Clockwise or Anticlockwise, That is the Question: Phonons with Angular Momentum in Chiral Crystals
© The Physical Society of Japan
This article is on
Theory of Energy Dispersion of Chiral Phonons
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 92, 023601 (2023).
Chiral crystals have lattice structures with no mirror or inversion symmetries.
A few basic questions about their unique phonon excitations with intrinsic angular momentum are answered.
Propagating lattice vibration in crystals resembles light, and atomic displacement corresponds to the electric or magnetic field. One can easily control light by using a waveplate to convert its polarization from linear to circular and vice versa without changing the frequency of light. However, this type of conversion is not possible for lattice vibrations in chiral crystals, such as quartz and tellurium. This is because some vibration modes must have a distorted circular (i.e., elliptic) polarization, and their eigenfrequency depends on whether displacements rotate clockwise or anticlockwise in contrast to the behavior of ordinary light. The quanta of these vibration modes are called chiral phonons, and the corresponding eigenfrequency determines their energy. Chiral crystals have a structure with definite handedness. The mirror image of the structure is not identical to the original, as the right-handed screw differs from the mirror-reflected left-handed screw. The abovementioned split of the chiral phonon energy is an important consequence of the chiral lattice structure.
The crystals have multiple branches of lattice vibrations, i.e., phonons; three are acoustic branches, while the others are optical. One crucial question is which branch is most sensitive to the chiral structure. By studying a chiral model with a screw structure, the authors have shown that the optical branches are considerably more sensitive than the acoustic branches and obtained a simple formula for the energy splitting between chiral phonons rotating oppositely. The splitting is proportional to the product of the phonon’s linear momentum and the intrinsic angular momentum representing the rotation direction. The proportionality constant Γ is an important order parameter characterizing the system’s chirality, and its explicit form has been obtained in this work as a function of microscopic material parameters. As the chiral systems lack mirror and related inversion symmetries, polar vectors such as linear momentum can directly couple to axial vectors such as angular momentum, and their coupling constants have the symmetry of pseudoscalar. The nonvanishing value of the obtained Γ demonstrates this situation and is related to the electric toroidal monopole G0. As for the chiral acoustic branches, there has been an argument that they may have different sound velocity values depending on the phonon’s angular momentum. The authors have proved that these sound velocities should be identical under general conditions. They have also obtained conditions for microscopic models, which are expected to be useful for model building based on the first-principle calculations.
(written by H. Tsunetsugu on behalf of all the authors.)
Theory of Energy Dispersion of Chiral Phonons
(JPSJ Editors' Choice)
J. Phys. Soc. Jpn. 92, 023601 (2023).
Share this topic
Fields
Related Articles
-
Evaluation of the Exchange Stiffness Constants of Itinerant Magnets from the First-Principles Calculations
Electron states in condensed matter
Structure and mechanical and thermal properties in condensed matter
2024-6-5
Using first-principles calculations, we evaluated the exchange stiffness constants of ferromagnetic metals at finite temperatures. The constants can be used as parameters in the Landau–Lifshitz–Gilbert equation.
-
Which is Moving?—Pinning Down the Origin of Fluctuations in Muon Spin Relaxation—
Structure and mechanical and thermal properties in condensed matter
Cross-disciplinary physics and related areas of science and technology
2024-3-28
The study demonstrated that we can distinguish between the diffusion motion of the muon itself and the motion of the surrounding ions in muon spin relaxation.
-
Variety of Mechanically Induced Spin Currents in Rashba Systems
Electronic transport in condensed matter
Magnetic properties in condensed matter
Structure and mechanical and thermal properties in condensed matter
2024-3-22
Various types of spin currents, including unconventional types, are generated in Rashba spin-orbit coupled systems by dynamic lattice distortions associated with, for example, surface acoustic waves.
-
Relation between Mean-Field Theory and Atomic Structures in Chalcogenide Glasses
Structure and mechanical and thermal properties in condensed matter
2024-2-1
The authors conducted various of X-ray and neutron scattering experiments on typical chalcogenide glasses and clarified the relationship between the atomic structure and simple rigidity percolation theory.
-
Possible Origin of High Thermoelectric Power Factor in Ultrathin FeSe: A Two-band Model
Electronic structure and electrical properties of surfaces and nanostructures
Structure and mechanical and thermal properties in condensed matter
Cross-disciplinary physics and related areas of science and technology
2023-12-21
The high thermoelectric power factor observed in ultrathin FeSe can be theoretically explained by a two-band model with chemical potential between upper and lower band bottoms.