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TT: Tiefe Temperaturen

TT 3: Spin-Systeme und itinerante Magnete

TT 3.13: Vortrag

Montag, 8. März 2004, 12:30–12:45, H18

Magnetic excitations in the low-dimensional quantum spin systems CaCu₂O₃ and Ba₂Cu₃O₄Cl₂ — •Eva Benckiser¹, Markus Grüninger¹, Tamara Nunner², Thilo Kopp², Chinnathambi Sekar³, and Gernot Krabbes³ — ¹II. Physikalisches Institut, Universität zu Köln, Germany — ²Experimentalphysik VI, Universität Augsburg, Germany — ³IFW Dresden, Germany

The magnetic excitations of CaCu₂O₃ and Ba₂Cu₃O₄Cl₂ are studied by means of phonon-assisted infrared absorption. CaCu₂O₃ has been discussed as a two-leg S=1/2 ladder with J_⊥≪ J_∥. An analysis of the optical conductivity σ_leg(ω) for polarization parallel to the legs at 4 K indeed yields J_∥≈ 165 meV and a much smaller J_⊥. However, comparison of σ(ω) for polarization parallel to the rungs and perpendicular to the ladders with the result of a DMRG calculation shows that the coupling perpendicular to the ladders is comparable to J_⊥, i.e. that CaCu₂O₃ is not a ladder but rather a 3D system of weakly coupled chains.

In the 2D cuprates, the large spectral weight in σ(ω) above the bi-magnon-plus-phonon peak and the nature of the high-energy excitations are still under discussion. The 2D Cu₃O₄ layers in Ba₂Cu₃O₄Cl₂ contain the CuO₂ square lattice plus a second Cu square lattice with a much weaker coupling constant J₂. The spins of the two subsystems are decoupled from each other on the mean-field level. For ω ≫ J₂ the compound represents a clean realization of an undoped CuO₂ layer. We find that the high-energy spectral weight is larger than assumed previously. The temperature dependence of σ(ω) and a comparison with 1D systems strongly indicate a strong contribution from local, incoherent excitations.