Great video intro from PBS Space Time:
https://youtu.be/le_ORQZzkmE?si=ylKXLkx5D_AfzGdE
Quasiparticles are collective excitations in a system that behave like particles, even though they're not fundamental particles. Think of them as emergent phenomena that arise from the complex interactions within materials.
Phonons are among the most important quasiparticles. They represent quantized vibrations in a crystal lattice - essentially, they're the particles of sound and heat in solids. When atoms in a crystal vibrate, they create wave-like patterns of motion. These waves can be thought of as particles (phonons) with specific energies and momenta. Phonons play a crucial role in:
Thermal conductivity (they carry heat through materials)
Sound propagation in solids
Electron-phonon interactions that are vital for superconductivity
(Top) acoustic phonons, where the movement of atoms is in phase (Bottom) optical phonons, where the movement of atoms is out-of-phase. Optical phonons can be excited by electromagnetic waves. https://www.researchgate.net/figure/Two-types-of-phonon-modes-in-crystalline-structure-up-acoustic-phonons-where-the_fig16_333337278
Both types of phonons play crucial roles in conventional superconductivity, but in different ways.
Optical Phonons:
These are often more important for superconductivity because they're directly involved in the electron-phonon coupling mechanism that forms Cooper pairs
The strong out-of-phase motion of ions creates local charge fluctuations that can mediate electron pairing
In many conventional superconductors, optical phonon modes are the primary mechanism for electron-electron attraction
MgB2 is a famous example where high-frequency optical phonon modes are crucial for its relatively high Tc (39K)
Acoustic Phonons:
While generally less important for the pairing mechanism, they still contribute to the overall electron-phonon interaction
They can affect the coherence length of Cooper pairs
They're important for understanding the thermal properties of the superconducting state
They can influence the phase stability of the superconducting state
The interplay between both types becomes especially interesting when considering:
The total electron-phonon coupling strength
The competition between electron-phonon attraction and Coulomb repulsion
The overall stability of the superconducting state
The temperature dependence of superconducting properties
This relationship between different phonon modes and superconductivity is part of why designing new superconductors is so challenging - you need to optimize multiple competing factors simultaneously.
Cooper pairs are another fascinating type of quasiparticle, central to our understanding of superconductivity. They form when two electrons overcome their natural repulsion and become bound together in a special way. This happens because:
One electron slightly distorts the crystal lattice as it moves through
This distortion creates a small positive charge region
A second electron is attracted to this region
Through this phonon-mediated interaction, the electrons become paired
https://courses.physics.illinois.edu/phys498art/sp2018/PHYS489ART_SC_KP.pdf
The remarkable thing about Cooper pairs is that they behave like bosons (particles that can occupy the same quantum state), unlike individual electrons which are fermions. This allows them to form a collective state called a condensate, which enables electrical current to flow without resistance - the defining feature of superconductivity.