Scientists Discovered Chiral Phonons in Nano Tungsten Selenide

Tungsten diselenide nanosheet is a kind of graphene-like two-dimensional material. Its main structure is composed of a layer of selenium atoms from top to bottom connected with a layer of tungsten atoms from the middle. This material is becoming a research hotspot in the scientific community. Because scientists have found that the structure of atomic-level thin (2-D) tungsten diselenide material has a spontaneous circular rotation, i.e. pseudospin.

chiral phonons image

In February this year, Chinese and American scientists jointly published an academic achievement, Observation of Chiral Phonons, in the international leading academic journal Science, in which they claimed to have observed the characteristics of chiral phonons in tungsten selenide materials.

What is a chiral phonon? In classical physics theory, phonons are widely regarded as linear polarized, non-angular momentum, and collective linear motion of atoms. However, in recent studies, scientists have found that phonons can carry non-zero angular momentum in magnetic systems with spin-phonon interaction. At zero temperature, phonons have zero angular momentum as well as zero angular momentum.

Monolayer nano tungsten diselenide is one of the lowest heat conductivity materials in the world. The phonons of this material vibrate collectively in an atomic crystal and naturally rotate in a certain direction. This property is called chirality - similar to human left and right hands, which mirror each other but are not exactly the same. Controlling the direction of rotation will provide a stable mechanism to carry and store information. This rotation may become the cornerstone of new information technology, or the molecular rotor design to drive micro motors and machines.

One of the greatest advantages of chiral phonons is that rotation is locked in with the momentum of particles and is not easily disturbed. In the study, scientists found that atomic energy moves in a loop in the atomic monolayer crystal of tungsten diselenide. In the phonon modes studied, selenium atoms seem to rotate clockwise in general, while tungsten atoms do not show movement. Researchers prepared a "sandwich" in which four centimeter-sized single-layer WSe2 samples were placed between thin sapphire crystals. They synchronize ultrafast lasers to record time-dependent movements.

Two laser sources focus on samples with a diameter of only 70 parts per million. One of the lasers switches precisely between two different tuning modes to perceive the difference in left-handed and right-handed phonon activity. The researchers then captured high-energy luminescence in the samples, which is characteristic of this rare absorption event. By using this technique, known as transient infrared spectroscopy, researchers have not only confirmed the existence of chiral phonons, but also obtained their rotational frequencies accurately.

So far, only a small number of chiral phonons have been produced in this process. The next step will be to generate a large number of rotating phonons and to understand whether the intense stirring in crystals can be used to flip the spin of electrons or significantly change the bottom properties of materials. Spin is an intrinsic property of electrons and can be regarded as its compass - if it can be turned north or south, it can be used to transmit information in a new form of Electronics called spintronics.

So what is the practical use of this research? Scientists believe that the same principle applies to all two-dimensional periodic structures with three fold symmetry and inversion asymmetry. The same principle covers a huge series of natural materials. New materials with these properties will become the cornerstone of future industrial manufacturing of computers, robots and so on.

 

 

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