in 1971 mikhail dyakonov and vladimir perel of the ioffe institute in leningrad proposed a new transport phenomenon (1) if one applies an electric field to the ends of a semiconducting strip they reasoned electrons would scatter off impurities in a spin-dependent way as the electrons make their way along the strip those with up spins would veer to one side while those with down spins would veer to the other side the result is a transverse flow of spin a spin hall effect unlike the traditional hall effect and its younger quantum cousins the spin hall effect doesnt cause a transverse voltage nor does it need an external magnetic field the polarization relies instead on the relativistic coupling of the electrons spin and orbital angular momentum but the predicted polarization is modest for two decades after its debut dyakonov and perels proposal languished as a curiosity all but ignored by theorists and experimenters alike interest picked up when physicists began looking at ways to exploit electron spins to carry store and process information although magnetically controlled ferromagnets offer the easiest path to spin-based applications electrically controlled semiconductors may have the greater potential electric fields can be produced and controlled on far shorter time scales and in far smaller volumes than magnetic fields can and semiconductors through gating doping and other techniques are more adaptable materials than ferromagnets in the envisioned world of semiconductor spintronics the spin hall effect looks like a useful tool to have in 1999 unaware of dyakonov and perels work jorge hirsch of the university of california santa cruz came up with the same idea (2) then in 2003 shuichi murakami and naoto nagaosa of the university of tokyo and shou-cheng zhang of stanford university claimed that a potentially stronger and more useful spin hall effect occurs even without impurities (3) independently and in parallel jairo sinova of texas a&m university allan macdonald of the university of texas at austin and their collaborators made a similar claim (4) macdonald coined the terms intrinsic spin hall effect for the impurity-free version and extrinsic spin hall effect for the impurity-dependent version the intrinsic effect proved controversial more than 50 papers appeared arguing for or against its existence but now at last the spin hall effect has been observed two independent groups one based at the university of california santa barbara; the other based at the hitachi laboratory in cambridge england have reported convincing evidence that an electric field can indeed cause spins of opposite sign to accumulate on opposite sides of a semiconductor (5 6) intriguingly the effect seen in santa barbara seems extrinsic whereas the effect seen in cambridge seems intrinsic kerr probes and leds the santa barbara team consists of david awschalom his graduate students yuichiro kato and roberto myers and art gossard to observe the spin hall effect they opted to use a kerr probe when linearly polarized light reflects off a magnetized surface the lights polarization axis rotates by an amount proportional to the magnetization: the kerr effect mapping the polarization of light bouncing back from a strip of semiconductor would in principle reveal the telltale accumulation of up and down spins near the edges the team fashioned a strip of n-doped gallium arsenide 300 µm long 77 µm wide and 23 µm thick to map the strips polarization they imaged each 1 µm × 1 µm patch of the strip figure 1 shows part of the setup one full scan takes 30 hours the kerr signal from the polarized spins turned out to be feeble to raise it above the noise the team forced the signal to vary at two predetermined frequencies the electric field that drives the spin hall effect was turned on and off at a frequency fe of 1169 khz at the same time a weak magnetic field perpendicular to both the electric field and the polarization of the accumulated spins was applied at a frequency fb of 33 hz when not equal to zero the magnetic field reoriented the accumulated spins and quenched their kerr signal the spin hall effect shows up therefore as a fluctuating signal of frequency fe ± 2fb the map of the peak amplitude shown in figure 2 constitutes direct proof that polarized spins accumulate at the strips edges the hitachi team consists of experimenters jörg wunderlich and bernd kästner of the hitachi lab and theorists sinova and tomas jungwirth of the czech institute of physics in prague and university of nottingham in england they also exploited the coupling of spins to light but used light-emitting diodes rather than a kerr probe to observe it an led consists of n- and p-doped layers stuck together across a narrow gap when the led is biased electrons and holes recombine to emit radiation which passes through the material if the electrons and holes are polarized the light itself is circularly polarized leds have been used before to observe polarized spins but their usual sandwich configuration is unsuitable for spotting the accumulation of spins at the edges of a sample wunderlich and kästner realized they could observe the spin hall effect by reconfiguring an led in their device a p-doped channel lies above n-doped material but not directly above when spin-polarized holes accumulate at the edges of the channel they recombine with electrons that jump up and sideways from the n-doped material to meet them the resulting circularly polarized light originates from the edges of the led the chip-like device is shown in figure 3 leds including the novel hitachi device emit a spectrum of light with several peaks careful calibration identified the appropriate peak (at 1512 mev in the experiment) the results appear in figure 4 as expected when the direction of the electric field is reversed the polarization signal at one edge switches sign (top panels) and just as expected when the field direction stays the same the polarization at the two edges is opposite (bottom panels) weak and strong coupling spin-orbit coupling the key ingredient of the spin hall effect originates from paul diracs famous union of special relativity and quantum mechanics in 1929 two years after diracs theory appeared nevill mott applied it to the problem of scattering spinning electrons off atoms (7) roughly speaking as an electron swings by an atom it sees an effective magnetic field -(v/c) × e due to the atoms coulomb field the coupling of an electrons spin to the magnetic field sends spin-up electrons one way and spin-down electrons the other this skew scattering as mott termed it is used to produce spin-polarized particle beams at accelerators in dyakonov and perels extrinsic theory impurities serve as skew scatterers in the intrinsic theories spin-orbit coupling of electrons to the lattice atoms acts with the applied electric field to change the band structure as a result the electrons group velocity acquires a spin-dependent transverse component that leads to a transverse spin current the stronger the spin-orbit coupling the stronger the spin hall effect for semiconductors that means heavier atoms with their larger coulomb potentials are better than light atoms in addition valence-band electrons which occupy p-like states have far stronger coupling than conduction band electrons which occupy s-like states a further boost to the coupling comes from breaking the up-down symmetry of the lattice silicon has inversion symmetry but gaas does not confining the charge carriers in a thin layer with a strong electric field also breaks up-down symmetry and strengthens the coupling in the santa barbara experiment the spins belong to conduction-band electrons whose spin-orbit coupling to the atoms of the gaas lattice is weak despite the lack of inversion symmetry here the spin hall effect most likely originates extrinsically from skew scattering off impurities rather than intrinsically from the gaas lattice the santa barbara team checked this hypothesis by repeating their experiment on a strained sample if the effect originated in the lattice then it should depend on the strain direction it doesnt the charge carriers in the hitachi experiment are holes whose already strong coupling to the gaas lattice is increased by their confinement in a narrow layer having measured the impurity concentration in their sample the hitachi researchers believe it to be too low to account for the polarization they observe the spin hall effect in their sample is they believe intrinsic in origin the controversy surrounding the intrinsic effect doesnt stem from the effect itself but from what happens when impurities are present when the electric field is switched on the spins polarize and head off to different edges of the strip in the absence of impurities the electrons would continue to accelerate down the strip but in a real impurity-ridden lattice the electrons must decelerate to stabilize the current either by scattering off impurities or by slamming against the sample boundary and when the electrons decelerate the intrinsic effect reverses direction and could in principle cancel itself out in some models the steady acceleration caused by the field doesnt necessarily cancel the abrupt deceleration that occurs when electrons slam head-on into a few sparsely distributed impurities but in other models the cancellation is exact and in some cases originates from the lattice as well as impurities even the notion of a spin current is somewhat problematic as an electron travels through the bulk to reach the edge its charge remains the same but its spin thanks to spin-orbit coupling is not conserved relating spin accumulation observed at the edges to a spin current through the bulk is not straightforward sankar das sarma of the university of maryland in college park has been closely following the controversy as its unfolds the theoretical situation is a complete mess he says references 1 m i dyakonov v i perel jetp lett 13 467 (1971); phys lett a 35 459 (1971) 2 j hirsch phys rev lett 83 1834 (1999) 3 s murakami n nagaosa s c zhang science 301 1348 (2003) 4 j sinova d culcer q niu n a sinitsyn t jungwirth a h macdonald phys rev lett 92 126603 (2004) 5 y k kato r c myers a c gossard d d awschalom science 306 1910 (2004) 6 j wunderlich b kästner j sinova t jungwirth phys rev lett (in press) 7 n f mott proc r soc london ser a 124 425 (1929)