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Direct and unambiguous observation of charge order in the cuprates has been an outstanding problem. One of the key questions is what other ordered phases compete with superconductivity in high-Tc superconductors. Possible candidates are charge density waves such as stripes and or checkerboards separating regions of antiferromagnetism. A related but lesser known system is cuprate quantum spin ladder. A doped ladder can be thought of as a high-Tc material with lower dimensionality. Independently, investigation of the charge and spin dynamics of spin 1/2 quasi one-dimensional Sr14Cu24O41 ladder compound has attracting attention because of the critical nature of its magnetic ground state.
We use ultra-low frequency Raman spectroscopy as well as linear and nonlinear electrical response over about 10 decades of frequency to identify the insulating state of self-doped Sr14Cu24O41 ladders as a weakly pinned, sliding spin/charge density wave with non-linear conductivity and a giant dielectric response (ε ~ 106) that persists to remarkably high temperatures [1]. We also performed structural studies in the density wave state by anomalous X-ray diffraction at the O K- and Cu L- edges. At sharp resonance with O K- pre-edge we observed peak that corresponds to charge density modulation with period of five ladder steps that is commensurate with the lattice structure. Intriguingly, the density wave peak intensity drops rapidly when excitation energy is detuned from the pre-edge resonance suggesting that the lattice almost does not respond to the charge modulation and therefore the modulation is driven by anti-ferromagnetic interactions [2]. Our results highlight the importance of long-range interactions for the physics of lightly doped copper-oxides. Similar density wave correlations with three ladder steps period were found in ladders with higher hole concentration that show metallic-like conductivity and are superconducting under pressure. Our results demonstrate that the superconducting state in cuprates is competing with a crystalline charge ordered state and suggest that the transport in metallic ladders, which is similar to transport in underdoped high-Tc cuprates, is driven by a collective electronic response [3]. * Work done in collaboration with A. Gozar, B.S. Dennis, P. Littlewood, E. Isaacs, P. Abbamonte, A. Rusydi, P. Evans, T. Siegrist, N. Motoyama, H. Eisaki, S. Uchida. 1. G. Blumberg et al, Science 297, 584 (2002). 2. P. Abbamonte, G. Blumberg et al, Nature 431, 1078 (2004). 3. A. Gozar, G. Blumberg et al, PRL91, 087401 (2003); PRL87, 197202 (2001). |