Narrowband emission feature of the selected samples of the single pulses. For each single pulse, the observed PPAs are included in the top panel, and the profile of the total intensity (Stokes I in the black), the linear polarization intensity ($L=\sqrt{Q^2+U^2}$ in the red), and the circular polarization intensity (Stokes V in the blue) are plotted in the middle panel. To investigate the emission feature of the pulse window in a single pulse further, we plot the dynamic spectra of the flux density as a function of frequency and pulse longitude in the bottom panel. The color bar represents the flux density and is measured in arbitrary units. The horizontal white lines denote the RFI frequency channels and are set to zero. The white areas indicate the noise floor. We chose the errors in the observed PPAs lower than 5° in each single pulse to limit the random error due to the system noise.