Electrons have a hidden feature — spin — that could revolutionize technology. Magnets can control it, but researchers are now exploring chiral molecules as an alternative. These uniquely shaped molecules might help direct electron spin just as well, opening new possibilities for future electronics.
Electrons are well known for their negative charge, which plays a key role in electric currents. However, they also possess another important property: spin, or magnetic moment. This characteristic has significant potential for improving data storage technologies, but controlling electron spin has proven challenging.
Specifically, isolating electrons with a particular spin direction, such as spin-up, is difficult. One established method involves passing an electric current through a ferromagnetic material, like iron. This process aligns the spin polarization of the electrons with the material’s magnetic field.
Chiral Molecules and Spin Polarization
An alternative approach, explored in recent years, involves using chiral molecules — structures that lack a superimposable mirror image, such as helix structures. Research suggests that these molecules can induce spin polarization at levels comparable to ferromagnetic materials, around 60 to 70 percent. However, this method is still under investigation and remains a topic of debate in the scientific community.
A Hybrid Gold-Chiral System Unveiled
Researchers at Johannes Gutenberg University Mainz (JGU) have recently been able to confirm the existence of this so-called chiral-induced spin selectivity (CISS) effect.
“Our group investigated the influence of chiral molecules using spintronic methods,” emphasized Professor Angela Wittmann of the JGU Institute of Physics. “We did not pass the charge current directly through the chiral molecules themselves. Instead, we created a hybrid system that consisted of a thin film of gold with chiral molecules on it. Although the major part of the current flows through the gold film, the presence of the chiral molecules alters the state of the gold.”
How Chiral Molecules Influence Spin Current
The researchers were interested in how the spin current was converted to a charge current. In a film consisting of pure gold, some three percent of the spin current is converted to charge, irrespective of whether the spin of the electrons is oriented up or down.
In the hybridized system of a gold layer with chiral molecules, however, the result is quite different. If the molecules on the surface of the gold are right-handed, currents with electron spin-up are converted much more efficiently to charge than those with spin-down. The outcome is exactly the opposite if molecules on the gold surface are left-handed. The extent to which a spin current is converted into a charge current thus depends on the chirality of the molecules on the gold surface.
“Moreover, the effect is vectorial,” explained Wittmann. If the helix structure of a chiral molecule is directed upwards, this effect occurs only if the spin is more or less in the same direction or completely counter to this.”
On the other hand, if the direction of spin is not aligned with the direction in which the helix structure is arranged, the effect does not occur. Consequently, the direction of the spin and the helix axes must either correspond or be exactly counter to each other.
The Significance of Spin Selectivity
“Our results are an important contribution to the acceptance of the spin selectivity effect and thus the influence of chiral molecules on spins,” concluded Wittmann.
Reference: “Chiral-induced unidirectional spin-to-charge conversion” by Ashish Moharana, Yael Kapon, Fabian Kammerbauer, David Anthofer, Shira Yochelis, Hadar Shema, Elad Gross, Mathias Kläui, Yossi Paltiel and Angela Wittmann, 1 January 2025, Science Advances.
DOI: 10.1126/sciadv.ado4285
