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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. Ammerahl2,3, P. Ribeiro 4, B. Büchner4, and A. Revcolevschi31II. 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 Sr14−xCaxCu24O41 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 σa(ω) and σc(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.

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