Ginzburg Criterion: Why Choose Volumes of Correlation Length?

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SUMMARY

The discussion centers on the Ginzburg Criterion, specifically the rationale for selecting a volume equal to the correlation length in the context of superconductivity. It is established that the coherence length, derived from the Ginzburg-Landau (GL) equation, serves as a natural scale for fluctuations of the order parameter. Additionally, the penetration depth is identified as a critical scale for electromagnetic field phenomena, emphasizing that if the system size is significantly smaller than this depth, the electromagnetic field can be disregarded. Thus, choosing a volume equal to the correlation length is essential for accurate modeling of superconducting behavior.

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  • Understanding of Ginzburg-Landau theory in superconductivity
  • Familiarity with coherence length and penetration depth concepts
  • Knowledge of fluctuations in order parameters
  • Basic principles of electromagnetic fields in condensed matter physics
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wdlang
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indeed, the fluctuations should be smaller than the average value of the order parameter

but why choose a volume with size of the correlation length?

why cannot we choose a larger volume? as the volume increases, surely the fluctuations will

decreases relative to the average value
 
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wdlang said:
but why choose a volume with size of the correlation length?

If your question is in the context of superconductivity then:

The coherence length is the "natural scale" set by parameters of GL equation. Likewise the penetration depth sets a "natural scale" for phenomena related to EM field. For example if the size of your system is much less than penetration depth you can forget about the presence of EM field. The same argument for your question.
 

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