In-Plane Anisotropy of the Penetration Depth in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and Y${\mathrm{Ba}}_2$${\mathrm{Cu}}_4$${\mathrm{O}}_8$ Superconductors

The superfluid response of single-crystal $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-x}$ and Y${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_8$ shows a strong anisotropy in the $a-b$ plane as revealed by far infrared spectroscopy. The value of the London penetration depth in both systems is considerably smaller in the direction along the Cu-O chains, suggesting that a substantial portion of the superconducting condensate is on the chains.