Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
Plenarvorträge - DPG-Tagungen
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Tiefe Temperaturen Montag<br />
are composite spinon-phonon objects. Second, the change of the magnetization<br />
and the corresponding change of the wave vector of the spinons<br />
strongly affects the way in which various umklapp processes can relax<br />
the heat current, leading to a characteristic fractal-like spiky behavior of<br />
κ(T, h) as a function of h.<br />
[1] E. Shimshoni, N. Andrei and A. Rosch, Phys. Rev. B 68, 104401<br />
(2003).<br />
TT 3.11 Mo 12:00 H18<br />
ESR study of the low-dimensional quantum spin system TiOCl<br />
— •V. Kataev 1,2 , J. Baier 2 , A. Möller 3 , L. Jongen 3 , G. Meyer 3<br />
und A. Freimuth 2 — 1 Leibniz-Institut für Festkörper- und Werkstoffforschung<br />
PF D-01171 Dresden — 2 II. Physikalisches Institut, Universität<br />
zu Köln, D-50937 Köln — 3 Institut für Anorganische Chemie, Universität<br />
zu Köln, D-50939 Köln<br />
We present electron spin resonance (ESR) measurements of Ti 3+ (3d 1 )<br />
ions in single crystals of the novel layered quantum spin magnet TiOCl<br />
[1]. The ESR data give evidence that the single electron in the d-shell occupies<br />
predominantly the dxy orbital which lies in the bc crystallographic<br />
plane. This result suggests a formation of a spin-1/2 chain along the b<br />
direction, owing to the overlap of the orbital states, and supports recent<br />
LDA+U calculations of the band structure [2]. The ESR signal vanishes<br />
at Tc=67 K signalling the transition to a non-magnetic, possibly a spin-<br />
Peierls state, as proposed in Ref. [2]. Our x-ray powder diffraction data<br />
give however no evidence for the structural transition associated with the<br />
spin-gap transition. A pronounced dependence of the linewidth and the<br />
g factor on temperature suggests a strong coupling of spins to the lattice<br />
which may be decisive for the magnetic properties of TiOCl. In connection<br />
with the peculiar temperature dependence of the ESR parameters<br />
we discuss the role of spin and probably also orbital fluctuations above<br />
Tc for the opening of the spin gap.<br />
[1] For details see, V. Kataev et al., Phys. Rev. B 68, 140405(R) (2003).<br />
[2] A. Seidel et al., Phys. Rev. B 67, 020405(R) (2003).<br />
supported by DFG through SFB 608<br />
TT 3.12 Mo 12:15 H18<br />
Thermodynamic Properties of Layered Ruthenates — •J.<br />
Baier 1 , T. Zabel 1 , M. Kriener 1 , M. Reuther 1 , T. Lorenz 1 ,<br />
A. Freimuth 1 , P. Steffens 1 , O. Friedt 1 , M. Braden 1 , A.<br />
Revcolevschi 2 , S. Nakatsuji 3 , and Y. Maeno 3 — 1 II. Physikalisches<br />
Institut, Universität zu Köln, Germany — 2 Laboratoire de<br />
Physico-Chimie de l‘Etat Solide, Universite Paris-Sud, Orsay, France —<br />
3 Department of Physics, Kyoto University, Japan<br />
The phase diagram of Ca2−xSrxRuO4 presents a rich spectrum of distinct<br />
structural distortions, which are accompanied by drastic changes of<br />
the magnetic and electronic properties [1]. Ca-Doping drives the system<br />
from the spin-triplet superconductor Sr2RuO4 to the antiferromagnetic<br />
Mott-insulator Ca2RuO4. At x = 0.5, the low-temperature magnetic susceptibility<br />
is considerably increased and an unusually large Cp/T-value<br />
due to the evolution of a nearly ferromagnetic heavy-mass Fermi liquid<br />
is observed [2]. We find an anomalous, anisotropic thermal expansion for<br />
this compound. In a magnetic field, the anomalous thermal expansion<br />
and the large Cp/T-value are suppressed with an anisotropic field dependence.<br />
For lower x, an additional tilting of the RuO6 octahedra sets in and<br />
at x = 0.2 the susceptibility is again strongly reduced [2]. In high magnetic<br />
fields this compound is driven through a metamagnetic transition<br />
and the large susceptibility recovers. We observe a large magnetostriction<br />
during this metamagnetic transition.<br />
[1] O. Friedt et al., Phys. Rev. B 63 (2001)<br />
[2] S. Nakatsuji et al., Phys. Rev. Lett. 90 (2003)<br />
Supported by the DFG through SFB 608<br />
TT 3.13 Mo 12:30 H18<br />
Heat Transport in Low Dimensional Spin Systems — •C. Hess 1 ,<br />
H. ElHaes 2 , P. Ribeiro 3 , B. Büchner 3 , F. Heidrich-Meisner 4 ,<br />
W. Brenig 4 , M. Hücker 5 , U. Ammerahl 5 , and A. Revcolevschi 5<br />
— 1 Department of Condensed Matter Physics, University of Geneva,<br />
Switzerland — 2 II. Physikalisches Institut, RWTH Aachen, Germany<br />
— 3 IFW Dresden, Germany — 4 Institut für Theoretische Physik, TU-<br />
Braunschweig, Germany — 5 Laboratoire de Physico-Chimie des Solides,<br />
Université Paris-Sud, France<br />
We present experimental results for the magnon thermal conductivity<br />
κmag of several low dimensional spin systems, like antiferromagnetic<br />
planes, ladders and chains. We focus on the quasi two dimensional antiferromagnet<br />
La2CuO4 and the spin ladder system (Sr, Ca, La)14Cu24O41,<br />
where scattering of the magnetic excitations on static defects, phonons,<br />
magnons and holes is analyzed on basis of a kinetic model. The analysis<br />
yields for example an intimate relation of magnetic and electronic transport<br />
channels in the spin ladders and in both systems a strict limitation<br />
of the magnon mean free path lmag by artificially induced non magnetic<br />
defects, i.e., lmag ≈ dZn−Zn at low temperatures, where dZn−Zn is the<br />
mean distance of doped Zn-ions in the low dimensional structures. The<br />
latter result is in stark contrast to recent model calculations for the heat<br />
transport in the spin ladders.<br />
TT 3.14 Mo 12:45 H18<br />
Magnetic excitations in the low-dimensional quantum spin systems<br />
CaCu2O3 and Ba2Cu3O4Cl2 — •Eva Benckiser 1 , Markus<br />
Grüninger 1 , Tamara Nunner 2 , Thilo Kopp 2 , Chinnathambi<br />
Sekar 3 , and Gernot Krabbes 3 — 1 II. Physikalisches Institut,<br />
Universität zu Köln, Germany — 2 Experimentalphysik VI, Universität<br />
Augsburg, Germany — 3 IFW Dresden, Germany<br />
The magnetic excitations of CaCu2O3 and Ba2Cu3O4Cl2 are studied<br />
by means of phonon-assisted infrared absorption. CaCu2O3 has been discussed<br />
as a two-leg S=1/2 ladder with J⊥ ≪ J�. An analysis of the<br />
optical conductivity σleg(ω) for polarization parallel to the legs at 4K indeed<br />
yields J� ≈ 165 meV and a much smaller J⊥. However, comparison<br />
of σ(ω) for polarization parallel to the rungs and perpendicular to the<br />
ladders with the result of a DMRG calculation shows that the coupling<br />
perpendicular to the ladders is comparable to J⊥, i.e. that CaCu2O3 is<br />
not a ladder but rather a 3D system of weakly coupled chains.<br />
In the 2D cuprates, the large spectral weight in σ(ω) above the bimagnon-plus-phonon<br />
peak and the nature of the high-energy excitations<br />
are still under discussion. The 2D Cu3O4 layers in Ba2Cu3O4Cl2 contain<br />
the CuO2 square lattice plus a second Cu square lattice with a much<br />
weaker coupling constant J2. The spins of the two subsystems are decoupled<br />
from each other on the mean-field level. For ω ≫ J2 the compound<br />
represents a clean realization of an undoped CuO2 layer. We find that the<br />
high-energy spectral weight is larger than assumed previously. The temperature<br />
dependence of σ(ω) and a comparison with 1D systems strongly<br />
indicate a strong contribution from local, incoherent excitations.<br />
TT 4 Quantenflüssigkeiten, Bose-Einstein Kondensate, ultrakalte Atome, ...<br />
Zeit: Montag 10:00–12:15 Raum: H19<br />
Hauptvortrag TT 4.1 Mo 10:00 H19<br />
Kalte Atome in optischen Gittern — •Wilhelm Zwerger — Institut<br />
für Theoretische Physik, Universität Innsbruck, Technikerstr. 25,<br />
A-6020 Innsbruck,<br />
Kalte Atome in optischen Gittern stellen eine neue Klasse von Vielteilchensystemen<br />
dar, in denen die Stärke der Wechselwirkung durch Variation<br />
der Gittertiefe oder der Streulängen über einen weiten Bereich abstimmbar<br />
ist. Paradigmatische Modelle der Vielteilchenphysik, wie das<br />
Hubbard-Modell oder Luttinger-Flüssigkeiten sind daher experimentell<br />
realisierbar und geben Zugang zu komplexen Vielteilchenzuständen und<br />
den damit verbundenen Quantenphasenübergängen. Aufbauend auf dem<br />
Beispiel eines Mott-Hubbard Übergangs für Bosonen von Greiner et.al.<br />
wird ein Überblick über aktuelle Entwicklungen in diesem Gebiet ge-<br />
geben, insbesondere eindimensionale Systeme und die Perspektiven für<br />
Vielteilchenphysik mit Fermionischen kalten Atomen.<br />
TT 4.2 Mo 10:30 H19<br />
Fluctuations and stability of superfluid turbulence in He-4 at<br />
mK temperatures — •Wilfried Schoepe — Fakultaet fuer Physik,<br />
Universitaet Regensburg<br />
Turbulent flow of superfluid helium-4 at mK temperatures around an<br />
oscillating microsphere is known to be unstable at low driving forces,<br />
switching intermittently between turbulent and laminar phases. The lifetimes<br />
of the turbulent phases are exponentially distributed and the mean<br />
lifetimes grow exponentially with the square of the driving force. These<br />
experimental results are attributed to statistical fluctuations of the vor-