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Dangling-bond electron-paramagnetic-resonance spectra and relaxation rates have been measured in the (0.3 -4)-K temperature range on samples of amorphous silicon produced by sputtering, vacuum evaporation, and ion implantation of si1icon, argon, neon, oxygen, and nitrogen into crystalline silicon. Intensity measurements of the dangling-bond resonance associated with silicon made amorphous by Si implantation suggest that the Curie temperature is essentially zero (0& 8 &0.03 K) for temperatures down to 0.4 K. The relaxation rates follow unusual temperature dependencies that cannot be explained on the basis of conventional spin-phonon interactions. Instead, the relaxation rates obey a simple T" power law in temperature where n falls within two ranges: 2.09 -2.36 and 3.26 -3.47. A comparison of rates at microwave frequencies of 9.3 and 16.5 6Hz indicates no magnetic field dependence. A relaxation model involving spin coupling to a distribution of two-level states is consistent with the observed T" dependence.

Electron spin relaxation data from five ferric proteins are analyzed in terms of the fractal model of protein structures. Details of this model are presented. The results lead to a characterization of protein structures by a single parameter, the fractal dimension, d. This structural parameter is shown to determine the temperature dependence of the Raman electron spin relaxation rate, which varies as T3 + 2d. Computations of d are made using x-ray data for 17 proteins. The results range from d = 1.76 for lysozyme to d = 1.34 for ferredoxin. These values are compared with values of d obtained from the present electron spin relaxation data on five ferric proteins. Typical results are d = 1.34 +/- 0.06 from relaxation data and 1.34 +/- 0.05 from x-ray data for ferredoxin; d = 1.67 +/- 0.03 from relaxation data and 1.66 +/- 0.05 from x-ray data for ferricytochrome c. The data thus support the theoretical model. Applications of this spin resonance technique to the study of changes in protein conformation are discussed.