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Ukr. J. Phys. 2015, Vol. 59, N 10, p.993-1000
doi:10.15407/ujpe59.10.0993    Paper

Stasyuk I.V., Velychko O.V., Dulepa I.R.

Institute for Condensed Matter Physics, Nat. Acad. of Sci. of Ukraine
(1, Svientsitskii Str., Lviv 79011, Ukraine)

Investigation of the Bosonic Spectrum of Two-Dimensional Optical Graphene-Type Lattices. Superfluid Phase

Section: Solid matter
Original Author's Text: Ukrainian

Abstract: The energy spectrum of a system of Bose atoms in the superfluid phase in an optical lattice of the graphene type has been studied. The dispersion laws for the energy bands and the single particle spectral densities are calculated in the random phase approximation and in the framework of the hard-core boson formalism, and their changes at the transition from the normal phase to the superfluid one are described. As a result of this transformation, the number of subbands doubles. In the case of the subband energetic equivalence, the Dirac points in the spectrum survive, and their number becomes twice as much. When the subbands are energetically nonequivalent, the Dirac points are absent. The shape of spectral densities is shown to be sensitive to the changes in the temperature and the chemical potential position.

Key words: optical lattice, honeycomb lattice, phase transition, spectral density, hard-core bosons, Dirac points.