Under many circumstances, the only important two-body interaction between atoms in ultracold dilute atomic vapors is the short-ranged isotropc s-wave collision. This interaction can be accurately described using a delta-function pseudo-potential with the s-wave scattering length being the only parameter. Situations however may arise where the dipolar interaction between atomic magnetic or electric dipole moments can play a significant role. The long-range anisotropic nature of the dipolar interaction greatly enriches the static and dynamic properties of ultracold atoms.
In the case of dipolar Bose-Einstein condensate, studies have shown that dipolar interaction may induce a roton feature in the collective excitation spectrum of the condensate. Previously, roton excitations were observed in superfuid helium (He II). We are interested in how the roton excitation can manifest itself in atomic condensates. Our recent studies have indicated that roton excitation may result in oscillations in condensate density profile (charge density wave) when impurities exist in the system. Here impurities may take the form of a vortex line, the ‘wall’ of the trapping potential, or maybe an alien species of atom.
Vortex lattice of a rotating dipolar BEC. The density ripples near vortex cores are linked to the roton excitation.
Experimental effort in creating dipolar condensate has been focused on atoms with relatively large magnetic dipole moment or polar molecules which possess huge electric dipole moment. While we do not yet have a condensate of polar molecules, dipole effects have been observed in chromium atoms. It is usualy thought that dipolar interaction is too weak in more conventional atomic alkali condensates. This however may not be true when atomic spin degrees of freedom is concerned. Our studies have shown that magnetic dipolar interaction, although rather weak, plays a crucial role in the spin dynamics of alkali spinor condensate. This is particularly true for the F=1 hyperfine state of Rb atoms. Dipolar interaction in spinor condensates couples the dynamics of the orbital and spin degrees of freedom, and may give rise to rich spin textures.
In the case of Fermi gas, the anisotropic dipolar interaction deforms the Fermi surface. This is mainly the work of the Fock exchange energy, which is absent in dipolar condensates. Since Fermi surface controls the low-energy properties of the Fermi gas, this deformation may lead to interesting phenomena.
Finally, dipolar quantum gases in optical lattice potentials represent an intersting system with extremely rich properties. It has compelling applications in quantum information science and quantum magnetism.
Related Publications
- Yi andH. Pu,Spontaneous Spin Textures in Dipolar Spinor Condensates, Phys. Rev. Lett. 97, 020401 (2006).
- Yi andH. Pu,Vortex Structures in Dipolar Condensates, Phys. Rev. A 73, 061602(R) (2006).
- Yi andH. Pu,Magnetization, Squeezing, and Entanglement in Dipolar Spin-1 Condensates, Phys. Rev. A 73, 023602 (2006).
- Yi, L. You andH. Pu,Quantum Phases of Dipolar Spinor Condensates, Phys. Rev. Lett. 93, 040403 (2004).
- Gross, C. P. Search,H. Pu, W. Zhang and P. Meystre,Magnetism in a Lattice of Spinor Bose-Einstein Condensates, Phys. Rev. A 66, 033603 (2002).
- Pu, W. Zhang and P. Meystre,Macroscopic Spin Tunneling and Quantum Critical Behavior of a Condensate in a Double-Well Potential,Phys. Rev. Lett. 89, 090401 (2002).
- Pu, C. P. Search, W. Zhang and P. Meystre,Atom Optics — From de Broglie Waves to Heisenberg Ferromagnets,Fortschritte der Physik 50, 664 (2002).
- Zhang,H. Pu, C. P. Search and P. Meystre,Spin Waves in a Bose-Einstein Condensed Atomic Spin Chain, Phys. Rev. Lett. 88, 060401 (2002).
- Pu, W. Zhang and P. Meystre, Ferromagnetism in a Lattice of Bose-Einstein Condensates,Phys. Rev. Lett. 87, 140405 (2001).