A team of scientists from Switzerland, Germany and India report the first ever observation of a quantum phenomenon in engineered atomic wire. Called the 'spin-orbit density wave', this phenomenon was just a theoretical speculation so far, and an experimental observation eluded scientists around the globe. This new discovery could potentially help build electronic devices that are much faster than what we have now.

Dr Tanmoy Das, Assistant Professor, Department of Physics, Indian Institute of Science, was part of the international team which reported this ground breaking observation. The other members of the team are from the University of Zurich, Paul Scherrer Institute, Ecole Polytechnique F ed ́rale de Lausanne (all from Switzerland), and University of Hannover, Germany.

Atoms are mainly made of protons, neutrons and electrons. Protons and neutrons are bundled together to form the nucleus, and the electrons go around them. Electrons, in addition to orbiting round the nucleus, also spin with its own axis. Sometimes, the orbital motion and the spinning motions can become interlocked with each other such as right and left moving electrons possess definite but opposite spinning axis.

“The orbital motion of an electron creates a magnetic field which influences the magnetic field due to the spinning motion. The result is that the axis of the magnetic field due to spin gets aligned with that of the field due to orbital motion. This is interesting because, without spin-orbit interaction, the axis of the magnetic field due to spin is changing continuously, averaging to zero net effect”, explains Dr Tanmoy Das.

In a crystalline solid, the atoms are arranged in an order, as if someone has taken care to put them side by side. In such materials, each atom can have its own spin-orbit interaction, which often cannot be controlled externally. Dr Tanmoy Das suggested a mathematical formalism for a new form of spin-orbit interaction, called 'spin-orbit density wave', in which spin can be fixed to the motion of the electron by Coulomb interaction and other external knobs. The net effect propagates like a wave, with say, forward moving wave has one definite spin, while the backward moving wave has opposite spin. This is like a clockwise spinning tornedo is fixed to move only in one direction, while the counter clockwise spinning tornedo goes in opposite direction. Dr. Das had proposed this idea in a paper in the journal Physical Review Letters, in 2012. Three years later, he and his co-workers have experimentally verified that such waves do exist in crystalline solids.

To observe this phenomenon, the team used a 'hand made', ultra thin, lead wire on a patch of silicon. Careful experimentation and measurements showed that spin-orbit effect peaked at certain points, and remained zero at a few other points, forming a spin-orbit density wave. An ultra thin material was necessary to make these sensitive observations.

“In ultra thin wires, the Coulomb interactions entangling the orbital and spinning motions of the electron are stronger. Also, they offer better control than thicker wires”, says Dr Tanmoy Das, while explaining the advantages offered by extremely small wires.

This discovery, though fundamental in nature, can have implications in emerging technologies like spintronics. Spintronic devices, when realised, can give us memory chips that are a lot more faster than what we have now. “Unlike conventional electronic devices that leverage only the charge of an electron, spintronics devices use spin too. However, making the electron to retain its spin as moves has been a challenge. Our observations may help overcome that challenge”, says Dr Tanmoy Das.