Spin-orbit interaction in semiconductor nanowires

I have been studying spin-orbit interaction in semiconductors for the last six years. Looking back I realize that at some moment along the way… I fell in love with it. Mathematically speaking, spin-orbit interaction is nothing but an energy which is a product of spin and momentum. It is to this day remarkable to me that such a simple term in the Hamiltonian can have such profound consequences. On the one hand, it can be used to rotate spins by controlling the motion of electrons. On the other hand, spin itself can be used as a steering wheel to move electrons around.

For decades spin-orbit interaction was viewed as a nuisance, since its only known effect was to cause the loss of spin coherence in disordered systems. With the development of nanotechnology many creative ways were found to employ spin-orbit interaction in electronic devices. Most notoriously, it brought us the concept of a spin transistor, the spin Hall effect, topological insulators etc.

In our group we were able to define and manipulate spin-orbit quantum bits, which are carried by single electrons. Over the past several months we used spin-orbit qubits to measure the strength and the orientation of spin-orbit interaction in the host material – semiconductor nanowires. The paper on this is now posted in open access: http://arxiv.org/abs/1201.3707

The idea behind the measurement is very simple. Since spin-orbit interaction couples electron’s orbits and their spins, it results in the interaction between singlet and triplet levels in a two-electron system (an artificial helium). We detected singlet-triplet coupling of two spin-orbit qubits and mapped out the strength of this coupling for different orientations of magnetic field. Importantly, we have concluded that spin-orbit interaction in our nanowires is strong enough to create new fundamental quasiparticles – Majorana fermions – but this deserves a separate blog entry.

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