Observation of a strongly ferromagnetic spinor Bose-Einstein condensate

We report the observation of strongly ferromagnetic $F=1$ spinor Bose-Einstein condensates of ${}^7$Li atoms. The condensates are generated in an optical dipole trap without using magnetic Feshbach resonances, so that the condensates have internal spin degrees of freedom. Studying the non-equilibrium spin dynamics, we have measured the ferromagnetic spin interaction energy and determined the $s$-wave scattering length difference among total spin $f$ channels to be $a_{f=2}-a_{f=0}=-18(3)$ Bohr radius. This strong collision-channel dependence leads to a large variation in the condensate size with different spin composition. We were able to excite a radial monopole mode after a spin-flip transition between the $\mathrm{\mid }{m}_F=0⟩$ and $\mathrm{\mid }{m}_F=1⟩$ spin states. From the experiments, we estimated the scattering length ratio $a_{f=2}\mathrm{/}{a}_{f=0}=0.27(6)$, and determined $a_{f=2}$ = 7(2) and $a_{f=0}$ = 25(5) Bohr radii, respectively. The results indicate the spin-dependent interaction energy of our system is as large as 46$\mathrm{\%}$ of the condensate chemical potential.