Finding the maxima or minima of band-edges?

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SUMMARY

The discussion focuses on determining the maxima and minima of band-edges in the context of solid-state physics, specifically using the condition ∇KE=0. This condition indicates that the gradient in 3D k-space must be zero to find the k values corresponding to the maxima and minima. If the k values for the maximum and minimum are identical, a direct band gap is present; otherwise, it is an indirect band gap. Understanding this concept is crucial for analyzing band structure in materials.

PREREQUISITES
  • Understanding of calculus, particularly concepts related to gradients.
  • Familiarity with solid-state physics and band theory.
  • Knowledge of direct and indirect band gaps in semiconductor physics.
  • Basic comprehension of k-space and its significance in electronic properties.
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  • Study the mathematical derivation of gradients in k-space.
  • Learn about the implications of direct vs. indirect band gaps in semiconductor applications.
  • Explore computational tools for band structure calculations, such as Quantum ESPRESSO.
  • Investigate the role of k-values in determining electronic properties of materials.
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Students in physics or materials science, researchers studying semiconductor properties, and anyone involved in electronic materials characterization.

LaVela
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I'm working on a homework assignment and I'm completely stuck on this last problem. I'm not even sure what ∇KE=0 even means to begin with. I understand the difference between indirect and direct bandgap but I'm just confused on how to find the maxima and minima to determine this. Any help would be greatly appreciated.

Direct Indirect Bandgap.PNG
 
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This really should be posted in the HW/Coursework forum.

First of all, don't you remember from your calculus class that a maximum and minimum will have zero gradient? ∇KE=0 is exactly that, except that this is obviously a gradient in 3D k-space. So find the k values for each one of them where the gradient is zero. If the k values you found for each band is the same, then the max and min occurs at the same k values and so, you have a direct band gap. If not, you have an indirect gap.

Zz.
 

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