Ballistic Spintronics

   Our experiments combine nano-electronics and spin manipulation for spintronics in semiconductors . The spin of electrons and holes in semiconductors holds promise to realize new spin-based electronic device concepts and quantum computational schemes. In semiconductors, spin-orbit interaction (SOI) leads to a spin-splitting in energy levels, and hence in the Fermi contours. SOI can lead to short spin-coherence lengths. However, semiconductor heterostructures can be fabricated with a long carrier mean free path, longer than lateral dimensions within reach of present lithographic techniques. If the mean free path is longer than the lateral dimensions, charge transport in the geometry occurs ballistically, i.e. the preponderant scattering events involve the device boundaries. If in such mesoscopic devices the spin coherence length is also longer than the lateral dimensions, then SOI, together with the device geometry, can be used for spin manipulation, and for the preparation of spin-polarized carrier states.

   In the triangular devices pictured, electrons enter the triangles from one of the apertures in the side walls (triangles ~ 3 micron side). The triangles were wet etched into n-type InSb/InAlSb heterostructures after electron beam lithography. The beam of two-dimensional electrons is injected towards the sidewall, acting as a barrier. Both energy and the momentum parallel to the barrier are conserved during the scattering event off the barrier. If the Fermi contours are spin-split, spin-flip scattering events result in different reflection angles for different spin polarizations. The interaction with the barrier gives rise to two spin-polarized side beams and one unpolarized specular beam. The reflected beams can be captured through suitably positioned apertures. The multi-beam reflection process can be utilized to create spin-polarized electron populations, without the use of ferromagnetic contacts. Similar experiments were perfomed on InAs/AlGaSb heterostructures.

   The spin part of the wave function can give rise to quantum mechanical spin interference effects . We study these effects in ring geometries. Pictured is an Aharonov-Bohm interference ring fabricated on an InSb/InAlSb heterostructure. The spin interference phenomena clearly appear in Altshuler-Aronov-Spivak oscillations in ring networks on InSb/InAlSb and InAs/AlGaSb. The oscillations allow us to study this unique quantum mechanical effect, and also give values for the spin coherence length in mesoscopic geometries. Support from NSF grant Career DMR-0618235.