Phase coherence effect on the normal state internal friction in underdoped YBa₂Cu₃O_7-δ

Internal friction (IF) measurement in the kilohertz frequency region has been performed for underdoped YBa₂Cu₃O_7-δ. A rapid decrease of IF with decreasing temperature is observed at a temperature T_g (> T_c), which is considered as the opening of pseudogap. Based on the feature of pseudogap described by V.J. Emery and the BCS theory, a power-law decrease of IF with temperature at T_g is derived and the estimated index is found to be consistent with the experimental results. The decrease of the spin-lattice relaxation rate 1/T₁ for NMR near 150 K in the underdoped YBCO, similar to that of IF, can be explained in the same way. Furthermore, in view of the different phase coherence effects on IF and 1/T₁, it can be predicted that another decrease in IF will occur near T_c but not for 1/T₁. This conclusion is confirmed by the experiments.