The superconducting dome with a dip in infinite-layer Nd1−xSrxNiO2 interpreted with the free volume concept

The superconducting nickelates have the common features observed in many unconventional superconductors, and offer several unique characteristics such as a superconducting single-dome or a double-dome with a dip observed in Nd 1 − x Sr x NiO 2 at x about 1/5. The transition from electron-doped to hole-doped superconductors is believed to be responsible for the dip, however, the mechanism has not been fully understood yet. Less attention has been paid to the lattice volume change accompanying such a transition. Here, the resistivity data of Nd 1 − x Sr x NiO 2 showing a double superconducting dome with a dip reported previously have been analyzed using a universal conductivity equation derived from the Eyring’s rate process theory and the free volume concept. The results reveal a clear correlation of the dip to a specific packing structure and the free volume of electrons in the system. The distinct atomic size and electronegativity differences between Sr and Nd may result in the electron number and free volume decrease, which in turn facilitates the transition from an electron-type to a hole-type dome. Such a physical picture can explain the characteristic dip in the superconducting dome and other accompanied behaviors. This work provides a unique perspective to interpret the superconducting dome in nickelates, and may also offer an alternative framework to understand the superconductivity in other unconventional superconductors.