Does anyone have a key understanding of why these extreme pressures enable superconductivity?
I'm trying to imagine how these extreme pressures would modify bond angles, nuclei spacing, and constraints on motion. And also tring to understand how that's affecting the behavior and creation of the Cooper pairs.
Also a handwavy explanation aimed at people who aren't familiar with a lot of the concepts of condensed matter physics. Please salt with the knowledge that current theory can't fully explain how high temperature superconductors work. And that I'm not an expert in the field.
First concept, virtual particles vs real particles. When we talk about "an electron flowing through metal" it is not actually a single electron. As it moves, the electron will move into an atom, another gets knocked out. But in aggregate it "acts like" a single particle with possibly different properties from a real electron. For example it likely has a different mass. A virtual photon will travel slower than a real one. And so on.
Virtual particles can even correspond to things that aren't particles at all! For example sound is a wave, and quantum mechanically is carried by virtual particles known as phonons. These act exactly like any other particle, even though they are actually aggregate behavior of lots of other things!
A Cooper pair is a pair of things (eg electrons) that are interacting enough that they have a lower energy together than they would apart. Electrons are fermions, with half spin. They have a variety of properties, such as the Fermi exclusion principle. A bound pair of electrons becomes a virtual particle with an integer spin. Which makes it a boson, which behaves differently.
Superconductivity happens when charge is carried by bosons.
In high temperature superconductors, it looks like the electrons are at least partially bound by interaction with phonons. The high pressures change the speed of sound, and therefore change how easily Cooper pairs form.
Everything that I said above was based on what was known a couple of years ago.
However https://phys.org/news/2019-04-mechanism-high-temperature-sup... claims that there is now a theoretical explanation for high temperature superconductors, and the best guess above doesn't seem to be the real explanation. The real explanation being that the feature/TIQ-7651_unique_schema_version
Remember what I said about particles having a different mass moving through materials? The binding together of electrons through interaction with phonons seems to depend on the mass of the electrons. When you squeeze the lattice, that mass decreases.
>In high temperature superconductors, it looks like the electrons are at least partially bound by interaction with phonons. The high pressures change the speed of sound, and therefore change how easily Cooper pairs form.
Interesting. Do we know if it possible to disrupt superconductivity with sound at just the right frequency? And the converse, has anyone tried to enhance superconductivity by using sound (i.e. increase either the critical temperature, increase the current density, etc)?
HTS will stop superconducting once a certain amount of energy is added. This energy can be in the form of heat, magnetic field, electric current, or mechanical strain. If you keep the HTS colder you can accommodate more of the other forms of energy. I do not know if sound would disrupt superconductivity but since sound is a form of energy it is very likely.
Like another poster already said, both lower temperatures and higher pressures confine the movements of the atoms, so either of them can cause the same phase transitions.
Besides this new example with superconductivity, there are other more familiar phase transitions with the same behavior.
For example, with most liquids, in order to solidify them you may either cool them or compress them.
The same if you want to liquefy gases, either cooling or compressing has the same effect.
Room-temperature superconductivity at very high pressures has been predicted many years ago, but it is very nice to have an experimental confirmation.
Handwavey explaination: the particles pair up because of vibrations in the crystal. It's modeled like a bunch of metal balls on with springs between them and you can imagine tapping one end and sending a wave of vibrations through. However, these springs are a bit non-linear and so I imagine that if you pack the atoms closer together then you will change the spring constant.
The other knob you can use to change the vibrations is the mass of the balls. This can be done by using different isotopes of the same element and the critical temperature goes down with mass.
the particles can't pair up, because equal charges repell. That's still the virtual model.
I don't quite remember my intro to electrical components, though it's a quick read for the basics. The GP obviously knows about atom models and band gap.
The paradox bit is that, as far as I can tell pressure is roughly equivalent to heat, and heat equals decreased intrinsic conductivity. But if I imagine that high preasure restricts the absolute motion of particles, that would equal decreased resistance (like an idealized fixed suspension for your swing, that doesn't take energy out of the system).
Since Hydrogen is involved, I suppose there's a channel of Hydrogen rumps without any electrons, and the high preassure is needed to keep the hydrogen from moving apart and recombining outside the ensemble. Surely this involves some form of entanglement? Which I imagine as a kind of clockwork, all cores spinning in unison.
Type I superconductors (the ones people understand) happen because electrons pair up.
The equal charges participate on the problem, but do not stop the electrons from pairing up. There is a lot of virtual particle exchange between them, but that's how forces happen. It's more correct to say that the crystal mechanically constrains the electrons into pairs than that the electrons pair with virtual particles.
(IANAP, but this one topic I have studies a little.)
I have a different explanation. Think of a material as a sponge for heat. When I squeeze the material, I raise the temperature, and that causes heat to leak out. The temperature of the material doesn't really tell me how much heat is in it, so this experiment is suggesting that it is the heat itself that prevents superconductivity.
Now a superconductor is just a conduit for electrons that doesn't generate heat. We know from Landauer's principle that heat is only generated when you destroy information. If I take a pair of entangled electrons, those electrons contain exactly one bit of information (in the von neumann sense). If I cannot add energy in excess of the energy required to disentangle them, then that bit of information is never destroyed.
Whether or not a given interaction between the electron pair and the substrate has enough energy to disentangle them is not a function of temperature, it is a function of the actual energy that may be imparted to my pair. Which is proportional to the actual heat in my material, rather than its temperature.
I'm trying to imagine how these extreme pressures would modify bond angles, nuclei spacing, and constraints on motion. And also tring to understand how that's affecting the behavior and creation of the Cooper pairs.