Spin dynamics in semiconductor nanocrystals

Time-resolved Faraday rotation is used to study both transverse and longitudinal spin relaxation in chemically-synthesized CdSe nanocrystals (NCs) 22-80 Angstroms 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 NCs occurs on several distinct timescales ranging from 100 ps-10 microseconds and exhibits markedly different dependencies on temperature and field in comparison to transverse spin relaxation.