I did a condensed matter experiment PhD, but high Tc is not my field and I haven’t spent much time thinking about this. [Edit: I didn’t see Charlie Steiner’s comment until I had written most of this comment and started editing for clarity. I think you can treat this as mostly independent.] Still, some thoughts on Q1, maybe starting with some useful references:
Bednorz and Müller, “Possible High Tc Superconductivity in the Ba—La—Cu—O System” (1986) was the inciting paper for the subsequent discoveries of high-Tc superconductors, notably the roughly simultaneous Wu, Ashburn, Torng, et al. (1987) and Cava et al. (1987) establishing superconductivity in YBCO with Tc > 90K. The first paper is more cautious in its claims on clearer evidence than the present papers. The latter two show dispositive measurements.
I might also recommend Ekin’s Experimental Techniques for Low-Temperature Measurements for background on how superconductors are usually measured (regardless of temperature). It discusses contacts, sample holders, instrumentation, procedures, and analysis for electrical transport measurements in superconductors, with a focus on critical current measurements. (I don’t think skimming it to find a graph or statement that you can apply to the present case will be very helpful, though.)
One person alleges an online rumor that poorly connected electrical leads can produce the same graph. Is that a conventional view?
It’s well understood that a jump in the I-V curve does not imply superconductivity. Joule heating at high currents and thermal expansion, for example, can cause abrupt changes in contact resistance. I’m not sure exactly what it would take to reproduce that graph, but contact physics is gnarly enough that there’s probably a way, together with other experimental complications.
If this material is a superconductor, have we seen what we expected to see? Is the diminishing current capacity with increased temperature usual?
In the roughest sense, yes. The critical current density for a superconductor decreases as temperature increases and as magnetic field increases. Quantitatively, maybe. There are evidently other things going on, at least. (In another sense, the papers aren’t really what I’d expect a lab that thought they had a new superconductor to present. But I think that can be explained between reproducibility issues, the interpersonal issues rushing publication, and the fact that they’re somewhat outside the community that usually thinks about superconducting physics.)
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?
In an impure sample you would see high residual resistance below Tc (I think the authors do, but I’m not confident at a glance particularly given paper quality problems) and broad transitions due to a spread of transition temperatures over superconducting domains (it seems to me that the authors see very sharp transitions, although data showing the width in temperature from the April paper is omitted from the arXiv papers). The worse these are, the more mundane explanations are viable (roughly speaking), which is part of why observing the Meissner effect is important. But this is a good question. To some extent people are “vibing” rather than getting a story straight.
For what it’s worth, the 2022 CTA homicides were a huge outlier. The years 2001-2019 had 0-2 homicides each (filtering “CTA” in “Location”), then 4 in 2020 and 4 in 2021. Meanwhile, leading up to the pandemic, the National Transit Database says Passenger Miles Traveled was approaching 2 billion (https://www.transit.dot.gov/ntd/transit-agency-profiles/chicago-transit-authority), was down to 800 million for 2020 and 2021, and presumably came back up a lot in 2022 (no profile available yet).
I agree it’s not infinitely safe, but I do suspect transit comes out safer by most analyses.