College Park, Maryland      June 6 - 10 , 2004

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TP56: Spin fluctuations in superconducting YBa₂Cu₃O_6.35 near the transition to antiferromagnetism

C. Stock (Physics Department, University of Toronto, Toronto, Ontario, Canada), W. J. L. Buyers (National Research Council, Chalk River, Ontario, Canada), R. Liang, D. Peets, D. Bonn, W.N. Hardy (Physics Department, University of British Columbia, Vancouver, B. C., Canada), R. J. Birgeneau (Physics Department, University of Toronto, Toronto, Ontario, Canada)

The discovery of superconductivity in the cuprates has resulted in a vast amount of interest in the spin fluctuations of these nearly two-dimensional materials. Despite the extensive research that has occured, there is no generally accepted theory of the pairing mechanism in these materials. Due to the strong interplay between antiferromagnetism and superconductivity, spin fluctuations are drastically altered by doping of the parent insulator and are strongly correlated with superconductivity. Understanding the spin fluctuations in the cuprates is an important step to understanding superconductivity.

Despite the vast amount of attention that the bilayer YBa₂Cu₃O_6+x superconductor system has received, little work has been done in investigating the spin fluctuations near the boundary of antiferromagnetism and superconductivity. We present neutron elastic and inelastic scattering results, utilizing both polarized and unpolarized beams, on the YBCO_6.35 superconductor (T_c=18 K). In contrast to the Ortho-II YBCO_6.5 system (T_c=59 K), we find that the spectral weight of YBCO_6.35 has been shifted to very low-energies resulting in a broad over-damped peak at around 2 meV. In comparison to higher oxygen concentrations, we speculate that this mode may be interpreted as a resonance peak. We also observe a broad central peak indicative of slowly fluctuating copper spins with correlation lengths of about 20 Å in the CuO₂ plane and about 15 Å along the c-axis. By comparing data taken with E_f=5 meV and E_f=14.5 meV, we suggest that the properties observed in YBCO_6.35 are analogous to the spin-glass phase observed in the LSCO system. This suggests that the spin-glass phase may be a common characteristic of cuprate systems near the transition from antiferromagnetism to superconductivity.

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