Spin Waves in Paramagnetic Fermi Gases

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Abstract
The nature of spin-type zero sound, "paramagnetic spin waves" (PSW's), in free-electron gases is studied by means of the time-dependent Hartree-Fock approximation. The importance of attractive interactions is emphasized. A short-wavelength cutoff in the spectrum at microwave or low infrared frequencies is found. The effect of the application of magnetic fields is studied. The frequencies are shifted by the Larmor precession. Some branches of the spectrum are effectively eliminated; others are strengthened and split. Spin wave fronts travel in helices about magnetic field lines. PSW's are damped by electron collisions with crystal imperfections. The relaxation time is identical with the relaxation time as determined by the dc conductivity. Electron-electron collision damping is also studied and found to be negligible by comparison. The observation of Landau-damped modes is considered and conditions where this is possible are indicated. A boundary condition is found: A surface must be at an antinode of the spin density. A microwave, "thin"-film resonance experiment is suggested. The strength of the resonance lines is considered. An expression for power dissipation at PSW resonance is derived.

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