STRUCTURE OF THE Cu-NQR SPECTRUM IN Hg-1223 BETWEEN 4.2 K AND 145 K

Abstract

An extremely low noise RF preamplifier cooled with liquid helium has been developed 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 the spectra are centered at 15 MHz with a spectral width of approximately 2 MHz, similar to the Cu-NQR spectra of the other members of the HgBa 2 Ca n -1 Cu n O 2 n +2+δ series. The Cu-NQR spectra in this temperature range exhibit some features indicating an unresolved splitting but they can still be fitted quite well by a set of two pairs of 63/65 Cu lines, as expected for the 1223 structure. The spectra at 40 K and above clearly show a structure that can not be fitted suitable by a set of two pairs of 63/65 Cu lines. However, a fit with a set of four pairs of 63/65 Cu lines is in good agreement with 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 for other high-T c compounds, instead of a simple disorder in the crystal lattice.