The 237Np nucleus possesses a non-vanishing electrical quadrupole moment and thus provides useful information about the electronic distribution around the nucleus through the interaction with the electric-field gradient (EFG) at the nucleus position. In NpN, which is known to be a ferromagnet, a positive EFG has been observed experimentally in contrast to the negative EFG observed in the isolated Np3+ ion. The first-principles calculations using the FFLCAO method explains the fact successfully.
The orbital magnetic moments in UX are estimated experimentally using the XMCD sum rules. It is then important to check the accuracy of the sum rules from a theoretical point of view using the first-principles calculations with the FFLCAO method. The results of calculations show that the agreement between the orbital magnetic moments calculated using the sum rules and those calculated using the conventional formula is fairly good.
There has been an unresolved problem that a large discrepancy exists between the experimentally measured magnetic moments and the theoretically calculated ones. One possible reason is that the conventional method of calculating the orbital magnetic moments has a large error in the accuracy of calculations. We thus carried out the first-principles calculations with the FFLCAO method based on the original definition of the magnetic moment, i.e, the integral of the vector product between the position vector and the Dirac current over a well-defined spatial region, As a result, the calculated magnetic moments were found to be in a better agreement with the experimentally observed values.
Although the agreement between the experimentally observed and theoretically calculated magnetic moments is considerably improved by the method with the Dirac current, there still remains a discrepancy that cannot be ignored. We study a possibility that this discrepancy is originated in the exchange-correlation energy functionals employed in the first-principles calculations with the FFLCAO method. As a result, it is found that, although the lattice constants, the bulk moduli, and the cohesive energies depend on the energy functionals, the magnetic moments show almost no dependence.