A magneto-optical study of the 1.4 eV Ni color center in boron-free synthetic diamond, grown at high pressure and high temperature, has been performed in magnetic fields up to 56 T. The data is interpreted using the effective spin Hamiltonian of Naza
re, Nevers and Davies [Phys. Rev. B 43, 14196 (1991)] for interstitial Ni$^{+}$ with the electronic configuration $3d^{9}$ and effective spin $S=1/2$. Our results unequivocally demonstrate the trigonal symmetry of the defect which preferentially aligns along the [111] growth direction on the (111) face, but reveal the shortcomings of the crystal field model for this particular defect.
The electron hole asymmetry has been measured in natural graphite using magneto-optical absorption measurements. A splitting is observed for the transitions at both the $K$-point and the $H$-point of the Brillouin zone of graphite where the effect of
trigonal warping vanishes. This result is fully consistent with the SWM Hamiltonian providing the free electron kinetic energy terms are taken into account. An identical electron-hole asymmetry should be present in graphene.
