RPA approach to supersolidity

Max-Planck-Institute for the Physics of Complex Systems - D-01187 Dresden, Germany, EU and Nonlinear Physics Centre, Australian National University - Canberra, ACT 0200, Australia

Corresponding author: gulacsi@pks.mpg.de

Received: 9 October 2008
Accepted: 5 January 2009

Abstract

We investigate the newly discovered supersolid phase by solving in random phase approximation the anisotropic Heisenberg model of the hard-core boson lattice. We include nearest- and next-nearest-neighbor interactions and calculate exactly all pair correlation functions in a cumulant expansion scheme. Here we clarify the controversy over the role of the vacancies and defects, which have long been proposed to have a crucial role in the formation of a SS phase. We show that vacancies and interstitials will be present even at zero temperature in the supersolid phase. This phase is characterized by Bose condensation of the vacancies as well as the interstitials and may be regarded as a bond-ordered wave as it exhibits alternating strength of the expectation value of the kinetic energy term on bonds. We also show that the superfluid-to-supersolid transition is triggered by a collapsing roton minimum, however, the supersolid phase is stable against spontaneously induced superflow.