  We study the formation of dipolar excitons and their superfluidity in a black
phosphorene double layer. The analytical expressions for the single dipolar
exciton energy spectrum and wave function are obtained. It is predicted that a
weakly interacting gas of dipolar excitons in a double layer of black
phosphorus exhibits superfluidity due to the dipole-dipole repulsion between
the dipolar excitons. In calculations are employed the Keldysh and Coulomb
potentials for the interaction between the charge carriers to analyze the
influence of the screening effects on the studied phenomena. It is shown that
the critical velocity of superfluidity, the spectrum of collective excitations,
concentrations of the superfluid and normal component, and mean field critical
temperature for superfluidity are anisotropic and demonstrate the dependence on
the direction of motion of dipolar excitons. The critical temperature for
superfluidity increases if the exciton concentration and the interlayer
separation increase. It is shown that the dipolar exciton binding energy and
mean field critical temperature for superfluidity are sensitive to the electron
and hole effective masses. The proposed experiment to observe a directional
superfluidity of excitons is addressed.

Mesoscale and Nanoscale Physics