[HN Gopher] Physicists Puzzle over Emergence of Electron Aggregates
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Physicists Puzzle over Emergence of Electron Aggregates
Author : westurner
Score : 102 points
Date : 2024-05-30 14:33 UTC (2 days ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| pmayrgundter wrote:
| I wonder if the moire materials can tuned as a dynamic
| metamaterial to absorb or dump a lot of charge, based on their
| rotational geometry. That charge plateau seems like a quite
| powerful trick
| johncarlosbaez wrote:
| I've never seen a good clear explanation of "fractionalization" -
| the way that that systems can contain excitations that act like
| particles with charges that are a fraction of the electron
| charge. A typical non-explanation:
|
| https://scitechdaily.com/quantum-revolution-redefining-physi...
| octonion137 wrote:
| Couldn't an effective theory/mean-field theory have almost any
| emergent behavior depending on the underlying configuration
| space/system? Kind of like how one can build a domain-specific
| language out of an expressive enough underlying language it's
| implemented in
| johncarlosbaez wrote:
| Maybe, but getting fractional charges out of a system with
| charges that are integer multiples of the electron charge is
| no mean feat, and I want to understand the math behind this
| in detail. Someone knows, but I haven't found any clear
| explanations.
| gradschoolfail wrote:
| The way my inner child handwaves it away: you have the electron
| wavefunction spread out, say, it's equally likely to be in one
| of 2 points in space. If you only look at one of these 2
| points, you are likely to measure only half an electron.. until
| an adult (say, you) corrects me (using the Feynman Dirac
| hand/belt trick)
| johncarlosbaez wrote:
| That explains having a 50% chance of seeing an electron
| somewhere, not seeing an entity of charge 1/2. It's like a
| weather report saying there's a 50% chance of rain doesn't
| mean you're going to see little raindrops cut in half.
| gradschoolfail wrote:
| If you look at the experiments, they don't mention
| observing a single entity of fractional charge, it is
| always in terms of aggregate behavior under EM fields:
| conductance(1) inferred from shot noise (2), or density (3)
|
| (1) https://arxiv.org/pdf/0912.4868
|
| (2) https://n.ethz.ch/~marnikm/files/shotNoise.pdf
|
| (3) https://www.researchgate.net/profile/Gerhard-
| Abstreiter/publ...
|
| Personally, I find it curious that people talk about
| detecting single photons, but in these fractional charge
| experiments, nobody mentions detecting a single
| quasiparticle.
|
| As for the math, nobody says it outright, or even in a
| single paragraph, but a fractional charge ("filling
| fraction") of p/q does correspond to p "normal" charges
| distributed over q degenerate states (q=2 equivalent
| locations I used in the naive example)
|
| https://xgwen.mit.edu/sites/default/files/documents/topWN.p
| d...
| zmgsabst wrote:
| > Personally, I find it curious that people talk about
| detecting single photons, but in these fractional charge
| experiments, nobody mentions detecting a single
| quasiparticle.
|
| You detect a single photon when it perturbs an apparatus
| like a photon multiplier; you detect a single
| quasiparticle when it perturbs a split stream of
| electrons.
|
| The apparent difference is that photons can travel
| through free space and strike such an apparatus from
| afar; while quasiparticles definitionally cannot.
| However, I've read about experiments that measure a
| single anyon on a dot by wrapping electron interferometry
| around it, which is measuring the lone quasiparticle on
| that dot.
|
| So I don't follow your point.
| marcosdumay wrote:
| There is only a difference if you observe the position.
|
| When interactions happen only between particles that are
| all spread through that same space, both look the same.
| dustingetz wrote:
| do fractional electrons when perturbed radiate fractional
| photons?
| boothby wrote:
| I'm no physics expert but I do make programmable crystals for a
| living. One of my favorite applications of adiabatic quantum
| computers is the simulation of physical materials such as [1]
| wherein we see emergent phenomena like fractional
| magnetization. My contribution to this paper was to find the
| configuration of qubits equivalent to the material in question.
| What's really cool about experiments like this is that you can
| directly observe the spin configurations that give rise to the
| fractional states -- if you're sufficiently motivated, you
| could reproduce the experiment and paw through the data
| yourself (disclaimer: that may require you to pay my employer
| for QPU access). For the less motivated, see figure 3.
|
| [1, lay coverage] https://csmd.ornl.gov/highlight/quantum-
| computing-enables-si...
|
| [1, free PDF] https://arxiv.org/pdf/2003.01019
| mensetmanusman wrote:
| " The potential of non-abelian anyons for quantum computing has
| driven decades of research in academia and industry."
|
| I love sentences like these. Reminds me that the world is far
| more complicated than a human can grasp in their lifetime. Great
| work!
| visarga wrote:
| > the world is far more complicated than a human can grasp in
| their lifetime
|
| Language plays a big part in this "grasping" process. Without
| language we would not have been able to build upon past
| experience and get to where we are now. It now preserves more
| knowledge than any one human can ever learn. We often think
| intelligence is an individual trait, but it is made of concepts
| and methods discovered by a social process based on language. A
| single human can't bootstrap to that level on his own.
| yummypaint wrote:
| The behavior of electrons in a material depend on the material.
| Crystals and sheets of graphene are special because of their
| periodicity, i.e. the inability to distinguish one location in
| the lattice from another over long distances. The wavefuntions of
| electrons are to first order due to this periodic potential.
|
| Crystals are also interesting mathematically because there is a
| finite number of possible lattices that can exist, and it isn't
| even that many. All the useful and interesting effects of doping
| semiconductors is the result of throwing some contaminants in to
| disrupt the perfect periodicity of the silicon crystal and
| therebey make junctions that can ultimately be used for
| computation etc. "Holes" are sort of emergent quasiparticles with
| charge +1.
|
| I think a key term in the article is "moire". When two regular
| patterns are overlayed and then twisted or offset, they interfere
| to produce a new emergent pattern, sometimes with radically
| different periodicity than the original. The electrons must now
| inhabit this very artificial pattern of pottentials that is
| neither a crystal or a doped crystal and may not occur in nature
| at all. Plus there are probably effects caused by the material
| being nearly 2d.
|
| Very exciting stuff, emergent complexity seems to be a major
| theme in physics so far this century.
| westurner wrote:
| "Catalog of topological phonon materials" (2024)
| https://www.science.org/doi/10.1126/science.adf8458
|
| "Non-trivial quantum geometry and the strength of electron-
| phonon coupling" (2024)
| https://www.nature.com/articles/s41567-024-02486-0 :
|
| > _Here, we devise a theory that incorporates the quantum
| geometry of the electron bands into the electron-phonon
| coupling, demonstrating the crucial contributions of the
| Fubini-Study metric or its orbital selective version to the
| dimensionless electron-phonon coupling constant. We apply the
| theory to two materials, that is, graphene and MgB2, where the
| geometric contributions account for approximately 50% and 90%
| of the total electron-phonon coupling constant, respectively.
| The quantum geometric contributions in the two systems are
| further bounded from below by topological contributions_
|
| > _Here, we devise a theory that incorporates the quantum
| geometry of the electron bands into the electron-phonon
| coupling, demonstrating the crucial contributions of the
| Fubini-Study metric or its orbital selective version to the
| dimensionless electron-phonon coupling constant. We apply the
| theory to two materials, that is, graphene and MgB2, where the
| geometric contributions account for approximately 50% and 90%
| of the total electron-phonon coupling constant, respectively.
| The quantum geometric contributions in the two systems are
| further bounded from below by topological contributions_
|
| There is emergence in complex fluid attractor systems.
|
| Superhydrodynamics models fluid attractor systems; for example
| with vorticity in "Gravity as a fluid dynamic phenomenon in a
| superfluid quantum space. Fluid quantum gravity and
| relativity." (2015) https://hal.science/hal-01248015/ ..
| https://news.ycombinator.com/item?id=38871054
|
| Vorticity in Electron-Electron interactions:
| https://news.ycombinator.com/item?id=40360691
|
| Is there emergence from just vorticity?
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