Increase of the chemical potential and phase transitions in four-component exciton condensates subject to magnetic fields

We analyze the spin structure of the ground state of four-component exciton condensates in coupled quantum wells as a function of spin-dependent interactions and applied magnetic field. The four components correspond to the degenerate exciton states characterized by $\ifmmode\pm\else\textpm\fi{}2$ and $\ifmmode\pm\else\textpm\fi{}1$ spin projections to the axis of the structure. We show that in a wide range of parameters, the chemical potential of the system increases as a function of magnetic field, which manifests a pseudo-diamagnetism of the system. The transitions to polarized two- and one-component condensates can be of the first-order in this case. The predicted effects are caused by energy-conserving mixing of $\ifmmode\pm\else\textpm\fi{}2$ and $\ifmmode\pm\else\textpm\fi{}1$ excitons.