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Difference between revisions of "Phase of a modulation"

From Online Dictionary of Crystallography

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[[Phase of a Modulation]]
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<font color="blue">Phase d'une modulation</font> (''Fr''). <font color="red">Modulationsphase</font> (''Ge''). <font color="black">Fase di une modulazione</font> (''It''). <font color="purple">変調の位相</font> (''Ja'').
  
Phase de la modulation (Fr.)
 
  
'''Definition'''
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== Definition ==
  
A modulation (see [[modulated crystal structure]]) is a periodic or quasiperiodic, scalar or vector function. In the former case, the phase measures
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A [[modulated crystal structure|modulation ]] is a periodic or [[quasiperiodicity|quasiperiodic]], scalar or vector function. In the former case, the phase measures the progress along one periodic direction. The periodic or quasiperiodic function may be developed into plane waves.
the progress along one periodic direction. The periodic or quasiperiodic function may be developed
 
into plane waves.
 
 
The  ''phase(s) of the modulation'' is (are) the phase(s) of elementary plane waves which describe the modulation.
 
The  ''phase(s) of the modulation'' is (are) the phase(s) of elementary plane waves which describe the modulation.
  
'''Details'''
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== Details ==
  
 
A  ''displacive modulation'' may be written as follows. For the ''j''th atom in the unit cell '''n''' the displacement
 
A  ''displacive modulation'' may be written as follows. For the ''j''th atom in the unit cell '''n''' the displacement
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and the displacement is given by
 
and the displacement is given by
  
<math>u_{ n j\alpha}~=~\sum_{ k\in M^*} \hat{u}_{j\alpha}( k)\exp \left( 2\pi i \sum_m h_m  a_m^*.(n+r_j)+\phi_{j\alpha}\right),~~~(h_1,\dots,h_n\neq 0,\dots,0).</math>
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<math display="block">u_{n j\alpha} = \sum_{k\in M^*} \hat{u}_{j\alpha}( k) \exp \Big( 2\pi i \sum_m h_m  a_m^*.(n+r_j)+\varphi_{j\alpha}\Big), \quad(h_1,\dots,h_n\neq 0,\dots,0).</math>
  
For the simplest case with one modulation vector, one polarization direction and one atom per unit cell this becomes
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For the simplest case, with one modulation vector, one polarization direction and one atom per unit cell, this becomes
  
<math> u_n~=~\hat{ u} ( k)\exp (2\pi i k. n+\phi)+c.c..</math>
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<math> u_n~=~\hat{ u} ( k)\exp (2\pi i k. n+\varphi)+c.c.</math>
  
 
Here &phi; is the  ''phase of the modulation''.  
 
Here &phi; is the  ''phase of the modulation''.  
 
The embedded structure in superspace is
 
The embedded structure in superspace is
  
<math>( n+\hat{ u}( k) \exp (2\pi i  k.n+\phi +r_I)+c.c,~r_I).</math>
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<math>( n+\hat{ u}( k) \exp (2\pi i  k.n+\varphi +r_I)+c.c,~r_I).</math>
  
 
r<sub>''I''</sub> is the internal coordinate, which changes the phase of the modulation. (In the literature
 
r<sub>''I''</sub> is the internal coordinate, which changes the phase of the modulation. (In the literature
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reciprocal lattice in  [[superspace]], and this has an external and an internal component:
 
reciprocal lattice in  [[superspace]], and this has an external and an internal component:
  
<math> k~=~\pi k_s~=~\sum_{i=1}^n h_i  a_{Ei}^*,~ a_{Ii}^*).</math>
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<math> k~=~\pi k_s~=~\sum_{i=1}^n h_i  (a_{Ei}^*,~ a_{Ii}^*).</math>
  
 
Then the embedding has components
 
Then the embedding has components
<math>
 
\left(  n_{j\alpha}+ r_{j\alpha}+\sum_{ k\in M^*} \hat{\rho}_{j\alpha}( k)\exp \left[ 2\pi i \sum_m h_m  a_m^*.( n+ r_j)+\phi_{j\alpha}+\sum_m h_m a_{Im}^*.r_{I}\right],~~{\rm r}_{I}\right)</math>
 
  
Each plane wave for the modulation has a phase &phi;<sub>j&alpha;</sub> which is changed by changing the internal component  ''r''<sub>''I''</sub>, an ''$n-m''= ''d''-dimensional vector in internal space.
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<math>\Big( n_{j\alpha}+ r_{j\alpha}+\sum_{k\in M^*} \hat{u}_{j\alpha}( k)
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        \exp [ 2\pi i \sum_m h_m  a_m^*.( n+ r_j)+  \varphi_{j\alpha}+\sum_m h_m a_{Im}^*.r_{I} ],\;\mathrm{r}_{I} \Big)</math>
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Each plane wave for the modulation has a phase <math>\phi_{j\alpha}</math> which is changed by changing the internal component  <math>r_I</math>, an (''n-m'')-dimensional vector in internal space.
  
 
For an occupation modulation, the situation is quite similar, but simpler because the occupation function is a scalar.
 
For an occupation modulation, the situation is quite similar, but simpler because the occupation function is a scalar.
 
For the simplest case one has
 
For the simplest case one has
  
<math>p( n)~=~\hat{p} ( k)\exp (2\pi i  k. n+\phi)+ c.c.</math>
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<math>p( n)~=~\hat{p} ( k)\exp (2\pi i  k. n+\varphi)+ c.c.</math>
  
 
The embedding is the function
 
The embedding is the function
  
<math>p({\bf n}, r_I)~=~\hat{\rho}( k) \exp (2\pi i  k.n+\phi +r_I)+c.c. </math>
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<math>p( n, r_I)~=~\hat{\rho}( k) \exp (2\pi i  k.n+\varphi +r_I)+c.c. </math>
  
 
In the general case
 
In the general case
  
<math>p({\bf n})_{j}~=~\sum_{{\bf k}\in M^*} \hat{p}_{j}({\bf k})\exp \left( 2\pi i \sum_m h_m {\bf a}_m^*.({\bf n}+{\bf r}_j)+\phi_{j}\right),~~~(h_1,\dots,h_n\neq 0,\dots,0).</math>
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<math display="block">p( n)_{j} = \sum_{k\in M^*} \hat{p}_{j}( k)\exp \Big( 2\pi i \sum_m
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  h_m a_m^*.( n+ r_j)+\varphi_{j}\Big),\quad (h_1,\dots,h_n\neq 0,\dots,0)</math>
  
 
and the embedding is
 
and the embedding is
  
<math>p({\bf n},{\bf r}_I)_{j}~=~\sum_{{\bf k}\in M^*} \hat{p}_{j}({\bf k})\exp \left[ 2\pi i \sum_m h_m {\bf a}_m^*.({\bf n}+{\bf r}_j)+\phi_{j}+\sum_m h_m{\bf a}_{Im}^*.{\bf r}_{I}\right].</math>
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<math>p( n, r_I)_j~=~\sum_{ k\in M^*} \hat{p}_{j}( k)\exp \left[ 2\pi i \sum_m h_m a_m^*.( n+ r_j)+\varphi_{j}+\sum_m h_m a_{Im}^*.r_{I}\right].</math>
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 +
By a change of the internal coordinate ''r''<sub>''I''</sub> the phases <math>\phi_j</math> of the modulation functions change.
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== See also ==
  
By a change of the internal coordinate ''r''<sub>''I''</sub> the phases &phi;<sub>j</sub> of the modulation functions change.
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*[[Incommensurate modulated structure]]
  
'''See also''':  [[Modulation function]], [[incommensurate modulated crystal structure]].
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[[Category: Fundamental crystallography]]

Latest revision as of 09:42, 17 November 2017

Phase d'une modulation (Fr). Modulationsphase (Ge). Fase di une modulazione (It). 変調の位相 (Ja).


Definition

A modulation is a periodic or quasiperiodic, scalar or vector function. In the former case, the phase measures the progress along one periodic direction. The periodic or quasiperiodic function may be developed into plane waves. The phase(s) of the modulation is (are) the phase(s) of elementary plane waves which describe the modulation.

Details

A displacive modulation may be written as follows. For the jth atom in the unit cell n the displacement has m components un, where α=x,y,z in three dimensions. Then for a modulation of finite rank the Fourier module M* consists of the reciprocal vectors

[math] k~=~\sum_{i=1}^n h_i a_i^*,[/math]

and the displacement is given by

[math]u_{n j\alpha} = \sum_{k\in M^*} \hat{u}_{j\alpha}( k) \exp \Big( 2\pi i \sum_m h_m a_m^*.(n+r_j)+\varphi_{j\alpha}\Big), \quad(h_1,\dots,h_n\neq 0,\dots,0).[/math]

For the simplest case, with one modulation vector, one polarization direction and one atom per unit cell, this becomes

[math] u_n~=~\hat{ u} ( k)\exp (2\pi i k. n+\varphi)+c.c.[/math]

Here φ is the phase of the modulation. The embedded structure in superspace is

[math]( n+\hat{ u}( k) \exp (2\pi i k.n+\varphi +r_I)+c.c,~r_I).[/math]

rI is the internal coordinate, which changes the phase of the modulation. (In the literature the internal coordinate rI is sometimes denoted by t.)

For the general case, a vector k from the Fourier module is the projection of a vector of the reciprocal lattice in superspace, and this has an external and an internal component:

[math] k~=~\pi k_s~=~\sum_{i=1}^n h_i (a_{Ei}^*,~ a_{Ii}^*).[/math]

Then the embedding has components

[math]\Big( n_{j\alpha}+ r_{j\alpha}+\sum_{k\in M^*} \hat{u}_{j\alpha}( k) \exp [ 2\pi i \sum_m h_m a_m^*.( n+ r_j)+ \varphi_{j\alpha}+\sum_m h_m a_{Im}^*.r_{I} ],\;\mathrm{r}_{I} \Big)[/math]

Each plane wave for the modulation has a phase [math]\phi_{j\alpha}[/math] which is changed by changing the internal component [math]r_I[/math], an (n-m)-dimensional vector in internal space.

For an occupation modulation, the situation is quite similar, but simpler because the occupation function is a scalar. For the simplest case one has

[math]p( n)~=~\hat{p} ( k)\exp (2\pi i k. n+\varphi)+ c.c.[/math]

The embedding is the function

[math]p( n, r_I)~=~\hat{\rho}( k) \exp (2\pi i k.n+\varphi +r_I)+c.c. [/math]

In the general case

[math]p( n)_{j} = \sum_{k\in M^*} \hat{p}_{j}( k)\exp \Big( 2\pi i \sum_m h_m a_m^*.( n+ r_j)+\varphi_{j}\Big),\quad (h_1,\dots,h_n\neq 0,\dots,0)[/math]

and the embedding is

[math]p( n, r_I)_j~=~\sum_{ k\in M^*} \hat{p}_{j}( k)\exp \left[ 2\pi i \sum_m h_m a_m^*.( n+ r_j)+\varphi_{j}+\sum_m h_m a_{Im}^*.r_{I}\right].[/math]

By a change of the internal coordinate rI the phases [math]\phi_j[/math] of the modulation functions change.

See also