Implementations of DFT Used in This Work

3.2.1 The Elk FP-LAPW code

Elk is an open-source full potential linearized augmented plane waves (FP-LAPW, FLAPW) code [70]. FLAPW treats core and valence electrons simul-taneously, and is generally considered the most accurate method to solve the Kohn-Sham problem.

The FLAPW method starts from the muffin-tin partitioning. The unit cell is devided into spheres, centered on the nuclei (the muffin-tins) and a region in between (the interstitial). The basis set is built from spherical harmonics in the muffin-tin spheres and plane waves in the interstitial. This is referred to as an augmented plane waves (APW) basis, which was originally suggested by Slater. Matching conditions on the muffin-tin boundary can be imposed to arbitrary order. The basis set used by Elk is a linearized version of the APW+lo method [71]. It can be expressed as

φ^k(r) =







∑

G

cGe^i(G+k)·r r∈interstitial

∑

lm

α^k_lmu_l(r,E¹_l)Y_lm(rˆ) r∈muffin-tin (3.19) wherer = |r|and ˆr = r/r. The plane wave coefficientscGare variational quantities, and the α^k_lm are determined by the matching conditions at the muffin-tin boundary. Matching to zeroth order (i.e., only the value of the wave function) and obtaining the solutionsul(r,E¹_l)(one per angular momentuml) of radial Schr¨odinger equations at fixed energyE¹_l is sufficient, if local orbitals are added to the APW set. The local orbitals (lo/LO) are represented by radial functions and spherical harmonics in the muffin-tin spheres and are forced to zero on the muffin-tin boundary. They do not depend onk. Two types of local orbitals are added to the basis set:

φ^lo_lm(r) =β_lmul(r,E¹_l) +γ_lmu⁰_l(r,E_l¹)Ylm(rˆ) (3.20) φ^LO_lm(r) =δ_lmul(r,E¹_l) +e_lmu⁰_l(r,E¹_l) +ζ_lmul(r,E²_l)Ylm(rˆ) (3.21)

3.2 Implementations of DFT Used in This Work

The local orbital coefficientsβ_lmandγ_lmare determined by the condition to have the local orbital wave function zero at the muffin-tin boundary and its normalization. Similarly,δ_lm,e_lm, andζ_lmare determined by the wave function and its derivative being zero at the muffin-tin boundary, and its normalization.

The second type of local orbitals have atomic-like wave functions and are used to describe semi-core states. The local orbitals greatly improve the flexibility of the basis set at very low computational cost.

The radial functions and derivativesu_l(r,E¹_l),u⁰_l(r,E¹_l),u_l(r,E²_l) are solu-tions of the radial Schr¨odinger equation at fixed energiesEⁱ_l, which results in a standard linear eigenvalue problem. The linearization energiesE¹_l have to be chosen approximately in the center of the valence bands. The lineariza-tion energiesE_l²are at the approximate energy of the semi-core state, and are searched automatically. The variational coefficientscGare obtained from the Rayleigh-Ritz variational principle.

Core level electrons are treated separately in a fully relativistic way with the radial Dirac equation. Spin-orbit coupling can be included for the valence states in a second-variational step by adding aσ·Lterm to the Hamiltonian.

The crystal potentialV(r)is expanded similar to the wave functions,

V(r) =







∑G

V_Ge^i(G+k)·r r∈interstitial

∑

lm

Vlm(r)Ylm(rˆ) r∈muffin-tin. (3.22) This constitutes the full potential treatment, which is to be contrasted with a spherical approximation (usually called atomic spheres approximation). It corresponds to a truncation of the potential expansion atl = 0 andG = 0.

Thus, the potential in the muffin-tins would be spherically averaged, and the potential in the interstitial would be constant. The potential expansion of (3.22) allows to treat the full potential without shape approximations.

3.2.2 The Munich SPRKKR package

The Munich SPRKKR package is a spin polarized relativistic implementation of the Korringa-Kohn-Rostoker Green’s function method. It determines the eletronic structure of a periodic solid by means of multiple scattering theory.

The method is described in detail in a review article by the authors of the code [72]. Another very instructive introduction is given by Mavropoulos and Papanikolaou [73]. Here, the main ideas are summarized in short.

One starts from a formal introduction of the Green function G(r,r⁰,E) through the Schr¨odinger equation:

(E−H)G(r,r⁰,E) =δ(r−r⁰). (3.23) G(r,r⁰,E)has the following spectral representation:

G(r,r⁰,E) = lim

η→+0

∑

ν

ψ_ν(r)ψ_ν^∗(r⁰)

E−E_ν+iη, (3.24) whereE_νare the eigenvalues of the HamiltonianH, andηis a small positive real number. From the Green function, the density of statesρ(E)and the charge densityn(r)are obtained as

ρ(E) =−¹

πIm^Z d³r G(r,r,E), (3.25) n(r) =−¹

πIm^{Z EF}dE G(r,r,E). (3.26) The Green function contains all information which is given by the eigenfunc-tions, both are equivalent. All physical properties of the system can be found, if the Green function is known.

There are several ways of calculating the Green function, the most important and flexible of which is multiple scattering theory (MST). The solution of the electronic structure problem is broken up in two parts, a potential related one and a geometry related one.

In the full-potential formulation, the unit cell is divided into Wigner-Seitz polyhedra, centered on the nuclei. The potential of site n is expanded in spherical harmonics,Vⁿ(r) =_∑LV_Lⁿ(r)YL(rˆ), withL:= (l,m). The potential of site nis zero outside its polyhedron. In contrast to FLAPW there is no interstitial region.

In a first step, the single-site scattering problem, i.e. the scattering of a plane wave on the potential of siten, is solved individually for all sites. The scattering solutionsψⁿ(r,E)for the isolated potential wellsVⁿ(r)are obtained from the Lippmann-Schwinger equation, an integral form of the Schr¨odinger equation:

ψⁿ(r,E) =ψ₀(r,E) +

Z d³r⁰ G₀(r,r⁰,E)Vⁿ(r)ψⁿ(r,E), (3.27) with the free-electron wave functionψ₀(r,E) =e^ik·rand the corresponding Green function

G0(r,r⁰,E) =−^e

−i√ E|r−r⁰|

4π|r−r⁰|^{. (3.28)}

3.2 Implementations of DFT Used in This Work

The scattering behaviour of the potentialVⁿ(r)can be expressed in terms of a tⁿ-operator,

tⁿ =Vⁿ+VⁿG0tⁿ (3.29)

=Vⁿ(1−G0Vⁿ)⁻¹, (3.30) where the arguments have been dropped for clarity. It is related to the radial part of the scattering solution outside the polyhedron of siten.

Instead of working with the Lippmann-Schwinger equation, one can write a Dyson equation (in operator form) for the single-site scattering problem:

Gˆⁿ(E) =G^ˆ0(E) +G^ˆ0(E)V^ˆnGˆⁿ(E) (3.31)

=G^ˆ₀(E) +G^ˆ₀(E)t^ˆn(E)G^ˆ₀(E). (3.32) Analogous equations are found in the multiple-scattering case:

Gˆ(E) =G^ˆ0(E) +G^ˆ0(E)V^ˆGˆ(E) (3.33)

=G^ˆ₀(E) +G^ˆ₀(E)T^ˆ(E)G^ˆ₀(E), (3.34) where the multiple-scatteringT-matrix operator has been introduced. It can be expanded as

Tˆ(E) =

∑

nn⁰

ˆ τⁿⁿ

0(E). (3.35)

The scattering path operator ˆτⁿⁿ⁰(E)is defined to transfer an electron wave incoming at siten⁰into a wave outgoing from sitenwith all possible scattering events in between incorporated. In an angular momentum basis (denoted by underlines), ˆτⁿⁿ⁰(E)has the following equation of motion:

τⁿⁿ

0(E) =tⁿ(E)δ_nn⁰+tⁿ(E)

∑

m6=n

G^nm₀ τ^mn

0(E). (3.36) For a finite system, this equation is solved by matrix inversion,

τ(E) =^ht(E)⁻¹−G₀(E)ⁱ⁻¹. (3.37) The double underlines denote matrices with respect to angular momentum and sites. The matrix in square brackets is known as the real-space KKR matrix. For a periodic solid with sitesnat positionsR_n, one finds by Fourier transformation

τⁿⁿ

0(E) = ¹

Ω_BZ Z

ΩBZ

d³kh

t(E)⁻¹−G₀(k,E)ⁱ⁻¹e^{ik·(Rn−Rn0)}, (3.38)

with the (reciprocal space) structure constants matrixG₀(k,E)being the Fourier transformed of the real-space structure constants matrixG₀(E).

The formalism outlined above is very general with respect to the Hamilto-nianH. In practice, the Kohn-Sham equations are solved in the usual iterative way, to self-consistency.

A major advantage of the Green’s function formalism is the connection of a perturbed system and a reference system through the Dyson equation:

Gˆ =G^ˆref+G^ˆrefHˆ^pertG.ˆ (3.39) This equation gives also the formal background for the scheme described above, in which the free-electron system is the reference system, and the pertur-bation Hamiltonian is given by the potential of the system under investigation.

Because MST seperates the electronic structure problem into a geometric and a potential part, it is easy to treat impurities in a perfect host material without using supercells or large clusters, as in other methods:

τ^imp =^h(τ^host)⁻¹−(t^host)⁻¹+ (t^imp)⁻¹ⁱ⁻¹. (3.40) Similarly, disordered systems are treated within the so-called coherent potential approximation (CPA). An auxiliary CPA medium is introduced, in which the concentration average of the constituents causes no additional scattering. For a binary alloy with concentrations x_A, x_B, this can be expressed with the scattering path operator matrices:

x_Aτ^Ann+x_Bτ^Bnn=τ^CPAnn. (3.41) In analogy to the impurity problem, the component projected scattering path operator matrices are given as

τ^αnn =^h(τ^CPA)⁻¹−(t^CPA)⁻¹+ (t^α)⁻¹ⁱ⁻¹, α=A, B. (3.42) The matrix dimension of the multiple-scattering problem isN_scatterers·(l_max+ 1)². Therefore, one tries to keep the angular momentum cutoff as small as possible, typicallylmax = 3 ford-electron systems. In principle, one would have to take the angular momentum expansion to infinity to obtain the charge density correctly. Due to the truncation, the charge density is somewhat incomplete, leading to a slight miscalculation of the Fermi energy. This problem can be resolved by an analytically exact expression to obtain a correct charge normalization, the Lloyd formula [74, 75].

Im Dokument Ferrimagnetic Heusler compounds : from first principles to thin films (Seite 30-35)