17 ms·

New quasi-particle discovered: Introducing the Pi-ton

77 points by lelf 7y ago

dieselerator 7y ago
tl;dr The authors studied simulations of electrical conduction in a semi-conductor crystal. They found "bound states" of two electrons with two holes, which they call "pi-tons" a kind of quasi-particle that only exists in the crystal. They hope to find experimental evidence for this. This looks like a more complicated version of a one electron one hole bound state called an "exciton" [1]. (I have no expertise in this topic.) [1] https://en.wikipedia.org/wiki/Exciton https://en.wikipedia.org/wiki/Exciton
- srl 7y ago
  And apparently, "pi-ton" is not a pun on "pion" at all, but rather derived from a characteristic momentum that comes up... somewhere. (I didn't read the article so closely -- just enough to be horribly disappointed that the pun I was expecting, was absent.)
- SiempreViernes 7y ago
  I also don't have much expertise in condensed matter physics, but this summary looks convincing enough that it saves me the trouble of reading what is most likely a reworded press release. Many thanks!
srl 7y ago
The preprint is here on arXiv for those who want a look at the original work and don't have access to PRL: https://arxiv.org/abs/1902.09342 https://arxiv.org/abs/1902.09342 (Really, it's a little annoying that phys.org doesn't link to that directly -- a good proportion of physicists prefer to look at the arXiv version instead of the official published version even when they have access to the paywall!)
- SiempreViernes 7y ago
  Being accepted for publication still serves as a useful filter (not everything on arxiv is gold), and in this context it makes sense to link to the published version to demonstrate there is some credibility. They could put in two links though.
AstralStorm 7y ago
So, electromagnetic field would actually be mediated by excitons and pi-tons not photons? (I always found it fishy they were called electromagnetic, photons alone have no charge, only when they actually hit something does the field appear.) That would mean during Compton scattering experiment, electric field would show discrepancy due to spontaneous formation of excitons and/or pi-tons, especially if you used coherent standing wave beams of electrons and photons and vary the phases.
- perlgeek 7y ago
  > So, electromagnetic field would actually be mediated by excitons and pi-tons not photons? No. This discovery doesn't change how electromagnetism works. This is a discovery a layer above electromagnetism. Quasi particles are a useful model, because you can use them more easily to explain stuff than through the fundamental forces. For example phonons (another quasi particle found in solid stat physics) explain heat transfer more easily than if you tried to solve all the wave equations from scratch. (I like to think of quasi particles as analogous to chairs, desks and other every-day objects: they don't follow separate laws of physics, so nothing fundamentally new, but talking about them instead of just wave functions of electrons and nuclei has a much greater explanatory power).
- majewsky 7y ago
  > I always found it fishy they were called electromagnetic, photons alone have no charge Being the particles that mediate the electromagnetic force, they don't have charge themselves, but they only interact with particles that have charges. Think of it this way: When you have a charged particle, it will produce an electric field. And when you have a moving charged particle, it produces a magnetic field. Photons are how energy gets deposited into these fields. And by interacting with those photons, other particles can draw energy from this field.
  - AstralStorm 7y ago
    So moving charged particles in vacuum produce photons on their own. How do we know there are photons produced and not something else with right energy that happens to act on our sensor? Specifically, the sensors that use photoelectric effect do not react in the right wavelength, while standard radio sensors are also sensitive to charged particles. And presumably also use this exciton/pi-tons interface. How can we exclude it? Presumably fire photons from putative EM field at neutrons, observe emitted photons with wavelength change? (Essentially Compton scattering experiment with neutral particles.) My whole problem here is there's a lot of "seems as though" in those experiments... Like when you try to measure center of charge with it.
unnouinceput 7y ago
Quote": Instead of describing the motion of constantly moving electrons, it is easier to study the motion of the hole. If the electrons move to the right, the hole moves to the left—and this movement follows certain physical rules, just like the movement of an ordinary particle." That's precisely how my physics professor in 11th grade explained to us how PNP transistors works, more then 25 years ago
- gus_massa 7y ago
  Yep. That's the part we all know and is in the article only as background information. It is also known that and electron and a hole can be bound and form an "exciton" that is stable enough to be called a quasiparticle. It's somewhat similar to positronium. [1] The novel part of the article is that (in some materials?) two electrons and two holes can bound together and form something that is stable enough to be called a quasiparticle. [1] Why nobody call positronium a quasiparticle?
  - ncmncm 7y ago
    You may ask, Why does nobody call atoms quasi-particles? The original quasi-particle is the phonon. Ultimately, it comes down to convenience. It's a zoo down there. Physicists have to impose some order to be able to say anything about what goes on.
  - fhars 7y ago
    Quasiparticles need a substrate to live in, they usually are localised collective excitations of the particles that make up a solid that behave like a particle. The positronium is a bound state of two real particles.
- kmm 7y ago
  I know all analogies, especially those explaining complicated physics to 11th graders, have to oversimplify some aspects of a phenomenon, but this one always rubbed me the wrong way. The hole moves with the electrons, if they move right, the hole also moves right. The crucial point is that the electrons at the top of a valence band have effective negative mass, i.e. they move opposite to an applied force. Of course their true mass hasn't changed, this is simply a consequence of the fact that we're considering a system of uncountably many electrons moving in unison. This means a full band carries no current (which is the situation in isolators), but if a band has some electrons missing at the top, these holes will be transported in the opposite direction of the usual electron current, which happens to be the correct direction for positive charges. Hence, we can talk about positive charge carries. A better analogy would be a bubble of air underwater in a river. It moves with the flow of the water, not against it. I know it doesn't really matter, but I feel an analogy must at least have a core of truth.
einpoklum 7y ago
And now for something quasi-different: Introducing the Pi-Ton by Cleese, Idle, Palin, Jones (Posthumous) & al.
kaliali 7y ago
I assume this a video by Sixty Symbols on the same subject https://www.youtube.com/watch?v=8k6dKfmOufI https://www.youtube.com/watch?v=8k6dKfmOufI
- tiborsaas 7y ago
  I don't think it's related. Why there's no mention of X17 neither in the article nor in the paper?
mfer 7y ago
I’m fascinated by the language we use. They have not observed the particle. It was found in computer simulations. Yet, the word used for this is discovered. At first I had the impression the observed it. I hope they do.

77 points by lelf 7y ago

tl;dr The authors studied simulations of electrical conduction in a semi-conductor crystal. They found "bound states" of two electrons with two holes, which they call "pi-tons" a kind of quasi-particle that only exists in the crystal. They hope to find experimental evidence for this. This looks like a more complicated version of a one electron one hole bound state called an "exciton" [1]. (I have no expertise in this topic.) [1] https://en.wikipedia.org/wiki/Exciton https://en.wikipedia.org/wiki/Exciton

srl 7y ago
And apparently, "pi-ton" is not a pun on "pion" at all, but rather derived from a characteristic momentum that comes up... somewhere. (I didn't read the article so closely -- just enough to be horribly disappointed that the pun I was expecting, was absent.)
SiempreViernes 7y ago
I also don't have much expertise in condensed matter physics, but this summary looks convincing enough that it saves me the trouble of reading what is most likely a reworded press release. Many thanks!

srl 7y ago

The preprint is here on arXiv for those who want a look at the original work and don't have access to PRL: https://arxiv.org/abs/1902.09342 https://arxiv.org/abs/1902.09342 (Really, it's a little annoying that phys.org doesn't link to that directly -- a good proportion of physicists prefer to look at the arXiv version instead of the official published version even when they have access to the paywall!)

SiempreViernes 7y ago
Being accepted for publication still serves as a useful filter (not everything on arxiv is gold), and in this context it makes sense to link to the published version to demonstrate there is some credibility. They could put in two links though.

AstralStorm 7y ago

So, electromagnetic field would actually be mediated by excitons and pi-tons not photons? (I always found it fishy they were called electromagnetic, photons alone have no charge, only when they actually hit something does the field appear.) That would mean during Compton scattering experiment, electric field would show discrepancy due to spontaneous formation of excitons and/or pi-tons, especially if you used coherent standing wave beams of electrons and photons and vary the phases.

perlgeek 7y ago
> So, electromagnetic field would actually be mediated by excitons and pi-tons not photons? No. This discovery doesn't change how electromagnetism works. This is a discovery a layer above electromagnetism. Quasi particles are a useful model, because you can use them more easily to explain stuff than through the fundamental forces. For example phonons (another quasi particle found in solid stat physics) explain heat transfer more easily than if you tried to solve all the wave equations from scratch. (I like to think of quasi particles as analogous to chairs, desks and other every-day objects: they don't follow separate laws of physics, so nothing fundamentally new, but talking about them instead of just wave functions of electrons and nuclei has a much greater explanatory power).
majewsky 7y ago
> I always found it fishy they were called electromagnetic, photons alone have no charge Being the particles that mediate the electromagnetic force, they don't have charge themselves, but they only interact with particles that have charges. Think of it this way: When you have a charged particle, it will produce an electric field. And when you have a moving charged particle, it produces a magnetic field. Photons are how energy gets deposited into these fields. And by interacting with those photons, other particles can draw energy from this field.
- AstralStorm 7y ago
  So moving charged particles in vacuum produce photons on their own. How do we know there are photons produced and not something else with right energy that happens to act on our sensor? Specifically, the sensors that use photoelectric effect do not react in the right wavelength, while standard radio sensors are also sensitive to charged particles. And presumably also use this exciton/pi-tons interface. How can we exclude it? Presumably fire photons from putative EM field at neutrons, observe emitted photons with wavelength change? (Essentially Compton scattering experiment with neutral particles.) My whole problem here is there's a lot of "seems as though" in those experiments... Like when you try to measure center of charge with it.

unnouinceput 7y ago

Quote": Instead of describing the motion of constantly moving electrons, it is easier to study the motion of the hole. If the electrons move to the right, the hole moves to the left—and this movement follows certain physical rules, just like the movement of an ordinary particle." That's precisely how my physics professor in 11th grade explained to us how PNP transistors works, more then 25 years ago

gus_massa 7y ago
Yep. That's the part we all know and is in the article only as background information. It is also known that and electron and a hole can be bound and form an "exciton" that is stable enough to be called a quasiparticle. It's somewhat similar to positronium. [1] The novel part of the article is that (in some materials?) two electrons and two holes can bound together and form something that is stable enough to be called a quasiparticle. [1] Why nobody call positronium a quasiparticle?
- ncmncm 7y ago
  You may ask, Why does nobody call atoms quasi-particles? The original quasi-particle is the phonon. Ultimately, it comes down to convenience. It's a zoo down there. Physicists have to impose some order to be able to say anything about what goes on.
- fhars 7y ago
  Quasiparticles need a substrate to live in, they usually are localised collective excitations of the particles that make up a solid that behave like a particle. The positronium is a bound state of two real particles.
kmm 7y ago
I know all analogies, especially those explaining complicated physics to 11th graders, have to oversimplify some aspects of a phenomenon, but this one always rubbed me the wrong way. The hole moves with the electrons, if they move right, the hole also moves right. The crucial point is that the electrons at the top of a valence band have effective negative mass, i.e. they move opposite to an applied force. Of course their true mass hasn't changed, this is simply a consequence of the fact that we're considering a system of uncountably many electrons moving in unison. This means a full band carries no current (which is the situation in isolators), but if a band has some electrons missing at the top, these holes will be transported in the opposite direction of the usual electron current, which happens to be the correct direction for positive charges. Hence, we can talk about positive charge carries. A better analogy would be a bubble of air underwater in a river. It moves with the flow of the water, not against it. I know it doesn't really matter, but I feel an analogy must at least have a core of truth.

einpoklum 7y ago

And now for something quasi-different: Introducing the Pi-Ton by Cleese, Idle, Palin, Jones (Posthumous) & al.

kaliali 7y ago

I assume this a video by Sixty Symbols on the same subject https://www.youtube.com/watch?v=8k6dKfmOufI https://www.youtube.com/watch?v=8k6dKfmOufI

tiborsaas 7y ago
I don't think it's related. Why there's no mention of X17 neither in the article nor in the paper?

mfer 7y ago

I’m fascinated by the language we use. They have not observed the particle. It was found in computer simulations. Yet, the word used for this is discovered. At first I had the impression the observed it. I hope they do.