Measurement of the ab plane anisotropy of microwave surface impedance of untwinned YBa2Cu3O6.95 single crystals.

Abstract

The temperature dependence of the surface resistance ${R}_{s}$, and the penetration depth $\ensuremath{\lambda}$ 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.