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We calculate the full density response function, and from it thelong-wavelength acoustic dispersion for a two-dimensional system of stronglycoupled point dipoles interacting through a 1/r^3 potential at arbitrarydegeneracy. Such a system has no RPA limit and the calculation has to includecorrelations from the outset. We follow the Quasi-Localized Charge (QLC)approach, accompanied by Molecular Dynamics (MD) simulations. Similarly to whathas been recently reported for the closely spaced classical electron-holebilayer [G. J. Kalman et al. Phys. Rev. Lett. 98, 236801 (2007)] and in markedcontrast to the RPA, we report a long-wavelength acoustic phase velocity thatis wholly maintained by particle correlations and varies linearly with thedipole moment p. The oscillation frequency, calculated both in an extended QLCapproximation and in the Singwi-Tosi-Land-Sjolander approximation, is invariantin form over the entire classical to quantum domains all the way down to zerotemperature. Based on our classical MD-generated pair distribution functiondata and on ground-state energy data generated by recent quantum Monte Carlosimulations on a bosonic dipole system [Astrakharchik et al, Phys. Rev. Lett.98, 060405 (2007)], there is a good agreement between the QLCA kinetic soundspeeds and the standard thermodynamic sound speeds in both the classical andquantum domains.

Acoustic dispersion in a two-dimensional dipole system