Mar 5, 2024 09:58 PM
Great short account of superconductivity by Richard Behiel from X.
https://twitter.com/RBehiel/status/1764752537150988338
The context was a discussion of the recent Chinese claims about high-temperature superconductivity.
Superconductivity requires a spin-up electron to pair with a spin-down electron, so that the pair has a net spin of zero and therefore obeys Bose-Einstein statistics, not Fermi-Dirac. This breaks them free from the Pauli exclusion principle, so you can have a great many of them occupying a low energy quantum state that can slide effortlessly through the material like a ghost. Every superconductor relies on exactly that effect.
What differs between materials is the way in which the electrons pair. Electrons don’t like to pair, because of their mutual Coulomb repulsion, but in some materials they can both jiggle the crystal lattice in a way that weakly brings them together, like an arranged marriage for a couple that’s not really into each other. This is called phonon coupling, and can be modeled with BCS theory, at least… for simple crystal structures. That model predicts that superconductivity can only occur up to like 30 K, beyond which the lattice is jostling around too much because of temperature, so the pairing mechanism is disrupted.
However, there are some materials with complicated crystal structures, that get around the assumptions of BCS, and that are more robust when it comes to the effect of temperature on the pairing mechanism. For example, ReBCO, which I work with, can go up to ~90 K. This is a shockingly high number from the perspective of BCS, which is why these materials are called “high temperature” superconductors, despite still having to be pretty cold (liquid nitrogen boils at 77 K).
Anyway, the high T superconductors have more complicated and exotic pairing mechanisms. It’s no longer “simple” phonon coupling, but something that involves longer-range and more geometrically complicated details, and to this day it’s still not fully understood.
Although, the way supercurrent flows is well-understood, in all superconductors. It’s called the Ginsburg-Landau model, and is based on the idea that there’s some energy associated with electron pair formation, and some energy associated with pair-to-pair interactions, in addition to the usual electromagnetic terms. This model works extremely well, and is the basis for all superconducting technology.
One of the predictions of GL is that, when the condensation energy is nearly zero (around the Tc), there will be fluctuations where electrons temporarily pair and un-pair. These oscillations are extremely rapid, so in experiential measurements they will average out into a state that seems to be kinda superconducting, but not completely. But it’s really just fluctuating, a second-order phase transition. So that’s one cause of the transition width.
Another cause of the transition width is the purity of the sample, or lack thereof. A sample with varying quality throughout will have a wider transition, as some parts of it “turn on” before others.
With ReBCO, transition widths are typically a few K, with tails noticeable out to like a 10 K span, sometimes more. And dirty samples might have double or triple transitions, if different parts of it are oriented different ways.
Anyway, sorry for the novel of a response, but I hope that provides some context. Any Tc curve on a new superconducting sample is probably going to have quite a wide transition, for lack of sample purity, and also due to the fluctuations that thermodynamically should be there no matter what. So it seems extremely unlikely that a material which somehow superconducts at 4X the temperature of ReBCO would also have a much cleaner Tc transition.
So bottom line, he's expressing some skepticism about the published details of the claims.
https://twitter.com/RBehiel/status/1764752537150988338
The context was a discussion of the recent Chinese claims about high-temperature superconductivity.
Superconductivity requires a spin-up electron to pair with a spin-down electron, so that the pair has a net spin of zero and therefore obeys Bose-Einstein statistics, not Fermi-Dirac. This breaks them free from the Pauli exclusion principle, so you can have a great many of them occupying a low energy quantum state that can slide effortlessly through the material like a ghost. Every superconductor relies on exactly that effect.
What differs between materials is the way in which the electrons pair. Electrons don’t like to pair, because of their mutual Coulomb repulsion, but in some materials they can both jiggle the crystal lattice in a way that weakly brings them together, like an arranged marriage for a couple that’s not really into each other. This is called phonon coupling, and can be modeled with BCS theory, at least… for simple crystal structures. That model predicts that superconductivity can only occur up to like 30 K, beyond which the lattice is jostling around too much because of temperature, so the pairing mechanism is disrupted.
However, there are some materials with complicated crystal structures, that get around the assumptions of BCS, and that are more robust when it comes to the effect of temperature on the pairing mechanism. For example, ReBCO, which I work with, can go up to ~90 K. This is a shockingly high number from the perspective of BCS, which is why these materials are called “high temperature” superconductors, despite still having to be pretty cold (liquid nitrogen boils at 77 K).
Anyway, the high T superconductors have more complicated and exotic pairing mechanisms. It’s no longer “simple” phonon coupling, but something that involves longer-range and more geometrically complicated details, and to this day it’s still not fully understood.
Although, the way supercurrent flows is well-understood, in all superconductors. It’s called the Ginsburg-Landau model, and is based on the idea that there’s some energy associated with electron pair formation, and some energy associated with pair-to-pair interactions, in addition to the usual electromagnetic terms. This model works extremely well, and is the basis for all superconducting technology.
One of the predictions of GL is that, when the condensation energy is nearly zero (around the Tc), there will be fluctuations where electrons temporarily pair and un-pair. These oscillations are extremely rapid, so in experiential measurements they will average out into a state that seems to be kinda superconducting, but not completely. But it’s really just fluctuating, a second-order phase transition. So that’s one cause of the transition width.
Another cause of the transition width is the purity of the sample, or lack thereof. A sample with varying quality throughout will have a wider transition, as some parts of it “turn on” before others.
With ReBCO, transition widths are typically a few K, with tails noticeable out to like a 10 K span, sometimes more. And dirty samples might have double or triple transitions, if different parts of it are oriented different ways.
Anyway, sorry for the novel of a response, but I hope that provides some context. Any Tc curve on a new superconducting sample is probably going to have quite a wide transition, for lack of sample purity, and also due to the fluctuations that thermodynamically should be there no matter what. So it seems extremely unlikely that a material which somehow superconducts at 4X the temperature of ReBCO would also have a much cleaner Tc transition.
So bottom line, he's expressing some skepticism about the published details of the claims.