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The question of paramagnetic relaxation at low temperatures and the "phonon bottleneck" is discussed in terms of the analogy with Holstein's theory of trapping of resonance radiation in gases. It is observed that the diffusion of phonon energy to the wings of the line is the limiting process, and a new mechanism for this in dilute cases is proposed. Since this mechanism involves the spin-spin interactions as well as phonons, it provides a reason for the observed increase of relaxation rate with concentration.
P. W. Anderson (Fri,) studied this question.