Microsecond spin-flip times for localized donors in GaAs

One of the central tasks in developing spin-based quantum computing is the development of materials which have long spin lifetimes. Observations of long electron spin lifetimes (hundreds of ns) in n-type GaAs dating back to Kikkawa and Awschalom in 1998 [1] have stimulated much excitement in the field, and many groups have similarly made observations of the inhomogeneous T$_{2}^{\ast }$ lifetime of electrons in GaAs in the ns regime. The homogeneous dephasing time, T$_{2}$, has not yet been measured, although it is expected to be much longer. Here, a series of measurements of lifetimes [2] are described for donors in lightly n-type GaAs doped at 3E14, 1E15, and 3E15 cm$^{-3}$ that mimic spin memory in doped quantum dots. Hanle effect measurements yield T$_{2}^{\ast }$ at close to 0T, magnetic resonance measurements provide T$_{2}$* at 40 mT, and Kerr rotation measurements provide T$_{2}^{\ast }$at higher fields. The measured T$_{2}^{\ast }$ values for the 3E14 sample are consistent with full electron localization. A new pump-probe technique using electronic delays between pulses has been used to measure spin lifetimes into the $\mu $s range. This time-resolved technique provides measurements of the spin-flip time (often labeled T$_{S}$, which is essentially the same as T$_{1})$ for two of the samples at a range of fields and temperatures. T$_{S}$ is greater than 1 $\mu $s for B$>$0.6T at 1.5K and for B$>$2.5T at 6 K. Since T$_{2}$ is limited by the spin-flip time, these measurements show the range of temperature and magnetic field where very long T$_{2}$'s are possible. [1] Phys Rev Lett \textbf{80}, 4313 (1998). [2] J.S. Colton et al., Phys Stat Sol B \textbf{233}, 445 (2002); Phys Rev B \textbf{67}, 165315 (2003); Phys Rev B \textbf{69}, 121307(R) (2004); Solid State Comm \textbf{132}, 613 (2004).