Quantum metallic state in the titanium sesquioxide heterointerface superconductor

The emergence of the quantum metallic state marked by a saturating finite electrical resistance in the zero-temperature limit in a variety of two-dimensional superconductors injects a new momentum to the realm of unconventional superconductivity. Despite much research efforts over last few decades, there is not yet a general consensus on the nature of this unexpected quantum metal. Here, we report the unique quantum metallic state within the hallmark of Bose-metal characterized by the saturated resistance and simultaneously vanished Hall resistance in the titanium sesquioxide heterointerface superconductor Ti$_2$O$_3$/GaN. Strikingly, the quantum bosonic metallic state proximate to the two-dimensional superconductivity-metal transition tuned by magnetic fields persists in the normal phase, suggesting that the existence of composite bosons formed by electron Cooper pairs survives even in the normal phase. Our work marks the observation of the preformed electron Cooper pairs in heterointerface superconductor and sheds new light on understanding the underlying pairing mechanism of unconventional superconductivity.