Evidence for the formation of magnetic moments in the cuprate superconductor Hg$_{0.8}$Cu$_{0.2}$Ba$_2$Ca$_2$Cu$_3$O$_{8+\delta}$ below $T_c$ seen by NQR

摘要

We report pure zero field nuclear magnetic resonance (NQR) measurements onthe optimally doped three layer high-$ T_{c} $-compounds HgBaCaCuO andHgBaCaCuO(F) with $T_c$ 134 K. Above $ T_{c}$ two Cu NQR line pairs areobserved in the spectra corresponding to the two inequivalent Cu lattice sites.Below $ T_{c}$ the Cu NQR spectra show additional lines leading to the extremebroadened Cu NQR spectra at 4.2 K well known for the HgBaCaCuO compounds. Thespin-lattice relaxation curves follow a triple exponential function withcoefficients depend onto the saturation time (number of saturation pulses),whereas the spin-spin relaxation curve is described by a single exponentialfunction. From the spin-lattice relaxation we deduced a complete removal of theKramers degeneracy of the Cu quadrupole indicating that the additional linesare due to a Zeemann splitting of the $^{63/65}$Cu lines due to the spontaneousformation of magnetic moments within the CuO layers. Below 140 K, the spectraare well fitted by a number of 6 $^{63/65}$Cu line pairs. From the number ofthe Cu lines, the position of the lines relative to each other and the completeremoval of the Kramers degeneracy we deduced an orientation of the magneticmoments parallel to the symmetry axis of the electric field gradient tensorwith magnitudes of the order of 1000 G. We also discuss the possiblemicroscopic origin of the observed internal magnetic fields.