Superconducting Dome Underlying Bipolaronic Insulating State in Charge-doped Ti4O7 Epitaxial Films

The bipolaronic insulating (BI) state can cause the emergent superconducting phase when the strength of electron–phonon interaction is deliberately reduced. In this report, we have investigated electronic phase diagram of the well-known BI compound Ti4O7 by analyzing the transport properties of over 40 epitaxial-film samples. The diagram captures the systematic suppression of the BI state with increasing the number of doped electrons and a superconducting dome located under the BI state. The coexistence of the BI and superconducting states suggests the bipolaronic superconductivity in Ti4O7 epitaxial films.