Dresden 2003 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 7: Niedrigdimensionale Systeme

TT 7.1: Hauptvortrag

Dienstag, 25. März 2003, 09:30–10:00, HSZ03

Optical Spectroscopy of low-dimensional quantum spin systems — •Markus Grüninger — II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln

In recent years, optical spectroscopy has been established as a powerful tool for the study of magnetic excitations with S=0. Although electric dipole absorption of such S=0 excitations is not allowed in the presence of inversion symmetry, one may effectively avoid this selection rule by the simultaneous excitation of a symmetry-breaking phonon. This allows to study magnetic excitations at high energies throughout the entire Brillouin zone, since the phonon takes care of momentum conservation. The line shape of the optical conductivity provides important information on the kinetics and on the interactions of the magnetic excitations.

Low-dimensional quantum spin systems show novel ground states and fascinating excitations. The magnetism of cuprate systems is particularly interesting due to the relevance for high T_c superconductivity. Here, we review the results of phonon-assisted magnetic absorption. In the 2D S=1/2 cuprates, the large spectral weight of the incoherent high-energy contribution underlines the pronounced importance of quantum fluctuations. Excellent agreement with theoretical results is obtained both for the spinon continuum of the 1D S=1/2 chain Sr₂CuO₃ and for the high-energy continuum of the two-leg S=1/2 ladders (La,Ca)₁₄Cu₂₄O₄₁. The latter strikingly resembles the data of the 2D cuprates. We focus on the recent observation of a two-triplet bound state in the ladders. We discuss the exchange parameters and demonstrate that the cyclic exchange is sizable. The effects of hole doping and charge order are addressed.