Spin spirals

Generalized Bloch condition

Spin spirals may be conveniently modeled using a generalization of the Bloch condition (set LNONCOLLINEAR=.TRUE. and LSPIRAL=.TRUE.):

$\left[{\begin{array}{c}\Psi _{\bf {k}}^{\uparrow }({\bf {r)}}\\\Psi _{\bf {k}}^{\downarrow }({\bf {r)}}\end{array}}\right]=\left({\begin{array}{cc}e^{-i{\bf {q\cdot {\bf {R/2}}}}}&0\\0&e^{+i{\bf {q\cdot {\bf {R/2}}}}}\end{array}}\right)\left[{\begin{array}{c}\Psi _{\bf {k}}^{\uparrow }({\bf {r-R)}}\\\Psi _{\bf {k}}^{\downarrow }({\bf {r-R)}}\end{array}}\right],$

i.e., from one unit cell to the next the up- and down-spinors pick up an additional phase factor of $\exp(-i{\bf {q}}\cdot {\bf {R}}/2)$ and $\exp(+i{\bf {q}}\cdot {\bf {R}}/2)$, respectively, where R is a lattice vector of the crystalline lattice, and q is the so-called spin-spiral propagation vector.

The spin-spiral propagation vector is commonly chosen to lie within the first Brillouin zone of the reciprocal space lattice, and has to be specified by means of the QSPIRAL-tag.

The generalized Bloch condition above gives rise to the following behavior of the magnetization density:

${\bf {m}}({\bf {r}}+{\bf {R}})=\left({\begin{array}{c}m_{x}({\bf {r}})\cos({\bf {q}}\cdot {\bf {R}})-m_{y}({\bf {r}})\sin({\bf {q}}\cdot {\bf {R}})\\m_{x}({\bf {r}})\sin({\bf {q}}\cdot {\bf {R}})+m_{y}({\bf {r}})\cos({\bf {q}}\cdot {\bf {R}})\\m_{z}({\bf {r}})\end{array}}\right)$

This is schematically depicted below: the components of the magnization in the xy-plane rotate about the spin-spiral propagation vector q.

Basis set considerations

The generalized Bloch condition redefines the Bloch functions as follows:

$\Psi _{\bf {k}}^{\uparrow }({\bf {r)=\sum _{\bf {G}}{\rm {C_{\bf {k{\bf {G}}}}^{\uparrow }e^{i({\bf {k+{\bf {G-{\frac {\bf {q}}{2}})\cdot {\bf {r}}}}}}}}}}}$

$\Psi _{\bf {k}}^{\downarrow }({\bf {r)=\sum _{\bf {G}}{\rm {C_{\bf {k{\bf {G}}}}^{\downarrow }e^{i({\bf {k+{\bf {G+{\frac {\bf {q}}{2}})\cdot {\bf {r}}}}}}}}}}}$

This changes the Hamiltonian only minimally:

$\left({\begin{array}{cc}H^{\uparrow \uparrow }&V_{\rm {xc}}^{\uparrow \downarrow }\\V_{\rm {xc}}^{\downarrow \uparrow }&H^{\downarrow \downarrow }\end{array}}\right)\rightarrow \left({\begin{array}{cc}H^{\uparrow \uparrow }&V_{\rm {xc}}^{\uparrow \downarrow }e^{-i{\bf {q\cdot {\bf {r}}}}}\\V_{\rm {xc}}^{\downarrow \uparrow }e^{+i{\bf {q\cdot {\bf {r}}}}}&H^{\downarrow \downarrow }\end{array}}\right),$

where in $H^{\uparrow \uparrow }$ and $H^{\downarrow \downarrow }$ the kinetic energy of a plane wave component changes to:

$H^{\uparrow \uparrow }:\qquad |{\bf {k}}+{\bf {G}}|^{2}\rightarrow |{\bf {k}}+{\bf {G}}-{\bf {q}}/2|^{2}$

$H^{\downarrow \downarrow }:\qquad |{\bf {k}}+{\bf {G}}|^{2}\rightarrow |{\bf {k}}+{\bf {G}}+{\bf {q}}/2|^{2}$

In the case of spin-spiral calculations the cutoff energy of the basis set of the individual spinor components is specified by means of the ENINI-tag.

Additionally one needs to set ENMAX appropriately: ENMAX needs to be chosen larger than ENINI, and large enough so that the plane wave components of both the up-spinors as well as the components of the down-spinor all have a kinetic energy smaller than ENMAX. This is the case when:

${\mathtt {ENMAX}}\geq {\frac {\hbar ^{2}}{2m}}\left(G_{\rm {ini}}+|q|\right)^{2}$

where

$G_{\rm {ini}}={\sqrt {{\frac {2m}{\hbar ^{2}}}{\mathtt {ENINI}}}}$

In most cases it is more than sufficient to set ENMAX=ENINI+100.

To judge whether ENMAX is chosen large enough one will always get a warning at runtime, e.g.

 ----------------------------------------------------------------------------- 
|                                                                             |
|           W    W    AA    RRRRR   N    N  II  N    N   GGGG   !!!           |
|           W    W   A  A   R    R  NN   N  II  NN   N  G    G  !!!           |
|           W    W  A    A  R    R  N N  N  II  N N  N  G       !!!           |
|           W WW W  AAAAAA  RRRRR   N  N N  II  N  N N  G  GGG   !            |
|           WW  WW  A    A  R   R   N   NN  II  N   NN  G    G                |
|           W    W  A    A  R    R  N    N  II  N    N   GGGG   !!!           |
|                                                                             |
|      To represent the spin spiral you requested, with a kinetic             |
|      energy cutoff of ENINI=  300.00 eV, choose ENMAX >  331.21 eV          |
|      Currently ENMAX=  400.00 eV                                            |
|                                                                             |
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Symmetry

Generally the introduction of a spin-spiral will lower the symmetry of the system. At present VASP can not correctly account for the presence of a spin-spiral in its symmetry analysis.

Therefore the use of symmetry has to be switched of completely:

ISYM = -1

Related Tags and Sections

LSPIRAL, QSPIRAL, LZEROZ, LNONCOLLINEAR, MAGMOM, ENINI, ENMAX, ISYM, I_CONSTRAINED_M, LAMBDA, M_CONSTR, RWIGS

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