In-plane charge dynamics in underdoped YBCO

We report on the in-plane optical spectra in underdoped YBa2Cu3O7 (YBCO) using a twin-free single crystal. At low temperatures, there exist two components in the optical conductivity which are considered as a Drude term by quasiparticles in a nodal direction and the pseudogap excitation in an antinodal direction. A large residual conductivity together with pseudogap feature is consistent with the picture of inhomogeneous in-plane electronic state observed by scanning tunneling microscopy (STM). We estimate a superfluid density ns from optical spectra and discuss a universal relation between Tc and ns. The value estimated from our results is considerably smaller than that of the μSR measurement. It is considered due to the inhomogeneity of in-plane, the difference of applying field, and a property on pseudogap.