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arXiv:1310.3384 · Physical Review A

Resonant dynamics of chromium condensates

Tomasz Świsłocki, Jarosław H. Bauer, Mariusz Gajda, Mirosław Brewczyk

Published Feb 14, 2014• 5 citations
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Abstract

We numerically study the dynamics of a spinor chromium condensate in low magnetic fields. We show that the condensate evolution has a resonant character revealing a rich structure of resonances similar to that already discussed in the case of alkali-metal-atom condensates. This indicates that dipolar resonances occur commonly in systems of cold atoms. In fact, they have already been observed experimentally. We further simulate two recent experiments with chromium condensates in which the threshold in spin relaxation and spontaneous demagnetization phenomena were observed. We demonstrate that both these effects originate in the resonant dynamics of chromium condensates.

DOI
10.1103/physreva.89.023622

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We numerically study the dynamics of a spinor chromium condensate in low magnetic fields. We show that the condensate evolution has a resonant character revealing a rich structure of resonances similar to that already discussed in the case of alkali-metal-atom condensates. This indicates that dipolar resonances occur commonly in systems of cold atoms. In fact, they have already been observed experimentally. We further simulate two recent experiments with chromium condensates in which the threshold in spin relaxation and spontaneous demagnetization phenomena were observed. We demonstrate that both these effects originate in the resonant dynamics of chromium condensates.
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Abstract. Ultra-cold atomic systems provide a new setting where to investigate the role of longrange interactions. In this paper we will review the basics features of those physical systems, in particular focusing on the case of Chromium atoms. On the experimental side, we report on the observation of dipolar effects in the expansion dynamics of a Chromium Bose-Einstein condensate. By using a Feshbach resonance, the scattering length characterising the contact interaction can be strongly reduced, thus increasing the relative effect of the dipole-dipole interaction. Such experiments make Chromium atoms the strongest candidates at present for the achievement of the strong dipolar regime. On the theoretical side, we investigate the behaviour of ultra-cold dipolar systems in the presence of a periodic potential. We discuss how to realise this situation experimentally and we characterise the system in terms of its quantum phases and metastable states, discussing in detail the differences with respect to the case of zero-range interactions.
I will discuss the spin dynamics of two magnetic atoms trapped in an external trap. Assuming that initially the atoms are in their orbital ground state I will discuss in details all possible elementary processes of spin flipping. Dipole-dipole interactions couple spin degrees of freedom of the two particles to their quantized orbital motion. Because of conservation of the total angular momentum if a magnetization of the system changes the system acquires an orbital angular momentum and atoms circulate around the center of the trap. I show that spin dynamics driven by dipolar interactions is possible only when the energies of the initial and the final sates are equal. In a case of weak magnetic moments of atoms this condition can be fulfilled utilizing a resonant external magnetic field, which tunes energies of involved states via the linear Zeeman effect. I will illustrate the resonant dynamics comparing the theoretical predictions to some results of experiments on Chromium atoms.
We study spin mixing dynamics in a chromium dipolar Bose-Einstein condensate, after tilting the atomic spins by an angle $\ensuremath{\theta}$ with respect to the magnetic field. Spin mixing is triggered by dipolar coupling, but, once dynamics has started, it is mostly driven by contact interactions. For the particular case $\ensuremath{\theta}=\ensuremath{\pi}/2$, an external spin-orbit coupling term induced by a magnetic gradient is required to enable the dynamics. Then the initial ferromagnetic character of the gas is locally preserved, an unexpected feature that we attribute to large spin-dependent contact interactions.
We have observed Bose-Einstein condensation of chromium atoms [1], whose large magnetic dipole moment is unique among all species that have been Bose-condensed so far. The arising magnetic forces are of anisotropic and long-range character and therefore introduce a novel type of interaction in the physics of ultracold quantum gases. In addition, it is expected that the character of the interaction present in a chromium BEC can be varied from mainly contact to purely dipolar utilizing one of the recently observed Feshbach resonances in 52Cr-collisions.
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