[spoiler=Author's Forward]
Hey everybody! I've been away from this site for various reasons, but during my absence, I've learned quite a bit about many electrical phenomena. I hope to share some of this knowledge with you, even if it flies above some heads.

I'll try to explain it in lay terms, but if you have any questions, please ask!

Cheers
- Gui

[/spoiler]

When I mention "particle physics," you might think of the electron, neutron, and proton we're taught about in school. Or maybe you think of the massive particle colliders that smash small things together at almost the speed of light.

Particle physics has a closer application than you think: computers and phones! These are built on the application of condensed matter physics, a flavor of particle physics that focuses on the collective behavior of particles under regular (ish) conditions.

In this series of blogs (in no particular order), I'll introduce exciting concepts from this branch of physics, starting with what we call "quasiparticles."

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I briefly mentioned the electron, proton, and neutron as the typical subatomic particles. Many more particles are in the so-called Standard Model, but those aren't our focus today. Instead, we'll describe things that behave like particles but aren't really.

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Brief Foray into Quantum Mechanics

To build an understanding of quasiparticles, we'll need to take two principles from quantum mechanics: quantization and particle-wave duality.

You might've heard before that light can act as a wave or a particle. This is because the energy from a beam of light can't be divided up infinitely - like how you can split a jelly bean into smaller parts until you reach the atom (or, if you want to get technical, quarks). At some point, you'll be unable to divide the thing into smaller parts. This is because many things in our universe (like energy or angular momentum) are quantized or formed out of discrete "packets." As such, you end up with the photon, which is one of those packets of energy for light.

At the same time, light comprises many photons that still act like a wave. For example, in the famous double-slit experiment, light traveling through two adjacent slots in a board leaves a pattern of light that isn't simply two bright spots directly behind the slots. Instead, they create an interference pattern: a repeating pattern of light and dark spots showing where the light from the two slots added and subtracted their strengths from each other, respectively. This is in line with our expectations that light acts as a wave.
However, the behavior persisted when the experiment was repeated with a single photon. That must mean that it's still a wave, but it was a particle. This is the essence of the particle-wave duality: a particle with a mass and velocity can also be described as a wave. (You can do this with things like electrons, too!)

Yes, you and I can be described as waves, too. What's different is that the wave's wavelength is inversely dependent on your mass, which means that even though electrons can tunnel through walls, you and I will have no chance of doing the same.

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Holes and Lattices

With that out of the way, we can start talking about our first quasiparticle: the hole.

The hole is essentially the lack of an electron. Why, you might ask, would that be important?

Holes are essential in understanding some electronic phenomena in a lattice. A lattice is a grid of atoms packed together in some regular pattern. In this lattice, we have electrons that dance around the lattice, mostly staying with an atom but sometimes not (this is due to thermal excitations that kick the electron away from the atom). When an electron leaves an atom behind, it takes a negative charge, leaving a positive charge in its wake. This is the hole.
Just like an electron can create a hole, one can destroy it. When an electron comes into the gap left behind by another electron, it is said to recombine with that hole, thus eliminating it. However, if that electron left a hole behind in another atom, we could say that the hole is "moving": that gap is moving around the lattice, much like an electron.

The hole's positive charge is exactly the opposite of the electron's negative charge, meaning it, too, can catch an electron and make it orbit it, creating something called the exciton.
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The Phonon and the Cooper Pair

We can apply some of the same logic we did to the photon to vibrations in the lattice, also known as heat. It has finite chunks of energy, like the photon, that we can superimpose, and it travels around a lattice like a ripple in a pond. As such, we can define the phonon as a new quasiparticle that represents these vibrations.

What's cool about these vibrations, if they're localized enough, is that the movement of the positively charged nuclei in the lattice can create enough attraction and repulsion to trap electrons. On the other hand, the electrical charge from the electron can also move the nuclei around, creating a phonon. (This is known as the phonon-electron interaction.)

This interplay, if tuned correctly, can cause one moving electron to move the lattice around and trap another electron right behind it. These electrons then move in lock-step with each other. This is a quasiparticle known as the "Cooper pair." This phenomenon of electrons moving through a material effortlessly has another name: superconductivity.
This kind of interaction is only stable in the absence of other vibrations in the lattice – which means that the material has to be very cold. This is why superconductors must be chilled close to absolute zero, with very few vibrations in the lattice.

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I hope you learned something from this brief blog, and please let me know if you'd like to see more!