What does this length represent?

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Discussion Overview

The discussion revolves around the relationship between a metal's Fermi temperature ##T_F## and Debye temperature ##\theta_D##, specifically examining the ratio and the implications of certain approximations related to Fermi momentum and electron concentration in a lattice structure.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant presents a formula relating ##T_F## and ##\theta_D##, suggesting a potential interpretation of the length ##\lambda## in terms of Fermi momentum and lattice spacing.
  • Multiple participants question the meaning of the variable ##c##, with one suggesting it refers to the averaged speed of sound.
  • Concerns are raised about the appropriateness of associating ##mc## with Fermi momentum, noting that electrons near the Fermi sphere have significantly higher speeds.
  • Another participant questions why electron concentration does not appear in the result, pointing out that Debye temperature does not account for free electron concentration.
  • There is a discussion about the number of atoms ##N## in the context of a face-centered cubic (FCC) lattice and its relation to free electrons.
  • One participant mentions having seen a similar expression in a textbook but notes a lack of explanation regarding the variable ##\lambda##.

Areas of Agreement / Disagreement

Participants express differing views on the appropriateness of certain approximations and the implications of electron concentration in the context of the discussed formula. The discussion remains unresolved with multiple competing perspectives on the interpretations of the variables involved.

Contextual Notes

There are limitations regarding the assumptions made about the lattice structure and the nature of the electrons involved, as well as the definitions of the variables used in the equations.

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I was doing some calculations earlier and tried the ratio between a metal's fermi temperature ##T_F## and debye temperature ##\theta_D##:
[tex]\frac{T_F}{\theta_D} = (6 \pi^2)^{\frac{1}{3}} \left( \frac{\lambda}{a} \right)[/tex]

where ##\lambda = \frac{\hbar}{2 m_e c}## and lattice spacing is ##a##.

I tried approximating fermi momentum ##p_F \approx m_e c## and we get ##\lambda \approx \frac{1}{2k_F}##. Does this mean anything?
 
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What is c?
 
nasu said:
What is c?
Averaged speed of sound
 
Oh, then it does not make sense to associate mc with the Fermi momentum. The electrons near the Fermi sphere have much higher speeds. At least one order of magnitude if not two.

How come that electron concentration does not show up in the result? There is no free electron concentration in the Debye temperature.
 
nasu said:
Oh, then it does not make sense to associate mc with the Fermi momentum. The electrons near the Fermi sphere have much higher speeds. At least one order of magnitude if not two.

How come that electron concentration does not show up in the result? There is no free electron concentration in the Debye temperature.
Oh I assumed it was a FCC lattice, so ##n = \frac{N}{a^3} = \frac{4}{a^3}##.
 
N would be the number of atoms, right?
The number of free electrons is not necessarily equal to the same N. It may be, though.
 
nasu said:
N would be the number of atoms, right?
The number of free electrons is not necessarily equal to the same N. It may be, though.

I think we can make that approximation for a mono-valent atom. I saw a similar expression in my textbook too, but they never explained what ##lambda## was which is why I'm trying to find out.
 
bumpp
 

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