Chiral Phonons Observed in Nano-Tungsten Diselenide

Tungsten diselenide nanosheet is a kind of two-dimensional graphene-like material. Its main structure is composed of two layers of selenium atoms connected with a layer of tungsten atoms in the middle. This material is a hot research topic in the scientific community. 

Because scientists have discovered that an atomically thin (2-D) tungsten diselenide material possesses a naturally occurring circular rotation, that is, pseudospin.

In February this year, Chinese scientists and American scientists jointly published an academic achievement called Observation of chiral phonons in the internationally authoritative journal Science, in which they claimed to have observed the properties of chiral phonons in tungsten diselenide materials.

What are chiral phonons? In the classical theory of physics, phonons are widely considered as a collective linear motion of atoms which are linear polarization and do not have any angular momentum, a linear collective motion of atoms. But scientists have recently discovered that in magnetic systems with spin-phonons interactions, phonons can carry non-zero angular momentum. At zero temperatures, phonons have zero energy and zero angular momentum.

Monolayer nano-tungsten diselenide is one of the materials which own the lowest thermal conductivity. The phonons of this material are collectively vibrating in atomic crystal and naturally rotating in a certain direction. This property is called chirality which is similar to a person's handedness. The left and right hand are a mirror image of each other but not identical. The control of the rotation direction can provide a stable mechanism to carry and store the information. This rotation could become the cornerstone for a new form of information technology and for the design of molecular-scale rotors to drive micro-motors and machines.

The monolayer material, tungsten diselenide (WSe2), is well-known for its unusual ability to sustain special electronic properties that are far more fleeting in other materials. Therefore, it is regarded as the most potential valleytronics material. What is valleytronics material? For example, the momentum and wavelike motion of electrons in a material can be sorted into opposite "valleys" in a material's electronic structure. Each of these valleys represents the ones and zeroes in conventional binary data, which forms the basis of all modern computer operations, the binary system.

One of the biggest advantages of chiral phonon is that the rotation is locked with the particle's momentum and not easily disturbed. The experiment discovered that atoms can move in circles in an atomic monolayer crystal of tungsten diselenide. When scientists studied the phonon mode, they found that the selenium atoms appear to collectively rotate in a clockwise direction, while the tungsten atoms showed no motion. Researchers prepared a "sandwich" which has 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 the samples with 70 millionths of a meter in diameter. One of the lasers was precisely switched between two different tuning modes to sense the difference of left and right chiral phonon activity. Later, the researchers discovered the high-energy luminescence in the sample. This is a feature of this rare absorption event. Through this technique called transient infrared spectroscopy, researchers not only verified 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. The next step is to produce a large number of rotating chiral phonons and to learn whether vigorous agitations in the crystal can be used to flip the spin of electrons or to obviously alter the valley properties of the material. Self-spin is an inherent property of electron which can be regarded as a compass. If it could be flipped and point to the north or south, it can transmit messages in a new electronic form which is called self-spin electronics. 

Are there any practical applications of this research result? Scientists believe that the same principle can be applied in all 2-D periodic structures with three-fold symmetry and inversion asymmetry. The same principle covers a huge family of natural materials which will become the cornerstone of the production of future computer, robots and other new industries.

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