  Investigating microwave absorption in asymmetric Si quantum wells in an
external magnetic field, we discover a spin dependent component of Joule
heating at spin resonance. We explain this effect in terms of Rashba spin-orbit
coupling which results in a current induced spin precession and Zitterbewegung.
Evidence is based on the observation of a specific dependence of the electron
spin resonance line shape and its amplitude on the experimental geometry which
in some range suggests a "negative" differential power absorption.
