What is the lower limit for experimentally measuring $T_c$ of superconductors?

Question

I'm pretty new to this topic, but find it interesting, so please bear with me.

Since superconductors with $T_c \approx 4.2\text{K}$ have been measured, I wonder, is it possible to have superconductors with much lower $T_c$.. So my question is, what is the lowest possible experimentally measurable $T_c$? Because I do understand that in theory, it is possible to have superconductors below $T_c < 1\text{nK}$, but could this be measurable and confirmed experimentally?

Would be cool if you could provide some papers or other sources to back-up your answers :D thnx

Answer 1

Here's an example of 0.5mK Tc in Bismuth: https://www.science.org/doi/abs/10.1126/science.aaf8227

Answer 2

As Xcheckr shared it is possible to measure superconductivity at much lower temperatures than 4.2K and a Dilution fridge is normally used for targeting milliKelvin scales and are commonly used for studying quantum computing and the associated superconductive circuits. If you are interested by using laser cooling and other methods it is possible to reach even lower temperatures like this 100 pK Rhodium metal (http://ltl.tkk.fi/wiki/LTL/World_record_in_low_temperatures) or Rb atoms in 2 dimensions at 50 pK (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.143004). Whilst not a superconductor phase, NASA is using the space station to try to reach 1 pK and study Bose-Einstein condensates (https://www.nasa.gov/mission_pages/station/research/news/cold_atom_lab). Other people are looking into laser cooling (this one has a system of 50000 yttrium monoxide molecules https://physics.aps.org/articles/v14/s163) and this group wanted to look at a quantum degenerate gases at 500 nK (https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.205302), where there would be a wide variety of exotic quantum states including superfluid crystals. So yeah who knows how low we will get but we might be a little bit away from say a solid being cooled to less than a nK scale and discovering it is a superconductor, however other gases and molecules have already gone quite low.