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.
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).
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.
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.
> 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.
Not that low in range though, you will end up seeing thermal noise that dwarfs your measurement.
> 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).
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.
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.
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.
