Gapped spectrum in pair-superfluid bosons

Abstract

We study the ground state of a bilayer system of dipolar bosons with dipoles oriented by an external field perpendicularly to the two parallel planes. By decreasing the interlayer distance, for a fixed value of the strength of the dipolar interaction, the system undergoes a quantum phase transition from an atomic to a pair superfluid. We investigate the excitation spectrum across this transition by using microscopic approaches. Quantum Monte Carlo methods are employed to obtain the static structure factors and intermediate scattering functions in imaginary time. The dynamic response is calculated using both the correlated basis functions (CBF) method and the approximate inversion of the Laplace transform of the quantum Monte Carlo imaginary time data. In the atomic phase, both density and spin excitations are gapless. However, in the pair-superfluid phase a gap opens in the excitation energy of the spin mode. For small separation between layers, the minimal spin excitation energy equals the binding energy of a dimer and is twice the gap value.

8 pages, 5 figures

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• The Multi-component Correlated Basis Function Method and its Application to Multilayered Dipolar Bose Gases
• Optimized correlations inspired by perturbation theory
• Nonlocal order parameter of pair superfluids
• Self-Assembled Chains and Solids of Dipolar Atoms in a Multilayer
• Ultradilute quantum liquid of dipolar atoms in a bilayer
• Quantum Monte Carlo study of few- and many-body Bose systems in one and two dimensions