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Comment by ajnin | Hacker News Reader

I'm puzzled by this, looking at the resistance versus temperature graph they're not demonstrating zero resistance at 110K, they're demonstrating that at 110K the resistance becomes so low that it reaches the noise floor of their instrument. It's a stretch to call that superconductivity because when it come to that it needs to be actually zero, as in "I can put a current in a loop made of the material and come back one year later and it will still be there". Exponential decay will quickly catch up with a non-zero value, even very very small. Still this exponentially decreasing resistivity looks interesting, I have no idea if that's unusual or if that means anything. The weird dip at around 230K will need to be explained as well. I'm just an interested bystander, that goes without saying.

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Well, that's one of the reasons demonstrating the Meissner effect is considered a better evidence of superconductivity. Of course any physical experiment always has a noise floor and a finite error. You can't exactly ever measure zero resistance because your probes and the probe-sample interface have nonzero resistance…

But looking at that graph, it would be an incredible coincidence if the resistance dropped like it does and then suddenly stabilized to some very small but nonzero value… That would probably require entirely new physics to explain and would be a much bigger news than "merely" a 110K Tc superconductor!

Groups that typically do resistivity measurements on regularly measure low-temperature resistivities on conducting materials of ~ 10s of micro Ohm m. So if you're measuring in this range on a conducting material and hitting the noise floor with a SHARP drop, that's a pretty big indicator of superconductivity (assuming you haven't just broken your contacts which is a concern when cooling things down). I'm not so familiar with PPMS systems, but I imagine it has some built in auto-adjust on the sensitivities to where you can be pretty confident your noise floor is below these values.

What is complicating the interpretation here is the log scale (and lack of conversion to resistivity): It is amplifying the impression of the noise below what they call Tc, and making it harder to interpret the approach of the material to the transition point. The behaviour at the approach to Tc also doesn't really look like a metal, which should scale as propto T, or a semiconductor which should increase with decreasing temperature. Possibly a result of it being some horrible mixed phase ceramic.

Really, really, really Stupid question (yes, capital S stupid). Several hypotheticas here that got me thinking:

If superconductor exists at room temperature, and if I understand you correctly, it can hold a charge, essentially, indefinitely.

Could I get a roll of it and charge it and throw it at someone to kill them? Or could I fry a passing car by throwing it at the car?

You certainly can use it as a batteryless inductor coil to zap people, similar to how you zap people with a charged capacitor. But a superconductor is limited by the magnetic field strength it can support. When the magnetic field strength goes up beyond the critical field by increasing the current, superconductivity is lost. In superconducting magnets, this causes a catastrophic failure called a "magnet quench". So a superconductor cannot store unlimited energy. In the original LK-99 claim, the critical current is 200 mA, it's not much.

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