Interfacial superconductivity between over- and under- doped cuprate Josephson junctions

Superconductivity in cuprates is constrained by two primary factors: the pairing energy scale and the superfluid stiffness. In hole-doped cuprates, as the system moves away from the Mott insulating state, the pairing energy scale weakens while the superfluid stiffness increases. Optimal doping represents a crossover between two regimes: stiffness-limited in the underdoped region and pairing-limited in the overdoped region. Theory suggests that at the interface between the underdoped and overdoped regions, enhanced superconducting transition temperature Tc may arise from the interplay between stronger pairing and improved superfluid stiffness.

To investigate this hypothesis, we fabricated heterostructures of atomically thin underdoped and overdoped BSCCO-2212 crystals using a cryogenic transfer stage in an inert atmosphere, ensuring minimal interface degradation. The resulting device shows Josephson coupling between two crystals, allowing us to probe the interfacial superconductivity. Resistance measurements across the devices as a function of temperature revealed an additional resistance drop in resistance, distinct from the Tc of the individual flakes. This behavior persists even after a mild thermal annealing, which shifts three transition temperatures all together, ruling out the possibility of oxygen diffusion as a cause. These experimental findings suggest an emergent interfacial superconductivity in the over-under doped cuprate, which is distinctly different from bulk superconductivity.