Acoustic attenuation study on the normal-state pseudogap in underdoped YBa₂Cu₃O₇-δ

Internal friction (IF) measurement in the kHz frequency region has been performed for underdoped YBa₂Cu₃O₇-δ. A rapid decrease of internal friction with decreasing temperature is observed at a temperature Τ_g (> Τ_c), which is considered as the opening of a pseudogap. Based on the feature of pseudogap described by V. J. Emery and BCS theory, a power law decrease of internal friction with temperature at Τ_g is derived and the index is estimated, which is consistent with the experiments. The decrease of the spin-lattice relaxation rate 1/Τ₁ for NMR near 150 K in the underdoped YBCO, similar to that of internal friction, can be explained in the same way. Furthermore, in view of the different phase coherence effect on IF and 1/Τ₁, it can be predicted that another decrease for internal friction will occur near Τ_c but not for 1/Τ₁. The conclusion is confirmed by the experiments. In addition, the reported anomaly of resistivity near Τ*(∼ 160 K) for the underdoped YBCO is discussed too.