6 ms·

I have heard people much more knowledgeable than me say that measuring true zero resistance is actually quite difficult and takes some degree of specialized equ

I have heard people much more knowledgeable than me say that measuring true zero resistance is actually quite difficult and takes some degree of specialized equipment, especially with such small samples. That may be part of it.

jacquesm 3y ago
Just getting the probes to connect reliably is tricky. depending on how large the superconducting features are it may be anywhere from just difficult to next to impossible to do accurately (for instance, if the size of the superconducting features is smaller than the probe size).
balaji1 3y ago
so none of the previous more esoteric superconductors were ever measure for zero resistance?
- whimsicalism 3y ago
  They were, but they had more time to resolve the difficulties
- scarmig 3y ago
  They were. E.g. see here for a major, good paper announcing a novel class of superconductors: https://sci-hub.ru/https://link.springer.com/article/10.1007/bf01303701 https://sci-hub.ru/https://link.springer.com/article/10.1007... The difference now is that we're seeing a premature preprint being replicated in real time. Even in that paper, the authors note: "The way the samples have been prepared seems to be of crucial importance: Michel et al. [21] obtained a single-phase perovskite by mixing the oxides of La and Cu and BaCOa in an appropriate ratio and subsequent annealing at 1,000 ~ in air. We also applied this annealing condition to one of our samples, obtained by the decomposition of the corresponding oxalates, and found no superconductivity." And you can see that in their resistivity/temperature graph of samples prepared using different protocols.
  - pyrale 3y ago
    > a premature preprint Considering how that preprint has sparked interest in other research institutions and multiplied the resources allocated to the problem, I would say this publication was not premature, it's most other research results that are late.
    - squeaky-clean 3y ago
      The paper was leaked early. The team wanted more time to get more attempts at producing it and improving their yield. The paper also wasn't generally up to their writing standards. It's essentially an early draft that was leaked.
      - gus_massa 3y ago
        It looks like this worked in this material for years, perhaps the would have taken a few more years.
- caconym_ 3y ago
  I'm not sure how you're getting that from my comment.
frisco 3y ago
Right, if our best regular conductors (used in your ohmmeter) are ~10^-8 and superconductivity is (by convention) less than 10^-11, one can see right away the simple regular methods won’t work and some cleverness is needed.
- someplaceguy 3y ago
  > superconductivity is (by convention) less than 10^-11, Ah, so you're saying that superconductivity is not actual zero resistance, but something close to it, and in fact only a factor of 1000x less resistive than the best conductor? If that is so, this is something that I had previously thought would make a lot more sense to me. But in that case it's not intuitive to me how SMES is possible with a 0% discharge rate. Shouldn't a significant fraction of the electrons looping around the coils be lost after many loops? (I know very little about electricity, as you can probably tell, never mind superconductors).
  - mort96 3y ago
    No, I believe it's literally zero but we don't have a measurement apparatus with infinite precision so we need some cut-off.
    - someplaceguy 3y ago
      Thanks, that makes sense.
    - deleted 3y ago
      [deleted]
    - fooker 3y ago
      It's only literal zero for superconductors close to 0K temperature. For high temperature superconductors (50-70+K), it's not literal zero for superconducting mechanisms discovered so far.
- crote 3y ago
  The conductors of your ohmmeter are not that important, though. You can work around that by using four-terminal sensing, and you can of course also calibrate your probes by directly touching them together. Even if your ohmmeter conductors have a resistance of several ohm, you could still get an accurate measurement if your tool has a high enough resolution. A bigger issue is going to be sample size. A 1mm-diameter 1mm-long rod of silver has a resistance of about 20 μΩ (or 2e-5) at room temperature. That's already getting tricky to measure with lab-grade equipment without pushing insane currents through it, let alone anything even smaller. If you want to measure a 1m-diameter 1m-long silver rod (which would be 0.02μΩ or 2e-8) you could just push a few thousand amps through it and reliably measure that using a household multimeter in the mV range - but do that with a small sample and it'll evaporate.

jacquesm 3y ago

Just getting the probes to connect reliably is tricky. depending on how large the superconducting features are it may be anywhere from just difficult to next to impossible to do accurately (for instance, if the size of the superconducting features is smaller than the probe size).

balaji1 3y ago

so none of the previous more esoteric superconductors were ever measure for zero resistance?

whimsicalism 3y ago
They were, but they had more time to resolve the difficulties
scarmig 3y ago
They were. E.g. see here for a major, good paper announcing a novel class of superconductors: https://sci-hub.ru/https://link.springer.com/article/10.1007/bf01303701 https://sci-hub.ru/https://link.springer.com/article/10.1007... The difference now is that we're seeing a premature preprint being replicated in real time. Even in that paper, the authors note: "The way the samples have been prepared seems to be of crucial importance: Michel et al. [21] obtained a single-phase perovskite by mixing the oxides of La and Cu and BaCOa in an appropriate ratio and subsequent annealing at 1,000 ~ in air. We also applied this annealing condition to one of our samples, obtained by the decomposition of the corresponding oxalates, and found no superconductivity." And you can see that in their resistivity/temperature graph of samples prepared using different protocols.
- pyrale 3y ago
  > a premature preprint Considering how that preprint has sparked interest in other research institutions and multiplied the resources allocated to the problem, I would say this publication was not premature, it's most other research results that are late.
  - squeaky-clean 3y ago
    The paper was leaked early. The team wanted more time to get more attempts at producing it and improving their yield. The paper also wasn't generally up to their writing standards. It's essentially an early draft that was leaked.
    - gus_massa 3y ago
      It looks like this worked in this material for years, perhaps the would have taken a few more years.
caconym_ 3y ago
I'm not sure how you're getting that from my comment.

frisco 3y ago

Right, if our best regular conductors (used in your ohmmeter) are ~10^-8 and superconductivity is (by convention) less than 10^-11, one can see right away the simple regular methods won’t work and some cleverness is needed.

someplaceguy 3y ago
> superconductivity is (by convention) less than 10^-11, Ah, so you're saying that superconductivity is not actual zero resistance, but something close to it, and in fact only a factor of 1000x less resistive than the best conductor? If that is so, this is something that I had previously thought would make a lot more sense to me. But in that case it's not intuitive to me how SMES is possible with a 0% discharge rate. Shouldn't a significant fraction of the electrons looping around the coils be lost after many loops? (I know very little about electricity, as you can probably tell, never mind superconductors).
- mort96 3y ago
  No, I believe it's literally zero but we don't have a measurement apparatus with infinite precision so we need some cut-off.
  - someplaceguy 3y ago
    Thanks, that makes sense.
  - deleted 3y ago
    [deleted]
  - fooker 3y ago
    It's only literal zero for superconductors close to 0K temperature. For high temperature superconductors (50-70+K), it's not literal zero for superconducting mechanisms discovered so far.
crote 3y ago
The conductors of your ohmmeter are not that important, though. You can work around that by using four-terminal sensing, and you can of course also calibrate your probes by directly touching them together. Even if your ohmmeter conductors have a resistance of several ohm, you could still get an accurate measurement if your tool has a high enough resolution. A bigger issue is going to be sample size. A 1mm-diameter 1mm-long rod of silver has a resistance of about 20 μΩ (or 2e-5) at room temperature. That's already getting tricky to measure with lab-grade equipment without pushing insane currents through it, let alone anything even smaller. If you want to measure a 1m-diameter 1m-long silver rod (which would be 0.02μΩ or 2e-8) you could just push a few thousand amps through it and reliably measure that using a household multimeter in the mV range - but do that with a small sample and it'll evaporate.