When it comes to low-power electronic devices, spintronics looks promising. The spin is a property of quantum electrons that can best be imagined as electrons spinning around their own axis, causing them to behave like tiny needles. A chain of electronic spins could be used in electronic devices. However, to generate a proper rotational current, you need a relatively large magnet. An alternative method that uses a special type of molecule has been proposed, but the big question is: does it work? University of Groningen Ph.D. The student Xu Yang has constructed a theoretical model that describes how to test this new method.
The turn may have two directions, generally referred to as "up" and "down." In a normal electron current, there are equal amounts of both directions of rotation, but if you want to use spin to transfer information, you need a surplus in one direction. This is usually done by injecting electrons into a spintronic device through a ferromagnet, which will favor the passage of one type of spin. "But the ferromagnets are bulky compared to the other components," says Yang.
That is why a breakthrough of 2011 published in Science is attracting more attention. "This article described how passing a current through a double-helix monolayer of DNA would favor a kind of spin." DNA molecules are chiral, which means they can exist in two forms that are the mirror image of each other. . The phenomenon has been dubbed the Chiral Induced Spin Selectivity (CISS), and in recent years, several experiments have been published that supposedly showed this CISS effect, even in electronic devices.
"But we were not sure about that," explains Yang. One type of experiment used a monolayer of DNA fragments, while another used an atomic force microscope to measure current through individual molecules. Different chiral helices were used in the experiments. "The models that explain why these molecules favor one of the spins made many assumptions, for example, about the shape of the molecules and the path the electrons took."
So Yang set out to create a generic model describing how spins would go through different circuits under a linear regime (ie the regime in which electronic devices operate). "These models were based on universal rules, independent of the type of molecule," explains Yang. One such rule is charge conservation, which states that every electron entering a circuit must eventually exit it. A second rule is reciprocity, which states that if you change the roles of voltage and current contacts in a circuit, the signal must remain the same.
Next, Yang described how these rules would affect the transmission and reflection of spins in different components, for example, a chiral molecule and a ferromagneto between two contacts. The universal rules allowed us to calculate what happened to the spins in these components. He then used the components to model more complex circuits. This allowed him to calculate what to expect if the chiral molecules showed the CISS effect and what to expect if they did not.
When he modeled the CISS experiments published so far, Yang found that some are, in fact, inconclusive. 'These experiments are not convincing enough. They do not show a difference between molecules with and without CISS, at least not in the linear regime of electronic devices. "In addition, any device that uses only two contacts will not be able to prove the existence of the CISS. The good news is that Yang has designed circuits with four contacts that will allow scientists to detect the CISS effect on electronic devices. "I'm also working on this circuit today, but since it's made of molecular blocks, that's a big challenge."
In publishing his model now, Yang hopes that more scientists will start building the circuits he has proposed and will finally be able to prove the existence of CISS in electronic devices. "This would be a great contribution to society as it could allow a whole new approach to the future of electronics."
University of Groningen. .