Session S2 - High-Tc Superconductors: Scattering.
INVITED session, Wednesday afternoon, March 24
517B, Palais des Congres
The dynamic spin response of ordered YBCO_6.50 shows fluid low-energy one-dimensional incommensurate fluctuations (stripes) rising to a sharp 33 meV resonance with a high-energy cutoff. The spectral form is similar to the dSC density of states. The resonance persists well above Tc=59 K indicating that local SC pairing fluctuations exist in the normal phase. The small 9larger, 50120 meV. When the doping is reduced to ~6SC-AF crossover in YBCO_6.37, Tc drops to 19 K and the resonance becomes overdamped with a relaxation rate of 2.5 meV. A new energy scale occurs where a central mode appears with slow dynamics yet with short-range correlations. We speculate that these dynamics originate from a texture where clusters of spins are separated by a hole-rich superconducting wrapping - the analog of stripes at large doping. The non-critical growth of spin correlations proceeds without regard to Tc indicating that spins and charge are at best weakly coupled.