What are quasiparticles?

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.

Quasiparticles in superconductors

Phonons

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

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:

1. The total electron-phonon coupling strength

2. The competition between electron-phonon attraction and Coulomb repulsion

3. The overall stability of the superconducting state

4. 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

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:

1. One electron slightly distorts the crystal lattice as it moves through

2. This distortion creates a small positive charge region

3. A second electron is attracted to this region

4. Through this phonon-mediated interaction, the electrons become paired

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.