Delta-temperatural electronic transportation achieved in metastable perovskite rare-earth nickelate thin films

The metal to insulator transition (MIT) in Mott-Hubbard systems is one of the most important discoveries in condensed matter physics, and results in abrupt orbital transitions from insulating to metallic phases by elevating the temperature across a critical point (T_MIT). Although the MIT was previously expected to be mainly driven by the orbital Coulomb repulsion energy, the entropy contribution to the orbital free energy, which also determines the relative stability of the metallic and insulating phases, was overlooked. Herein, we demonstrate an additional reversible electronic transition observed in chemically grown thin films of metastable rare-earth nicklate perovskites (ReNiO₃) on single crystalline substrates, in addition to their MIT. By elevating the temperature across another critical point (T_R-MIT) below T_MIT, the resistivity of the ReNiO₃/substrate system abruptly increases by 2-3 orders, the transition of which is named reverse temperature-dependence in electrical transportation compared to the metal to insulator transition (denoted herein as the R-MIT) and associated with entropy. T_R-MIT is shown to be enhanced via reducing the compositional complexity and size of Re or imparting bi-axial compressive strains, and meanwhile the transition sharpness of delta-temperatural transport is reduced. Combining the afterwards exponentially decreasing resistivity in the insulating phase of ReNiO₃ with further temperature elevation, a delta-temperatural transportation character is established. This functionality is potentially useful in locking the working temperature window for electric devices that cater for the demand in the fast developing automatic transmission or artificial intelligence.