  We observe a superfluid $\lambda$ transition in the $P-V$ criticality of
charged AdS black holes, within a holographic extended thermodynamics that
considers the variation of Newton's constant $G$. We calculate the critical
exponents and find that they coincide with those of a superfluid transition in
liquid $^4\text{He}$ and the Bose-Einstein condensation of hard-sphere Bose
gas. Moreover, the independence of entropy and thermodynamic volume in the
holographic framework allows us to construct a well-defined Ruppeiner metric.
The associated scalar curvature suggests that the black holes show similar
microscopic interactions with the hard-sphere Bose gas, where the superfluid
(condensed) phase is dominated by repulsive interactions, while the normal
(gas) phase is dominated by attractive interactions. These findings might
provide us with new insights into the quantum aspect of charged AdS black
holes.
