It is well known that high pressures are one of the tools for increasing the superconducting transition temperature (Tc) of superconductors due to the interatomic distances shortening. Suffice it to recall that the Tc record belonging to thallium cuprates is achieved at high pressures.
It is also known that the substitution of large cations by a smaller rare-earth ions in the not so long ago discovered family of superconducting iron chalcogenides (such doping, in fact, is simulation of pressure increase) is accompanied by Tc increase up to record 55 K. The dependence of Tc on doping level exhibits a bell-shaped curve – Tc drops monotonically after passing the optimal doping level (pressure).
A large group of Chinese scientists from China and the United States has investigated the effect of pressure on the superconducting transition temperature in some iron chalcogenides (Tl0.6Rb0.4Fe1.67Se2, K0.8Fe1.7Se2 and K0.8Fe1.78Se2) and found that, as expected, with pressure increase from 1 to 9 GPa Tc decreases from 32 K to zero, but with further pressure increasing at 11.5 GPa Tc abruptly increases up to 48,0-48,7 K. The studies were performed on single crystals (100 micrometers in diameter and 50 micrometers thick), placed between two diamond anvils.
It should be noted that the phenomenon of pressure-induced re-emergence of superconductivity has been not found in any families of high-temperature superconductors. The nature of this remains unclear, and for its establishment the additional structural studies (in particular neutron scattering experiments) are required. They will help determine whether the ordering of vacancies, changes in magnetism or another effect is behind the change in behaviour.