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Observation of spin Coulomb drag in a two-dimensional electron gas

Abstract

An electron propagating through a solid carries spin angular momentum in addition to its mass and charge. Of late there has been considerable interest in developing electronic devices based on the transport of spin that offer potential advantages in dissipation, size and speed over charge-based devices1. However, these advantages bring with them additional complexity. Because each electron carries a single, fixed value (- e) of charge, the electrical current carried by a gas of electrons is simply proportional to its total momentum. A fundamental consequence is that the charge current is not affected by interactions that conserve total momentum, notably collisions among the electrons themselves2. In contrast, the electron's spin along a given spatial direction can take on two values, ± /2 (conventionally ↑,↓), so that the spin current and momentum need not be proportional. Although the transport of spin polarization is not protected by momentum conservation, it has been widely assumed that, like the charge current, spin current is unaffected by electron–electron (ee) interactions. Here we demonstrate experimentally not only that this assumption is invalid, but also that over a broad range of temperature and electron density, the flow of spin polarization in a two-dimensional gas of electrons is controlled by the rate of ee collisions.

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Acknowledgements

We thank I. D'Amico and G. Vignale for sending us numerical evaluations of their integral expression for the spin drag resistance. This work was funded by the US DOE, DARPA, and NSFDMR. We also acknowledge support from the Fannie and John Hertz Foundation (C.P.W.) and the Hellman Foundation (J.E.M.).

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Correspondence to C. P. Weber.

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Reprints and permissions information is available at npg.nature.com/reprintsandpermissions. The authors declare no competing financial interests.

Supplementary information

Supplementary Figure 1

The q = 0 spin-relaxation rate as a function of temperature for the sample with Fermi temperature 400 K. (PDF 1061 kb)

Supplementary Discussion

A discussion of the measurement of the q = 0 spin-relaxation rate, and of the derivation of the non-interacting susceptibility of a two-dimensional electron gas. (PDF 46 kb)

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Further reading

Figure 1: Spin-grating decay at various wavevectors ( q ) and temperatures ( T ) for the sample with Fermi temperature 400 K.
Figure 2: Time dependence of the spin grating's amplitude.
Figure 3: Comparison of motion of spin and charge, for samples with the Fermi temperatures ( T F ) shown.
Figure 4: A representation of e e scattering that does not conserve spin-current.
Figure 5: Relation between suppression of spin diffusion and spin drag resistance.

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