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Spin waves in a Bose Ferromagnet

Qiang Gu·Kai Bongs·Klaus Sengstock
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Abstract

It is shown that the ferromagnetic transition takes place always above Bose-Einstein condensation in ferromagnetically coupled spinor Bose gases. We describe the Bose ferromagnet within Ginzburg-Landau theory by a "two-fluid" model below Bose-Einstein condensation. Both the Bose condensate and the normal phase are spontaneously magnetized. As a main result we show that spin waves in the two fluids are coupled together so as to produce only one mixed spin-wave mode in the coexisting state. The long wavelength spectrum is quadratic in the wave vector ${\bf k}$, consistent with usual ferromagnetism theory, and the spin-wave stiffness coefficient $c_s$ includes contributions from both the two phases, implying the "two-fluid" feature of the system. $c_s$ can show a sharp bend at the Bose-Einstein condensation temperature.

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Qiang Gu, Kai Bongs, Klaus Sengstock. 2004-05-05. Spin waves in a Bose Ferromagnet. https://doi.org/10.1103/physreva.70.063609

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