A new and highly efficient way of filtering electrons according to their spin has been built using double strands of DNA. The technique, which has been developed by physicists in Israel and Germany, is about three times more efficient than using magnet-based spin filters. The method could be used in spintronic circuits, which exploit both the spin and charge of electrons, and could even lead to a better understanding of the possible role that spin plays in biological processes.

Spintronics holds great promise for creating circuits that are faster and more energy efficient than standard semiconductor devices. This is because the energy required to transport and process spins is much less than that needed to create electron currents. Creating spins is not a problem as magnetic metals such as iron are full of them. The challenge, however, is extracting the spins to form a spin-polarized current and injecting them into a circuit without the polarization degrading along the way.

Today, spins are often made using a filter that exploits the phenomenon of giant magnetoresistance (GMR). This involves passing a current of unpolarized electrons through a material containing alternating layers of magnetic and non-magnetic material in the presence of a magnetic field. In principle, only electrons with their spin pointing in the "up" direction can pass through the filter, but the currents obtained by the device are never entirely pure, with a significant fraction of the electrons emerging spin "down".

Dense forest of DNA

Now, however, Ron Naaman and colleagues at the Weizmann Institute in Israel and the University of Münster in Germany have found that a 60% spin polarization at room temperature can be achieved by passing free electrons through a gold surface covered with a densely packed layer of DNA strands. Although DNA does not normally adhere to gold, the researchers treated one end of each strand with a sulphur compound to make it stick. The result is a dense forest of DNA chains all standing tall on the gold surface.

The researchers then shone a laser on to the gold, which liberates electrons via the photoelectric effect. Some of these electrons travel through the DNA forest and are fed into a device that measures their spin polarization. The team performed the experiment using linearly polarized laser light, which liberates unpolarized electrons. However, after travelling through the DNA, the electrons became polarized by as much as 60%.

The longer the better

The researchers found that the polarization was a strong function of the length of the DNA strands – with 80 base-pair-long strands giving 60% polarization but 25 base pairs only yielding about 10%. The team also found that the filter does not work when the DNA coverage is sparse, suggesting that the electrons are polarized by interactions with the lattice of strands, rather than individual strands.

Despite the strength of polarization effect, Naaman told physicsworld.com that the researchers are not certain why the effect occurs, but he believes that it is probably related to the "handedness", or "chirality" of the DNA double helix. While other physicists have shown that passage through a vapour of chiral molecules can affect the spin polarization of electrons, the effect is minuscule compared with what is seen with DNA. As a result the interaction at work in the vapour – spin–orbit coupling – is simply too weak to explain these recent results, according to Naaman.

Geert Rikken of the CRNS High Magnetic Field Laboratory in Toulouse, France, speculates that the effect could be a "Bragg-like resonance", which is a diffraction effect that occurs because the De Broglie wavelength of the electrons is about the same as the lattice spacing of the DNA strands. He points out that a similar spin-filtering of photons due to Bragg diffraction has been seen in cholesteric liquid crystals, which also have a helical structure. To gain a better understanding of the physics at work in the filter, the team is now studying the polarization of electrons that flow through the DNA strands, rather than the free electrons that travel past the strands.

Benefits of DNA

Looking ahead, Naaman believes that spin devices based on organic materials such as DNA could offer several benefits. One is that spin-polarized currents should travel further in such materials – compared with metals – because the strength of the spin–orbit coupling is much smaller and because the spins are less likely to interact with vibrations in the material. Another benefit is that the ends of the DNA can be modified with a wide range of chemicals, which could make it possible to connect DNA devices to spintronic circuits in such a way that the spin polarization is not degraded at the connection.

However, Rikken is more cautious about the work. "I do not think that DNA films would be a welcome component in spintronic devices," he says. But he does think that other chiral structures could find application in spintronics – if chirality is found to be the mechanism behind the filtering, that is.

Beyond spintronics, the discovery that DNA has a strong effect on electron spin suggests that spin interactions could also play a role in some biological processes. Indeed, Naaman believes that studies of spin in biomolecules could shed light on poorly understood low-energy biochemical processes that occur in nature.

The spin filter is described in Science 331 894.