(Redirected from XNCD)



Use either of these cards to calculate x-ray circular dichroism (the output will contain both magnetic and natural). The difference between the XMCD and XNCD cards is purely aesthetic. I.e., the code treats both cards exactly the same. This card takes no arguments. This card is not compatable with the POLARIZATION card or the ELNES card.

For nonmagnetic systems only XNCD will be present, while for magnetic materials with high symmetry only XMCD is present. Both will be present for magnetic materials with low symmetry, and x-ray direction (ELLIPTICITY card) must be used to disentangle the two contributions. The EXAFS region can also be used to determine the position of spins relative to magnetic field. The XMCD card has to be present in ‘feff.inp’ for these calculations with FEFF8.40.

The XNCD originates from cross dipole-quadrupole contributions for certain nonmagnetic materials, such as special types of crystals. It will change sign for opposite direction of propagation (use the ELLIPTICITY card to do this). We performed calculations for LiIO3 and found results very similar to previous multiple scattering XNCD calculations. XNCD requires that the XANES card also be used.

The XMCD (dipolar and quadrupolar) does not change sign under the change of direction of x-ray propagation, and is zero for nonmagnetic systems. The origin of the effect is that due to spin-orbit coupling, the right circular polarized light will produce more electrons with spin along or opposite to the direction of x-ray propagation. Thus it is important to use spin-dependent calculations for XMCD calculations. See the SPIN card and the section on Spin dependent calculations for more details and an example.

Note that the XMCD signal will only be contained in the output if the FEFF code has been compiled with nspx=2. For the (default) value nspx=1, you have to combine data from two ‘xmu.dat’ files. A simple program to do this, ‘spin.f’ is available on the feff web site, and printed in Section 4, where you can also find more details on the signal extraction process.

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