  We show that the electric field-induced thermal asymmetry between the
electron and lattice systems in pure silicon substantially impacts the identity
of the dominant spin relaxation mechanism. Comparison of empirical results from
long-distance spin transport devices with detailed Monte-Carlo simulations
confirms a strong spin depolarization beyond what is expected from the standard
Elliott-Yafet theory already at low temperatures. The enhanced spin-flip
mechanism is attributed to phonon emission processes during which electrons are
scattered between conduction band valleys that reside on different crystal
axes. This leads to anomalous behavior, where (beyond a critical field)
reduction of the transit time between spin-injector and spin-detector is
accompanied by a counterintuitive reduction in spin polarization and an
apparent negative spin lifetime.
