  The electromagnetic response to microwaves in the mixed state of
YBa2Cu3Oy(YBCO) was measured in order to investigate the electronic state
inside and outside the vortex core. The magnetic-field dependence of the
complex surface impedance at low temperatures was in good agreement with a
general vortex dynamics description assuming that the field-independent viscous
damping force and the linear restoring force were acting on the vortices. In
other words, both real and imaginary parts of the complex resistivity, \rho_1,
and \rho_2, were linear in B. This is explained by theories for d-wave
superconductors. Using analysis based on the Coffey-Clem description of the
complex penetration depth, we estimated that the vortex viscosity \eta at 10 K
was (4 \sim 5) \times 10^{-7} Ns/m^2. This value corresponds to \omega_0 \tau
\sim 0.3 - 0.5, where \omega_0 and \tau are the minimal gap frequency and the
quasiparticle lifetime in the vortex core, respectively. These results suggest
that the vortex core in YBCO is in the moderately clean regime. Investigation
of the moderately clean vortex core in high-temperature superconductors is
significant because physically new effects may be expected due to d-wave
characteristics and to the quantum nature of cuprate superconductors. The
behavior of Z_s as a function of B across the first order transition (FOT) of
the vortex lattice was also investigated. Unlike Bi2Sr2CaCu2Oy (BSCCO), no
distinct anomaly was observed around the FOT in YBCO. Our results suggest that
the rapid increase of X_s due to the change of superfluid density at the FOT
would be observed only in highly anisotropic two-dimensional vortex systems
like BSCCO. We discuss these results in terms of the difference of the
interlayer coupling and the energy scale between the two materials.
