Kohn-Sham potential with discontinuity for band gap materials

We model a Kohn-Sham potential with a discontinuity at integer particle numbers derived from the GLLB approximation of Gritsenko et al. We evaluate the Kohn-Sham gap and the discontinuity to obtain the quasiparticle gap. This allows us to compare the Kohn-Sham gaps to those obtained by accurate many-body perturbation theory based optimized potential methods. In addition, the resulting quasiparticle band gap is compared to experimental gaps. In the GLLB model potential, the exchange-correlation hole is modeled using a GGA energy density and the response of the hole to density variations is evaluated by using the common-denominator approximation and homogeneous electron gas based assumptions. In our modification, we have chosen the PBEsol potential as the GGA to model the exchange hole, and add a consistent correlation potential. The method is implemented in the GPAW code, which allows efficient parallelization to study large systems. A fair agreement for Kohn-Sham and the quasiparticle band gaps with semiconductors and other band gap materials is obtained with a potential which is as fast as GGA to calculate.Comment: submitted to Physical Review

Thermal collapse of spin-polarization in half-metallic ferromagnets

The temperature dependence of the magnetization and spin-polarization at the Fermi level is investigated for half-metallic ferromagnets. We reveal a new mechanism, where the hybridization of states forming the half-metallic gap depends on thermal spin fluctuations and the polarization can drop abruptly at temperatures much lower than the Curie point. We verify this for NiMnSb by ab-initio calculations. The thermal properties are studied by mapping ab-initio results to an extended Heisenberg model which includes longitudinal fluctuations and is solved by a Monte Carlo method

Spin-polarized tunneling between an antiferromagnet and a ferromagnet: First-principles calculations and transport theory

By combining first-principles calculations with transport theory we investigate the origin of the magnetoresistance of a magnetic tunnel junction consisting of a ferromagnetic and an antiferromagnetic lead. The (001) oriented Fe/vacuum/Cr planar junction serves as model junction. Even though the conduction electrons of antiferromagnetic Cr are spin-degenerate, it is possible to observe magnetoresistance due to two mechanisms: Firstly, the surface magnetism of Cr creates a spin-dependent potential barrier, and secondly, exchange-split surface states and resonances result in a tunneling conductance which depends on the relative orientation of the Fe and Cr magnetizations. Spin-dependent tunneling between a ferromagnet and an antiferromagnet happens frequently in tunneling setups such as in spin-polarized scanning tunneling microscopy or magnetic tunnel junctions for magnetic random access memory

Coexistence of ferro- and antiferromagnetic order in Mn-doped Ni2_2MnGa

Ni-Mn-Ga is interesting as a prototype of a magnetic shape-memory alloy showing large magnetic field induced strains. We present here results for the magnetic ordering of Mn-rich Ni-Mn-Ga alloys based on both experiments and theory. Experimental trends for the composition dependence of the magnetization are measured by a vibrating sample magnetometer (VSM) in magnetic fields of up to several tesla and at low temperatures. The saturation magnetization has a maximum near the stoichiometric composition and it decreases with increasing Mn content. This unexpected behaviour is interpreted via first-principles calculations within the density-functional theory. We show that extra Mn atoms are antiferromagnetically aligned to the other moments, which explains the dependence of the magnetization on composition. In addition, the effect of Mn doping on the stabilization of the structural phases and on the magnetic anisotropy energy is demonstrated.Comment: 4 pages, 3 figure

Magnetic anisotropy in Ni2MnGa

We study here, within the density-functional theory, the magnetic anisotropy energy (MAE) in Ni2MnGa which is a prototype of a magnetic shape-memory alloy. We calculate the MAE, which is a key property for the magnetic shape-memory effect, for tetragonal structure with different ratios of the c and a lattice constants, reproducing the experimental easy axes both in compression and elongation of the structure. Good agreement between the theory and the experiments in the actual values of the MAE is also found when the nonstoichiometry of the experimental samples is modeled with a simple rigid band approximation. In addition, we estimate the magnetostriction coefficient, confirming the difference between the ordinary magnetostriction and the magnetic shape-memory effect. Equally important, we study the microscopic origin of the MAE in Ni2MnGa with the spin density and the orbital moment anisotropy and extend the analysis of the orbital moment anisotropy to the ternary compounds. These results show that the largest contribution to the MAE comes from Ni, in spite of the larger magnetic moment in the Mn sites.Peer reviewe

A Self-consistent DFT+DMFT scheme in the Projector Augmented Wave : Applications to Cerium, Ce2O3 and Pu2O3 with the Hubbard I solver and comparison to DFT+U

An implementation of full self-consistency over the electronic density in the DFT+DMFT framework on the basis of a plane wave-projector augmented wave (PAW) DFT code is presented. It allows for an accurate calculation of the total energy in DFT+DMFT within a plane wave approach. In contrast to frameworks based on the maximally localized Wannier function, the method is easily applied to f electron systems, such as cerium, cerium oxide (Ce2O3) and plutonium oxide (Pu2O3). In order to have a correct and physical calculation of the energy terms, we find that the calculation of the self-consistent density is mandatory. The formalism is general and does not depend on the method used to solve the impurity model. Calculations are carried out within the Hubbard I approximation, which is fast to solve, and gives a good description of strongly correlated insulators. We compare the DFT+DMFT and DFT+U solutions, and underline the qualitative differences of their converged densities. We emphasize that in contrast to DFT+U, DFT+DMFT does not break the spin and orbital symmetry. As a consequence, DFT+DMFT implies, on top of a better physical description of correlated metals and insulators, a reduced occurrence of unphysical metastable solutions in correlated insulators in comparison to DFT+U.Comment: 19 pages, 9 figures. This is an author-created, un-copyedited version of an article accepted for publication in Journal of Physics: Condensed Matter. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at doi: 10.1088/0953-8984/24/7/07560

First-principles investigation of phonon softenings and lattice instabilities in the shape-memory system Ni2MnGa

Ferromagnetic Ni2MnGa has unique magnetoelastic properties. These are investigated by detailed computational studies of the phonon-dispersion curves for the non-modulated cubic L21 and tetragonal structures. For the L21 structure, a complete softening of the transverse-acoustic mode has been found around the wave vector q=[1/3,1/3,0](2π/a). The softening of this TA2 phonon mode leads to the premartensitic modulated superstructure observed experimentally. Further phonon anomalies, related to other structural transformations in Ni2MnGa, have also been found and examined. These anomalies appear to be due to the coupling of particular acoustic-phonon modes and optical modes derived from Ni.Peer reviewe

Analysis of energy gap opening in graphene oxide

The utilisation of graphene structures as photonics materials mandates that an optically active electronic energy gap be formed. Opening of a gap in graphene has been demonstrated by functionalisation with H, F, or O atoms, while experimental observations of graphene oxide have hinted at interesting optical properties, with the potential for absorption of visible light. As such, our analysis is focused on O functionalisation of graphene. We present results from extensive ab initio and hybrid DFT calculations, demonstrating the creation of an optically active gap

Structural, thermal, and magnetic properties of Ni[sub 2]MnGa

The two main effects underlying the magnetic shape memory effect in Ni2MnGa are martensitic transformations and magnetic anisotropy energies. Both issues are addressed here with first-principles calculations. First, we examine how the tetragonality in the martensitic phase varies with the composition. Then, the actual transformation is investigated by comparing the free energies of different phases. The transition from the cubic structure to the tetragonal structure with c/a=1.27 is driven by the vibrational free energy and occurs at a temperature of 200 K which is in the experimental range. Finally, we focus on the magnetic anisotropy energy for the tetragonal structure with c/a=0.94. It is shown to be a magnetically nearly ideal uniaxial system determined by the first-order anisotropy constant. However, it is estimated that the twinned microstructure can cause higher-order anisotropies to show up in the measured anisotropy.Peer reviewe