This diagram maps out atomic motion in separate phonon modes. At left, where ‘LO’ represents a longitudinal optical mode, selenium atoms exhibit a clockwise rotation while tungsten atoms stand still. At right, where ‘LA’ represents a longitudinal acoustic mode, tungsten atoms exhibit a clockwise rotation while selenium atoms rotate in a counter-clockwise direction. Image: Hanyu Zhu, et al.
This diagram maps out atomic motion in separate phonon modes. At left, where ‘LO’ represents a longitudinal optical mode, selenium atoms exhibit a clockwise rotation while tungsten atoms stand still. At right, where ‘LA’ represents a longitudinal acoustic mode, tungsten atoms exhibit a clockwise rotation while selenium atoms rotate in a counter-clockwise direction. Image: Hanyu Zhu, et al.

A research team from the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) has found the first evidence that a vibrating motion in the structure of an atomically thin, two-dimensional (2D) material possesses a naturally occurring circular rotation. This rotation could become the building block for a new form of information technology, as well as for the design of molecular-scale rotors to drive microscopic motors and machines.

The monolayer material, tungsten diselenide (WSe2), is already well-known for its unusual ability to sustain special electronic properties that occur far more fleetingly in other materials. It is considered a promising candidate for a sought-after form of data storage known as valleytronics, for example, in which the momentum and wave-like motion of electrons in a material can be sorted into opposite ‘valleys’ in a material's electronic structure. Each of these valleys can represent the ones and zeroes of conventional binary data.

Modern electronics typically rely on manipulations of the charge of electrons to carry and store information, though as electronics are increasingly shrinking they are more subject to problems associated with heat build-up and electric leaks. This latest study, published in a paper in Science, suggests a possible way to overcome these issues. It reports that some of the phonons in WSe2, with phonons describing collective vibrations in atomic crystals, are naturally rotating in a certain direction.

This property is known as chirality – similar to a person's handedness, where the left and right hand are a mirror image of each other but not identical. Controlling the direction of this rotation would provide a stable mechanism to carry and store information.

"Phonons in solids are usually regarded as the collective linear motion of atoms," said Xiang Zhang, the corresponding author of the paper and senior scientist of the Materials Science Division at Berkeley Lab and a professor at University of California, Berkeley. "Our experiment discovered a new type of so-called chiral phonons where atoms move in circles in an atomic monolayer crystal of tungsten diselenide."

"One of the biggest advantages of chiral phonon is that the rotation is locked with the particle's momentum and not easily disturbed," said Hanyu Zhu, the lead author of the paper and a postdoctoral researcher in Zhang's group.

In the phonon mode studied by the team, the selenium atoms appear to collectively rotate in a clockwise direction, while the tungsten atoms showed no motion. The researchers prepared a ‘sandwich’ with four sheets of centimeter-sized monolayer WSe2 samples placed between thin sapphire crystals. They synced ultrafast lasers to record the time-dependent motions.

The two laser sources converged on a spot on the samples measuring just 70 millionths of a meter in diameter. A so-called pump laser produced visible, red-light pulses that excited the samples, and a probe laser produced mid-infrared pulses that followed the first pump pulse within one trillionth of a second. This probe laser was precisely switched between two different tuning modes to sense the difference in left and right chiral phonon activity. About one mid-infrared photon in every 100 million is absorbed by WSe2 and converted to a chiral phonon.

The researchers were able to capture the high-energy luminescence from the sample, a signature of this rare absorption event. Through this technique, known as transient infrared spectroscopy, they not only confirmed the existence of a chiral phonon but also accurately obtained its rotational frequency.

So far, the process only produces a small number of chiral phonons. Next step in the research will be to generate larger numbers of rotating phonons, and to learn whether vigorous agitations in the crystal can be used to flip the spin of electrons or to significantly alter the valley properties of the material. Spin is an inherent property of an electron that can be thought of as its compass needle – if it could be flipped to point either north or south, it could be used to convey information in a new form of electronics called spintronics.

"The potential phonon-based control of electrons and spins for device applications is very exciting and within reach," Zhu said. "We already proved that phonons are capable of switching the electronic valley. In addition, this work allows the possibility of using the rotating atoms as little magnets to guide the spin orientation."

The chiral properties found in the study likely exist across a wide range of 2D materials based on a similar patterning in their atomic structure, Zhu noted. He added that the study could guide theoretical investigations of electron-phonon interactions and the design of materials to enhance phonon-based effects.

"The same principle works in all 2D periodic structures with three-fold symmetry and inversion asymmetry," Zhu said. "The same principle covers a huge family of natural materials, and there are almost infinite possibilities for creating rotors at the molecular scale."

This story is adapted from material from Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.