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TT: Tiefe Temperaturen

TT 30: Postersitzung IV: Kritische Ph
änomene, Quantenstörstellen, niederdimensionale Systeme

TT 30.39: Poster

Donnerstag, 11. März 2004, 14:30–19:00, Poster A

Optical study of the doped spin-ladder Sr_14−xCa_xCu₂₄O₄₁ — •C. Hilgers¹, M. Grüninger¹, A. Freimuth¹, U. Ammerahl^1,2, P. Ribeiro³, B. Büchner⁴, and A. Revcolevschi² — ¹II. Physikalisches Institut, Universität zu Köln — ²Laboratoire de Physico-Chimie de l’ Etat Solides, Université Paris-Sud, France — ³II. Physikalisches Institut, RWTH-Aachen — ⁴IFW Dresden

The phase-diagram of intrinsically hole-doped Sr_14−xCa_xCu₂₄O₄₁ shows e.g. superconductivity, charge order and magnetism, and the system offers a fascinating playground to study the interplay of spin and charge degrees of freedom. A key question is how the holes are distributed among the two subsystems, chains and ladders. We present the infrared reflectivity of Sr_14−xCa_xCu₂₄O₄₁ for x = 3, 5, 6 and 8 between 5 meV and 1.5 eV as a function of temperature for polarization along the legs (E||c) and along the rungs (E||a). The optical conductivity σ(ω) is derived via a Kramers-Kronig analysis and shows both a strong anisotropy and a strong temperature dependence. Between x = 6 and 8 we observe a qualitative change of σ(ω) for both polarization directions, and two additional strongly temperature dependent peaks develop in the far-infrared region, one in σ_a(ω) at about 400 cm⁻¹ and one in σ_c(ω) below 100 cm⁻¹. We discuss different scenarios, e.g. the suppression of charge order in the chains, the evolution of a charge-density wave in the ladders, magnetic excitations, the formation of hole pairs or a bonding-antibonding transition.