  Recently, a strongly enhanced xy anisotropy of magnetic excitations was
observed in YBa_2Cu_3O_y (YBCO_y) with y=6.45 and Tc=35 K [Science 319, 597
(2008)]. Unlike the observation in YBCO_6.6 and YBCO_6.85, the anisotropy grows
to be pronounced at lower temperature and at lower energy, and is not
suppressed by the onset of superconductivity. We propose that the effect of
singlet pairing is substantially reduced in YBCO_6.45. This reduction
concomitantly enhances an order competing with singlet pairing, a strong
tendency of the so-called d-wave Pomeranchuk instability, leading to the
magnetic excitations observed experimentally.
