A spintronic perspective on chiral molecule interactions

Berlin, Germany (SPX) Jan 31, 2025
Electrons are well known for their negative charge, a key factor in electrical currents. However, they also possess an intrinsic property known as spin, which has significant implications for data storage technologies. The challenge lies in selectively filtering electrons based on their spin orientation. Traditionally, passing an electric current through a ferromagnet, such as iron, aligns electron spin polarization with the magnetic field. However, an alternative method involving chiral molecules has been gaining attention in recent years.

Chiral molecules, which lack a superimposable mirror image and often adopt helical structures, have been shown to induce spin polarization at levels comparable to ferromagnetic materials-approximately 60 to 70 percent. While this effect, known as chiral-induced spin selectivity (CISS), has been debated, researchers continue to explore its potential applications.

Investigating spin selectivity through a hybrid gold film system

A research team at Johannes Gutenberg University Mainz (JGU) has now provided experimental confirmation of the CISS effect. "Our group investigated the influence of chiral molecules using spintronic methods," stated Professor Angela Wittmann of the JGU Institute of Physics. "Instead of directly passing a charge current through the chiral molecules, we designed a hybrid system featuring a thin film of gold with chiral molecules on its surface. While most of the current flows through the gold, the presence of the chiral molecules alters the electronic properties of the gold film."

A key focus of the study was how spin current transforms into charge current. In a pure gold film, approximately three percent of the spin current is converted to charge, independent of electron spin orientation. However, in the hybridized system with chiral molecules, the results varied significantly.

When right-handed chiral molecules coated the gold surface, electrons with spin-up were more efficiently converted into charge than those with spin-down. Conversely, left-handed molecules favored the conversion of spin-down electrons. This demonstrates that the efficiency of spin-to-charge conversion is directly influenced by the chirality of the molecules interacting with the gold film.

"Additionally, this effect is directional," Wittmann explained. "If the helix structure of a chiral molecule is oriented upwards, the spin selectivity effect manifests only when the spin is aligned either in the same direction or completely opposite to the helix. When the spin orientation deviates from this alignment, the effect does not occur." Thus, for the spin selectivity effect to take place, the electron's spin direction and the helical axis of the chiral molecule must either match or be completely opposite.

"Our findings significantly contribute to the broader acceptance of the spin selectivity effect and highlight the crucial role chiral molecules play in influencing spin behavior," Wittmann concluded.

Research Report:Chiral-induced unidirectional spin-to-charge conversion