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0 pointsqnleigh2y ago0 comments

That's right. The standard way to measure resistance without being sensitive to contact resistance is called the four-terminal method, see (https://en.m.wikipedia.org/wiki/Four-terminal_sensing). You drive a current using two outer wires, but then detect the voltage across the sample using a different pair of wires. You'll measure zero voltage if it's superconducting, since V=IR. Or if one of the probe wires became detached.

The first LK-99 paper used this method to claim zero resistivity, but people complained that if the inner probes lost contact, that would also be consistent with their data. This criticism doesn't totally make sense to me, since the apparent superconductivity came and went in the expected way as they changed an external magnetic field. I don't understand how a loose terminal could mimic that figure (I think it was in figure 1).

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EvgeniyZh2y ago

One case in which you can see something similar to SC transition is percolation, where the sample is mix of regular metal and insulator, and you (accidentally) connect current contacts to metal but voltage probes to insulator [1]

[1] https://arxiv.org/abs/1808.05871

fnordpiglet2y ago

It also doesn’t make sense to me because you would expect they didn’t just say “omg super conductivity!” on their first measurement and presumably measured more than once, verified connections, and generally assumed scrutiny would be high on any claim and (from what I’ve read of the history so far) the reason it’s from 1999 and published now is they didn’t believe it was real to begin with but believed it was a measurement mistake. This doesn’t mean others should be credulous and replication is crucial. But it seems to beggar reality to assume they made such an amateur mistake. I would assume outright fraud first.

stolsvik2y ago

Don’t you also measure the current of the “inner probes” (the ones “driving a current”), to verify that you do have contact?!

VygmraMGVl2y ago

You drive current through the outer probes. The inner probes measure voltage and are not driven. When measuring non-zero resistance, you can vary the driving current to confirm the voltage you measure also varies linearly (ex thermal effects, capacitance of the material etc.). When measuring zero resistance, you can't distinguish vs a voltage probe error.

99912y ago

Magnetic fields can move the probes.

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