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Dipolar chromium atoms: Spin dynamics in optical lattices and thermodynamics

Bruno Laburthe-Tolra (Laboratoire de Physique des Lasers — Univ. Paris XIII, Villetaneuse)
Quand ? Le 10/07/2015,
de 10:45 à 12:00
Où ? room 116
Participants Bruno Laburthe-Tolra
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Dipole-dipole interactions profoundly modify the magnetic properties of Bose-Einstein condensates made of strongly magnetic atoms such as Chromium. First, the anisotropy of the interaction introduces the possibility of magnetization-changing collisions, which creates an intrinsic coupling between the spin degrees of freedom and the orbital degrees of freedom. Second, dipole-dipole interactions are long-ranged, which leads to non-local spin-spin interactions, for example in an optical lattice. For these reasons, dipolar gases in optical lattices are a fascinating new platform to study quantum magnetism and quantum many-body physics. 

In this seminar, I will first describe an experiment where magnetic atoms in different sites of a 3D optical lattice undergo spin-exchange processes due to dipole-dipole interactions. A Bose-Einstein condensate of Chromium atoms is loaded into deep 3D optical lattices. After the atoms are transferred into a spin-excited state, we observe a non-equilibrium spinor dynamics resulting from inter-site Heisenberg-like spin-spin interactions provided by non-local dipole-dipole interactions. This spin dynamics is inherently many-body, as each atom is coupled to its many neighbors. Our experiment thus reveals the interest of chromium lattice gases for the study of quantum magnetism of high-spin systems. 

We have also studied the thermodynamics of chromium atoms at low magnetic field. Due to the anisotropy of dipolar interactions, magnetization is free and adapts to temperature. We observe that the BEC always forms in the lowest energy Zeeman state. By applying a magnetic field gradient, we introduce a selective loss of atoms in spin-excited states, which provides a specific loss channel for thermal atoms. This new cooling mechanism based on spin filtering results in purification of the BEC and an increased phase-space density.