Persistent sourcing of coherent spins for multifunctional semiconductor spintronics

Recent studies of n-type semiconductors have demonstrated spin-coherent tra nsport over macroscopic distances(1), with spin-coherence times exceeding 1 00 ns(2,3); such materials are therefore potentially useful building blocks for spin-polarized electronics ('spintronics'). Spin injection into a semi conductor (a necessary step for spin electronics(4)) has proved difficult(5 ,6); the only successful approach involves classical injection of spins fro m magnetic semiconductors(7,8). Other work has shown that optical excitatio n can provide a short (<500 ps) non-equilibrium burst of coherent spin tran sfer across a GaAs/ZnSe interface, but less than 10% of the total spin cros ses into the ZnSe layer, leaving long-lived spins trapped in the GaAs layer (ref. 9). Here we report a 'persistent' spin-conduction mode in biased sem iconductor heterostructures, in which the sourcing of coherent spin transfe r lasts at least 1-2 orders of magnitude longer than in unbiased structures . We use time-resolved Kerr spectroscopy to distinguish several parallel ch annels of interlayer spin-coherent injection. The relative increase in spin -coherent injection is up to 500% in the biased structures, and up to 4,000 % when p-n junctions are used to impose a built-in bias. These experiments reveal promising opportunities for multifunctional spin electronic devices (such as spin transistors that combine memory and logic functions), in whic h the amplitude and phase of the net spin current are controlled by either electrical or magnetic fields.