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## Main Question or Discussion Point

Considering the Nearly Free Electron model of solids, where we assume the valence electrons of some

We may derive (which I assume you are familiare with, and will not do here) the central equation as an algebraic reformulation of the time independent Schrödinger eq. corresponding to the model/potential at hand,

[itex]

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

[/itex]

where [itex] \lambda_k = (\hbar^2 k^2) / (2m_e) [/itex], [itex] G [/itex] is the set of possible reciprocal lattice vectors and [itex] C_k [/itex] is det fourier coefficients corresponding to the solution of the Schrödinger eq.:

[itex]

\psi_k = \Sigma_k C_k e^{ikx}

[/itex].

Say for instance I solve it and find the energy [itex] \epsilon_\pm = \lambda_k \pm U_0 [/itex] for some [itex]k[/itex]. Then

**one dimensional(!)**solid to move in a weak, periodic (with respect to the solids lattice constant) potensial.We may derive (which I assume you are familiare with, and will not do here) the central equation as an algebraic reformulation of the time independent Schrödinger eq. corresponding to the model/potential at hand,

[itex]

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

[/itex]

where [itex] \lambda_k = (\hbar^2 k^2) / (2m_e) [/itex], [itex] G [/itex] is the set of possible reciprocal lattice vectors and [itex] C_k [/itex] is det fourier coefficients corresponding to the solution of the Schrödinger eq.:

[itex]

\psi_k = \Sigma_k C_k e^{ikx}

[/itex].

**My problem**is that I do not understand what exactly we do find if we solve the central equation.Say for instance I solve it and find the energy [itex] \epsilon_\pm = \lambda_k \pm U_0 [/itex] for some [itex]k[/itex]. Then

**I am told**the energy gap, [itex] \epsilon_{gap} = \epsilon_+ - \epsilon_- [/itex], between two energy bands for this [itex]k[/itex] at hand. Please (dis)confirm!?**... and then WHICH two bands are this gap between?**(If that makes sense). And**is it possible to find values for [itex] C_k [/itex], how?**.. Assuming we know the periodicity of the potential and [itex] k [/itex].