We report a detailed neutron-scattering study of the dynamical spin susceptibility in a single crystal of YBa2Cu3O6+x, with x=0.6 and Tc=53 K. The measurements cover the energy range from 5 to 50 meV, and temperatures from 10 to 100 K. It is shown that antiferromagnetic correlations between nearest-neighbor CuO2 layers are quite strong. As a result, only in-phase bilayer spin fluctuations are observed at low energies, with the out-of-phase fluctuations making a weak appearance at 40 meV. Within the two-dimensional (2D) Brillouin zone corresponding to a single layer, the imginary part of the dynamical susceptibility χ''(Q,ω) exhibits a broad peak about the point corresponding to antiferromagnetic order QAF with a width that is mildly energy dependent. At 10 K, χ''(QAF,ω) has a rather sharp peak near 27 meV; integrating χ'' over the 2D magnetic Brillouin zone makes the peak in energy much broader. (We have verified that the falloff at high energies also occurs in a previously studied crystal with x=0.5, Tc=50 K.) Although the amplitude at 5 meV and 10 K is near zero, considerable spectral weight is observed at energies well below the weak-coupling limit for 2Δ, where Δ is the superconducting gap. The temperature dependence of the 2D Q-integrated χ'' is well described by a simple function containing a temperature-independent energy gap of 9 meV. A study of the Q dependence of χ'' at Latin small letter h with strokeω=15 meV indicates that the signal falls off rather abruptly on moving away from QAF (compared to a simple Gaussian distribution). Measurements along two different directions in the 2D zone suggest that the width of the distribution about QAF is anisotropic. These results are discussed in the context of current theoretical models.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics