
New quasi-particle discovered: Introducing the Pi-ton - lelf
https://phys.org/news/2020-02-quasi-particle-pi-ton.html
======
unnouinceput
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

~~~
kmm
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.

~~~
mkl
The hole only moves with the electrons if the electrons are moving in unison.
If they move one at a time, into the hole left by the previous electron, then
the hole moves the opposite way. E.g. say a, b, c, ... are electrons:

    
    
      1. abcde
      2. abcd e
      3. abc de
      4. ab cde
      5. a bcde
      6.  abcde
    

(I don't know much chemistry or electronics, but that's how I've always
understood the analogy.)

~~~
kmm
That is exactly the analogy I have a problem with, it is variously explained
with cars in parking lots, people on seats, .... It is wrong however, the
electrons do not move one at a time, they move in unison opposite to the
normal flow of current.

Apart from theoretical considerations, we know it has to be like this because
the voltage induced by the Hall effect flips for these kinds of
semiconductors, and since there are no positively charged free particles in
solids, this has to mean the electrons are now moving in the other direction,
giving a complete illusion of a positive charge carrier.

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mfer
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.

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dieselerator
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
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.)

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

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AstralStorm
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.

~~~
majewsky
> 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
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.

------
srl
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
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.

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einpoklum
And now for something quasi-different: Introducing the Pi-Ton

by Cleese, Idle, Palin, Jones (Posthumous) & al.

