Structure of the Cu-NQR spectrum in Hg-1223 between 4.2 K and 145 K

An extremely low noise RF preamplifier cooled with liquid helium has been d eveloped and is used to perform Cu-NQR measurements for Hg-1223 (T-c= 134 K )at temperatures from 4.2 K to 145 K. At temperatures from 4.2 K to 20 K th e spectra are centered at 15 MHz with a spectral width of approximately 2 M Hz, similar to the Cu-NQR spectra of the other members of the HgBa2Ca-1CuaO 2a+2+delta series. The Cu-NQR spectra in this temperature range exhibit som e features indicating an unresolved splitting but they can still be fitted quite well by a set of two pairs of Cu-63/65 lines, as expected for the 122 3 structure. The spectra at 40 K and above clearly show a structure that ca n not be fitted suitable by a set of two pairs of (CU)-C-63/65 lines. Howev er, a fit with a set of four pairs of Cu-63/65 lines is in good agreement w ith the experimental results. Furthermore, the spectra at 80 K, 85 K and 90 K show a strong variation with respect to each other which might indicate a structural change like the formation of a superstructure, as discussed fo r other high-T-c compounds, instead of a simple disorder in the crystal lat tice.