Symmetry sustains a spin helix in a quantum well

Spin–orbit coupling is a double-edged sword to physicists who want to exploit the spin degree of freedom in novel electronic devices. On the one hand, the coupling treats up spins differently from down spins, a necessary feature of any spintronic device. But on the other hand, the coupling nullifies the conservation of spin that prevails in free space. Although the net spin polarization averages to zero in a crystal, it fluctuates randomly and locally. Joseph Orenstein of the University of California, Berkeley, and Lawrence Berkeley National Laboratory and his collaborators have demonstrated a way to restore the conservation of spin in a semiconductor quantum well and extend the lifetime of a coherent spin structure. Two tricks are involved. The first is to build a quantum well in which two types of spin–orbit coupling, Rashba and Dresselhaus, are equal. The second trick is to create a particular kind of coherent signal, a spin helix, and send it through the quantum well with a particular wave vector. Mathematically, the spin helix in its specially tuned well shares the same SU(2) symmetry as an isolated spin. Empirically, the spin helix retains its coherence for 1 ns before it diffuses away. That lifetime may seem fleeting, and it's hardly infinite, but it's an order of magnitude longer than that of a coherent spin signal launched without symmetry's sustaining power. To learn more about the spin helix and two other recently observed spin textures, look for the news story on page 12 of the April issue. (J. D. Koralek et al., Nature, in press.) — Charles Day