Using Single Ion Module phonon

The module phonon allows to consider the phononic degrees of freedom in McPhas. The single ion input file for an oscillating atom has to have the following format:

# phonon
# MODPAR1=mass of atom in units of m0 (atomic mass unit=1.660539e-27 kg)
MODPAR1=57  # mass in(m0)
MODPAR2=7   # Kxx
MODPAR3=7   # Kyy
MODPAR4=8   # Kzz
MODPAR5=0   # Kxy  in (meV)
MODPAR6=0   # Kxz
MODPAR7=0   # Kyz

# Neutron Scattering Length (10^-12 cm) (can be complex)
#  ... note: - if an occupancy other than 1.0 is needed, just reduce 
#              the scattering length linear accordingly


Thus the single ion property file contains the matrix $\overline{K}(nn)$, the matrix $\overline{K}(nn')$ decribing the forces between different ions $n$ and $n'$ have to be given in the file mcphas.j, which could look like:

# Lattice Constants (A)
#! a=4.047 b=4.047 c=9.612 alpha=  90 beta=  90 gamma=  90
#! r1a=   1 r2a=   0 r3a= 0.5
#! r1b=   0 r2b=   1 r3b= 0.5   primitive lattice vectors [a][b][c]
#! r1c=   0 r2c=   0 r3c= 0.5
#! nofatoms=1  nofcomponents=3  number of atoms in primitive unit cell/number of components of each spin
#! da=   0 [a] db=   0 [b] dc=   0 [c] nofneighbours=2 diagonalexchange=1 sipffilename=phonon.sipf
#da[a]   db[b]     dc[c]       Jaa[meV]  Jbb[meV]  Jcc[meV]  Jab[meV]  Jba[meV]  Jac[meV]  Jca[meV]  Jbc[meV]  Jcb[meV]
0 0 1 1.1 1.1 1.1
0 0 -1 1.1 1.1 1.1 

It is planned that these files should be created automatically from the output of DFT programs. [to be done]

If crystal field - phonon coupling is to be input, magnetic ions may be added to mcphas.j. They should not be placed at exactly the same position as the phonon atoms - i.e. da db dc should be chosen (slightly) different to enable the mcphas.j loader to identify which is which.

The coupling between phononic and crystal field degrees of freedom should be derived from $\hat H_{cf-phon}=(\mathbf \nabla _{\hat \mathbf u(n')} B_l^m(n)) \mathbf u(n') \hat O_{lm}(\hat \mathbf J_n)$. This could be done using the program pointc and makenn applying small differential changes to the atomic positions in the unit cell in order to evaluate the gradient of the crystal field parameters. [efficient program to do this task is to be written]

Martin Rotter 2017-01-10