Abstract
One-component Coulomb crystals of ions with hexagonal close-packed (hcp) lattice likely form in the crust of strongly-magnetized neutron stars (magnetars). In this work we present a detailed study of vibration modes and thermodynamic properties of such crystals in a wide range of temperatures at zero magnetic field. In contrast to typically considered lattices, the phonon spectrum of the system exhibits a peculiar crossing of the acoustic modes near the Brillouin zone center in certain directions of the wavevector. It is demonstrated that in the field-free regime the Helmholtz free energy of the hcp Coulomb crystal is always higher than those of the Coulomb crystals with body-centered cubic and face-centered cubic lattices. The results of our numerical calculations are fitted by simple analytic expressions.
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Notes
Strictly speaking, in the inner neutron star crust, at densities above \(4.3 \times 10^{11}\ \mbox{g}\,\mbox{cm}^{-3}\), the appearance of dripped neutrons may impact the ion dynamics (e.g., Chamel 2012).
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This work was supported by RSF, grant No. 14-12-00316.
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Kozhberov, A.A., Baiko, D.A. Thermodynamic functions of the hcp Coulomb crystal lattice. Astrophys Space Sci 359, 50 (2015). https://doi.org/10.1007/s10509-015-2496-z
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DOI: https://doi.org/10.1007/s10509-015-2496-z