What is the zero-resistance transition temperature of monolayer FeSe on $\mathrm{SrTiO_3}$?
In plain words
Different methods give very different transition temperatures, from about 40 K in transport on protected films and about 65 K in magnetic measurements to a single report above 100 K, so even the basic number is uncertain.
Precise statement
Ex situ transport on capped monolayers gives onset $T_c \sim 40\,\mathrm{K}$ with lower zero-resistance temperatures, diamagnetic (Meissner) onset appears near 65 K (Zhang et al., Science Bulletin 60, 1301, 2015), photoemission gap closing occurs near 55-65 K, and one in situ four-probe study reported $T_c$ above 100 K (Ge et al. 2015) that has not been reproduced. Determine the onset and zero-resistance $T_c$ of uncapped monolayer FeSe/$\mathrm{SrTiO_3}$ in situ, and whether the gap-closing temperature marks phase coherence or only pairing.
What would settle it
In situ transport and mutual-inductance measurements on uncapped films by several groups, with the fluctuation regime characterized.
Status in the literature
Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.
Nernst measurements on ultrathin FeSe/$\mathrm{SrTiO_3}$ find superconducting fluctuations only below about 1.2 times the onset $T_c$ and suggest the gap seen above $T_c$ in photoemission may have a non-superconducting origin (Kobayashi, Ogawa and Maeda, Physical Review B 112, 094525, 2025).
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