Spontaneous Spin Transfer

Long electron spin lifetimes have been obtained in n-doped semiconductors, and coherent spin transport has been achieved over macroscopic distances in n-GaAs. In order to achieve spin dependent integrated electronics, it is necessary for spins to cross semiconductor interfaces without loss of quantum coherence. Based on our previous results (see electrons in semiconductors) we designed an experiment to test how spins cross interfaces between semiconductors with different band gaps, g-factors and doping concentrations:

Band alignment schematic of a GaAs/ZnSe heterostructure used for spontaneous spin transfer experiments. The diagram shows molecular beam epitaxy growth, layer thicknesses, carrier densities, and conduction band offsets that enable spin-polarized carrier transfer between GaAs and ZnSe.

The coherent spin transfer can be measured using a variation of the Time Resolved Faraday Rotation technique in in reflection (Kerr) using different lasers to pump and probe. Since the band gap of GaAs is nearly half that of the ZnSe epilayer, it is possible to tune the pump to excite spin polarization only in the GaAs and probe only in the ZnSe:

Illustration of spontaneous spin transfer across a GaAs/ZnSe interface. Spins are optically prepared in GaAs and subsequently probed in ZnSe, demonstrating coherent transfer of spin polarization between semiconductor materials.

As the energy of the pump is tuned from below to above the absorption threshold of GaAs, coherent spin signal starts accumulating in the adjacent ZnSe epilayer (B=0):

Time-resolved Kerr rotation signals in ZnSe measured at 5 K following spin excitation in GaAs. The traces show spontaneous spin transfer and coherent spin polarization detected in ZnSe for different optical excitation conditions.

Unlike the case when spins are pumped and probed in the same layer, the rise on the spin signal is not instant but it takes hundreds of picoseconds. From the data shown above, a characteristic spin accumulation time can be extracted and it is found to be 200-400 ps (depending o the substrate doping).

When a magnetic field B is applied in the Voigt geometry, spins start precessing about the applied magnetic field at the Larmor frequency. We therefore probe an oscillating spin signal in the ZnSe epilayer where the frequency is proportional to the nominal gZnSe and to B:

Kerr rotation measurements in ZnSe at room temperature under magnetic fields of 0 T, 0.1 T, and 1 T. Oscillatory spin precession signals demonstrate coherent spin transfer and long-lived spin coherence following spontaneous spin injection from GaAs.

From the previous figure we see that as B is increased, the maximum spin signal decreases. Also, as illustrated below a phase shift appears.

Time-resolved spin dynamics in ZnSe showing measured data and fitted oscillatory spin coherence after spontaneous spin transfer. The plot highlights coherent spin precession and phase evolution over nanosecond timescales.

By taking into account the difference in g-factors between GaAs and ZnSe and the distribution of spin arrival times.

Schematic illustration of spontaneous spin transfer and coherent spin precession between GaAs and ZnSe. Kerr rotation signals evolve differently depending on transfer timing t1 and t2 due to differing electron g-factors in the two semiconductor materials.

We see that two spins excited at the same time in the GaAs but crossing the interface at different times will end-up "dephased" with respect to each other. This results in a (field dependent) decrease of the total spin amplitude and a non-zero phase. Using the fits to the B=0 data and taking into account the added dephasing at the interface, predictions can be made on how the spin signal evolves as the magnetic field is increased:

Comparison of theoretical and experimental Kerr rotation signals during spontaneous spin transfer from GaAs to ZnSe. Multiple colored traces show coherent spin precession dynamics and phase evolution, demonstrating agreement between measured data and spin transfer theory.

To learn more about these results, look in:

"Coherent Transfer of Spin Through a Semicondutor Heterointerface", I. Malajovich, J.M. Kikkawa, D.D. Awschalom, J.J. Berry and N. Samarth, Phys. Rev. Lett., vol. 84, p. 1015 (2000).