12 CHAPTER 2. THE SPRKKR INPUT FILE

2.2.5 CPA-calculations for disordered alloy systems

For a system with substitutional disorder, the CPA is used. The listed variables control the CPA cycle specified by Eqs. (1.9) and (1.10).

2.2.6 Calculation mode

If not specified otherwise the programs of the SPRKKR-package assume that a magnetic system should be treated in a fully relativistic way. By setting the parameter SP-SREL in the section MODE a scalar relativistic calculation can be done instead for a magnetic system. This is useful when starting the SCF-cycle (see below), because it is somewhat faster than the fully relativistic mode. Because in the SP-SREL-mode the same representation is used as for the fully relativistic one, all types of calculations described in the next chapters can be done in this mode.

If it is known that the system considered is non-magnetic, one can make use of this by setting the switch PARA.

This leads to a higher symmetry for the system and accordingly in general to shorter runtime for the SCF-cycle because a smaller part of the Brillouin-zone has to be sampled.

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2.2.7 Orientation of the magnetisation

If magnetic systems are considered by the SPRKKR programs it is in general assumed that the magnetic moments of all atoms point parallel or antiparallel to the crystallographic z- axis. SPRKKR allows to consider different configurations as well. Using the parameter MDIR in section MODE the direction of the magnetisation can be oriented in an arbitrary direction. This implies that for all lattice sites the same orientation is assumed.

Internally, MDIR is converted to corresponding Euler angles ( ; ; ) (with redundant at the moment), that specify a local frame of reference. The Dirac equation for the single site problem is solved for this, because it reduces the spin dependent potential term in the Dirac equation to BBeff (r) z. Instead of using a common magnetisation direction for all lattice sites the direction may also be set individually for each lattice site (non-collinear spin structures). This is done using the parameter MDIR* (with * = 1; :::; NQ) in the section MODE.

MDIR* = fx; y; zg f0; 0; 1g orientation vector for the spin magnetisation on the lattice site *, with * standing for the site number IQ = 1; :::;NQ

2.2.8 Manipulating the spin-orbit coupling

When dealing with spin-orbit induced properties it is often interesting to demonstrate the connection of the investigated effect and the spin-orbit coupling. This can be done by manipulating the spin-orbit coupling. The SPRKKR package allows this in several ways [9, 10]. The most simple way to manipulate the spin-orbit coupling is to change the speed of light c. Because the most prominent relativistic corrections are proportional to 1=c2, one approaches the non-relativistic limit for c going to infinity. Accordingly, the SPRKKR input uses a scaling parameter C* = (c0=c)2 (with * = 1; :::; NT) that has to be put to small values for the nonrelativistic limit (for numerical reasons one should have C* 10 4), to 1 for a relativistic and > 1 for a ultra-relativistic calculation.

section MODE

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The scheme described above obviously manipulates all relativistic effects simultaneously. To scale exclusively the strength of the spin-orbit coupling, one may use the parameter SOC*

Setting SOC* = f0:0g suppresses the spin-orbit coupling completely and the calculation corresponds to a so-called scalar relativistic one. The parameter SOC* in addition allows to use only parts of the spin-orbit coupling. Setting SOC*= 1 only the spin-diagonal partzlz is used, while for SOC*= 2 only the spin-off-diagonal or spin-mixing part ( xlx +yly) is used in the calculations.

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