fermi dirac statistics in solids.ppt

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Applications of statistical physics to selected solid-state physics phenomena for metals “similar” models for thermal and electrical conductivity for metals, on basis of free electron gas, treated with Maxwell-Boltzmann statistics – i.e. as if it were an ideal gas Lorenz numbers, a fortuitous result, don’t be fooled, the physics (Maxwell-Boltzmann statistics) behind it is not applicable as we estimated earlier But Fermi-Dirac statistics gets us the right physics

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Metals have high conductivities for both electricity and heat. To explain both the high conductivities and the trend in this table we need to have a model for both thermal and electrical conductivity, that model should be able to explain empirical observations, i.e. Ohm’s law, thermal conductivity, Wiedemann- Franz law,

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Wiedemann and Franz Law, 1853, ratio K/σT = Lorenz number = constant  2.4 10-8 W Ω K -2 independent of the metal considered !! So both phenomena should be based on similar physical idea !!! Classical from Drude (early 1900s) theory of free electron gasToo small by factor 2, seems not too bad ???2.22.552.3300K

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To explain the high conductivities and the trend we need to have a model for both thermal and electrical conductivity, that model should be able to explainOhm’s law, empirical for many metals and insulators, ohmic solids J = σ E current density is proportional to applied electric field R = U / I for a wire R = ρ l / A J: current density A/m2 σ: electrical conductivity Ω-1 m-1, reciprocal value of electrical resistivity E: electric field V/m Also definition of σ: a single constant that does depend on the material and temperature but not on applied electric field represents connection between I and UConductivity , resistivity is its reciprocal value

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Figure 12.11 (a) Random successive displacements of an electron in a metal without an applied electric field. Gas of classical charged particles, electrons, moves through immobile heavy ions arranged in a lattice, vrms from equipartition theorem (which is of course derived from Boltzmann statistics)Between collisions, there is a mean free path length: L = vrms τ and a mean free time τ (tau)

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Figure 12.11 (b) A combination of random displacements and displacements produced by an external electric field. The net effect of the electric field is to add together multiple displacements of length vd  opposite the field direction. For purposes of illustration, this figure greatly exaggerates the size of vd compared with vrms. If there is an electric field E, there is also a drift speed vd (108 times smaller than vrms) but proportional to E, equal for all electrons

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Figure 12.12 The connection between current density, J, and drift velocity, vd. The charge that passes through A in time dt is the charge contained in the small parallelepiped, neAvd dt. Substituting for vdSo the correct form of Ohm’s law is predicted by the Drude model !!

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Proof of the pudding: L should be on the order of magnitude of the inter-atomic distances, e.g. for Cu 0.26 nmWith mean free time τ = L/vrmsWith vrms according to Maxwell-Boltzmann statisticsσCu, 300 K = 5.3 106 (Ωm)-1 compare with experimental value 59 106 (Ωm) -1, something must we wrong with the classical L and vrms

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Result of Drude theory one order of magnitude too small, so L must be much larger, this is because the electrons are not classical particles, but wavicals, don’t scatter like particles, in addition, the vrms from Boltzmann-Maxwell is one order of magnitude smaller than the vfermi following from Fermi-Dirac statistics

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Figure 12.13 The resistivity of pure copper as a function of temperature. So ρ ~T0.5 theory for all temperatures, but ρ ~T for reasonably high T , so Drude’s theory must be wrong !

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Phenomenological similarity conduction of electricity and conduction of heat, so free electron gas should also be the key to understanding thermal conductivity Ohm’s law with Voltage gradient, thermal energy conducted through area A in time interval Δt is proportional to temperature gradient Using Maxwell-Boltzmann statistics, equipartion theorem, formulae of Cv for ideal gas = 3/2 kB n Classical expression for K

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For 300 K and CuExperimental value for Cu at (300 K) = 390 Wm-1K-1, again one order of magnitude too small, actually roughly 20 times too smallLets continue

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Lorenz number classical K/σThis was also one order of magnitude too small, With Maxwell-BoltzmannWrong only by a factor of about 2, Such an agreement is called fortuitous

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replace Lfor_a_particle with Lfor_a_wavial and vrms with vfermi, For Cu (at 300 K), EF = 7.05 eV , Fermi energies have only small temperature dependency, frequently neglected

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two orders of magnitude larger than classical result for particleone order of magnitude larger than classical vrms for ideal gas

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So here something two orders of two magnitude too small (L) gets divided by something one order of magnitude too small (vrms), i.e. the result for electrical conductivity must be one order of magnitude too small, which is observed !! But L for particle is quite reasonable, so replace Vrms with Vfermi and the conductivity gets one order of magnitude larger, which is close to the experimental observation, so that one keeps the Drude theory of electrical conductivity as a classical approximation for room temperature

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Figure 12.11 (a) Random successive displacements of an electron in a metal without an applied electric field. (b) A combination of random displacements and displacements produced by an external electric field. The net effect of the electric field is to add together multiple displacements of length vd  opposite the field direction. For purposes of illustration, this figure greatly exaggerates the size of vd compared with vrms. .in effect, neither the high vrms of 105 m/s of the electrons derived from the equipartion theorem or the 10 times higher Fermi speed do not contribute directly to conducting a current since each electrons goes in any directions with an equal likelihood and this speeds averages out to zero charge transport in the absence of E

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Vrms was too small by one order of magnitude, Lclassical was too small by two orders of magnitude, the classical calculations should give a result 3 orders of magnitude smaller than the observation (which is of course well described by a quantum statistical treatment) so there must be something fundamentally wrong with our ideas on how to calculate K, any idea ???

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Wait a minute, K has something to do with the heat capacity that we derived from the equipartion theoremWe had the result earlier that the contribution of the electron gas is only about one hundredth of what one would expect from an ideal gas, Cv for ideal gas is actually two orders or magnitude larger than for a real electron gas, so that are two orders of magnitude in excess, with the product of vrms and Lfor particle three orders of magnitude too small, we should calculate classically thermal conductivities that are one order of magnitude too small, which is observed !!!

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fortunately L cancelled, but vrms gets squared, we are indeed very very very fortuitous to get the right order of magnitude for the Lorenz number from a classical treatment (one order of magnitude too small squared is about two orders of magnitude too small, but this is “compensated” by assuming that the heat capacity of the free electron gas can be treated classically which in turn results in a value that is by itself two order of magnitude too large- two “missing” orders of magnitude times two “excessive orders of magnitudes levels about out

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That gives for the Lorenz number in a quantum treatment

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Back to the problem of the temperature dependency of resistivityThis is due to Debye’s phonons (lattice vibrations), which are bosons and need to be treated by Bose-Einstein statistics, electrons scatter on phonons, so the more phonons, the more scattering Drude’s theory predicted a dependency on square root of T, but at reasonably high temperatures, the dependency seems to be linearNumber of phonons proportional to Bose-Einstein distribution function Which becomes for reasonably large T

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At low temperatures, there are hardly any phonons, scattering of electrons is due to impurity atoms and lattice defects, if it were not for them, there would not be any resistance to the flow of electricity at zero temperature Matthiessen’s rule, the resistivity of a metal can be written as σ = σlattice defects + σlattice vibrations

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Last Updated: 8th March 2018

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