Spin dynamics in semiconductor nanocrystals

Time-resolved Faraday rotation is used to study both transverse and longitudinal spin relaxation in chemically synthesized CdSe nanocrystals (NC's) 22-80 Angstrom in diameter. The precession of optically injected spins in a transverse magnetic field occurs at distinct frequencies whose assignment to electron and exciton spins is developed through systematic studies of the size-dependence and theoretical calculations. It is shown that the transverse spin lifetime is limited by inhomogeneous dephasing to a degree that cannot be accounted for by the NC size distribution alone. Longitudinal spin relaxation in these NC's occurs on several distinct time scales ranging from 100 ps to 10 mus and exhibits markedly different dependencies on temperature and field in comparison to transverse spin relaxation.