Dresden 2003 – wissenschaftliches Programm

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TT: Tiefe Temperaturen

TT 6: Postersitzung I (Amorphe- u. Tunnelsysteme, Niedrigdimensionale Systeme, Supraleitung: Elektronenstruktur, Phononen, Tunneln, Ordnungsparameter)

TT 6.28: Poster

Montag, 24. März 2003, 14:30–19:00, P2c, P2d

Magnetic excitations in low-dimensional quantum spin systems: an infrared study — •M. Windt¹, M. Grüninger¹, J. Puchalla¹, E. Benckiser¹, C. Hilgers¹, A. Freimuth¹, C. Knetter², K.P. Schmidt², G.S. Uhrig², T. Nunner³, T. Kopp³, U. Ammerahl^1,4, B. Büchner⁵, A. Revcolevschi⁴, C. Sekar⁶, G. Krabbes⁶, A. Löffert⁷, C. Gross⁷, and W. Assmus⁷ — ¹II. Phys. Inst., Uni. Köln — ²Inst. für Theo. Physik, Uni. Köln — ³EP VI, Uni. Augsburg — ⁴Lab. de Chimie des Solides, Uni. Paris-Sud, France — ⁵II. Phys. Inst., RWTH-Aachen — ⁶IFW Dresden — ⁷Phys. Inst.  Uni. Frankfurt

Recently, we reported the first observation of a two-triplet bound state in a two-leg S=1/2 ladder [1]. This was achieved by measuring the phonon-assisted two-triplet absorption in (La,Ca)₁₄Cu₂₄O₄₁. The optical conductivity σ(ω) agrees very well with the results of three different theoretical approaches (Jordan-Wigner fermions, continuous unitary transformations and DMRG). We find J_∥/J_⊥ ≈ 1.3, J_cyc / J_⊥≈ 0.2 and J_⊥≈ 1000 cm⁻¹, which demonstrates that the cyclic exchange is sizable [2]. With increasing temperature a dramatic change of the line shape of σ(ω) is observed. The effects of hole doping and charge order are studied by comparison with Sr₁₄Cu₂₄O₄₁, Sr₁₁Ca₃Cu₂₄O₄₁, and La₁Sr₁₃Cu₂₄O₄₁. We discuss a modulation of the exchange parameters [3] and the relationship between the S=1/2 ladder and an S=1 chain. Finally, we present results for CaCu₂O₃ and SrCu₂O₃.

[1] M. Windt et al., Phys. Rev. Lett. 87, 127002 (2001).

[2] T. Nunner et al., Phys. Rev. B 66, 180404(R) (2002).

[3] K. Schmidt et al., cond-mat/0210092.