Electron-Lattice Coupling in Correlated Materials of Low Electron Occupancy

In correlated materials including transition metal oxides, electronic properties and functionalities are modulated and enriched by couplings between the electron and lattice degrees of freedom. These couplings are controlled by external parameters such as chemical doping, pressure, magnetic and electric fields, and light irradiation. However, the electron-lattice coupling relies on orbital characters, i.e., symmetry and occupancy, of t_2g and e_g orbitals, so that a large electron-lattice coupling is limited to e_g electron system, whereas t_2g electron system exhibits an inherently weak coupling. Here, we design and demonstrate a strongly enhanced electron-lattice coupling in electron-doped SrTiO₃, that is, the t_2g electron system. In ultrathin films of electron-doped SrTiO₃ [i.e., (La_0.25Sr_0.75)TiO₃], we reveal the strong electron-lattice-orbital coupling, which is manifested by extremely increased tetragonality and the corresponding metal-to-insulator transition. Our findings open the way of an active tuning of the charge-lattice-orbital coupling to obtain new functionalities relevant to emerging nanoelectronic devices.