Awschalom Group

Biased Spin Transfer

When a net conduction band spin polarization is prepared by optical orientation in an n-GaAs substrate, about 5% of those polarized spins spontaneously transfer to an adjacent n-ZnSe epilayer.

Most of the spin polarized electrons, however, remain trapped in the substrate, which due to its long spin lifetime acts as a reservoir for spin coherence. When an electrical bias is applied to force the polarized electrons into the ZnSe layer, the reservoir acts as a source of spin coherence. This is illustrated in the figure below, where a schematic of the conduction band and spin transfer with and without bias is represented.

A "two color" time-resolved pump-probe technique can be used to prepare spins in GaAs and measure the spin transfer into an adjacent ZnSe epilayer. In this manner, changes in the spin transfer with electrical bias can be measured. From the Resonant Spin Amplification data taken by sweeping the external magnetic field at a fixed delay, it is possible to learn the characteristics of electron spins in a given material. Spin lifetimes and g-factors can be extracted from the resonance widths and peak separations, respectively. In the figure below the top spectrum (red) corresponds to resonance data taken in GaAs, while the bottom spectrum (blue) corresponds to data taken for spins in ZnSe.

The magnetic field response of the transferred spins (middle) evolves from ZnSe-like to GaAs-like with an electrical bias: as the bias increases, the resonance spectrum transforms from that characteristic ZnSe to one whose spacing more closely resembles that of GaAs. This measurements show that the resulting spin polarization in the ZnSe epilayer can be controlled by either electric or magnetic fields.

When the time resolved Kerr rotation is measured, we observe that the bias: (1) increases the intensity of the signal, thus total number of spins that cross the interface is also increased, (2) extends the duration of the spin polarization in the epilayer to the duration of the reservoir, and (3) results in the presence of a second precession frequency that is within 2% of the nominal GaAs precession value.

The biased and unbiased time progression of the spin transfer across the interface can be accurately modeled. From fits to this data, it is possible to separate three parallel channels of spin transfer: two spontaneous modes (SA and SB) and one persistent mode, SC, present only under a biased condition.

From fits to the data it is also possible to measure the total (time integrated) change in spin transfer as a function of bias:

The spin transfer increases with a positive bias (as much as 500%) and decreases with a negative bias, following the I-V characteristics of the structure. This demonstrate the spin-charge coupling during the transfer across the interface.

Finally, a built-in internal bias across a p-n junction can also be used to force spins to cross the interface. When this is done, the spin transfer is increased by a factor of 40. For internal biases, however, we do not obtain a persistent current because the p-GaAs substrate has very short spin lifetimes and does not act as a reservoir.

To learn more about these results, look in:

"Persistent sourcing of coherent spins for multifunctional semiconductor spintronics", I. Malajovich, J.J. Berry, N. Samarth and D.D. Awschalom, Nature, vol. 411, p. 770 (2000).