Electronic state of vortices in YBa2Cu3Oy investigated by complex surface impedance measurements - art. no. 184517

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 surf ace impedance at low temperatures was in good agreement with a general vort ex 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 sup erconductors. Using analysis based on the Coffey-Clem description of the co mplex penetration depth, we estimated that the vortex viscosity eta at 10 K was (4-5)x10(-7) Ns/m(2). This value corresponds to omega (o)tau similar t o0.3-0.5, where omega (o) and tau are the minimal gap frequency and the qua siparticle lifetime in the vortex core, respectively. These results suggest that the vortex core in YBCO is not in the deeply superclean regime but 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 quantu m 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 in vestigated. 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 i n 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.