New experiments set the record of the superconducting transition temperatures for a new family of iron-based selenide superconductors. These materials were recently found to superconduct below 30 Kelvin, but their transition temperatures decline until approaching absolute zero temperature with the application of pressure. Now Carnegie scientists Xiao-Jia Chen, Lin Wang, and Ho-Kwang Mao, in collaboration with scientists from from the National Institute of Standards and Technology, the Chinese Academy of Science, and Zhejiang University, have uncovered reemerging superconductivity above 48 Kelvin in iron selenides upon further compression. Their work was published online in Nature on February 22, 2021.

Iron selenides, iron arsenides, and cuprates are classified as three families of high temperature superconductors. Despite many different physical properties, these three families share a common structural unit with charge reservoir layers for electron or hole doping, as well as conducting layers responsible for superconductivity. The charge carriers are believed to solely come from the charge reservoir layers in the early discovered cuprate and iron arsenide superconductors. However, the carrier concentration in the iron selenium layers of the new iron selenide superconductors can be changed through charge diffusion from the two different occupied iron sites within the conducting layers themselves and/or charge transfer from the charge reservoirs to the conducting layers. properties. This novel feature makes iron selenides to stand out an interesting class of superconductors having close interplay among their structural, magnetic, and superconducting.

To test this idea, the researchers checked their early experimental data, which showed that superconductivity disappeared at around 9 GPa, and decided to extend their resistance and magnetic susceptibility measurements to higher pressures. The measurements on six single crystals yield the same re-entrance of superconductivity from its early disappearance but with an 18 K increase from the ambient-pressure value of 30 K. Their structure investigations confirm that the initial crystal structure persists throughout the pressure range studied. Therefore, the disappearance of superconductivity in the low-pressure cycle and the re-emergence of superconductivity with higher transition temperatures in the high-pressure cycle reflect detailed structural variances within the basic unit cell itself. The two superconducting domes were likely the result of different charge carriers. Finding the reentrance of superconductivity at 48 K in the new iron family of superconductors points to the possibility of achieving similar higher transition temperatures at ambient pressure through some structural modifications [L. Sun et al., Nature (2012)].