Turning Quantum Dots Into Spintronic Transistors

Spintronics is one of the technologies that is set to change the nature of computing and communications in the next few years. Until now, all electronic components such as transistors have exploited a single property of the electron: its charge. But electrons have another property, their spin, which can also be exploited to encode information.

So finding ways to manipulate electron spin is a priority in many labs around the world. And today, many spin-based devices are close to commercialization. That’s not to say there isn’t room for more and today E. C. Siqueira and Guillermo Cabrera at UNICAMP in Brazil put forward a new idea for a spintronic transistor based on the exotic phenomenon of Andreev reflection.

Place a magnetized lump of iron next to a superconductor and an unusual effect takes place. Single electrons cannot normally enter a superconductor. But if the spin of an electron in the iron is exactly opposite to that of another electron, they can combine to form a Cooper pair which can enter the superconductor. This process leaves a hole in the iron travelling away from the interface with the superconductor–the so-called Andreev reflection.

Of course, this process is highly dependent on the spin of the electrons. In theory, it is possible to switch the effect on and off by manipulating this spin.

What Siqueira and Cabrera suggest is a novel way to control this switching. They suggest using two ferromagnets to inject spin polarized electrons into the system. In one of these ferromagnets, the direction of the magnetic field is fixed while in the other it can vary. But instead of connecting these ferromagnets directly to the superconductor, the spin current first has to pass through two quantum dots.

This leads to complex pattern of currents. First, there are the spin polarised currents from each ferromagnetic traveling toward the superconductor. Then there are the Andreev reflections travelling away from the superconductor. And the melting pots in which these currents mix and interefere are the quantum dots.

With the help of some mathematical trickery, Siqueira and Cabrera show that the end result is a spin polarized current through the fixed ferromagnet that can be modulated and switched on and off by changing the bias on the other ferromagnet. In other words, a spintronic transistor.

Although, the physics involved is complex, the structure of this device is relatively simple and so should be straightforward to build and test

Whether it works as Siqueira and Cabrera suggest is the big question. They make a number of assumptions about the behavior of their system and how its components interact that will need to be tested experimentally before it can be developed further.

Still, it’s an exotic approach to an interesting problem.

Ref: arxiv.org/abs/1003.3688: Magnetoresistance And Transistor-Like Behavior Of Double Quantum Dots Connected To Ferromagnetic And Superconductor Leads