Physics

Spinning Randomly in the Hall

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Science  10 Sep 2010:
Vol. 329, Issue 5997, pp. 1259
DOI: 10.1126/science.329.5997.1259-c

The Hall effect occurs when a current running through a conductor is deflected by a perpendicular magnetic field, causing a transverse voltage to develop. In certain semiconductor structures where the motion of the charge carriers is confined to two dimensions, this Hall voltage is quantized. When the parameter of quantization (the so-called filling factor ν) is an integer, the effect can be easily explained through the formation of Landau levels. Fractional filling factors may also occur, requiring a more involved explanation in terms of non-interacting composite fermions. Α state with ν = 5/2 is yet more exotic as it eludes the composite fermion description and was predicted to exhibit a complicated kind of quantum statistics known as non-Abelian, making it especially interesting for quantum computing. However, one of the assumptions of that prediction was that the state was also spin-polarized. Stern et al. test that assumption by measuring the photoluminescence of a GaAs-AlGaAs quantum well and find that the state appears to be spin-unpolarized. Their conclusions agree with those extracted from Raman and tilted-field measurements, prompting further theoretical research into whether a spin-unpolarized state can still support a non-Abelian theory for quantum computing applications.

Phys. Rev. Lett. 105, 96801 (2010).

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