Hamburg 2001 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 11: Fachsymposium: Eindimensionale Übergangsmetalle

TT 11.11: Vortrag

Dienstag, 27. März 2001, 18:45–19:00, A

Infrared study of magnetic excitations in the two-leg spin 1/2 ladders of La_14−xCa_xCu₂₄O₄₁ — •M. Windt¹, M. Grüninger¹, U. Ammerahl¹, A. Freimuth¹, and A. Revcolevschi² — ¹II. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln — ²Laboratoire de Chimie des Solides, Université Paris-Sud, France

In order to determine the controversially discussed superexchange parameters we investigated the magnetic excitations of the two-leg spin 1/2 ladders of La_14−xCa_xCu₂₄O₄₁. We measured the mid-infrared transmission and reflectance between 4 K and 500 K for various x and for polarization of the electrical field parallel to the legs (E  ∥ c) and the rungs (“Sprossen”, E  ∥ a). The combination of both transmission and reflectance data allows us to calculate the optical conductivity directly. At low temperatures we observe three peaks at about 265, 350 and 500 meV for E  ∥ c which are attributed to phonon-assisted magnetic excitations. For E  ∥ a the peak frequencies are slightly larger and the spectral weight is smaller. With increasing Ca content the peaks shift to higher frequencies due to the shrinkage of the lattice in both a and c directions. We compare the ladder data with related results of the 2D cuprates such as YBa₂Cu₃O₆ where a peak at 470 meV has been attributed to strong quantum fluctuations [1].

[1] M. Grüninger et al., Phys. Rev. B 62, 12422 (2000).