On the Interplay Between Spin Polarization, Orbital Polarization and Spin-Orbit Coupling in Actinides from Pa to Cm

We present in this work a systematic investigation of the effects of spin polarization, orbital polarization and spin-orbit coupling in six actinide elements, namely protactinium (Pa), uranium (U), neptunium (Np), plutonium (both alpha-Pu and delta-Pu), americium (Am) and curium (Cm) using a relativistic full-potential augmented plane wave with local orbital basis method. For each element, these effects have been investigated for non-magnetic (NM), ferromagnetic (FM) and anti-ferromagnetic (AFM) arrangements. Also, each of these three magnetic orders has been studied at three levels of approximations, specifically without spin-orbit coupling (NSO), with spin-orbit coupling (SO) and with orbital polarization (SO+OP). Our calculation shows that the 5f electrons in lighter actinides (Pa, U, Np) are strongly itinerant with no magnetic effects. In heavier actinides (Am, Cm), the 5f electrons are strongly correlated and quite sensitive to spin polarization, although the difference between FM and AFM energies tends to be very small. The orbital polarization has strongest effect in NM order and nearly vanishes for spin polarized calculations in almost all cases. The 5f electrons in alpha-Pu are close to being delocalized and their behavior is similar to that of lighter actinides. Among all the elements studied in this work, delta-Pu exhibits the most complex behavior. All forms of correlation effects are comparable in delta-Pu and significantly affect volume, bulk modulus and ground state energy.