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.
[S2.002] Magnetic excitations in YBCO and BSCCO superconductors
Bernhard Keimer (Max-Planck-Institute for Solid State Research, Stuttgart, Germany)
We give an overview of magnetic neutron scattering experiments on superconducting copper oxides, with an emphasis on a "resonant mode" that dominates the spin excitation spectrum in the superconducting states of YBCO and BSCCO compounds. Recent results include observations of the primary resonant mode in single- [1] and trilayer high-Tc materials and of a predicted secondary mode in bilayer materials [2]. We also report a detailed map of the in-plane anisotropy of the incommensurate spin dynamics in the superconducting state of untwinned, optimally doped YBCO. This map shows that the incommensurability is fully two-dimensional, that is, the peak intensity forms a ring around (\pi/a, \pi/b). However, the peak along the a-axis is narrower and about a factor of two more intense than the one along b. Finally, we report new x-ray diffraction results on oxygen vacancy ordering throughout the YBCO phase diagram. These oxygen superstructures must be taken into account when interpreting spectral anomalies in this system.
[1] H. He et al., Science 295, 1045 (2002) [2] S. Pailhes et
al., cond-mat/0308394 (PRL, in press)
[S2.003] Signatures of inhomogeneous superconducting condensate in IR spectra of high-T_c cuprates
Sasa Dordevic (Brookhaven National Laboratory)
We have developed a new method to study local charge inhomogeneities, based on the effect they have on the Josephson plasma resonance (JPR). The analysis of the JPR mode provides information on the local variation of the superfluid density within the CuO_2 planes, thus empowering one with a tool for "microscopy" on the superconducting condensate. Analysis of c-axis IR spectra of LSCO samples has revealed presence of regions within CuO_2 planes with the length scale of ~ 100-200 Åwithin which superconductivity is strongly depressed or completely depleted. An examination of the doping trends suggests that development of these superconducting inhomogeneities is triggered by the formation of the unidirectional spin density waves (stripes) in LSCO at x=1/8. This picture is consistent with NMR and \muSR results, but also with neutron scattering, assuming the spin-ordered signal is coming from regions of depressed superconductivity. The analysis of Josephson plasmon in YBCO doped with Zn and Ni reveals important differences these impurities have on local charge dynamics.
This work is a collaboration with D.N.Basov (UCSD), Seiky
Komiya, Yoichi Ando (CRIEPI) and C.C.Homes (BNL).
[S2.004] From stripes, resonance and local pairing, to frozen spin clusters within SC order near the SC-AF crossover in YBCO
William JL Buyers (National Research Council Canada)
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.