NMR determination of the Edwards-Anderson order parameter in the deuterated pseudo-spin glass Rb1-x(ND4)xD2PO4: anisotropy and concentration dependence of the ND4 deuteron second moment

We measured the rotationally averaged N D 4 + deuteron second moment in deuterated mixed single crystals Rb1_x(ND4 ) xD2P04 with x = 0.22, 0.44, 0.78, and pure ND4D2P04 . We found that the linewidths of the mixed systems show a strong dependence on the crystal's orientation with respect to the external magnetic field. We interpret the temperature dependence of the N D 4 + second moment as due to the slowing down of the O-D. . .O intrabond motions. We also show that, in the temperature region above the glass transition, the second moment may be proportional either to the first or to the second powers of the Edwards-Anderson order parameter, depending on the crystal orientation. We were able to fit our experimentally obtained qEA to a theoretical value obtained by Pirc, Tadic, and Blinc from the random-field random-bond Sherrington-Kirkpatrick Ising model. A comparison between similar measurements in pure and in mixed systems verifies that the strong orientational dependence of the linewidth is a property unique to the mixed systems.