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z2h-a6n 5 days ago [-]
As a physicist studying cuprates, I would caution that many of the strongest claims in the Quanta article are basically clickbait, especially the title of the article, and the notion that this experiment has solved the problem "to nearly everyone's satisfaction".

The actual experiment and resulting paper [0] on the other hand are very good work, and may very well be right, but a substantial amount more work will be needed to determine how much of the rather unusual phenomenology of the cuprates is well-explained by superexchange interactions. I guess even more work will be needed to convince the majority of the field that this the right model for the cuprates, as opposed to any of the very many other models that have been proposed and studied in the past few decades. In particular:

- High temperature superconductivity is not the only strange thing about the cuprates. The normal state (i.e. above the superconducting transition temperature) also has some rather unusual properties that are the subject of much of the current research. Some of these properties can probably be explained as arising from superexchange interactions, but I expect not all of them can.

- BSCCO is one of quite a large family of cuprate compounds. In this case, it was almost certainly chosen because it cleaves easily, producing nice surfaces which are necessary for this type of experiment. On the other hand, it is quite structurally complicated, and in some sense "messy" compared to many of the other commonly-studied compounds. This is actually taken advantage of for the experiment discussed here, but it would be interesting (if possible) to see if a similar result can be replicated in any of the other cuprates.

Anyway, this is quite an interesting experiment, but I'm somewhat dissapointed in the Quanta article for such sensationalistic reporting. I suppose that's to be expected for a popular science article.


dav_Oz 5 days ago [-]
Thanks for your insight.

Most people reading sci-pop articles are by now hopefully used to sensationalism.

But I think in this case (which can be equally bad) the journalist only spoke to codensed matter physicists biased towards or having a stake in the superexchange explanation, normally one would at least incorporate one skeptical position (such as yours, even if it's a minority one) in yet unsettled cases rather than just letting decency speak out of the scientists themselves:

>But Yazdani and other researchers caution that there’s still a chance, however remote, that glue strength and ease of hopping move in lockstep for some other reason, and that the field is falling into the classic correlation-equals-causation trap. For Yazdani, the real way to prove a causal relationship will be to harness superexchange to engineer some flashy new superconductors.

z2h-a6n 5 days ago [-]
The people quoted in the article are all fairly well known in the field, and while I obviously can't speak for them (especially because each of them alone has much more experience in the field than I do) I expect that they generally have realistic rather than what I might call sensationalistic views of this result. I expect this is also true of the authors of the work in question, though of course they may be even more excited about the result (with good reason). To be clear, this is an interesting result, and is certainly food for thought, but I think it's too soon to tell how much of the "cuprate problem" it solves.

I my guess that the sensationalism in the article is just a combination of overexcitement and misinterpretation by the journalist. I think most people in the field would agree that this result is interesting, and could end up being quite important. If the journalist heard something like this from everyone he asked, he may have misinterpreted this as the matter being "settled" in the view of most of the community.

mikewarot 5 days ago [-]
It's like ties in a railroad track, if you find a stride that matches an integer multiple of the tie distance you can run like it was normal ground.

The natural "stride" of an electron might be more than 2.6 nanometers. It could be they swing side to side a bit if the distance isn't quite right.

Some experimentation is now in order.

2.6 Nanometers is about 5 atoms of silicon between peaks.

I'm trying to find out the size of the cell created between sheets of graphene at the "magic angle" of 1.56 degrees.

It may be that you simply need to construct the right geometry between nodes to get superconductivity.

Regardless of the underlying quantum mechanics, there's a large opening for experimental physics here. Are there edge effects, what about grain boundaries, etc.

upsidesinclude 5 days ago [-]
Electrons don't have legs, but the concept might.

How does the electron wavelength compare to the incidence of the geometry?

Gamma = h bar/ momentum

Where, m = m*v

In a DC circuit the momentum can be held fairly stable once in a state of superconductance, but v is increasing up to the breakover point. Which in turn is decreasing wavelength. Bit of a trouble for a static 'circuit'.

None of that is quantum anything, but it gets you thinking.

mikewarot 5 days ago [-]
Electrons normally flow very slowly in a conductor, at about a snail's crawl. For a superconductor, I suspect this is not the case... the quantity of electrons through a channel will go up, but the velocity is probably stable.
tomxor 5 days ago [-]
That's an interesting idea, it sounds a lot like nanotechnology that exploits various quantum effects.

My extremely layman understanding notices the currently known high temperature super conductors are crystalline, structure seems to matter, but whether a nanoscale structure above this could possibly have any significant effect? It would be nice to get the opinion of someone less vaguely educated on the subject :)

z2h-a6n 5 days ago [-]
To provide some rough intuition, I might point out that in order for a material to become superconducting, there must be some interaction in the material that allows the electrons to form Cooper pairs, and these interactions are more likely to occur in crystalline systems. In typical BCS superconductors for example, the electrons interact with atomic lattice vibrations (called phonons) to form these pairs. The work being discussed indicates that the relevant interactions are (magnetic) superexchange interactions between electrons are relevant in the cuprates (although I'd say more work is necessary to be sure). In both cases, the structure of the crystal lattice is important for determining how these interactions take place. That said, I'm aware of some amorphous (i.e. non-crystalline) superconductors, although I'm by no means an expert in these.

Another point to consider is that most chemically homogeneous solids are in principle crystalline, although in practice a chunk of matieral may be made of very many tiny crystals stuck together in random orientations.

As for whether nanoscale structure could have a significant effect, that question forms a pretty major part of my current research. The stuff I'm actively working on isn't ready to publish yet, but to repeat the example I mentioned elsewhere in the comments, my colleagues were able to increase the superconducting transition temperature of strontium titanate by almost a factor of 2 by plastic deformation (i.e. squeezing it until we start messing up the structure) [0][1] (full disclosure: I'm one of the authors of this paper). We think this basically occurs by modifying the nanoscale structure in a way that introduces mechanical strain in the crystal, and this strain in turn influences the electronic interactions in such a way as to locally increase the superconducting transition temperature. In a general sense, this is somewhat similar to the fact that in many cases, applying high pressure to many superconductors will increase their transition temperature.

[0] [1] (same paper as above, but in a non-peer-reviewed and paywall-free version)

mikewarot 5 days ago [-]
Long ago, there was a company that claimed to have a way of growing a "bipoloron chain" that wasn't superconducting, but was 10^6 times (or more) as conductive as copper or silver. They called it an "ultraconductor"[1]. I give it 50/50 odds of being a scam.

I've always wondered if there could be something to this, and now it sounds like there might be. I have no way to judge the merits of it personally, I just keep my eye out for things that indicate it might have been real.

Also, there's this patent[2] from 3 years ago that might have been using strain to make a superconductor that previously wasn't.



tomxor 5 days ago [-]
Thank you !
z2h-a6n 5 days ago [-]
> It may be that you simply need to construct the right geometry between nodes to get superconductivity.

I would say that the key thing is to construct the right interactions between electrons. That said, the geometry (or structure, as we would generally say in condensed matter physics) is one of the key parts of what determines the interactions in a material, so this is certainly one approach. In practice, it is difficult to tune the structure of most materials except by applying pressure, or by changing to a different (possibly related) material, so finely tuning the interactions by influincing the structure can be difficult (twisted bilayer graphene being a notable exception).

> Regardless of the underlying quantum mechanics, there's a large opening for experimental physics here. Are there edge effects, what about grain boundaries, etc.

I agree! Topological insulators and their surface states (which are in some sense "edge effects") is a pretty hot topic right now. Related to superconductivity, the research group I'm working in has been playing with introducing disorder to unconventional superconductors by plastically deforming them (i.e. squeezing them until we start to mess up the structure). Surprisingly, this has actually increased the superconducting transition temperature in some cases [0][1] (full disclosure: I'm one of the authors of this paper). As far as we can tell this "strain engineering" of the electronic properties of superconductors is basically a new subfield, so there's still a lot to learn. One of the difficulties is that in unconventional superconductors (which includes the high-Tc superconductors), theorists tend to prefer to work on idealized models of materials, since the calculations are difficult enough without introducing disorder, defects, etc. in addition.

[0] [1] (same paper as above, but in a non-peer-reviewed and paywall-free version)

dmitrybrant 5 days ago [-]
I assume this is already being done, but I've had the following idea kicking around for a while:

Create a simulation, down to the quantum states, of a lattice of molecules at a certain simulated temperature (say, room temperature), and induce a simulated current through the lattice, and see if it superconducts. Proceed by iterating through billions of permutations of compounds in the simulated lattice, until the simulation finds a room-temperature superconductor.

Assuming this is feasible, does anyone know of organizations that are doing this?

mkhorton 5 days ago [-]
Not for superconductivity specifically, but for a broad range of properties of crystals, this is what the Materials Project[0] does.

Materials Project is funded by the US Department of Energy and uses supercomputing to simulate hundreds of thousands of different crystal structures on the quantum mechanical level to try and find those which have useful properties for practical applications.

This line of research is broadly called “materials discovery”, “materials design” (often “high throughput”) or even “materials genomics” depending on who you ask. These terms are provided in case anyone wants to search and read more about it.


fsh 5 days ago [-]
The computational complexity of brute-force simulating many-body quantum systems scales exponentially in the number of particles. There is no supercomputer on earth that can simulate a realistic solid.
routeroff 5 days ago [-]
It is a very active domain, with lots and lots of techniques to do some clever approximations. Brute forcing is not feasible, even for the simplest systems.
pouulet 5 days ago [-]
This made me think about this nice little TV show:
aqme28 5 days ago [-]
And you can’t get out of this problem by just simulating e.g. two or three particles. That small of a system can’t simulate things like temperature in a meaningful way.
ncmncm 5 days ago [-]
It is anyway a lot easier to simulate a solid that has no defects. But many properties found that way turn up in defect-ridden, real materials too.

A truly periodic substrate lets you wrap it in a closed universe just big enough for what you hope to make live in it.

contravariant 5 days ago [-]
You can probably find a few condensed matter groups that are doing computational simulations, but I think full quantum mechanical simulations on that scale are computationally intractable so they'll have to use approximations of some sort. This is why quantum computers are predicted to exceed the computational power of classical computers.

Besides it's been fairly recent that simulations for classical thermodynamics with a decent number of particles became somewhat feasible. And even then it often uses lots of approximations as well as statistical tricks to get decent results for sizeable systems. For quantum mechanics quite a few tricks to explore the state-space stop working because the state-space is unimaginably huge, so I'm not even sure how to begin to do similar simulations, but I reckon they'll be disastrously slow for all but the simplest of systems.

The level of detail you need for a simulation that allows you to see cooper pairs come into existence is downright insane, you might be better of trying to predict next years weather.

routeroff 5 days ago [-]
That is essentially the idea of Random Structure Searching [0], but the precision of quantum simulation (better known as ab initio) for solids is not perfect and very time consuming.


helpm33 5 days ago [-]
I don't know the state of the art but I think that we can't even predict the static crystal structure of simple substances---e.g. when iron is BCC vs FCC. The first-principles simulation of dynamic properties of large quantum many body systems is just not feasible. It is possible that this may change dramatically when quantum computers arrive---currently we describe quantum systems by modeling them with discrete, classical computers, and quantum computers might turn out to model the relevant quantum processes directly.
melonrusk 5 days ago [-]
As others have pointed out, the computations are currently infeasable. The rest of the plan works though, it would just need to be done with actual atoms.
durranh 5 days ago [-]
It’s stories like these that keep me interested in physics
RobertoG 5 days ago [-]
You can stop being interested in physics, but physics never stop being interested in you! ;-)
whatshisface 5 days ago [-]
Or impartially disinterested in you, as the case may be.
sportstuff 5 days ago [-]
You may be interested in Physics of Pranayama(Breath). I hope someone would write up or research, Physics of Cancer. Physics is everywhere and all the time.