Wannier-function approach to spin excitations in solids

Abstract

We present a computational scheme to study spin excitations in magnetic materials from first principles. The central quantity is the transverse spin susceptibility, from which the complete excitation spectrum, including single-particle spin-flip Stoner excitations and collective spin-wave modes, can be obtained. The susceptibility is derived from many-body perturbation theory and includes dynamic correlation through a summation over ladder diagrams that describe the coupling of electrons and holes with opposite spins. In contrast to earlier studies, we do not use a model potential with adjustable parameters for the electron-hole interaction but employ the random-phase approximation. To reduce the numerical cost for the calculation of the four-point scattering matrix we perform a projection onto maximally localized Wannier functions, which allows us to truncate the matrix efficiently by exploiting the short spatial range of electronic correlation in the partially filled d or f orbitals. Our implementation is based on the full-potential linearized augmented-plane-wave method. Starting from a ground-state calculation within the local-spin-density approximation ͑LSDA͒, we first analyze the matrix elements of the screened Coulomb potential in the Wannier basis for the 3d transition-metal series. In particular, we discuss the differences between a constrained nonmagnetic and a proper spin-polarized treatment for the ferromagnets Fe, Co, and Ni. The spectrum of single-particle and collective spin excitations in fcc Ni is then studied in detail. The calculated spin-wave dispersion is in good overall agreement with experimental data and contains both an acoustic and an optical branch for intermediate wave vectors along the ͓1 0 0͔ direction. In addition, we find evidence for a similar double-peak structure in the spectral function along the ͓1 1 1͔ direction. To investigate the influence of static correlation we finally consider LSDA+ U as an alternative starting point and show that, together with an improved description of the Fermi surface, it yields a more accurate quantitative value for the spin-wave stiffness constant, which is overestimated in the LSDA.

Excitation of spin waves by tunneling electrons in ferromagnetic and antiferromagnetic spin-12Heisenberg chains

“…The use of the Heisenberg Hamiltonian to describe a magnetic structure can be questioned in particular for itinerant magnetism but should be appropriate for the case of magnetic moments well localized on atomic sites (see, e.g., the recent discussion and spin-wave treatment in Ref. 38 and references therein).…”

Efficient computation of magnon dispersions within time-dependent density functional theory using maximally localized Wannier functions

“…Corrections schemes presented earlier involve modifying the exchange-correlation kernel itself in order to satisfy the acoustic condition. 4,25,29 However, in TDDFT, the ground-state density is computed first and the exchange-correlation kernel is a given functional of this density; modifying the kernel a posteriori breaks the consistency between these two quantities. Here, a correction scheme that does not break this consistency is developed.…”

“…Although this is a problem of current interest, there are relatively few fully ab initio calculations available in the literature given the large computational cost. [2][3][4][5][6][7] The paper is organized as follows. In Sec.…”

Competition of lattice and spin excitations in the temperature dependence of spin-wave properties

“…Finally, other corrections to the DFT/KKR+ASD framework are needed in general for a quantitative prediction of T c , e.g., many-body effects in the J ρη ij [46].…”