Seebeck effect in the mixed state of the 60-K phase of single-crystal ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{y}}$

The resistivity and the Seebeck effect in the mixed state were investigated for the 60-K phase ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{\mathrm{y}}$ single crystal. The measured Seebeck coefficient clearly shows an additional contribution at low temperatures, which may be evidence that there are two components (quasiparticles and vortices) carrying entropy in the mixed state. The ratio of the Seebeck coefficient to resistivity shows a remarkable field dependence, from which the lifetime enhancement of the quasiparticle below ${\mathrm{T}}_{\mathrm{c}}$ was derived. The enhancement is found to be less rapid in the 60-K phase than in the 90-K phase, which might come from the opening of the pseudogap above ${\mathrm{T}}_{\mathrm{c}}$.