Berlin 2005 – scientific programme

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TT: Tiefe Temperaturen

TT 32: Correlated Electrons - Low-dimensional Materials II

TT 32.6: Talk

Tuesday, March 8, 2005, 17:30–17:45, TU H2053

Evidence for bound holes in the doped spin ladders of (Sr,Ca)₁₄Cu₂₄O₄₁ — •C. Hilgers¹, M. Grüninger¹, A. Freimuth¹, U. Ammerahl^2,3, P. Ribeiro ⁴, B. Büchner⁴, and A. Revcolevschi³ — ¹II. Physikalisches Institut, Universität zu Köln — ²II. Physikalisches Institut, RWTH-Aachen — ³Laboratoire de Physico-Chimie de L’Etat Solides, Université Paris-Sud, France — ⁴IFW Dresden

In the telephone-number compounds (Sr,Ca)₁₄Cu₂₄O₄₁ the interplay of spin and charge degrees of freedom gives rise to a competition between charge-density wave (CDW) and superconducting (SC) ground states. Superconductivity was found in Sr_14−xCa_xCu₂₄O₄₁ under external pressure for x ≥ 11.5, whereas for x ≤ 8 a charge ordering in the chains and a CDW ground state in the ladders were observed. However, the nature of the CDW ground state is still unresolved. We present a detailed study of the doping dependence of the optical conductivity in the far-infrared range as a function of temperature and polarization (E∥a,c). Between x = 6 and x = 8, we observe a qualitative change of σ_a(ω) and σ_c(ω) with new collective modes for x = 8. We interpret these modes as evidence for bound holes in the ladders. For x ≤ 8, we observe a new optical phonon mode at low temperatures which can be attributed to a Raman-active ladder mode activated by the CDW. We report an interesting correlation between transition temperatures of the CDW ground state in the ladders and charge ordering in the chains.