Absence of anisotropic universal transport in YBCO

There exists significant in-plane anisotropy between a and b axes for vario us properties in YBCO. However, recent thermal conductivity measurement by Chiao et al. [M. Chiao et al., to be published], which confirms previous mi crowave conductivity measurement by Zhang et al. [K. Zhang et al., Phys. Re v. Lett. 73 (1994) 2484], shows no obvious anisotropy in the context of uni versal transport, We give a possible explanation of why the anisotropy is s een in most properties, but not seen in the universal transport. (C) 1999 E lsevier Science B.V. All rights reserved.