Superfluidity in two-dimensional Bose Coulomb liquids

Two alternative interaction models have been studied in the literature for a fluid of charged Bose particles (BCF) moving in a plane: a strictly two-dimensional fluid with logarithmic interactions (2D-BCF) and a quasi-two-dimensional fluid with inverse-first-power repulsions (Q2D-BCF). A study by W.R. Magro and D.M. Ceperley [Phys. Rein Lett. 73, 826 (1994)] has shown that the ground state of the 2D-BCF is not Bose-condensed, but exhibits algebraic off-diagonal order in the single-particle density matrix rho(r). We have used Popov's hydrodynamic Hamiltonian expressed in terms of the density and phase fluctuation operators, in combination with an f-sum rule on the superfluid fraction, to reproduce these results and to extend them to finite temperature T This approach allows us to treat the liquid as a superfluid in the absence of a condensate and to show that the exponent in the power-law decay of rho(r) is determined by the superfluid density n(s)(T). These properties are contrasted with those of the Q2D-BCF, which parallels the neutral two-dimensional Bose fluids treated in the work of Popov and of Yu. Kagan and coworkers: a genuine condensate is present at T = 0, and algebraic off-diagonal order at finite T can be associated with a quasi-condensate.