4 research outputs found
Relativistic corrections in magnetic systems
We present a weak-relativistic limit comparison between the Kohn-Sham-Dirac
equation and its approximate form containing the exchange coupling, which is
used in almost all relativistic codes of density-functional theory. For these
two descriptions, an exact expression of the Dirac Green's function in terms of
the non-relativistic Green's function is first derived and then used to
calculate the effective Hamiltonian, i.e., Pauli Hamiltonian, and effective
velocity operator in the weak-relativistic limit. We point out that, besides
neglecting orbital magnetism effects, the approximate Kohn-Sham-Dirac equation
also gives relativistic corrections which differ from those of the exact
Kohn-Sham-Dirac equation. These differences have quite serious consequences: in
particular, the magnetocrystalline anisotropy of an uniaxial ferromagnet and
the anisotropic magnetoresistance of a cubic ferromagnet are found from the
approximate Kohn-Sham-Dirac equation to be of order , whereas the
correct results obtained from the exact Kohn-Sham-Dirac equation are of order
. We give a qualitative estimate of the order of magnitude of these
spurious terms
Local Charge Excesses in Metallic Alloys: a Local Field Coherent Potential Approximation Theory
Electronic structure calculations performed on very large supercells have
shown that the local charge excesses in metallic alloys are related through
simple linear relations to the local electrostatic field resulting from
distribution of charges in the whole crystal.
By including local external fields in the single site Coherent Potential
Approximation theory, we develop a novel theoretical scheme in which the local
charge excesses for random alloys can be obtained as the responses to local
external fields. Our model maintains all the computational advantages of a
single site theory but allows for full charge relaxation at the impurity sites.
Through applications to CuPd and CuZn alloys, we find that, as a general rule,
non linear charge rearrangements occur at the impurity site as a consequence of
the complex phenomena related with the electronic screening of the external
potential. This nothwithstanding, we observe that linear relations hold between
charge excesses and external potentials, in quantitative agreement with the
mentioned supercell calculations, and well beyond the limits of linearity for
any other site property.Comment: 11 pages, 1 table, 7 figure