Term
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Definition
1. Ionic
2. Covalent
3. Metallic
4. Van Der Waals (secondary)
5. Hydrogen |
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Term
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Definition
transfer of valence electrons (NaCl)
Spherically Symmetric
Coulomb force |
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Term
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Definition
Sharing valence electrons (diamond)
isolinear (favors certain directions) |
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Term
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Definition
Delocalized electrons
explains conductivity/ heat ductility |
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Term
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Definition
| electron stays with original molecule (solid nitrogen |
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Term
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Definition
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Term
| Main weakness of the Einstein model? |
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Definition
| at low temps, the model suggests the specific heat approaches 0 exponentially while experiments show that it is T^3 |
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Term
| Assumptions of the Debye model? |
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Definition
Assumed that lattice vibrations behaved like sound waves obeying the dispersion relation.
Treats the lattice as a set of modes which vibrate independently, but atoms still interact. |
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Term
| at high T, which modes are excited |
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Definition
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Term
| at low T which modes are excited? |
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Definition
only long-wavelength phonons
g(w)~w^2
T^3 |
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Term
| What are degenerate branches |
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Definition
| there are two or more (acoustic/optical) branches which are the same |
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Term
| What are the flaws of the Debye model for Specific Heat? |
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Definition
it approximated using the linear dispersion relation to describe all vibration modes. This only works near the center of the Brillouin Zone in reality. Doesn't work near the edges because of dispersion.
Also it completely ignores the optical branches |
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Term
| How is heat transferred in metals? |
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Definition
| mostly by electrons, also by lattice waves or phonons |
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Term
| How is heat transferred in insulators? |
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Definition
| entirely by phonons because there are no conduction electrons |
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Term
| In the so called "phonon gas" what factors determine the mean free path? |
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Definition
1. T
2. phonons colliding with phonons: "anharmonic interaction" producing scattering
3. collisions with imperfections in the crystal - destroy periodicity causing scattering
4. collisions with the boundary - primarily for low T |
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Term
| Why is there not a lot of scattering going on at low T? |
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Definition
| There aren't too many phonons |
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Term
| What is the standard method for measuring dispersion curves |
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Definition
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Term
| What is a disadvantage of using the x-ray method |
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Definition
| determining frequency shift (hard to get good resolution) |
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Term
| what is the energy of an x-ray vs phonon? |
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Definition
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Term
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Definition
only long wavelength phonons participate in scattering
only good for small k phenomena (optical branch) |
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Term
| Types of scattering yielding information about the mode |
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Definition
1. x-ray scattering
2. neutron scattering
3. light scattering |
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Term
| Applications of scattering: |
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Definition
1. Microwave ultrasonics
2. Coherent phonons (laser)
3. Magnetism
4. Phonon Amplifier
5. Surface phonons |
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Term
| What is so good about using phonons in magnestism? |
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Definition
can study spin-phonon interaction
gives the coupling strength between spins and phonons |
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Term
| why is the free electron gas model valid? |
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Definition
the coulomb attractions is offset by quantum effects which negate the interactions between electrons and ions
also electrons with parallel spins tend to stay away from due to the Pauli exclusion principle
and electrons tend to minimize the energy of the system thus trying to avoid other electrons. |
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Term
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Definition
| it is a space around the electron where no other electrons are likely to be found |
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Term
| differences between the electron gas and regular gas models? |
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Definition
1. charged, so it behaves like a plasma
2. N for electrons ~ 10^29
N for gas ~ 10^25
its way denser |
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Term
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Definition
| Theory states that the electron would travel farther that we would expect if the electron collided with an ion. This can be explained using quantum mechanics |
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Term
| Concerning the wave nature of electrons |
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Definition
waves don't scatter when passing through a lattice,
atoms absorb energy from the wave and radiate it back which
results in no change in direction or intensity
velocity propagation changes
if it is a perfect lattice, should be no collision at all
in reality there is an observed discrepancy |
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Term
| What is the observed discrepancy in electron scattering in a lattice due to? |
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Definition
1. Lattice vibrations due to thermal excitation
2. foreign impurities and crystal defects |
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Term
| what is the Kondo Effect? |
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Definition
| deviation from the accepted model for electrical resistivity vs. temperature (seen in Fe for example) at low T. This is due to additional scattering of electrons by magnetic moments on impurity centers |
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