Posted in | Quantum Physics

Molecular Mystery May Be Resolved Using New Theoretical Model

Spintronics shows potential when it comes to the creation of low-power electronic gadgets. Spin is a quantum-mechanical property of electrons that can best be visualized as electrons spinning around their own axis, making them act like small compass needles. A current of electron spins could be utilized in electronic gadgets. However, to produce an appropriate spin current, a moderately large magnet is needed. A substitute technique that uses a distinct type of molecule has been suggested, but the real question is: is it workable? University of Groningen PhD student Xu Yang has designed a theoretical model which explains how to put to test this new method.

Xu Yang. (Image credit: Yang/University of Groningen)

Spin can possess two directions, typically designated as “up” and “down”. In a standard electron current, there are same quantities of both spin directions, but if spin is used for transferring data, then a surplus of one direction is needed. This is typically achieved by injecting electrons into a spintronic device through a ferromagnet, which will favor the route of one type of spin. “But ferromagnets are bulky compared to the other components”, says Yang.


That is why a 2011 innovation that was reported in Science is garnering a lot of attention. “This paper described how passing a current through a monolayer of DNA double helices would favour one type of spin.” The DNA molecules are chiral, which means they can occur in two forms which are each other’s mirror image—similar to the left and right hand. The phenomenon was labeled Chiral Induced Spin Selectivity (CISS), and in recent years, several experiments were reported which supposedly demonstrated this CISS effect, even in electronic gadgets.

“But we were not so sure”, explains Yang. One kind of experiment employed a monolayer of DNA fragments, while another measured the current through single molecules using used an atomic force microscope. Various chiral helices were applied in the experiments.

The models explaining why these molecules would favor one of the spins made lots of assumptions, for example about the shape of the molecules and the path the electrons took.

Xu Yang, PhD Student, University of Groningen


Therefore, Yang began to design a generic model which could illustrate how spins would travel through various circuits under a linear regime (i.e. the regime that electronic gadgets function in). “These models were based on universal rules, independent of the type of molecule”, explains Yang. One such rule is charge conservation, which defines that every electron that enters a circuit should ultimately exit it. Reciprocity is the second rule, and it states that if the roles of the voltage and current contacts are swapped in a circuit, the signal should stay the same.

Subsequently, Yang explained how these rules would impact the transmission and reflection of spins in various components, for instance, a ferromagnet and a chiral molecule between two contacts. The universal rules allowed him to calculate what happened to the spins in these components. He then employed the components to model more-complex circuits. This permitted him to calculate what to anticipate if the chiral molecules exhibited the CISS effect and what to assume if they did not.


When he modeled the CISS experiments reported thus far, Yang learned that a few are, truly, questionable.

These experiments aren’t convincing enough. They do not show a difference between molecules with and without CISS, at least not in the linear regime of electronic devices.

Xu Yang, PhD Student, University of Groningen.

Moreover, any device using merely two contacts will not be able to demonstrate the presence of CISS. The positive news is that Yang engineered circuits with four contacts that will enable researchers to detect the CISS effect in electronic gadgets. “I am currently also working on constructing such a circuit, but as it is made up of molecular building blocks, this is quite a challenge.”

By publishing his model at this time, Yang is optimistic that more scientists will begin constructing the circuits he has suggested, and will lastly be able to substantiate the existence of CISS in electronic gadgets.

This would be a great contribution to society, as it may enable a whole new approach to the future of electronics.

Xu Yang, PhD Student, University of Groningen.

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