Central Equation Derivation: NxN Matrix?

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Discussion Overview

The discussion revolves around the derivation and interpretation of a central equation related to NxN matrices in the context of solid state physics, specifically regarding wave functions and their coefficients in reciprocal space. Participants explore the nature of the equations involved, their matrix representation, and the relationships between various coefficients.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant questions the classification of the central equation as a NxN matrix, suggesting it appears to be a one-line equation with summation across reciprocal lattice vectors.
  • Another participant points out that the central equation is actually a set of equations that couple coefficients for different wave vectors, indicating a more complex structure than initially perceived.
  • There is a discussion about the specific forms of the equations and how they relate to the coefficients, with references to external materials for clarification.
  • One participant expresses confusion about the nature of C(k), questioning whether it is a vector or simply a Fourier coefficient of the wavefunction.
  • A later reply asserts that C(k) is not a vector but emphasizes the necessity of writing the equations in matrix form for solving them.

Areas of Agreement / Disagreement

Participants express differing views on the representation of the central equation and the nature of the coefficients involved. There is no consensus on whether C(k) should be considered a vector or a Fourier coefficient, and the discussion remains unresolved regarding the interpretation of the NxN matrix structure.

Contextual Notes

Participants reference various equations and external materials, indicating that there may be assumptions or definitions that are not fully articulated within the discussion. The relationship between the coefficients and the potential terms is also a point of contention, with some steps in the derivation remaining unclear.

semc
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I went through the derivation for the central equation

$$(\lambda_k - \epsilon)C_k + \Sigma_G U_G C_{k-G} = 0$$

and everywhere I look everybody just says this is a NxN matrix. I don't see how this is true. Isn't this just a one line equation with summation across all reciprocal lattice? I don't understand why at one value of k we get NxN matrix. Any help is greatly appreciated!
 
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Yeah this is what I found but the central equation is
$$ \frac{\hbar^2k^2}{2m} C_k + \sum_{G}^{} V_GC_{k-G}=EC_k$$
so shouldn't I just have
$$ \frac{\hbar^2k^2}{2m} C_k + V_0C_{k-g} + V_0C_{k+g}=EC_k$$
for G=ng and $$ V=V_0e^{igx} +V_0e^{-igx} ?$$
How do you get the other equations
$$ \frac{\hbar^2(k-g)^2}{2m} C_{k-g} + V_0C_{k-2g} + V_0C_{k}=EC_{k-g}$$
 
The "central equation" is a set of equations. For a fixed k in the first Brillouin zone, this set of equations for all reciprocal lattice vectors G couples those coefficients Ck , CkG , CkG' , CkG'' ,... whose wave vector differ from k by a reciprocal vector. Equation (107) in [1] which follows the presentation in the textbook “Solid State Physics” by Neil W. Ashcroft and N. David Mermin should illustrate the meaning of the "central equation". Have a look at [2], too.

[1] [PDF]Introduction to Solid State Physics
[2] [PDF]7.6 The Schrödinger equation of electron in a periodic potential
 
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Well I read the book by ashcroft and mermin and [2] before. Everybody just says that the C(k) couples to C(k+G). Are they referring to the coupling via V0? I really don't see where the other equations come from
 
I guess I am asking why is C(k) a vector? Isn't C(k) just the Fourier coefficient of the wavefunction?

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semc said:
I guess I am asking why is C(k) a vector?

C(k) isn't a vector. You simply write the set of equations which have to be solved in a matrix form.
 
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