Parth G | Why Do Physicists Believe In These Particles That DON'T Exist? Quasiparticles by Parth G @ParthGChannel | Uploaded 2 years ago | Updated 5 hours ago
The answer: these "Quasiparticles" make physics much easier to study!
In this video we'll be studying 3 quasiparticles (sometimes known as collective excitations). They don't actually exist, in that they are not fundamental particles themselves, but can be thought of as mathematical simplifications of more complex systems.
The first quasiparticle we'll look at is the phonon. We look at a sound wave passing through a neatly arranged grid of atoms in a solid. Transferring energy to one end of the solid, we see that the atoms will oscillate in a special way so that the energy is transferred through the solid to the other side. This oscillation of the atoms is known as a sound wave.
If we want to pass multiple sound waves through the material at the same time, then in order to study these in detail, we have to look at the movement of each individual particle. This becomes extremely time-consuming. Instead, we can treat each sound wave as a made-up quasiparticle. Since each sound wave is represented by a "phonon", we can study how multiple sound waves interact by looking at how multiple phonons interact. And this is simpler than studying the motion of millions of atoms, because phonons follow some "common sense" physics laws such as conservation of momentum. We can also study the interaction of phonons with other particles, like photons (which are considered to be real particles)! I've made a video discussing phonons in more detail, you can find it here if you're interested: https://www.youtube.com/watch?v=_axrpVnGHpk
The next quasiparticle we'll discuss in this video is the electron hole. Atoms can form covalent bonds with each other by sharing electrons in order to have full outer shells. This is a stable configuration. However if we provide some amount of energy to our atoms, this can cause an electron to leave a covalent bond and break it. This free electron moves away through the lattice and leaves behind an "electron hole". This hole can be filled by another electron from a nearby bond, which means the hole moves to this second bond.
Sometimes, free electrons can come and fill the hole. This process is known as recombination, and we don't discuss that in this video. Instead, we focus more on bound electrons moving to fill a hole, resulting in the movement of this hole through the grid of atoms. Studying the movement of a hole is easier than looking at each of the individual electrons that move around the lattice in order to fill where the hole was previously. And on top of this, we can give some properties to the hole that form the entire foundation of semiconductor physics.
For example, when a hole is formed due to an electron gaining enough energy to leave the covalent bond, which then travels to other parts of the solid, the number of protons in the nuclei surrounding the hole is larger than the number of electrons in the surrounding area. There is an excess of positive charge, and this positive charge can actually be assigned to the hole. As the hole moves around, we can see (roughly) the motion of excess positive charge - not because the positive charges are moving, but because the regions of missing negative charge (i.e. holes) are moving.
The third quasiparticle we will look at is the electron quasiparticle. This involves looking at a free electron moving around a periodic potential (i.e. regular arrangement of nuclei / charged particles). In a real scenario, the electron will be affected by the periodic (regular) arrangement of charges, so will not move in a straight line at a constant speed. Instead, we can devise a new particle that moves through an assumed vacuum, with similar properties to the electron but with a different mass. This is useful is because we can assign the periodic accelerations of the electron (from the surrounding charges) to the quasiparticle's different mass. In other words, we imagine the quasiparticle doesn't experience any forces, and its average motion is the same as the electron's average motion. In certain systems where the periodic potential varies depending on the direction in which the electron is moving, the "effective mass" of the quasiparticle is direction-dependent!
Thanks so much for watching - please do check out my socials here:
Instagram - @parthvlogs
Patreon - patreon.com/parthg
Music Chanel - Parth G's Shenanigans
Merch - https://parth-gs-merch-stand.creator-spring.com/
Many of you have asked about the stuff I use to make my videos, so I'm posting some affiliate links here! I make a small commission if you make a purchase through these links.
A Quantum Physics Book I Enjoy: https://amzn.to/3sxLlgL
My Camera (Sony A6400): https://amzn.to/2SjZzWq
ND Filter: https://amzn.to/3qoGwHk
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Timestamps:
0:00 - Physicists Make Up Imaginary Particles
0:24 - Phonons
2:06 - Electron Holes
6:06 - Electron Quasiparticles
The answer: these "Quasiparticles" make physics much easier to study!
In this video we'll be studying 3 quasiparticles (sometimes known as collective excitations). They don't actually exist, in that they are not fundamental particles themselves, but can be thought of as mathematical simplifications of more complex systems.
The first quasiparticle we'll look at is the phonon. We look at a sound wave passing through a neatly arranged grid of atoms in a solid. Transferring energy to one end of the solid, we see that the atoms will oscillate in a special way so that the energy is transferred through the solid to the other side. This oscillation of the atoms is known as a sound wave.
If we want to pass multiple sound waves through the material at the same time, then in order to study these in detail, we have to look at the movement of each individual particle. This becomes extremely time-consuming. Instead, we can treat each sound wave as a made-up quasiparticle. Since each sound wave is represented by a "phonon", we can study how multiple sound waves interact by looking at how multiple phonons interact. And this is simpler than studying the motion of millions of atoms, because phonons follow some "common sense" physics laws such as conservation of momentum. We can also study the interaction of phonons with other particles, like photons (which are considered to be real particles)! I've made a video discussing phonons in more detail, you can find it here if you're interested: https://www.youtube.com/watch?v=_axrpVnGHpk
The next quasiparticle we'll discuss in this video is the electron hole. Atoms can form covalent bonds with each other by sharing electrons in order to have full outer shells. This is a stable configuration. However if we provide some amount of energy to our atoms, this can cause an electron to leave a covalent bond and break it. This free electron moves away through the lattice and leaves behind an "electron hole". This hole can be filled by another electron from a nearby bond, which means the hole moves to this second bond.
Sometimes, free electrons can come and fill the hole. This process is known as recombination, and we don't discuss that in this video. Instead, we focus more on bound electrons moving to fill a hole, resulting in the movement of this hole through the grid of atoms. Studying the movement of a hole is easier than looking at each of the individual electrons that move around the lattice in order to fill where the hole was previously. And on top of this, we can give some properties to the hole that form the entire foundation of semiconductor physics.
For example, when a hole is formed due to an electron gaining enough energy to leave the covalent bond, which then travels to other parts of the solid, the number of protons in the nuclei surrounding the hole is larger than the number of electrons in the surrounding area. There is an excess of positive charge, and this positive charge can actually be assigned to the hole. As the hole moves around, we can see (roughly) the motion of excess positive charge - not because the positive charges are moving, but because the regions of missing negative charge (i.e. holes) are moving.
The third quasiparticle we will look at is the electron quasiparticle. This involves looking at a free electron moving around a periodic potential (i.e. regular arrangement of nuclei / charged particles). In a real scenario, the electron will be affected by the periodic (regular) arrangement of charges, so will not move in a straight line at a constant speed. Instead, we can devise a new particle that moves through an assumed vacuum, with similar properties to the electron but with a different mass. This is useful is because we can assign the periodic accelerations of the electron (from the surrounding charges) to the quasiparticle's different mass. In other words, we imagine the quasiparticle doesn't experience any forces, and its average motion is the same as the electron's average motion. In certain systems where the periodic potential varies depending on the direction in which the electron is moving, the "effective mass" of the quasiparticle is direction-dependent!
Thanks so much for watching - please do check out my socials here:
Instagram - @parthvlogs
Patreon - patreon.com/parthg
Music Chanel - Parth G's Shenanigans
Merch - https://parth-gs-merch-stand.creator-spring.com/
Many of you have asked about the stuff I use to make my videos, so I'm posting some affiliate links here! I make a small commission if you make a purchase through these links.
A Quantum Physics Book I Enjoy: https://amzn.to/3sxLlgL
My Camera (Sony A6400): https://amzn.to/2SjZzWq
ND Filter: https://amzn.to/3qoGwHk
Microphone and Stand (Fifine): https://amzn.to/2OwyWvt
Gorillapod Tripod: https://amzn.to/3wQ0L2Q
Timestamps:
0:00 - Physicists Make Up Imaginary Particles
0:24 - Phonons
2:06 - Electron Holes
6:06 - Electron Quasiparticles
