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Half of all the transistors in your iPhone use positively-charged ‘holes’, rather than negatively-charged electrons to operate.
At university, we teach undergraduates that holes are quasiparticles, basically ‘missing electrons’ – a bit like the bubble in a spirit level, or the missing chair in a game of musical chairs.
But that isn’t the whole story: holes also have very different ‘spin’ properties than electrons. (A particle’s spin is its intrinsic angular momentum.)
These unique spin properties of holes makes them very attractive for ultra-low powered spin transistors, high-speed quantum bits, and fault-tolerant topological quantum bits.
The problem is that until recently we didn’t have a good understanding of the spin properties of holes in nanoscale transistors. In fact, the best theories predicted the opposite behaviour to that observed in experiments.
Now, a team of physicists led by UNSW’s Alex Hamilton and Oleg Sushkov have solved the mystery by identifying a new term in the equations that had previously been overlooked.
This reconciles experiments and theory, and paves the way for future quantum electronic and quantum computing devices.
What They Did
Key to the issue is that a hole behaves very differently when confined to only two-dimensions, compared with its behaviour in a normal, three-dimensional solid.
A transistor is manufactured with two semiconductor materials of slightly different electronic properties, pressed together. At the interface of those two materials, an effectively two-dimensional zone exists, in which a thin sheet of electrons or holes can be controlled to perform the necessary logical functions.
But while the behaviour of holes in three dimensions has been well understood for many decades, their confinement to two dimensions introduces new factors that cause otherwise-unpredictable responses to an applied magnetic field. Namely, this confinement introduces a new ‘spin–orbit interaction’.
Spin-orbit interaction (SOI), is the coupling of the hole’s movement through space (for example in orbit around an atom or along a current carrying path) and its spin. This spin-orbit interaction changes how holes respond to a magnetic field and is key to the function of topological materials, which are studied at FLEET for their potential to form ultra-low resistance pathways for electrical current.
The new study is the first time that these new spin-orbit effects for holes confined to one dimension have been properly classified.
Solving A Ten-Year Mystery
In 2006, UNSW experiments found a result that did not match existing theory:
Experimenters were looking at the effects of an external magnetic field applied to a one-dimensional, charge-carrying path known as a quantum wire.
The applied magnetic field separates, or splits, the energy levels of holes with different spins. Experiments showed that the spin-splitting was extremely sensitive to the direction of the magnetic field, unlike electrons which are insensitive to the field direction.
Furthermore, the spin-splitting was found to be largest when the magnetic field was applied along the quantum wire – a result that was completely contrary to existing theories. This disagreement between experiment and theory remained unexplained for the past decade.
The most recent study identified a new spin-orbit interaction factor caused by the holes’ confinement to one dimension, and found that this new factor explained the 2006 experimental result.