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arXiv:2512.07493 · Journal of Physics Condensed Matter

Quasiparticle spectra of mixtures of dipolar and non-dipolar condensates at zero and finite temperatures

Harsimranjit Kaur, Kuldeep Suthar

Published May 22, 2026• 0 citations
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Abstract

We examine the low-lying collective quasiparticle modes of a mixture of flattened infinite-pancake Bose-Einstein condensates having dipolar and non-dipolar atomic species. The dipolar atomic species have permanent magnetic dipolar moments. We employ Hartree-Fock-Bogoliubov theory to investigate the distinct axial collective spectra at zero and finite temperatures corresponding to phase separation phenomena stemming from the dipole-dipole interaction of dipolar atomic species. When the dipolar interaction is tuned to be repulsive, the number of zero-energy axial modes decreases, reflecting the system's tendency towards mixing. For a large number of atoms per unit area, we show that the attractive (repulsive) dipolar interaction strengths lead to ground states with non-dipolar (dipolar) atomic species at the periphery, and this leads to a discontinuity in quasiparticle mode evolution. We finally reveal that miscibility driven by thermal fluctuations at finite temperatures exhibits dipole mode hardening of axial excitations, confirmed by the loss of long-range phase coherence through the correlation function. The mode mixing in the dispersion relations ascertains a dipolar strength-dependent miscibility transition and the low-lying quasiparticle mode evolution.

DOI
10.1088/1361-648x/ae722f

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We examine the low-lying collective quasiparticle modes of a mixture of flattened infinite-pancake Bose-Einstein condensates having dipolar and non-dipolar atomic species. The dipolar atomic species have permanent magnetic dipolar moments. We employ Hartree-Fock-Bogoliubov theory to investigate the distinct axial collective spectra at zero and finite temperatures corresponding to phase separation phenomena stemming from the dipole-dipole interaction of dipolar atomic species. When the dipolar interaction is tuned to be repulsive, the number of zero-energy axial modes decreases, reflecting the system's tendency towards mixing. For a large number of atoms per unit area, we show that the attractive (repulsive) dipolar interaction strengths lead to ground states with non-dipolar (dipolar) atomic species at the periphery, and this leads to a discontinuity in quasiparticle mode evolution. We finally reveal that miscibility driven by thermal fluctuations at finite temperatures exhibits dipole mode hardening of axial excitations, confirmed by the loss of long-range phase coherence through the correlation function. The mode mixing in the dispersion relations ascertains a dipolar strength-dependent miscibility transition and the low-lying quasiparticle mode evolution.
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We study two-species Bose-Einstein condensates confined in quasi-two-dimensional (quasi-2D) optical lattices at finite temperatures, employing the Hartree-Fock-Bogoliubov theory with the Popov approximation. We examine the role of thermal fluctuations in the ground-state density distributions and the quasiparticle mode evolution. At zero temperature, the geometry of the ground state in the immiscible domain is side by side. Our results show that the thermal fluctuations enhance the miscibility of the condensates, and at a characteristic temperature the system becomes miscible with rotationally symmetric overlapping density profiles. This immiscible-miscible transition is accompanied by a discontinuity in the excitation spectrum, and the low-lying quasiparticle modes such as slosh mode become degenerate at the characteristic temperature.
Abstract The ultracold atoms are an ideal platform to implement atomtronics and Josephson junctions analogous to superconducting circuits. The collective modes of a Bose gas split by a potential barrier have been known. However, the role of barriers on the collective excitation spectra of ultracold atomic mixtures has not been examined. Here, we examine the low-lying collective modes of (an)harmonically trapped quasi-one-dimensional Bose–Einstein condensates (BECs) in a Josephson barrier by employing the variational approach and Bogoliubov theory. We first show that the anharmonicity of the external potential leads to an increase in the critical barrier strength of mode softening in a single-species condensate. The Josephson barrier drives the softening of in-phase and out-of-phase dipole modes of two-species BECs, and consequently leads to two additional zero-energy Goldstone modes in the miscible phase, in agreement with the variational approach. Furthermore, the sandwich immiscible state results in an additional Goldstone mode due to the barrier, in contrast to the spatially symmetry-broken side-by-side profile. Our results unveil the distinct collective response of the Josephson barrier in binary mixtures owing to interspecies atomic correlations.
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By considering symmetric- and asymmetric-dipolar coupled mixtures (with dysprosium and erbium isotopes), we report a study on relevant anisotropic effects, related to spatial separation and miscibility, due to dipole-dipole interactions (DDIs) in rotating binary dipolar Bose-Einstein condensates. The binary mixtures are kept in strong pancake-like traps, with repulsive two-body interactions modeled by an effective two-dimensional (2D) coupled Gross-Pitaevskii equation. The DDI are tuned from repulsive to attractive by varying the dipole polarization angle. A clear spatial separation is verified in the densities for attractive DDIs, being angular for symmetric mixtures and radial for asymmetric ones. Also relevant is the mass-imbalance sensibility observed by the vortex-patterns in symmetric- and asymmetric-dipolar mixtures. In an extension of this study, here we show how the rotational properties and spatial separation of these dipolar mixture are affected by a quartic term added to the harmonic trap of one of the components.
Quantum Monte Carlo simulations of a two-component Bose mixture of trapped dipolar atoms of identical masses and dipole moments, provide numerical evidence of demixing at low finite temperatures. Demixing occurs as a consequence of quantum statistics, which results in an effective attraction between like bosons. Spatial separation of two components takes place at low temperature with the onset of long exchanges of identical particles, underlying Bose-Einstein condensation of both components. Conversely, at higher temperature the system is miscible due to the entropy of mixing. Exchanges are also found to enhance demixing in the case of mixtures of nonidentical and distinguishable species.