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Physica Scripta

Symmetries and the miscible-immiscible transition in Bose–Einstein condensates confined in optical lattice

Shu-Song Wang, Suying Zhang

Published Nov 1, 2025• 0 citations
Publisher

Abstract

Abstract We present a systematic investigation of symmetries and ground state phase transitions in two-dimensional Bose–Einstein condensates with rotational and spin–orbit couplings confined within one-dimensional optical lattices. For lattice phases φ = π 2 , the vortex arrangement exhibits mirror symmetry about the y-axis, and the density distribution in momentum space maintains mirror symmetry about the k x -axis. For φ = 0, in contrast, the system exhibits spontaneous symmetry breaking, leading to ground state degeneracy. The optical lattice divides the condensate into distinct units. With a fixed spin–orbit coupling strength, the two-component condensates in different units successively undergo immiscible to miscible transitions as the rotational angular frequency increases. For φ = 0, in particular, the unit of condensate located in the center of the external potential consistently remains in the phase separation.

DOI
10.1088/1402-4896/ae1d32

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Abstract We present a systematic investigation of symmetries and ground state phase transitions in two-dimensional Bose–Einstein condensates with rotational and spin–orbit couplings confined within one-dimensional optical lattices. For lattice phases φ = π 2 , the vortex arrangement exhibits mirror symmetry about the y-axis, and the density distribution in momentum space maintains mirror symmetry about the k x -axis. For φ = 0, in contrast, the system exhibits spontaneous symmetry breaking, leading to ground state degeneracy. The optical lattice divides the condensate into distinct units. With a fixed spin–orbit coupling strength, the two-component condensates in different units successively undergo immiscible to miscible transitions as the rotational angular frequency increases. For φ = 0, in particular, the unit of condensate located in the center of the external potential consistently remains in the phase separation.
We study the ground-state phases of a two-dimensional rotating spin-orbit–coupled Bose-Einstein condensate loaded in a one-dimensional spin-dependent optical lattice. In the absence of rotation, the system undergoes a phase transition from the stripe phase to the supersolid phase gradually for weak spin-orbit coupling strength and directly for strong spin-orbit coupling strength as the depth of the optical lattice increases. With increasing the rotation strength, the stripe phase transforms to the ringlike state, and the domain number of the ringlike state increases when the Bose-Einstein condensate is in the weak depth of the optical lattice. The supersolid phase transforms to the phase with a single domain of a vortex line for small rotation strength, and alternating vortex lines along the x -direction appear for large rotation strength when the Bose-Einstein condensate is in the strong depth of the optical lattice. The spin textures of the ground-state phases are also discussed.
We consider a two-component immiscible Bose-Einstein condensate with dominating intra-species repulsive density-density interactions. In the ground-state phase of such a system only one condensates is present. This can be viewed as a spontaneous breakdown of Z2\mathbb{Z}_2Z2​ symmetry. We study the phase diagram of the system at finite temperature beyond mean-field approximation. In the absence of rotation, we show that the system undergoes a first order phase transition from this ground state to a miscible two-component normal fluid as temperature is increased. In the presence of rotation, the system features a competition between vortex-vortex interaction and short range density-density interactions. This leads to a rotation-driven "mixing" phase transition in a spatially inhomogeneous state with additional broken U(1)\mathrm{U}(1)U(1) symmetry. Thermal fluctuations in this state lead to nematic two-component sheets of vortex liquids. At sufficiently strong inter-component interaction, we find that the superfluid and Z2\mathbb{Z}_2Z2​ phase transitions split. This results in the formation of an intermediate state which breaks only Z2\mathbb{Z}_2Z2​ symmetry. It represents two phase separated normal fluids with density imbalance.
We study the miscible-immiscible quantum phase transition in a linearly coupled binary Bose-Hubbard model in one dimension that can describe the low-energy properties of a two-component Bose-Einstein condensate in optical lattices. With the quantum many-body ground state obtained from the density matrix renormalization group algorithm, we calculate the characteristic physical quantities of the phase transition controlled by the linear coupling between the two components. Furthermore we calculate the Binder cumulant to determine the critical point and construct the phase diagram. The strong-coupling expansion shows that in the Mott insulator regime the model Hamiltonian can be mapped to a spin-1/21/21/2 XXZ\mathit{XXZ}XXZ model with a transverse magnetic field.
Abstract We introduce a discrete model for binary spin–orbit-coupled Bose–Einstein condensates trapped in a deep one-dimensional optical lattice. Two different types of the couplings are considered, with spatial derivatives acting inside each species, or between the species. The discrete system with inter-site couplings dominated by the spin–orbit coupling (SOC), while the usual hopping is negligible, emulates condensates composed of extremely heavy atoms, as well as those with opposite signs of the effective atomic masses in the two components. Stable localized composite states of miscible and immiscible types are constructed. The effect of the SOC on the immiscibility–miscibility transition in the localized complexes, which emulates the phase transition between insulating and conducting states in semiconductors, is studied.
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.
Theoretical study is presented for a spinor Bose-Einstein condensate, whose two components are coupled by copropagating Raman beams with different orbital angular momenta. The investigation is focused on the behavior of the ground state of this condensate, depending on the atom-light coupling strength. By analyzing the ground state, we have identified a number of quantum phases, which reflect the symmetries of the effective Hamiltonian and are characterized by the specific structure of the wave function. In addition to the well-known stripe, polarized, and zero-momentum phases, our results show that the system can support phases whose wave functions contain a complex vortex molecule. Such a molecule plays an important role in the continuous phase transitions of the system. The predicted behavior of vortex-molecule phases can be examined in cold-atom experiments using currently existing techniques.