Dynamic nuclear Overhauser shifts in Larmor beats from a quantum well

The significance of nuclear spin polarisation in time-resolved optical studies of III-V semiconductors is addressed. Electron Larmor beats in pump-probe reflectivity from a GaAs/AlGaAs quantum well show Overhauser shift of 0.7 T due to accumulated nuclear polarisation 〈I〉/I=0.065. This leads to precision values of electron g-factor, elucidates nuclear spin pumping and diffusion mechanisms in quantum wells and informs discussion of implications for spin-electronics and transport. The proposed application of electron spin transport and coherence in semiconductors to spin electronics and perhaps quantum computing [1, 2] will involve a high degree of conduction electron spin polarisation and extended electron spin lifetimes [3]. Recently free Larmor precession continuing for ≥ 100 ns [4] and spin transport over microns [1, 5] were observed in time-resolved studies of ntype GaAs and quantum wells using circularly polarised optical pulses. On the other hand, since all naturally occurring isotopes in III-V semiconductors have non-zero nuclear spin [6], hyperfine interaction AI.S, can be expected to result in an equivalent level of polarisation of the nuclear spins [7] with, conversely, profound effects on the behaviour of the electrons. These include built-in effective magnetic fields (Overhauser effects) and nonlinear or bistable response to applied external fields [7]. Such effects have long been known in cw optical phenomena in bulk semiconductors [7] and in quantum wells [8, 9] and in quantum magneto-transport [11], but consequences for time-resolved optical measurements and spin applications have been neglected. In this letter we report measurements of nuclear Overhauser shift [7] in ps time-resolved Larmor beats [12, 13] in a GaAs/AlGaAs quantum well, revealing the dynamics and long timescales of nuclear spin polarisation by optical pumping and the change of electron spin precession frequency induced by a polarised nuclear spin system. In this experiment the effective internal field approaches 1 T although the nuclear polarisation is only 6.5%, emphasising the significance of nuclear phenomena. Proper account of the Overhauser shift gives excellent agreement between measured conduction electron Larmor frequencies and anisotropic k.p theory [14], while the dynamics provides new insights to nuclear spin diffusion