Paul Larson

Electronic structure and magnetism of europium chalcogenides in comparison with gadolinium nitride

Electronic structure calculations were carried out for the europium chalcogenides (EuO, EuS, EuSe, EuTe) using the LSDA+U approach, in which orbital-dependent Coulomb and exchange effects are added to the local spin density approximation (LSDA) for the f electrons. The usual LSDA gap underestimate is also corrected by adding Ud terms, which shift up the empty d states. While both GdN and EuO are found to be magnetic semiconductors, there are significant differences in the electronic structure, in particular in the location of the filled f bands. The origin of these differences and their effect on various other aspects of the band structure are discussed. The exchange coupling between magnetic moments is studied by mapping the energy differences of different magnetic configurations to a Heisenberg Hamiltonian with first-xa0and second-nearest-neighbour interactions. The exchange interactions are in fair agreement with experimental values and explain the trends in magnetic properties.

Ab Initio Construction of Magnetic Phase Diagrams in Alloys: The Case of Fe(1-x)Mn(x)Pt.

A first-principles approach to the construction of concentration-temperature magnetic phase diagrams of metallic alloys is presented. The method employs self-consistent total energy calculations based on the coherent potential approximation for partially ordered and noncollinear magnetic states and is able to account for competing interactions and multiple magnetic phases. Application to the Fe(1-x)Mn(x)Pt magnetic chameleon system yields the sequence of magnetic phases at T=0 and the c-T magnetic phase diagram in good agreement with experiment, and a new low-temperature phase is predicted at the Mn-rich end. The importance of non-Heisenberg interactions for the description of the magnetic phase diagram is demonstrated.

Gadolinium and Oxygen co-doping of Gallium Nitride: an LSDA + U study

Results of first-principles supercell calculations for Gd impurities, with and without O-impurities in the same cell are presented. The possibility of colossal magnetic moments, as reported by Dhar et al., [Phys. Rev. Lett. 94 , 037205 (2005)] is discussed in view of the results. Particular attention is paid to the size of the conduction band spin splitting, induced by Gd. It is argued that O plays a more active role than merely providing the electrons leading to the magnetic moment. Estimates are made of the splitting of the conduction band required to explain the occurrence of colossal magnetic moments.

Effect of K/Bi ordering on the electronic structure of K2Bi8Se13

K 2 Bi 8 Se 13 belongs to a class of complex chalcogenides which show potential for superior thermoelectric performance. This compound forms in two distinct phases, α and β. The β-phase, which has several sites with mixed K/Bi occupancy is a better thermoelectric. To understand the origin of this difference we have carried out electronic structure calculations within ab initio density functional theory using full potential linearized augmented plane wave (FLAPW) method. The generalized gradient approximation was used to treat the exchange and correlation potential. Spin-orbit interaction (SOI) was incorporated using a second variational procedure. The α-phase is found to be a semiconductor with an indirect band gap of 0.47eV compared to 0.76eV for the observed direct optical gap. For the β-phase we have chosen two different ordered structures with full occupancies of K and Bi atoms at the “mixed sites”. The system is a semi-metal for both the ordered structures. To incorporate the effect of mixed occupancy we have chosen a 1x1x2 supercell with an alternative K/Bi occupancy at the “mixed sites”. The superlattice ordering gives a semiconductor with an indirect gap of 0.38eV. Mixed occupancy is crucial for the system to be a semiconductor because the Bi atoms at the “mixed sites” stabilize the p orbitals of the neighboring Se atoms by lowering their energy, and opening up a gap at the chemical potential.