Temperature dependence of the Cu(2) NQR line width in YBa2Cu3O7−y

Systematic measurements of the 63Cu(2) NQR line width were performed in underdoped YBa2Cu3O7−y samples over the temperature range 4.2 K<T<300 K. It was shown that the copper NQR line width monotonically increases with decreasing 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<Tc. Quantitative estimates of the line broadening at T<Tc agree with the measurement results.