Session S2 - High-Tc Superconductors: Scattering.
INVITED session, Wednesday afternoon, March 24
517B, Palais des Congres
Spin ladders are important low-dimensional analogues of high temperature superconductors that exhibit close competition between charge density wave and superconducting order. We report the observation of a commensurate, sinusoidal, five-lattice-parameter valence modulation in the spin ladder plane of Sr_14Cu_24O_41 with anomalous soft x-ray scattering at the Oxygen K (530 eV) and Copper L (950 eV) edges. This CDW appears to form without participation of phonons, and has an ordering vector close to 4k_F, suggesting a possible Wigner crystalline origin. CDW correlations set in below T = 250 K and have non-MFT temperature dependence with a short coherence length (210 Åalong the ladder and 190 Åperpendicular), which we explain in terms of impurity-induced phason fluctuations. The modulation resonates strongly at the CuL_3/2 edge, but not at the L_1/2, which we conjecture to be evidence for locally broken time reversal symmetry. Our results demonstrate that ``hidden'' many-body phases may be uncovered through their valence fluctuations even in the absence of a structural signature.