In 1879 Edwin Hall observed the effect that bears his name. He found that a voltage is created perpendicular to an electric current as it flows through a conductor in a magnetic field. That’s because the magnetic field deflects the moving charges to the side of the conductor, resulting in a voltage due to the concentration of charge. In addition to charge, electrons have spin, and this plays a role in the Hall effect. For example, in the anomalous Hall effect in ferromagnets, the intrinsic magnetic field scatters spin-up and spin-down electrons along a direction perpendicular to the electric field. Thus in addition to the Hall voltage, the spin polarization causes opposite polarity on the two sides of the conductor. This is the extrinsic anomalous Hall effect (AHE), which does not require an external magnetic field. The AHE is used in devices that are sensitive to the spin polarization of electrons. Spintronic sensors based on ferromagnetic metals have reinvented the hard-disk industry in the past decade.

Practical spintronics in semiconductors has seemed to depend on the injection of a spin current from ferromagnetic metal, or the invention of semiconductor that is ferromagnetic at room temperature, a major focus of current semiconductor research. But Hot Paper #6 now suggests a new, third direction for semiconductor research. A small team based at Texas A & M University and the University of Texas at Austin, and led by Jairo Sinova, made a theoretical study of the mobility of a two-dimensional electron system with substantial spin-orbit coupling.

Their startling finding is that in paramagnetic spin orbit coupled systems there is an intrinsic Hall effect, dependent solely on the electron structure. The effect is intrinsic because it does not rely on internal scattering by impurities. Their prediction is that a sizeable intrinsic spin Hall effect will occur in any paramagnetic material with strong spin-orbit coupling.

The proposal immediately generated a strong debate, motivated by the potential as a spin injection tool. Despite needing to invent new devices, experimenters quickly confirmed the new effect by using optical techniques to detect the spin accumulation at the edges of examples. David Awschalom’s group at the Center for Spintronics (University of California, Santa Barbara) is responsible for Hot Paper #9, which describes optical detection in thin films of the semiconductors GaAs and InGaAs.

The phenomenon is quite remarkable. In a recent interview for Thomson Scientific’s online in-cites, the editorial component of Essential Science Indicators, Tomas Jungwirth (University of Nottingham, U.K.), a member of Sinova’s team said: "Achieving comparable levels of spin polarization in non-magnetic semiconductors by external magnetic fields requires laboratory apparatus a million times larger than those of a spin Hall effect device." The observed spin Hall effect provides new opportunities for manipulating electron spins in non-magnetic semiconductors without the application of magnetic fields. A key remaining challenge is the detection of the effect by electrical (rather than optical) means, because that is a crucial step on the path to actual useful devices.