Why Do Physicists Believe In These Particles That DON'T Exist? Quasiparticles by Parth G

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.

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!

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Timestamps:
0:00 - Physicists Make Up Imaginary Particles
0:24 - Phonons
2:06 - Electron Holes
6:06 - Electron Quasiparticles

[ParthGChannel]

Hey everyone, thanks for watching! As always please let me know what topics you'd like me to cover in future videos :)

[UsamaThakurr]

Can you do a video on how Pure Mathematics such as Topology plays a role in Physics?

[AndrewDotsonvideos]

Thought you were gonna discuss ghosts in the context of a gauge theory lol. If anyone could do it it be you 😂nice video!

[unvergebeneid]

If something is full of holes on a quantum level, does that make it quantum cheese?

[fniksnorthboy]

He is the teacher i never had but always needed

[gseeker_anew]

I read about holes before, but could not get my mind around them, but your explanation just made it so clear. This is genius.

[rahuldevarakonda]

Hey Parth! love the explanation on electron quasiparticles! it would be interesting if you could explain about Majorana Fermions, cause I think your audience might find it interesting, considering how they can be explained in the context of neutrinos and ising anyons. Quantum Computing AND particle physics in one video? Seems like a clickable video idea :)

[TJOsborne]

Currently studying a condensed matter course. So this was really helpful, especially your description of effective mass. More condensed matter videos would be great!

[AtulKumar-cgrl]

If it looks like a duck, swims like a duck, and quacks like a duck, then it probably is a duck.

[Thekilldbera]

Could you make a video of fundamental of superconductivity, as your created video are too much good for its good visualisation effect... Thank you...

[practicecoach]

Kind of unrelated, my dad lives on a hill in a city, there's strong wifi signal all around the area, but he lives in a 'pocket' of very bad signal. All the internet companies have the same problem in that pocket. You get signal there for 30 minutes, then next to none for the next hour or two. Why is this?? Thanks

[LeMinhHoang_]

You are my saver. sorry can i have a question about the electron holes, so when i think about these particles and the place of these particle take place, does it take place in vacuum?, and can we see the assumed version of this current in the real life at the eye, i got despressed for this situation for the long time, i have loved this video so much and i hope you can see my questions, thank you so much Parth G

[kayrstar]

Will they be massless ?...as there is no charge for hole but to balance the excess charge present at the nucleus, it is given to hole ....similarly does it acquire mass with similar logic?

[pritmukherjee]

Ghost particle = hole ????
If not, then what's the difference between them?

[recramorcenlemniscate]

Aren't holes only useful because of the insistence of people to use conventional current & not use the proper flow of charges?

[praharmitra]

I really like that you slow down when you explain difficult ideas so listeners can understand it in one go. I'd also like to add that in the video you discuss quasi-particles, but in the thumbnail of the video you mentioned ghost particles. Ghosts are a completely different type of fake particle than the quasi-particles you discuss in the video (they play a completely different role in physics). I strongly suggest clarifying this in a comment or changing the thumbnail to not cause confusion! Great video as always!

[narfwhals]

I read that we can model light in a medium as a quasiparticle with an effective mass instead of going through the wave mechanics of phase and group velocity. Is that accurate? And is it more or less useful than saying the "speed of light in the medium" is different than c.

[Sidthu]

Parth G can u make a video about post doc. I really wanna know some basics of it.

[internationalfatherinlaw]

4:02

But wouldn't the electrons delocalise as all those structures would act like resonance structures.

[jamestickle]

Aren’t all of our fundamental particles also just models or concepts of what is physically happening? Like we have quarks and we assign 1/3 or 2/3 charges etc but this is not physical reality. It is just handy for calculations.