  The confliction between the stable interface in phase-separated active
Brownian particles and its negative surface tension, obtained mechanically via
the active pressure, has sparked considerable debate about the formula of
active surface tension. Based on the intrinsic pressure of active system, we
here derive a proper mechanical expression of active surface tension by
calculating the work required to create a differential interface area, while
remaining interfacial profile invariant (not considered previously). Our
expression yields a significantly positive surface tension that increases with
the particle actvity, which is supported by mechanical analysis on the
interface. Our results thus fundamentally resolve the contradiction related to
active surface tension.
