Theory Of Reduced Singlet Pairing Without The Underlying State Of Charge Stripes In The High-temperature Superconductor Yba_2cu_3o_(6.45)

摘要

Recently, a strongly enhanced xy anisotropy of magnetic excitations was observed in YBa_2Cu_3O_y (YBCO_y), with y = 6.45 and T_c=35 K [V. Hinkov et al. 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.