Fermi-liquid-based theory for the in-plane magnetic anisotropy in untwinned high-Tc superconductors.

Using a generalized RPA-type theory we calculate the in-plane anisotropy of the magnetic excitations in hole-doped high-Tc superconductors. Extending our earlier Fermi-liquid-based studies on the resonance peak by inclusion of orthorhombicity we still find two-dimensional spin excitations, however, being strongly anisotropic. This reflects the underlying anisotropy of the hopping matrix elements and of the resultant superconducting gap function. We compare our calculations with new experimental data on fully untwinned YBa2Cu3O6.85 and find good agreement. Our results are in contrast to earlier interpretations on the in-plane anisotropy in terms of stripes [H. Mook, Nature (London) 404, 729 (2000)], but reveal a conventional solution to this important problem.