A polaron is a quasiparticle formed when an electron (or hole) in a crystal interacts with the vibrations of the surrounding atomic lattice (phonons), effectively dressing the charge carrier with a cloud of lattice distortion.
Key Features:
- Electron–phonon interaction: As an electron moves through an ionic crystal, its electric field distorts the positions of nearby atoms. This deformation creates a region of local polarization that follows the electron, modifying its properties.
- Effective mass: The polaron behaves like a heavier particle than a bare electron because it carries the lattice distortion with it, increasing its effective mass.
- Mobility: Due to its increased mass and interaction with the lattice, a polaron generally has lower mobility than a free electron.
Types of Polarons:
- Small polaron: The lattice distortion is confined to a small region around the electron. These are typically found in materials with strong electron–phonon coupling and low dielectric constants. Small polarons tend to hop from site to site.
- Large polaron: The lattice distortion extends over many lattice sites. These are more delocalized and occur in materials with weak to moderate coupling.
Significance:
- Conductivity: Polarons can dominate charge transport in materials like transition metal oxides, organic semiconductors, and perovskites.
- Thermoelectric and photovoltaic devices: Understanding polaron dynamics helps in designing materials with optimized charge transport properties.
- High-temperature superconductors: Polarons may play a role in the pairing mechanisms behind unconventional superconductivity.
Polarons illustrate how charge carriers do not move through solids in isolation — their motion is intricately connected to the medium they move through.