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- Scientists have found that while phonons can’t be deflected by the magnetic field – they have no electric charge – they are affected by the electrons that are deflected by the magnetic field.
What are phonons?
- Technically, phonons aren’t particles; they’re quasiparticles – packets of energy that behave like particles in a system.
- A phonon is a quasiparticle of vibrational energy. When the grid of atoms that make up the material vibrates, it releases this energy, and physicists encapsulate it in the form of photons.
- In the presence of a magnetic field, electrons are deflected from their paths in a perpendicular direction. The phonons aren’t directly affected by the magnetic field, but when they scatter off the deflected electrons, they are deflected in a perpendicular direction as well.
Note: The Thermal Hall effect arises when the particles carrying energy have chirality, in that they deviate more clockwise than anticlockwise, or vice versa, in some circumstances. This lateral scattering mechanism is called skew-scattering.
- To see the thermal Hall effect, phonons need to be deflected perpendicular to their direction of motion, so skew-scattering is important.
- Phonons are an example of collective excitations: “they are essentially a well-defined wave packet” of vibrational energy.
THERMAL HALL EFFECT
The thermal Hall effect is the thermal analog of the electrical Hall effect. Instead of a transverse voltage induced by a perpendicular magnetic field in the presence of an electric current, a transverse temperature difference is induced in the presence of a heat current.
The principle of the Hall effect states that when a current-carrying conductor or a semiconductor is introduced to a perpendicular magnetic field, a voltage can be measured at the right angle to the current path. This effect of obtaining a measurable voltage is known as the Hall effect.