Spin-1 atomic condensates in magnetic fields

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Zhang, Wenxian
You, Li
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In this thesis we investigate the static, dynamic, and thermodynamic properties of atomic spin-1 Bose gases in external magnetic fields. At low magnetic fields the properties of single-component, or scalar condensates, are essentially unaffected but can become significantly altered for spinor Bose condensates as shown by our studies. We first study the Bose-Einstein condensation of trapped spin-1 Bose gases by employing the Hartree-Fock approximation and the two-fluid model within a mean field approximation. Our detailed investigation reveals that the ferromagnetically interacting spin-1 condensates exhibit triple condensations while the antiferromagnetically interacting ones show double condensations. The ground state structure of homogeneous and trapped spin-1 Bose condensates with ferromagnetic and antiferromagnetic interactions at zero temperature in magnetic fields are then investigated systematically. We further illuminate the important effect of quadratic Zeeman shift which causes a preferred occupation of the $|m_F=0 angle$ state through spin exchange collisions, $2|m_F=0 angle leftrightarrow |m_F=1 angle + |m_F=-1 angle$. We also present detailed studies of the off-equilibrium coherent dynamics of spin-1 Bose condensates in magnetic fields within the single spatial mode approximation. Dynamical instabilities of the off-equilibrium oscillations are shown to be responsible for the formation of multiple domains as recently observed in several $^{87}$Rb experiments. Finally, we discuss briefly excited condensate states, or soliton-like states, in cigar-shaped spin-1 Bose condensates with an effective quasi-1D description, using the developed nonpolynomial Schr"odinger equation.
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