Fermi–Dirac statistics

Statistical mechanics Thermodynamics · Kinetic theory Fermi–Dirac statistics is a part of the science of physics that describes the energies of single particles in a system comprising many identical particles that obey the Pauli Exclusion Principle. It is named after Enrico Fermi and Paul Dirac, who each discovered it independently.^{[1]}^{[2]}
Fermi–Dirac (F–D) statistics applies to identical particles with halfoddinteger spin in a system in thermal equilibrium. Additionally, the particles in this system are assumed to have negligible mutual interaction. This allows the manyparticle system to be described in terms of singleparticle energy states. The result is the F–D distribution of particles over these states and includes the condition that no two particles can occupy the same state, which has a considerable effect on the properties of the system. Since F–D statistics applies to particles with halfinteger spin, they have come to be called fermions. It is most commonly applied to electrons, which are fermions with spin 1/2. Fermi–Dirac statistics is a part of the more general field of statistical mechanics and uses the principles of quantum mechanics.
Contents
History
Before the introduction of Fermi–Dirac statistics in 1926, understanding some aspects of electron behavior was difficult due to seemingly contradictory phenomena. For example, the electronic heat capacity of a metal at room temperature seemed to come from 100 times fewer electrons than were in the electric current.^{[3]} It was also difficult to understand why the emission currents, generated by applying high electric fields to metals at room temperature, were almost independent of temperature.
The difficulty encountered by the electronic theory of metals at that time was due to considering that electrons were (according to classical statistics theory) all equivalent. In other words it was believed that each electron contributed to the specific heat an amount on the order of the Boltzmann constant k. This statistical problem remained unsolved until the discovery of F–D statistics.
F–D statistics were first published in 1926 by Enrico Fermi^{[1]} and Paul Dirac.^{[2]} According to an account, Pascual Jordan developed in 1925 the same statistics which he called Pauli statistics, but it was not published in a timely manner.^{[4]} Whereas according to Dirac, it was first studied by Fermi, and Dirac called it Fermi statistics and the corresponding particles fermions.^{[5]}
F–D statistics was applied in 1926 by Fowler to describe the collapse of a star to a white dwarf.^{[6]} In 1927 Sommerfeld applied it to electrons in metals^{[7]} and in 1928 Fowler and Nordheim applied it to field electron emission from metals.^{[8]} Fermi–Dirac statistics continues to be an important part of physics.
Fermi–Dirac distribution
For a system of identical fermions, the average number of fermions in a singleparticle state i, is given by the Fermi–Dirac (F–D) distribution,^{[9]}
where k is Boltzmann's constant, T is the absolute temperature, is the energy of the singleparticle state i, and is the chemical potential. At T = 0 K, the chemical potential is equal to the Fermi energy. For the case of electrons in a semiconductor, is also called the Fermi level.^{[10]}^{[11]}
The F–D distribution is valid only if the number of fermions in the system is large enough so that adding one more fermion to the system has negligible effect on .^{[12]} Since the F–D distribution was derived using the Pauli exclusion principle, which allows at most one electron to occupy each possible state, a result is that .^{[13]}
Distribution of particles over energy
The above Fermi–Dirac distribution gives the distribution of identical fermions over singleparticle energy states, where no more than one fermion can occupy a state. Using the F–D distribution, one can find the distribution of identical fermions over energy, where more than one fermion can have the same energy.^{[15]}
The average number of fermions with energy can be found by multiplying the F–D distribution by the degeneracy (i.e. the number of states with energy ),^{[16]}
When , it is possible that since there is more than one state that can be occupied by fermions with the same energy .When a quasicontinuum of energies has an associated density of states (i.e. the number of states per unit energy range per unit volume ^{[17]}) the average number of fermions per unit energy range per unit volume is,
where is called the Fermi function and is the same function that is used for the F–D distribution ,^{[18]}so that,
 .
Quantum and classical regimes
The classical regime, where Maxwell–Boltzmann (M–B) statistics can be used as an approximation to F–D statistics, is found by considering the situation that is far from the limit imposed by the Heisenberg uncertainty principle for a particle's position and momentum. Using this approach, it can be shown that the classical situation occurs if the concentration of particles corresponds to an average interparticle separation that is much greater than the average de Broglie wavelength of the particles,^{[19]}
where h is Planck's constant, and m is the mass of a particle.
For the case of conduction electrons in a typical metal at T=300K (i.e. approximately room temperature), the system is far from the classical regime since . This is due to the small mass of the electron and the high concentration (i.e. small ) of conduction electrons in the metal. Thus F–D statistics is needed for conduction electrons in a typical metal.^{[19]}
Another example of a system that is not in the classical regime is the system that consists of the electrons of a star that has collapsed to a white dwarf. Although the white dwarf's temperature is high (typically T=10,000K on its surface^{[20]}), its high electron concentration and the small mass of each electron, precludes using a classical approximation, and again F–D statistics is required.^{[6]}
Two derivations of the Fermi–Dirac distribution
Derivation starting with canonical distribution
Consider a manyparticle system composed of N identical fermions that have negligible mutual interaction and are in thermal equilibrium.^{[12]} Since there is negligible interaction between the fermions, the energy E_{R} of a state R of the manyparticle system can be expressed as a sum of singleparticle energies,
where n_{r} is called the occupancy number and is the number of particles in the singleparticle state r with energy . The summation is over all possible singleparticle states r.
The probability that the manyparticle system is in the state R, is given by the normalized canonical distribution,^{[21]}
where = 1 / kT, k is Boltzmann's constant, T is the absolute temperature, e^{} is called the Boltzmann factor, and the summation is over all possible states R' of the manyparticle system. The average value for an occupancy number is^{[21]}
Note that the state R of the manyparticle system can be specified by the particle occupancy of the singleparticle states, i.e. by specifying so that
and the equation for becomes
where the summation is over all combinations of values of which obey the Pauli exclusion principle, and n_{r} = 0 or 1 for each r. Furthermore, each combination of values of satisfies the constraint that the total number of particles is N,
 .
Rearranging the summations,
where the ^{(i)} on the summation sign indicates that the sum is not over n_{i} and is subject to the constraint that the total number of particles associated with the summation is N_{i} = N − n_{i} . Note that Σ^{(i)} still depends on n_{i} through the N_{i} constraint, since in one case n_{i} = 0 and Σ^{(i)} is evaluated with N_{i} = N, while in the other case n_{i} = 1 and Σ^{(i)} is evaluated with N_{i} = N − 1. To simplify the notation and to clearly indicate that Σ^{(i)} still depends on n_{i} through N − n_{i} , define
so that the previous expression for can be rewritten and evaluated in terms of the Z_{i},
The following approximation^{[22]} will be used to find an expression to substitute for Z_{i}(N) / Z_{i}(N − 1) .
where
If the number of particles N is large enough so that the change in the chemical potential is very small when a particle is added to the system, then ^{[23]} Taking the base e antilog^{[24]} of both sides, substituting for , and rearranging,
 .
Substituting the above into the equation for , and using a previous definition of to substitute 1 / kT for , results in the Fermi–Dirac distribution.
Derivation using Lagrange multipliers
A result can be achieved by directly analyzing the multiplicities of the system and using Lagrange multipliers.^{[25]}
Suppose we have a number of energy levels, labeled by index i, each level having energy ε_{i} and containing a total of n_{i} particles. Suppose each level contains g_{i} distinct sublevels, all of which have the same energy, and which are distinguishable. For example, two particles may have different momenta (i.e. their momenta may be along different directions), in which case they are distinguishable from each other, yet they can still have the same energy. The value of g_{i} associated with level i is called the "degeneracy" of that energy level. The Pauli exclusion principle states that only one fermion can occupy any such sublevel.
The number of ways of distributing n_{i} indistinguishable particles among the g_{i} sublevels of an energy level, with a maximum of one particle per sublevel, is given by the binomial coefficient, using its combinatorial interpretation
For example, distributing two particles in three sublevels will give population numbers of 110, 101, or 011 for a total of three ways which equals 3!/(2!1!). The number of ways that a set of occupation numbers n_{i} can be realized is the product of the ways that each individual energy level can be populated:
Following the same procedure used in deriving the Maxwell–Boltzmann statistics, we wish to find the set of n_{i} for which W is maximized, subject to the constraint that there be a fixed number of particles, and a fixed energy. We constrain our solution using Lagrange multipliers forming the function:
Using Stirling's approximation for the factorials, taking the derivative with respect to n_{i}, setting the result to zero, and solving for n_{i} yields the Fermi–Dirac population numbers:
By a process similar to that outlined in the MaxwellBoltzmann statistics article, it can be shown thermodynamically that and where μ is the chemical potential, k is Boltzmann's constant and T is the temperature, so that finally, the probability that a state will be occupied is:
See also
References
 Reif, F. (1965). Fundamentals of Statistical and Thermal Physics. McGraw–Hill. ISBN 9780070518001.
 Blakemore, J. S. (2002). Semiconductor Statistics. Dover. ISBN 9780486495026. http://books.google.com/?id=cc4HE2YM1FIC&pg=front.
 Kittel, Charles (1971). Introduction to Solid State Physics (4th ed.). New York: John Wiley & Sons. ISBN 0471142867. OCLC 300039591.
Footnotes
 ^ ^{a} ^{b} Fermi, Enrico (1926). "Sulla quantizzazione del gas perfetto monoatomico" (in Italian). Rendiconti Lincei 3: 145–9., translated as Zannoni, Alberto (transl.) (19991214). "On the Quantization of the Monoatomic Ideal Gas". arXiv:condmat/9912229 [condmat.statmech].
 ^ ^{a} ^{b} Dirac, Paul A. M. (1926). "On the Theory of Quantum Mechanics". Proceedings of the Royal Society, Series A 112 (762): 661–77. Bibcode 1926RSPSA.112..661D. doi:10.1098/rspa.1926.0133. JSTOR 94692.
 ^ (Kittel 1971, pp. 249–50)
 ^ "History of Science: The Puzzle of the Bohr–Heisenberg Copenhagen Meeting". ScienceWeek (Chicago) 4 (20). 20000519. OCLC 43626035. http://scienceweek.com/2000/sw000519.htm. Retrieved 20090120.
 ^ Dirac, Paul A. M. (1967). Principles of Quantum Mechanics (revised 4th ed.). London: Oxford University Press. pp. 210–1. ISBN 9780198520115. http://books.google.com/?id=XehUpGiM6FIC&pg=PA210.
 ^ ^{a} ^{b} Fowler, Ralph H. (December 1926). "On dense matter". Monthly Notices of the Royal Astronomical Society 87: 114–22. Bibcode 1926MNRAS..87..114F.
 ^ Sommerfeld, Arnold (19271014). "Zur Elektronentheorie der Metalle". Naturwissenschaften 15 (41): 824–32. Bibcode 1927NW.....15..825S. doi:10.1007/BF01505083.
 ^ Fowler, Ralph H.; Nordheim, Lothar W. (19280501). "Electron Emission in Intense Electric Fields" (PDF). Proceedings of the Royal Society A 119 (781): 173–81. Bibcode 1928RSPSA.119..173F. doi:10.1098/rspa.1928.0091. JSTOR 95023. http://rspa.royalsocietypublishing.org/content/119/781/173.full.pdf.
 ^ (Reif 1965, p. 341)
 ^ (Blakemore 2002, p. 11)
 ^ Kittel, Charles; Kroemer, Herbert (1980). Thermal Physics (2nd ed.). San Francisco: W. H. Freeman. pp. 357. ISBN 9780716710882. http://books.google.com/?id=c0R79nyOoNMC&pg=PA357.
 ^ ^{a} ^{b} (Reif 1965, pp. 340–2)
 ^ Note that is also the probability that the state i is occupied, since no more than one fermion can occupy the same state at the same time and .
 ^ (Kittel 1971, p. 245, Figs. 4 and 5)
 ^ These distributions over energies, rather than states, are sometimes called the Fermi–Dirac distribution too, but that terminology will not be used in this article.
 ^ Leighton, Robert B. (1959). Principles of Modern Physics. McGrawHill. pp. 340. ISBN 9780070371309.
Note that in Eq. (1), and correspond respectively to and in this article. See also Eq. (32) on p. 339.  ^ (Blakemore 2002, p. 8)
 ^ (Reif 1965, p. 389)
 ^ ^{a} ^{b} (Reif 1965, pp. 246–8)
 ^ Mukai, Koji; Jim Lochner (1997). "Ask an Astrophysicist". NASA's Imagine the Universe. NASA Goddard Space Flight Center. Archived from the original on 20090120. http://www.webcitation.org/5dxyeNTKh.
 ^ ^{a} ^{b} (Reif 1965, pp. 203–6)
 ^ See for example, Derivative  Definition via difference quotients, which gives the approximation f(a+h) ≈ f(a) + f '(a) h .
 ^ (Reif 1965, pp. 341–2) See Eq. 9.3.17 and Remark concerning the validity of the approximation.
 ^ By definition, the base e antilog of A is e^{A}.
 ^ (Blakemore 2002, pp. 343–5)
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FermiDirac statistics — /ferr mee deuh rak /, Physics. quantum statistics defining the possible arrangements of particles in a given system in terms of the exclusion principle. Also, Fermi statistics. Cf. fermion. [1925 30; named after E. FERMI and A. M. DIRAC] * * * In … Universalium
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FermiDirac statistics — /ˌfɜmi dəræk stəˈtɪstɪks/ (say .fermee duhrak stuh tistiks), /ˌfɛəmi / (say .fairmee ) noun the branch of quantum statistics used with systems of identical particles whose wave function changes sign if any two particles are interchanged. {from… … Australian English dictionary
FermiDirac statistics — noun (physics) law obeyed by a systems of particles whose wave function changes when two particles are interchanged (the Pauli exclusion principle applies) • Topics: ↑physics, ↑natural philosophy • Hypernyms: ↑law, ↑law of nature … Useful english dictionary
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