Posted on 08/13/2021 11:14:21 AM PDT by Red Badger
In 1934, Eugene Wigner, a pioneer of quantum mechanics, theorized a strange kind of matter — a crystal made from electrons. The idea was simple; proving it wasn’t. Physicists tried many tricks over eight decades to nudge electrons into forming these so-called Wigner crystals, with limited success. In June, however, two independent groups of physicists reported in Nature the most direct experimental observations of Wigner crystals yet.
“Wigner crystallization is such an old idea,” said Brian Skinner, a physicist at Ohio State University who was not involved with the work. “To see it so cleanly was really nice.”
To make electrons form a Wigner crystal, it might seem that a physicist would simply have to cool them down. Electrons repel one another, and so cooling would decrease their energy and freeze them into a lattice just as water turns to ice. Yet cold electrons obey the odd laws of quantum mechanics — they behave like waves. Instead of getting fixed into place in a neatly ordered grid, wavelike electrons tend to slosh around and crash into their neighbors. What should be a crystal turns into something more like a puddle.
One of the teams responsible for the new work found a Wigner crystal almost by accident. Researchers in a group led by Hongkun Park at Harvard University were experimenting with electron behavior in a “sandwich” of exceptionally thin sheets of a semiconductor separated by a material that electrons could not move through. The physicists cooled this semiconductor sandwich to below −230 degrees Celsius and played around with the number of electrons in each of the layers.
The team observed that when there was a specific number of electrons in each layer, they all stood mysteriously still. “Somehow, electrons inside the semiconductors could not move. This was a really surprising find,” said You Zhou, lead author on the new study.
Zhou shared his results with theorist colleagues, who eventually recalled an old idea of Wigner’s. Wigner had calculated that electrons in a flat two-dimensional material would assume a pattern similar to a floor perfectly covered with triangular tiles. This crystal would stop the electrons from moving entirely.
In Zhou’s crystal, repulsive forces between electrons in each layer and between the layers worked together to arrange electrons into Wigner’s triangular grid. These forces were strong enough to prevent the electron spilling and sloshing predicted by quantum mechanics. But this behavior happened only when the number of electrons in each layer was such that the top and bottom crystal grids aligned: Smaller triangles in one layer had to exactly fill up the space inside bigger ones in the other. Park called the electron ratios that led to these conditions the “telltale signs of bilayer Wigner crystals.”
After they realized that they had a Wigner crystal on their hands, the Harvard team made it melt by forcing the electrons to embrace their quantum wave nature. Wigner crystal melting is a quantum phase transition — one that is similar to an ice cube becoming water, but without any heating involved. Theorists previously predicted the conditions necessary for the process to occur, but the new experiment is the first to confirm it through direct measurements. “It was really, really exciting to see what we actually learned from textbooks and papers in experimental data,” Park said.
Past experiments found hints of Wigner crystallization, but the new studies offer the most direct evidence because of a novel experimental technique. The researchers blasted the semiconductor layers with laser light to create a particle-like entity called an exciton. The material would then reflect or re-emit that light. By analyzing the light, researchers could tell whether the excitons had interacted with ordinary free-flowing electrons, or with electrons frozen in a Wigner crystal. “We actually have direct evidence of a Wigner crystal,” Park said. “You can actually see that it’s a crystal that has this triangular structure.”
The second research team, led by Ataç Imamoğlu at the Swiss Federal Institute of Technology Zurich, also used this technique to observe the formation of a Wigner crystal.
The new work illuminates the infamous problem of many interacting electrons. When you put a lot of electrons into a small space, they all push on each other, and it becomes impossible to keep track of all the mutually intertwined forces.
Philip Phillips, a physicist at the University of Illinois, Urbana-Champaign who was not involved with the experiment, described Wigner crystals as an archetype for all such systems. He noted that the only problem involving electrons and electric forces that physicists know how to solve with just pen and paper is that of a single electron in the hydrogen atom. In atoms with even one more electron, the problem of predicting what the interacting electrons will do becomes intractable. The problem of many interacting electrons has long been considered one of the most difficult in physics.
Going forward, the Harvard team plans on using their system to answer outstanding questions about Wigner crystals and strongly correlated electrons. One open question is what happens, exactly, when the Wigner crystal melts; competing theories abound. Additionally, the team observed Wigner crystals in their semiconductor sandwich at higher temperatures and for larger numbers of electrons than theorists predicted. Investigating why this was the case could lead to new insights about strongly correlated electron behavior.
Eugene Demler, a theorist at Harvard who contributed to both new studies, believes that the work will settle old theoretical debates and inspire new questions. “It’s always much easier to work on a problem when you can look up the answers at the end of a book,” he said. “And having extra experiments is like looking up the answer.”
(Color) 19-electron quantum Wigner "crystal" (left), radially ordered crystal (center) and mesoscopic fermionic liquid (right). From left to right quantum melting at constant temperature occurs. Dots correspond to the probability density ρ of the electrons in the 2D plane which varies
Ping!.................
Kinda neat, but can it be used for anything?
You could use it to store electrons until they are needed..................
A non-nuclear EMP device.
You just know someone is going to either mis-hear or mispronounce this.
ISWYM....................
They’re using excitons once they form, to indicate the presence of these electrons. If you overload the capacitance of an exciton, you can have LENR.
https://www.mail-archive.com/vortex-
[Vo]:Excitonic Collapse as the proximate cause of gain in LENR
Jones Beene Mon, 23 Jun 2014 08:08:55 -0700
To put this topic under its own subject heading …
An article turned up (“before its time”, literally) in Journal of
Electroanalytical Chemistry, Volume 727, 1 August 2014, Pages 53–58 which
could have relevance to LENR insofar as understanding the mechanics for gain
in some types of experiments – especially those where significant local
voltage fluctuations exist, since the voltage swings can be a function of
SPP formation or decay.
http://www.sciencedirect.com/science/article/pii/S1572665714002276
“Electrochemical supercapacitor behavior of α-Ni(OH)2 nanoparticles…” by
Vijayakumar and Muralidharan. The authors claim that Ni(OH)2 nanoparticles
exhibit specific capacitance of over 500 F g−1 (paywall prohibits more
detail).
The relevant analogy would be electron/positron annihilation, which are much
higher energy. In the case of excitons, SPP, and LENR, it would be
electron/electron-hole collapse. The SPP during either formation or decay
would overload the capacitance of the exciton. The net energy is expected to
be similar to bandgap energy in the range of 2-4 eV per collapse in the form
of photons. The ultimate source of energy is still in dispute, but can
related to the Dirac sea of negative energy. An eV value that is often seen
or surmised is the violet or near UV photon of 3.4 eV since it is a Rydberg
fraction of positronium binding energy.
In the context of LENR, Ni-O coated nanospheres are available, and would
form nickel hydroxide on hydrogen exposure. Here is an image of a 10 uF cap
exploding (10 microfarad)
http://i591.photobucket.com/albums/ss355/bill2009_photos/cap1.jpg
Presumably, a microgram of Ni hydroxide would have 50 times greater
explosive power than this image suggests, but of greater interest would be
to engineer the overloading of individual excitons, sequentially and in a
way that does not result in failure of the structure.
AFAIK no one has ever proposed before now that one form of LENR is built
upon the process of sequential voltage overloading of capacitive
nanoparticles in the form of excitons. In an exciton there is a “free
electron” and a “hole”. A positively-charged electron hole is generally
considered to be an abstraction for the location from which an electron was
moved. However, perhaps the electron hole is something more than abstraction
in LENR – for instance: being an interface with the Dirac sea of negative
energy where the “holes” therein may share more than the same name. The
Dirac hole and the exciton hole would thus be identical or connected by a
coupling mechanism.
Exciton collapse is known to happen. “Excitonic Collapse in Semiconducting
Transition Metal Dichalcogenides” by Rodin and Neto concerns semiconducting
transition metal crystals characterized by electron volt size band gaps,
spin-orbit coupling (SOC), and d-orbital character of its valence and
conduction bands. “We show that these materials carry unique exciton
quasiparticles (electron-hole bound states) with energy within the gap but
which can collapse in the strong coupling regime by merging into the band
structure continuum,
Another beauty of “excitonic collapse” for LENR theory is that this route to
gain merges well with the known and hypothetical features of the Dirac sea,
as explicated by the late Don Hotson.
Jones
It'd be interesting to study the (as it were) vibrational modes of the Wigner Crystal.
Or hey, for a large enough Crystal, whatever corresonds to "phonons"? (...or is that what the excitons are?)
It might be used for storing electricity in space?
"The team observed that when there was a specific number of electrons in each layer, they all stood mysteriously still. “Somehow, electrons inside the semiconductors could not move. "
OTOH, back in the early 80s the few times I rode the Red Line of the DC metro, I would observe much the same effect during rush hour...
The stench (worse than the hog pens in Coinjock, NC or the cattle pens in the Imperial Valley) made my wife just throw all the clothes away...
Virtually the required description for "real" discovery. Cool (unintentional pun)!
This is such an exciting story about Wigmer Crystals. I will post it on Instant-Gram.
Good question. If they can form electron sized insulating crystals, that can be quickly turned on and off... Well, that’s basically all our modern transistors are. The heart of every computer processor out there.
Maybe Moore’s Law isn’t quite dead... yet.
You can make a superconductor non-conductive under certain conditions...plus whatever else the research reveals.
bkmk
Whatever happened to Wigner’s sister?
She married Paul Dirac. It was her second marriage. She lived to age 97.
Wigner married the sister of physicist John Wheeler. It was his second marriage. His first wife died.
It would have been perfect if Dirac's sister married Wheeler, but that didn't happen.
Disclaimer: Opinions posted on Free Republic are those of the individual posters and do not necessarily represent the opinion of Free Republic or its management. All materials posted herein are protected by copyright law and the exemption for fair use of copyrighted works.
