce compound, the nature of this ferromagnetic state
has been studied intensively in the past. Here we report a high pressure
study of the magnetisation of a new generation of CeSi2−x single crystals
grown with an optical floating zone furnace under ultrapure conditions. In
CeSi1.81 where TC = 9.5K, moderate pressures are sufficient to suppress
ferromagnetism. The easy-plane ferromagnetic to paramagnetic quantum
phase transition is compared with the predictions of weak-coupling spin
fluctuation theory.
TT 24.32 Mi 14:30 Poster A
Investigation of the Low Ordered Moment in CeBiPt —
•G. Goll 1 , O. Stockert 2 , M. Prager 3 , N. Stüßer 4 , and T.
Takabatake 5 — 1 Physikalisches Institut, Universität Karlsruhe, 76128
Karlsruhe — 2 Max-Planck-Institut CPfS, 01187 Dresden — 3 Institut
für Festkörperforschung, Forschungszentrum Jülich, 52425 Jülich —
4 Hahn-Meitner-Institut, 14109 Berlin — 5 Department of Quantum
Matter ASDM, Hiroshima University, Higashi Hiroshima, 739-8526,
Japan
The electronic properties of CeBiPt show semimetallic behavior with
low charge carrier concentration nh = 7.7 ·10 17 cm −3 . Shubnikov-de Haas
experiments reveal a Fermi surface consisting mainly of a small hole
pocket around the Γ point with a volume of 1.5 · 10 −4 of the volume
of the first Brillouin zone [1]. To explain these result by band-structure
calculation a local character of the 4f electrons is required. The magnetic
behavior supports a local character as well. The magnetic susceptibility
follows a Curie-Weiss law above T = 100K with the full moment of free
Ce 3+ ions and magnetic order was found below TN = 1.1K by specificheat
measurements [2]. However, previous neutron-diffraction data suggested
an ordered moment well below 1µB. Crystal-electric fields (CEF)
in the cubic crystal symmetry can only partly account for the reduced
moment. Inelastic neutron scattering as well as neutron diffraction on
powder were used to investigate the CEF ground state and the magnetically
ordered moment. The measurements suggest additional magnetic
excitations with a characteristic temperature of 10 - 20K being responsible
for the low ordered moment.
[1] G. Goll et al., Europhys. Lett. 57, 233 (2002).
[2] T. Pietrus et al., Physica B 281 & 282, 745 (2000).
TT 24.33 Mi 14:30 Poster A
Successful growth of the intermetallic compound CeCu2(Si,Ge)2
by high-temperature solution — •M. Deppe, H. S. Jeevan, R.
Borth, C. Geibel, and F. Steglich — Max-Planck-Institute for
Chemical Physics of Solids, Nöthnitzer Str. 40, 01187 Dreden, Germany
Among the intermetallic compounds the system CeCu2(Si,Ge)2 is of
high interest because of the interplay between heavy-fermion superconductivity
and magnetism. For an intensive microscopic study large single
crystals of high quality are necessary. Large single crystals of this
compound were not available because of the incongruent formation of
CeCu2Si2 and substantial loss of copper during the typical growing process.
To overcome this difficulty the typical procedure is to use an offstochiometric
starting melt with an excess of copper. Four different
ground states exist for CeCu2Si2 which are very sensitiv to the exact
Cu/Si ratio. This makes it difficult to grow single crystals with a predetermind
ground state. To overcome this problem we started the growth
process with higher amount of Germanium because this reduced the
variety of physical ground states to only one. Later on we reduce the
Germanium content. We will discuss the optimal growth conditions for
our modified Bridgman technique using copper as flux. For the physical
characterisation we used resistivity and specific heat measurements.
These measurements show that our crystal growth method gives reproducible
results and evidence a systematic development of the magnetic
state with increasing Germanium content in the concentration range x
≤ (0 - 0.5). The discussion will focus on the future optimization of our
growth methods and the comparison with other crystal grow techniques.
TT 24.34 Mi 14:30 Poster A
Transferred hyperfine fields in the heavy fermion compounds
CeCu6−xAux — •Max Winkelmann, Gerda Fischer, and Elmar
Dormann — Physikalisches Institut, Universitaet Karlsruhe, D-76128
Karlsruhe
CeCu6 is one of the best known heavy-fermion paramagnets, CeCu5Au1
is a heavy-fermion antiferromagnet with TN = 2.3 K, and for x ≈
0.1 signatures of non-Fermi-liquid behaviour were reported. We present
temperature dependent studies of 63 Cu- and 65 Cu-NMR line shifts for
CeCu6−xAux (x=0;0.1;1) single-crystal powder samples. These are different
to the earlier[1] presented line shifts for polycrystalline powder
samples. Via the correlation with the temperature dependence of the
static magnetic susceptibility, Cu-site dependent Knight shift and transferred
hyperfine field coupling constants are derived. Local polarization
varies in sign and is larger by a factor of 10 than indicated by the earlier
published[2] high-temperature average value.
[1] M. Winkelmann, et al. Eur. Phys. J. B 26, (2002) 199
[2] I. Pop, et al. Czech. J. Phys. B 24 (1974) 1398