Synopsis: Flip-Flopping the Bands

A pair of semiconductor quantum wells with an inverted band structure hosts electrons whose spins are almost all in the same quantum state.  
Synopsis figure
F. Nichele et al. Phys. Rev. Lett. (2017)

Spintronics aims to make devices that carry information via an electron’s spin, rather than its charge. The quest rests on finding materials in which more electrons have their spins in one spin state than in another. A promising option might be a stack of two semiconducting nanolayers, according to Fabrizio Nichele of the University of Copenhagen, Denmark, and colleagues. The team found that nearly 100% of the conducting electrons in such a composite device had their spins in the same quantum state, a spin polarization more than 5 times that exhibited by either semiconductor layer on its own.

The researchers sandwiched two “quantum wells”—a 5-nm-thick layer of gallium antimonide on top of a thicker layer of indium arsenide—between two insulating layers. Both semiconductors contain heavy elements, which enhances the spin-orbit effect that ties the direction of an electron’s spin to its momentum. The effect can lead to a surplus of electrons in one spin state, but this spin polarization tops out at around 20% in most semiconductors, including gallium antimonide.

The team proved experimentally and with simulations that they could obtain a higher value if they raised the energy of the valence band in the gallium antimonide layer above that of the conduction band in the indium arsenide layer. To achieve this “inverted” band structure, they applied a voltage that pulled electrons out of the indium arsenide and injected holes into the gallium antimonide. With a spin-sensitive technique that detects the orbits of electrons in a magnetic field, they determined the voltage at which nearly all mobile electrons share the same spin state.

This research is published in Physical Review Letters.

–Jessica Thomas

Jessica Thomas is the Editor of Physics.


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