Absence of anisotropic universal transport in YBCO
There exists significant in-plane anisotropy between a and b axes for various properties in YBCO. However, recent thermal conductivity measurement by Chiao et al. [M. Chiao et al., to be published], which confirms previous microwave conductivity measurement by Zhang et al. [K. Zhang et al., Phys. Rev. Lett. 73 (1994) 2484], shows no obvious anisotropy in the context of universal transport. We give a possible explanation of why the anisotropy is seen in most properties, but not seen in the universal transport.