  We study the orbital diamagnetic susceptibility in excitonic condensation
phase using the meanfield approximation for a two-band model defined on a
square lattice. We find that, in semiconductors, the excitonic condensation
acquires a finite diamagnetic susceptibility due to spontaneous hybridization
between the valence and the conduction bands, whereas in semimetals, the
diamagnetic susceptibility in the normal phase is suppressed by the excitonic
condensation. We also study the orbital diamagnetic and Pauli paramagnetic
susceptibilities of Ta2NiSe5 using a two-dimensional three-band model and find
that the calculated temperature dependence of the magnetic susceptibility is in
qualitative agreement with experiment.
