▎ 摘　　要

The origin of the singular diamagnetic susceptibility at the Dirac point is probed through the study of effects of band-gap opening and spatially varying magnetic field. In the presence of a band gap, the susceptibility is nonzero only inside the band gap and exhibits a discrete jump at the band edges down to zero in the conduction and valence bands. The jump height is understood in terms of the pseudo-spin paramagnetism arising from valley degree of freedom. In spatially varying magnetic field with wave vector q, the susceptibility becomes nonzero only in a finite energy region containing the Dirac point, determined by q. This behavior is understood in terms of electronic states numerically calculated in periodic magnetic field. (C) 2011 Elsevier Ltd. All rights reserved.