Instability and stratification of a two-component Bose-Einstein condensate in a trapped ultracold gas

The thermodynamic stability of a trapped Bose gas with a two-component condensate, e.g., a gas of atoms in two distinct hyperfine states or a binary gaseous mixture, is considered on the basis of modified Gross-Pitaevskii equations. Under certain conditions the system becomes unstable with respect to stratification, i.e., to spatial separation between the two condensates. One condensate assumes a donutlike spatial distribution, while the other retains a centered distribution. Possible applications of the theory to recent experimental data on two overlapping condensates in ${}^{87}$Rb are discussed.