Me: PhD in condensed matter experiment, brief read-through of the 3-person paper a few days ago, went and checked out the 6-person paper just now, read some other links as needed.
EDIT: If I’m reading their figure 4 correctly, I missed how impossible their magnetic susceptibility data was if not superconducting. My bad—I’ve sprinkled in some more edits as necessary for questions 1, 2, and 4.
Q1
One person alleges an online rumor that poorly connected electrical leads can produce the same graph. Is that a conventional view?
Electrical leads can explain almost arbitrary phenomena. They measured resistivity with a four point probe, where you flow a current between two outer wires and then check the voltage between two inner wires. If the inner wires for some reason don’t allow current to pass at small voltage (e.g. you accidentally made a schottky diode, a real thing that sometimes happens), that can cause a spurious dip in resistivity.
Alternatively: If this material is a superconductor, have we seen what we expected to see?
The data isn’t particularly clean, and there are several ways it differs from what you’d expect. Here’s what a nice clean I-V curve looks like—symmetrical, continuous, flat almost to the limit of measurement below Tc, all that good stuff. Their I-V data is messier in every way. It’s not completely implausible, but if it’s real, why didn’t they take some better-looking data?
Is the diminishing current capacity with increased temperature usual?
Yes, critical current changing with temperature is normal. In fact, if this is a superconductor, we can learn interesting things about it from the slope of critical current as a function of temperature, near the critical temperature (does it look like √Tc−T?).
How does this alleged direct measurement of superconductivity square up with the current-story-as-I-understood-it that the material is only being very poorly synthesized, probably only in granules or gaps, and hence only detectable by looking for magnetic resistance / pinning?
The resistivity and levitation might be possible if only a tiny fraction of the material is superconducting, so long as there are 2D superconducting planes (a pattern that seems likely in a high-temperature superconductor) that can percolate through the polycrystalline material. However, I don’t see how this would work with the apatite structure (also the Griffin DFT paper says the band structure is 3D, and the Cu-Pb chains of claimed importance are 1D), so I think it’s more likely you would indeed have to have a high fraction of superconductor.
EDIT: I think their magnetic susceptibility data for sample 2, if correct, implies that the sample is at least 20% superconductor.
Q2
The video shows a surprising amount of diamagnetism, but it doesn’t really look like the Meissner effect, and isn’t so strong that it’s impossible to explain without it (especially since most of the weight of the sample is resting on the magnet). Locking in place isn’t strictly necessary, but especially in an impure material we should see a lot of pinning that prevents it from easily rotating. Russian catgirls are often untrustworthy.
EDIT: Actually, if I’m reading this right, figure 4a actually is pretty impossible without superconductivity. Score one for YES-Y. Although the data looks very ugly (where’s the above-Tc region with no diamagnetism?)
The diamagentism is still evidence that it’s a superconductor! It’s just even better evidence that it’s a non-superconducting strong diamagnet. The moderate difference they show between field cooled and zero-field cooled magnetization curves is likewise evidence either that it’s a superconductor, or evidence that it’s an ordered diamagnetic material.
Q3
Somewhere between YES-X and NO-C. First, DFT calculations are a good starting point but always require a grain of salt. Second, I think calling this a “flat band” is overhyping it—the density of states enhancement that makes flat bands so hype-worthy isn’t there as far as I can tell. Third, the hints of charge and spin waves in the material bear further study (if this is a superconductor they almost certainly are doing something interesting) but aren’t all that surprising given that you’ve jammed a bunch of heavy atoms together in a nontrivial crystal structure.
Q4
If getting it to conduct current at 0 resistance is as easy as they make it sound, they’ve probably replicated it a hundred times in three different ways. However, what if it’s tricky to get it hooked up to show superconductivity—you have to put the leads on just right, in some hard-to-understand way, and usually it doesn’t look superconducting… then wishful thinking has a lot more room to operate.
EDIT: The extreme diamagnetism measurement for sample 2 could just be a calibration error on a sensitive measurement, requiring neither fraud nor superconductivity.
Q5
No idea. They clearly know physics. They’re not maximally clear about everything, and I think they sweep data issues under the rug, but not in a way that makes me more suspicious conditional on the data.
(Phd in condensed matter simulation) I agree with everything you wrote where I know enough (for readers, I don’t know anything about lead contacts and several other experimental tricky points, so my agreement should not be counted too much).
I just add on the simulation side (Q3): this is what you would expect to see in a room-T superconductor unless it relies on a completely new mechanism. But, this is something you see also in a lot of materials that superconduct at 20K or so. Even in some where the superconducting phase is completely suppressed by magetism or structural distortions or any other phase transition. In addition, DFT+U is a quick-and-dirty approach for this kind of problem, as fits the speed at which the preprint was put out.
So from the simulation bayesian evidence in favor but very weak
If it’s possible that the polycrystalline structure is what determines superconductivity, and so this is a purity issue?
Could we perhaps find suitable alternative combinations of elements that are more inclined to form these ordered polycrystalline arrangements (superlattice)?
For example finding alloys that have atom A that attracts to atom B more than it attracts to atom A, and atom B that attracts to atom A more than it attracts to atom B, where these particular elements are also good candidates for materials that are likely to exhibit superconductivity, and are heavy elements so they’re likely to more stable at room-temperature, so they have higher Tc?
Or is this a dead-end way of trying to find a room temp superconductor?
Yeah, things are more complicated—atoms aren’t interchangeable, they have complicated effects on what the electrons are doing. If you want to understand, I can only recommend a series of textbooks (e.g. Marder’s Condensed matter physics, Phillips’ Advanced solid state physics, Tinkham’s Introduction to superconductivity).
Me: PhD in condensed matter experiment, brief read-through of the 3-person paper a few days ago, went and checked out the 6-person paper just now, read some other links as needed.
EDIT: If I’m reading their figure 4 correctly, I missed how impossible their magnetic susceptibility data was if not superconducting. My bad—I’ve sprinkled in some more edits as necessary for questions 1, 2, and 4.
Q1
Electrical leads can explain almost arbitrary phenomena. They measured resistivity with a four point probe, where you flow a current between two outer wires and then check the voltage between two inner wires. If the inner wires for some reason don’t allow current to pass at small voltage (e.g. you accidentally made a schottky diode, a real thing that sometimes happens), that can cause a spurious dip in resistivity.
The data isn’t particularly clean, and there are several ways it differs from what you’d expect. Here’s what a nice clean I-V curve looks like—symmetrical, continuous, flat almost to the limit of measurement below Tc, all that good stuff. Their I-V data is messier in every way. It’s not completely implausible, but if it’s real, why didn’t they take some better-looking data?
Yes, critical current changing with temperature is normal. In fact, if this is a superconductor, we can learn interesting things about it from the slope of critical current as a function of temperature, near the critical temperature (does it look like √Tc−T?).
The resistivity and levitation might be possible if only a tiny fraction of the material is superconducting, so long as there are 2D superconducting planes (a pattern that seems likely in a high-temperature superconductor) that can percolate through the polycrystalline material. However, I don’t see how this would work with the apatite structure (also the Griffin DFT paper says the band structure is 3D, and the Cu-Pb chains of claimed importance are 1D), so I think it’s more likely you would indeed have to have a high fraction of superconductor.
EDIT: I think their magnetic susceptibility data for sample 2, if correct, implies that the sample is at least 20% superconductor.
Q2
The video shows a surprising amount of diamagnetism, but it doesn’t really look like the Meissner effect, and isn’t so strong that it’s impossible to explain without it (especially since most of the weight of the sample is resting on the magnet). Locking in place isn’t strictly necessary, but especially in an impure material we should see a lot of pinning that prevents it from easily rotating. Russian catgirls are often untrustworthy.
EDIT: Actually, if I’m reading this right, figure 4a actually is pretty impossible without superconductivity. Score one for YES-Y. Although the data looks very ugly (where’s the above-Tc region with no diamagnetism?)
The diamagentism is still evidence that it’s a superconductor! It’s just even better evidence that it’s a non-superconducting strong diamagnet. The moderate difference they show between field cooled and zero-field cooled magnetization curves is likewise evidence either that it’s a superconductor, or evidence that it’s an ordered diamagnetic material.
Q3
Somewhere between YES-X and NO-C. First, DFT calculations are a good starting point but always require a grain of salt. Second, I think calling this a “flat band” is overhyping it—the density of states enhancement that makes flat bands so hype-worthy isn’t there as far as I can tell. Third, the hints of charge and spin waves in the material bear further study (if this is a superconductor they almost certainly are doing something interesting) but aren’t all that surprising given that you’ve jammed a bunch of heavy atoms together in a nontrivial crystal structure.
Q4
If getting it to conduct current at 0 resistance is as easy as they make it sound, they’ve probably replicated it a hundred times in three different ways. However, what if it’s tricky to get it hooked up to show superconductivity—you have to put the leads on just right, in some hard-to-understand way, and usually it doesn’t look superconducting… then wishful thinking has a lot more room to operate.
EDIT: The extreme diamagnetism measurement for sample 2 could just be a calibration error on a sensitive measurement, requiring neither fraud nor superconductivity.
Q5
No idea. They clearly know physics. They’re not maximally clear about everything, and I think they sweep data issues under the rug, but not in a way that makes me more suspicious conditional on the data.
(Phd in condensed matter simulation) I agree with everything you wrote where I know enough (for readers, I don’t know anything about lead contacts and several other experimental tricky points, so my agreement should not be counted too much).
I just add on the simulation side (Q3): this is what you would expect to see in a room-T superconductor unless it relies on a completely new mechanism. But, this is something you see also in a lot of materials that superconduct at 20K or so. Even in some where the superconducting phase is completely suppressed by magetism or structural distortions or any other phase transition. In addition, DFT+U is a quick-and-dirty approach for this kind of problem, as fits the speed at which the preprint was put out. So from the simulation bayesian evidence in favor but very weak
If it’s possible that the polycrystalline structure is what determines superconductivity, and so this is a purity issue?
Could we perhaps find suitable alternative combinations of elements that are more inclined to form these ordered polycrystalline arrangements (superlattice)?
For example finding alloys that have atom A that attracts to atom B more than it attracts to atom A, and atom B that attracts to atom A more than it attracts to atom B, where these particular elements are also good candidates for materials that are likely to exhibit superconductivity, and are heavy elements so they’re likely to more stable at room-temperature, so they have higher Tc?
Or is this a dead-end way of trying to find a room temp superconductor?
Yeah, things are more complicated—atoms aren’t interchangeable, they have complicated effects on what the electrons are doing. If you want to understand, I can only recommend a series of textbooks (e.g. Marder’s Condensed matter physics, Phillips’ Advanced solid state physics, Tinkham’s Introduction to superconductivity).