Berlin 2008 – wissenschaftliches Programm
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TT: Fachverband Tiefe Temperaturen
TT 32: Correlated Electrons: Poster Session
TT 32.54: Poster
Donnerstag, 28. Februar 2008, 14:00–18:00, Poster B
Magnetic excitations in S=1/2 chains and five-leg ladders: an optical study — •Eva Benckiser1, Markus Grüninger1, Marco Reuther1, Thomas Lorenz1, Pascal Ribeiro2, Christian Hess2, and Alexandre Revcolevschi3 — 1Institute of Physics II, University of Cologne, Germany — 2IFW Dresden, Germany — 3Laboratoire de Chimie des Solides, University Paris-Sud, France
The crossover from 1D chains via n-leg ladders to 2D planes is very interesting. However, hardly any experimental data exist for n > 2. We have grown single crystals of the 5-leg ladder La8Cu7O19 in an image furnace. A contribution of the magnetic excitations to the optical conductivity arises due to the simultaneous excitation of a phonon. Our optical data represent the first experimental result on the magnetic excitations of n-leg ladders (n > 2) at high energies. We find a surprising resemblance with the well-understood spectrum of a two-leg ladder. In particular, our data strongly suggest the existence of bound states of magnetic excitations also in the 5-leg ladder.
In the zig-zag double chain (Sr,Ca)CuO2, the observation of two-spinon excitations for polarization parallel to the chains enables us to determine the exchange coupling J = 227 ± 4 meV very accurately, which resolves an old controversy. We report the dependence of J on temperature and Ca substitution. Moreover, we discuss a new mechanism (without invoking a phonon) which explains the observation of magnetic excitations for polarization perpendicular to the chains (E ∥ b). The relevance of this contribution to the optical conductivity for other quantum spin systems is pointed out.