Systematic measurements of the $^{ 63}$Cu(2) NQR line width were performed in underdoped YBa$_2$Cu$_3$O$_{ 7-y}$ samples over the temperature range 4.2 K $<T<300$ K. It was shown that the copper NQR line width monotonically increases upon lowering temperature in the below-critical region, resembling temperature behavior of the superconducting gap. The observed dependence is explained by the fact that the energy of a condensate of sliding charge-current states of the charge-density-wave type depends on the phase of order parameter. Calculations show that this dependence appears only at $T<T_c$. Quantitative estimates of the line broadening at $T<T_c$ agree with the measurement results.
Categories: cond-mat.supr-con cond-mat.str-el