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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-

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