Band-filling effect on magnetic anisotropy using a Green's function method

We use an analytical model to describe the magnetocrystalline anisotropy energy (MAE) in solids as a function of band filling. The MAE is evaluated in second-order perturbation theory, which makes it possible to decompose the MAE into a sum of transitions between occupied and unoccupied pairs. The model enables us to characterize the MAE as a sum of contributions from different, often competing terms. The nitridometalates Li$_{2}$[(Li$_{1-x}$T$_{x}$)N], with $T$=Mn, Fe, Co, Ni, provide a system where the model is very effective because atomic like orbital characters are preserved and the decomposition is fairly clean. Model results are also compared against MAE evaluated directly from first-principles calculations for this system. Good qualitative agreement is found

Role of nematicity in controlling spin fluctuations and superconducting Tc in bulk FeSe

Bulk FeSe superconducts inside a nematic phase, that sets in through an orthorhombic distortion of the high temperature tetragonal phase. Bulk non-alloy tetragonal superconducting FeSe does not exist as yet. This raises the question whether nematicity is fundamental to superconductivity. We employ an advanced ab-initio ability and show that bulk tetragonal FeSe can, in principle, superconduct at almost the same Tc as the orthorhombic phase had that been the ground state. Further, we perform rigorous benchmarking of our theoretical spin susceptibilities against experimentally observed data over all energies and relevant momentum direction. We show that susceptibilities computed in both the tetragonal and orthorhombic phases already have the correct momentum structure at all energies, but not the desired intensity. The enhanced nematicity that simulates the correct spin fluctuation intensity can only lead to a maximum 10-15% increment in the superconducting Tc . Our results suggest while nematicity may be intrinsic property of the bulk FeSe, is not the primary force driving the superconducting pairing.Comment: 5 page, 4 figure

Spin excitations in K2_{2}Fe4+x_{4+x}Se5_{5}: linear response approach

Using \emph{ab initio} linear response techniques we calculate spin wave spectra in K$_{2}$Fe$_{4+x}$Se$_{5}$, and find it to be in excellent agreement with a recent experiment. The spectrum can be alternatively described rather well by localized spin Hamiltonian restricted to first and second nearest neighbor couplings. We confirm that exchange coupling between nearest neighbor Fe magnetic moments is strongly anisotropic, and show directly that in the ideal system this anisotropy has itinerant nature which can be imitated by introducing higher order terms in effective localized spin Hamiltonian (biquadratic coupling). In the real system, structural relaxation provides an additional source of the exchange anisotropy of approximately the same magnitude. The dependence of spin wave spectra on filling of Fe vacancy sites is also discussed

Interfacial contributions to spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers

The thickness dependence of spin-orbit torque and magnetoresistance in ferromagnet/heavy-metal bilayers is studied using the first-principles nonequilibrium Green’s function formalism combined with the Anderson disorder model. A systematic expansion in orthogonal vector spherical harmonics is used for the angular dependence of the torque. The dampinglike torque in Co/Pt and Co/Au bilayers can be described as a sum of the spin-Hall contribution, which increases with thickness in agreement with the spin-diffusion model, and a comparable interfacial contribution. The magnetoconductance in the plane perpendicular to the current in Co/Pt bilayers is of the order of a conductance quantum per interfacial atom, exceeding the prediction of the spin-Hall model by more than an order of magnitude. This suggests that the “spin-Hall magnetoresistance,” similarly to the dampinglike torque, has a large interfacial contribution unrelated to the spin-Hall effect

Ab-initio Prediction of Conduction Band Spin Splitting in Zincblende Semiconductors

We use a recently developed self-consistent $GW$ approximation to present systematic \emph{ab initio} calculations of the conduction band spin splitting in III-V and II-V zincblende semiconductors. The spin orbit interaction is taken into account as a perturbation to the scalar relativistic hamiltonian. These are the first calculations of conduction band spin splittings based on a quasiparticle approach; and because the self-consistent $GW$ scheme accurately reproduces the relevant band parameters, it is expected to be a reliable predictor of spin splittings. The results are compared to the few available experimental data and a previous calculation based on a model one-particle potential. We also briefly address the widely used {\bf k}$\cdot${\bf p} parameterization in the context of these results.Comment: 9 pages, 1 figur

All-electron self-consistent GW approximation: Application to Si, MnO, and NiO

We present a new kind self-consistent GW approximation (scGW) based on the all-electron, full-potential LMTO method. By iterating the eigenfunctions of the GW Hamiltonian, self-consistency in both the charge density and the quasiparticle spectrum is achieved. We explain why this form of self-consistency should be preferred to the conventional one. Then some results for Si are shown as a representative semiconductor, to establish agreement with a prior scGW calculation. Finally we consider many details in the electronic structure of the antiferromagnetic insulators MnO and NiO. Excellent agreement with experiment is shown for many properties, suggesting that a Landau quasiparticle (energy band) picture of MnO and NiO provides a reasonable description of electronic structure even in these correlated materials.Comment: 5 pages, 3 figure

GW method applied to localized 4f electron systems

We apply a recently developed quasiparticle self-consistent $GW$ method (QSGW) to Gd, Er, EuN, GdN, ErAs, YbN and GdAs. We show that QSGW combines advantages separately found in conventional $GW$ and LDA+$U$ theory, in a simple and fully \emph{ab initio} way. \qsgw reproduces the experimental occupied $4f$ levels well, though unoccupied levels are systematically overestimated. Properties of the Fermi surface responsible for electronic properties are in good agreement with available experimental data. GdN is predicted to be very near a critical point of a first-order metal-insulator transition.Comment: 5 pages,3 figures, 2 table

Quasiparticle self-consistent GWGW method; a basis for the independent-particle approximation

We have developed a new type of self-consistent scheme within the $GW$ approximation, which we call quasiparticle self-consistent $GW$ (QS$GW$). We have shown that QS$GW$ rather well describes energy bands for a wide-range of materials, including many where the local-density approximation fails. QS$GW$ contains physical effects found in other theories such as LDA$+U$, SIC and $GW$ in a satisfactory manner without many of their drawbacks (partitioning of itinerant and localized electrons, adjustable parameters, ambiguities in double-counting, etc.). We present some theoretical discussion concerning the formulation of QS$GW$, including a prescriptino for calculating the total energy. We also address several key methodological points needed for implementation. We then show convergence checks and some representative results in a variety of materials.Comment: v2:the same as previous version --but better tex file; v3:add appendix and modify introduction,mainly; v4 mainly, theoretical section (IB IC) are renewe