Difficulty in the Fermi-Liquid-Based Theory for the In-Plane Magnetic Anisotropy in Untwinned High-T_c Superconductor

Recently, Eremin and Manske [1] presented a oneband Fermi-liquid theory for the in-plane magnetic anisotropy in untwinned high-Tc superconductor YBa2Cu3O6:85 (YBCO). They claimed that they found good agreement with inelastic neutron scattering (INS) spectra. In this Comment, we point out that their conclusion on this important problem may be questionable due to an error in logic about the orthorhombicity delta_0 characterizing the lattice structure of YBCO. In Ref. [1], a single band at delta_0>0 is proved to be in accordance with the angle resolved photoemission spectroscopy (ARPES) on untwinned YBCO. But in their Erratum in PRL[3], they admit that delta_0= -0.03 was used to fit the INS data. Hence publications [1,3] contain errors that we believe invalidate their approach.Comment: This is a Comment on the paper of I. Eremin, and D. Manske, Phys. Rev. Lett. 94, 067006(2005

Analysis on the Invariant Properties of Constitutive Equations of Hydrodynamics in the Transformation between Different Reference Systems

The velocities of the same fluid particle observed in two different reference systems are two different quantities and they are not equal when the two reference systems have translational and rotational movements relative to each other. Thus, the velocity is variant. But, we prove that the divergences of the two different velocities are always equal, which implies that the divergence of velocity is invariant. Additionally, the strain rate tensor and the gradient of temperature are invariant but, the vorticity and gradient of velocity are variant. Only the invariant quantities are employed to construct the constitutive equations used to calculate the stress tensor and heat flux density, which are objective quantities and thus independent of the reference system. Consequently, the forms of constitutive equations keep unchanged when the corresponding governing equations are transformed between different reference systems. Additionally, we prove that the stress is a second-order tensor since its components in different reference systems satisfy the transformation relationship.Comment: Analyses with rigorous mathematical proofs on several classical subjects of hydrodynamic