An Extremum Principle for a Neutron Diffraction Experiment

An extremum principle was postulated by Horne, Finkelstein, Shull, Zeilinger, and Bernstein in order to derive the physically allowable parameters for sinusoidal standing waves governing a neutron in a crystal which is immersed in a strong external magnetic field: `̀ the expectation value of the total potential á V ñ is an extremum .’’ We show that this extremum principle can be obtained from the variational principle used by SchroÈ dinger to derive his nonrelativistic wave equation. We rederive the solutions found by the above-mentioned authors as well as some additional solutions .