Thursday, October 05, 2006

This week in cond-mat

Two papers this time around....
cond-mat/0610107 - Butenko et al., Electric field effect analysis of thin PbTe films on high-\epsilon SrTiO3 substrate
This paper is a nice example of using the three-terminal field-effect geometry as a way to probe the states of a material while keeping the disorder fixed. The authors use strontium titanate as the dielectric layer. Since SrTiO3 is almost a ferroelectric, it has an extremely high gateable polarization (gated charge density) at breakdown field. This means that the authors are able to shift the Fermi level over a very broad range, spanning the entire (relatively narrow compared to things like Si or GaAs) energy gap of the PbTe disordered film, and gate in either electrons or holes. They can see the effects of interface states, and the broadening of the conduction and valence bands due to disorder. Their main observation is that the mobility gap in the disordered case is actually larger than the standard band gap in PbTe. Pretty interesting, and written in a reasonably pedagogical style.

- Liu et al., Experimental observation of the inverse spin Hall effect at room temperature
The spin Hall effect is a neat concept that my friend Jairo Sinova at Texas A&M has been involved with heavily, as has Soucheng Zhang, who taught me many-body physics back in grad school. The basic idea is that, under the right conditions, it is possible for a dc longitudinal current to establish an unequal spin population on the transverse edges of a material (e.g. a GaAs heterostructure). That is, along the two edges of the sample that parallel the current flow, there will be an excess spin population (with no excess electronic population!), with one edge having an excess of spin-up, and the other edge having an excess of spin-down. Here, up and down are relative to the direction normal to the plane of the current flow. This spin population difference is analogous to the voltage difference that develops transverse to the current in the presence of a perpendicular magnetic field in the ordinary Hall effect. Anyway, the bottom line is that one can produce separated spin populations without actually injecting spins from a ferromagnet or something similarly difficult. The spin Hall effect can be intrinsic (due to spin-orbit coupling and a built-in electric field or lack of inversion symmetry in the material) or extrinsic (due to spin-dependent scattering off of disorder in the material). One of the first (the first?) observation of spin Hall was made by Awschalom's group at UCSB, using spatially resolved magneto-optic Kerr to map the spin density.

Anyway, in this paper the authors claim to observe the inverse spin Hall effect. That is, they establish an unequal spin population between edges of a sample using a spatially varying intensity of circularly polarized light to generate polarized carriers. Then, they observe a dc current transverse to the spin density gradient. The data look pretty convincing, though I'm no expert in photophysics of III-V materials.

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