engleski

Charge-Density Wave State in Ladder Planes of Sr14-xCaxCu24O41

The quantum spin chain and ladder system Sr14-xCaxCu24O41 provides a nice possibility to study spin and charge dynamics and their interplay in a spin-gapped environment. The compound possesses a complex layered structure of alternating Cu2O3 two-leg ladders (the subsystem responsible for the conductivity and superconductivity) and CuO2 chain sheets, which are separated by (Sr, Ca) layers. Independently of x, the average valence of copper is +2.25 so that the compound is inherently doped by six holes per formular unit. We have characterized the charge response of the ladders by dc resistivity, low frequency dielectric and optical spectroscopy on single crystals in all three crystallographic directions. For x=0 and at ambient pressure, the ground state in ladders was identified as a charge density wave insulator with a CDW gap almost three times larger than the spin gap [1]. Substituting Ca2+ for Sr2+ suppresses the CDW insulating phase fully for x>9 [2], while the spin gap remains finite when SC sets in under high pressure. The CDW is not of the standard nature since it sets in below an insulator-to-insulator phase transition and in some features resembles a charge order driven by correlations, it develops in the ladder planes with 2D long-range order, and posses an anisotropic phason-like dispersion [3]. We discuss possible candidates among exotic charge-ordered phases with strong electron-electron interactions and a most probable mechanism, which governs the CDW suppression by Ca-substitution. Finally a modified phase diagram is suggested. [1]B.Gorshunov et al., Phys.Rev.B66, 060508R (2002). [2]T.Vuletic et al., Phys.Rev.Lett. 90, 257002 (2003). [3]T.Vuletic et al., submitted to Phys.Rev.Lett.(2004).

spin ladders; charge-density wave; supraconductivity

nije evidentirano