Measurement of the $\mathrm{ab}$ Plane Anisotropy of Microwave Surface Impedance of Untwinned ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.95}$ Single Crystals

The temperature dependence of the surface resistance $R_s$, and the penetration depth $\lambda$ has been measured in both $a$ and $b$ directions in ${\mathrm{Yba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.95}$. The overall temperature dependences are similar and show a linear behavior at low $T$. This argues against the chains playing a direct role in the unconventional behavior of the surface impedance. The residual $R_s$ of these twin-free crystals is very small, implying a residual conductivity of the order of the minimum value predicted by Lee Phys. Rev. Lett. 71 1837 1993 for a $d$-wave superconductor.