NON-BLOCH DECAY OF TRANSIENT NUTATIONS IN S = 1 2 SYSTEMS - AN EXPERIMENTAL INVESTIGATION/

The decay of transient nutations has been experimentally investigated in S = 1/2 spin systems at microwave frequency: E' centers in silica a nd [AlO4]0 centers in quartz have been studied. We have found that the damping is well described by a single exponential decay function, as expected from a T1-T2 model (Bloch model). However, the agreement is o nly qualitative. In fact the measured decay rate GAMMA is faster than expected and depends on the driving-field amplitude: it tends to the B loch value GAMMA(B) = 1/2T2 in the low-power limit and becomes faster and faster on increasing the input power. In all the cases examined th e power dependence of the decay rate is fit well by a simple linear de pendence of GAMMA on the induced Rabi frequency chi. The observed powe r dependence of GAMMA cannot be ascribed to the inhomogeneity of chi o ver the sample volume nor to the radiation damping, since both effects are negligible in our experiments. Other mechanisms, which can, in pr inciple, yield a chi dependence of GAMMA, e.g., the direct interaction of the driving field with structural two-level systems or the spreadi ng of the spin-field coupling constant, are not compatible with the ex perimental conditions. So, our results suggest that the homogeneous de -phasing time of each isochromat contains an intrinsic term and a chi- dependent one. The latter may originate in a field-induced enhancement of the hyperfine or dipolar interaction; however, neither of these me chanisms completely fits the experimental features. The relationship w ith the decay properties of other coherent regimes is also discussed.