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Last Time. Debye Approximation. Free electron model. Fermi Surface. Fermi-Dirac Distribution Function. Density of states in 3D. Today. Heat Capacity. Electrons. Phonons. Measuring the occupied density of states Effective Mass Electrical Conductivity Thermal Conductivity

pearlie-kyrie
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Last Time

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  1. Last Time Debye Approximation. Free electron model Fermi Surface Fermi-Dirac Distribution Function Density of states in 3D

  2. Today Heat Capacity Electrons Phonons Measuring the occupied density of states Effective Mass Electrical Conductivity Thermal Conductivity Wiedemann-Franz Ratio

  3. Fermi-Dirac Distribution Function Becomes a step function at T=0. Low E: f ~ 1. High E: f ~ 0.  = chemical potential = “Fermi Level” (T=0)=F Fermi energy Right at the Fermi level: f = 1/2. Go play with the Excel file “fermi.xls” at: http://ece-www.colorado.edu/~bart/book/distrib.htm#fermi

  4. Density of Occupied States Density of states Fermi function Number of electrons per energy range Implicit equation for  N is conserved Shaded areas are equal  0.01% @ room temp

  5. Heat Capacity Width of shaded region ~ kT Room temp T ~ 300K, TF ~ 104 K  Small width  Few electrons thermally excited How many electrons are excited thermally? Shaded area  triangle. Area = (base)(height)/2 Number of excited electrons:  (g(F)/2)(kT)/2  g(F)(kT)/4 Excitation energy  kT (thermal) Total thermal energy in electrons: C ~ T Heat Capacity in a Metal

  6. Heat Capacity How you would do the real calculation: Implicit equation for   fully determines n(, T) Then In a metallic solid, Correct in simple metals. Electrons Phonons C ~ T is one of the signatures of the metallic state

  7. Measuring n(, T) n(, T) is the actual number of electrons at  and T X-ray Emission Bombard sample with high energy electrons to remove some core electrons Electron from condition band falls to fill “hole”, emitting a photon of the energy difference Measure the photons -- i.e. the X-ray emission spectrum

  8. Measuring n(, T) n(, T) is the actual number of electrons at  and T X-ray Emission Emission spectrum (how many X-rays come out as a function of energy) will look like this. Fine print: The actual spectrum is rounded by temperature, and subject to transition probabilities. Void in New Hampshire.

  9. EFFECTIVE MASS Real metals: electrons still behave like free particles, but with “renormalized” effective mass m* In potassium (a metal), assuming m* =1.25m gets the correct (measured) electronic heat capacity Physical intuition: m* > m, due to “cloud” of phonons and other excited electrons. At T>0, the periodic crystal and electron-electron interactions and electron-phonon interactions renormalize the elementary excitation to an “electron-like quasiparticle” of mass m* Fermi Surface

  10. Electrical Conductivity Collisions cause drag Electric Field Accelerates charge   mean time between collisions Steady state solution:  average velocity =mobility Electric current density (charge per second per area) current per area Units: n=N/V ~ L-3 v ~ L/S

  11. Electrical Conductivity Electric current density (charge per second per area) current per area OHM’s LAW (V = I R ) Electrical Conductivity n = N/V me = mass of electron e = charge on electron  = mean time between collisions

  12. What Causes the Drag?

  13. On average, I go about  seconds between collisions electron Bam! phonon Random Collisions with phonons and impurities

  14. Scattering It turns out that static ions do not cause collisions! What causes the drag? (Otherwise metals would have infinite conductivity) Electrons colliding with phonons (T > 0) imp is independent of T Electrons colliding with impurities

  15. Mathiesen’s Rule how often electrons scatter from impurities how often electrons scatter total how often electrons scatter from phonons Independent scattering processes means the RATES can be added. 5 phonons per sec. + 7 impurities per sec. = 12 scattering events per second

  16. Mathiesen’s Rule Resistivity If the rates add, then resistivities also add: Resistivities Add (Mathiesen’s Rule)

  17. Thermal conductivity Electric current density Heat current density Heat current density  = Energy per particle v = velocity n = N/V

  18. Thermal conductivity Heat current density jleft jright x Heat Current Density jtot through the plane: jtot = jright - jleft Heat energy per particle passing through the plane started an average of “l” away. About half the particles are moving right, and about half to the left. x

  19. Thermal conductivity Heat current density x Limit as l goes small:

  20. Thermal conductivity Heat current density x

  21. Thermal conductivity Heat current density x How does it depend on temperature?

  22. Thermal conductivity

  23. Wiedemann-Franz Ratio Fundamental Constants ! Cu: = 2.23  10-8 W/2 (Good at low Temp) Major Assumption: thermal = electronic Good @ very hi T & very low T (not at intermediate T)

  24. Homework Problem 3 “rs” Radius of sphere denoting volume per conduction electron Defines rs n=N/V=density of conduction electrons In 3D

  25. Solid State Simulations • http://www.physics.cornell.edu/sss/ Go download these and play with them! For this week, try the simulation “Drude”

  26. Today Heat Capacity Electrons Phonons Measuring the occupied density of states Effective Mass Electrical Conductivity Thermal Conductivity Wiedemann-Franz Ratio

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