Bao, CG (reprint author), Sun Yat Sen Univ, Sch Phys & Engn, State Key Lab Optoelect Mat & Technol, Guangzhou 510275, Guangdong, Peoples R China.
The variation of the hyperfine populations N rho(mu) of the ground state (g.s.) of spin-2 condensates against a magnetic field B is studied. A N-body theory which is rigorous in dealing with the spin degrees of freedom has been adopted. The conservation of the total magnetization M is assumed. A combined hyperfine population N rho(comb) = N(rho(0) - 3 rho(2) - 3 rho(-2)), which is an observable, is defined. The lower bound for this population has been derived analytically. It turns out that the lower bound is identical to the actual population when B = 0 and 8, and the population is monotonically uprising in between. Thus, via the analytical form of the lower bound, the variation of the population against B can be roughly known in advance. Numerical examples are given to demonstrate the applicability of the bound. When M = 0, the g.s. in the polar phase was found to be highly unstable against B because the neighboring level density is extremely high. When M = N, the g.s. in the cyclic phase was also found to be highly unstable. This is because the appearance of B, even if it is very weak, will break the high degeneracy of the g.s. and result in having numerous levels emerging in the neighborhood of the g.s.