Abstract
In order to gain insight into their degree of validity, certain commonly used approximate methods for finding dynamic susceptibilities are compared with the exact Kubo susceptibility for a linearly interacting spin-phonon system. Approximate methods investigated include: truncation or decoupling of equations of motion for the commutator and anticommutator Green's functions, and a memory-function method suggested by G\otze and W\olfle. It is shown that approximate methods, while adequate for weak dipole-phonon coupling, can lead to conspicuous deviations from the exact susceptibility in certain frequency ranges. Results are also sensitive to the shape of the phonon spectrum. Ergodic and nonergodic cases are discussed.
Published Version
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