The decay of transient nutations has been experimentally investigated in S=1/2 spin systems at microwave frequency: E’ centers in silica and [${\mathrm{AlO}}_4$${]}^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 ${\mathit{T}}_1$-${\mathit{T}}_2$ model (Bloch model). However, the agreement is only qualitative. In fact the measured decay rate Γ is faster than expected and depends on the driving-field amplitude: it tends to the Bloch value ${\mathrm{\Gamma }}_{\mathit{B}}$=1/2${\mathit{T}}_2$ in the low-power limit and becomes faster and faster on increasing the input power. In all the cases examined the power dependence of the decay rate is fit well by a simple linear dependence of Γ on the induced Rabi frequency χ. The observed power dependence of Γ cannot be ascribed to the inhomogeneity of χ over the sample volume nor to the radiation damping, since both effects are negligible in our experiments. Other mechanisms, which can, in principle, yield a χ dependence of Γ, e.g., the direct interaction of the driving field with structural two-level systems or the spreading of the spin-field coupling constant, are not compatible with the experimental conditions. So, our results suggest that the homogeneous dephasing time of each isochromat contains an intrinsic term and a χ-dependent one. The latter may originate in a field-induced enhancement of the hyperfine or dipolar interaction; however, neither of these mechanisms completely fits the experimental features. The relationship with the decay properties of other coherent regimes is also discussed.

• Received 19 April 1993

DOI:https://doi.org/10.1103/PhysRevB.48.7077