Magnetic Resonance and the Principle of Minimum Entropy Production

Abstract

It is shown by statistical mechanical methods that in the steady state attained by a system of magnetic moments in a resonance experiment the rate of entropy production is a minimum. It is assumed that the external field has a circularly polarized component perpendicular to a constant component, and that the contact between the spin system and the heat bath is provided only by strong collisions. Because of the coherent relations among the phases of the probability amplitudes, the average off-diagonal elements of the density matrix do not vanish in this case, and it is shown to be necessary to extend the usual statistical mechanical definition of entropy to include terms involving these elements.