Doping dependence of phonon and quasiparticle heat transport of pure and Dy-doped Bi_2Sr_2CaCu_2O_{8+\delta} single crystals

Abstract

The temperature and magnetic-field (H) dependences of thermal conductivity (\kappa) of Bi_2Sr_2CaCu_2O_{8+\delta} (Bi2212) are systematically measured for a broad doping range by using both pure Bi2212 single crystals with tuned oxygen contents and Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+\delta} (Dy-Bi2212) single crystals with different Dy contents x. In the underdoped samples, the quasiparticle (QP) peak below T_c is strongly suppressed, indicating strong QP scattering by impurities or oxygen defects, whereas the phonon conductivity is enhanced in moderately Dy-doped samples and a phonon peak at 10 K is observed for the first time in Bi2212 system, which means Dy^{3+} ions not only introduce the impurities or point defects but also stabilize the crystal lattice. The subkelvin data show that the QP heat conductivity gradually decreases upon lowering the hole doping level. The magnetic-field dependence of \kappa at temperature above 5 K is mainly due to the QP scattering off vortices. While the underdoped pure Bi2212 show very weak field dependence of \kappa, the Dy-doped samples present an additional "dip"-like term of \kappa(H) at low field, which is discussed to be related to the phonon scattering by free spins of Dy^{3+} ions. For non-superconducting Dy-Bi2212 samples with x \simeq 0.50, an interesting "plateau" feature shows up in the low-T \kappa(H) isotherms with characteristic field at 1 -- 2 T, for which we discuss the possible revlevance of magnon excitations.