Research into semiconductor quantum dots (QDs) is driven by their promise in exploring carrier behavior in the mesoscopic regime between bulk and molecular systems, and a variety of technological applications which exploit their size-tunable optical properties. Recent interest in manipulating semiconductor spins for applications ranging from spin-polarized magneto-electronics to quantum computation is based on the ability to control and maintain spin coherence over practical length and time scales. To this end, QDs have been suggested as potential elements for such devices due to control over the structural and electronic environment of localized carriers.
Time-resolved Faraday rotation is used to probe spin dynamics in chemically synthesized CdSe quantum dots (QDs) at temperatures from 6 to 282 Kelvin. The pump pulse used in the experiment excites spins into an ensemble of ~ 1010 QDs. The spin precession in a transverse magnetic field indicates that the measured relaxation lifetime of the spin polarization is dominated by inhomogeneous dephasing, ranging from ~3 nanoseconds at zero field to less than 100 picoseconds at 4 Tesla.
Single crystal CdSe QDs ranging from 22 - 80 Angstroms in diameter are synthesized by a solution-phase pyrolytic reaction of organometallic precursors. By controlling the reaction temperature and concentrations of reagents, size distributions from 5-10% can be achieved.
Atomic resolution Transmission Electron Micrographs indicate that the QDs are highly crystalline (with the wurtzite lattice structure), and nearly spherical in shape. Bright spots in the image correspond to spaces between individual atoms in the QD.
The figure below compares the spin lifetimes at temperatures of 6K (black) and 282K (red). The spin lifetimes drop from ~ 3 ns at 6K to 0.7 ns at 282K, although little change in the oscillation frequency is observed between these two temperature extremes. The robustness of spin coherence at room temperature is not predicted by conventional theories of spin-scattering in solid state systems.
The figure below shows the evolution of the Faraday Rotation as the magnetic field is increased from H = 0-3 T for 80 Å QDs. The oscillation frequency increases with H as expected from classical Larmor precession. Surprisingly, the spin lifetime also shows a strong field dependence. Spin precession becomes unobservable for t > 200ps at H = 3T, but continues for t > 1.7 ns at low fields. This field dependence suggests that the decay of the measured spin coherence is dominated by inhomogeneous dephasing within the ensemble of QDs.
The inset of the graph illustrates how inhomogeneous dephasing limits the measured spin lifetimes. Spins are initially aligned along the z-axis at t=0. Each spin is characterized by a g-factor, which determines the precession frequency. If these g-factors vary between QDs, then at a given time t, QDs with higher g-factors will have precessed an angle which is slightly greater than QDs with lower g-factors. The spins therefore, "walk" out of phase at a rate which is proportional to the field, thus producing the field-dependent spin lifetimes.