Thermal conductivity in the ab plane of untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$

We present measurements of thermal conductivity (κ) in the ab plane of untwinned ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (δ≊0.10) for T≤200 K. κ has been measured with heat flow along both the a and b axes and for two oxygen configurations of the same crystal. The conductivity anisotropy has its maximum value at the conductivity peak (T≊40 K), where we find ${\mathrm{\kappa }}_{\mathit{a}}$/${\mathrm{\kappa }}_{\mathit{b}}$≊1.15 before and after an oxygen anneal. Very little ab anisotropy in the magnitude of κ is observed in the normal state, though near ${\mathit{T}}_{\mathit{c}}$ the temperature dependences are noticeably different for the two directions: dκ/dT>0 along the b axis and dκ/dT<0 along the a axis. Near ${\mathit{T}}_{\mathit{c}}$ (T≤110 K) κ is enhanced slightly for both transport directions, features we attribute to the effect of superconducting fluctuations.