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1932 

Multi-time formalism
Dirac, Fock, and Podolsky formulate the so-called "multi-time formalism," the most popular form of the description of a field with particles until the appearance of Schwinger's papers.

P.A.M. Dirac, V.A. Fock, B. Podolsky, "On quantum electrodynamics," Phys. Zeits. Sowjetunion 2 (1932); 468-479.
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1934 

Fock functionals
Fock develops a method for describing a system with an indeterminate number of particles by means of a generating functional, a method now known as the method of Fock functionals. The first application (in quantum electrodynamics) of the apparatus of variational differentiation.

V. Fock, "Zur Quantenelektrodynamik," Phys. Zeits. Sowjetunion 6 (1934): 425-469.
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May 27, 1935 - Feb 04, 1937

The order parameter
Beginning with a paper on anomalous specific heats in lambda transitions, Lev Landau systematizes the notion of the "order parameter" (first employed by Felix Bloch). In his general theory of phase transitions Landau employs the order parameter as a measure of the degree of order in a physical system. The reliance on symmetries as indicators of common characteristics of otherwise distinct phenomena subsequently becomes an indispensible tool of the theory of critical phenomena.

L. Landau, "Zur Theorie der Anomalien der spezifischen Waerme," Phys. Zeits. Sowjetunion 8 (1935): 113-118.

L. Landau, "Zur Theorie der Phasenumwandlungen. I, II," Phys. Zeits. Sowjetunion 11 (1937): 26-47, 545-555; "K teorii fazovykh perekhodov I, II," ZhETF 7 (1937): 19-32, 627-632.
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1941 

2-D Ising model exhibits phase transition
Kramers and Wannier are the first to locate a transition point in the two-dimensional Ising model of the ferromagnet. Montroll shows that the lattice must be infinite in at least two dimensions in order for such a transition to take place. Wannier (Montroll's teacher) serves as the intermediary between these independent sets of calculations. In a sense these are classic "precursor" papers, since neither attempted an actual solution to the partition function. But they continue to be read even after the appearance of the Onsager solution. Kadanoff, for example, employed their transfer matrix method in his 1966 paper providing a theoretical justification for scaling. Are there other cases in which their techniques were retained and further exploited?

Primary: H. A. Kramers and G. H. Wannier, "Statistics of the two- dimensional ferromagnet. I, II," Phys. Rev. 60 (1941): 252- 262, 263-276. E. Montroll, "Statistical mechanics of nearest neighbor systems," J. Chem. Phys. 9 (1941): 706-721.

Secondary: Brush 1983, 244. Domb 1996, 132. Hoddeson et al. 1992, 528.
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1944 

The Onsager solution to the Ising model
The canonical paper offering an exact solution to the two- dimensional Ising model of a ferromagnet. This is usually taken as a moment of crisis for classical mean field theory, since the specific heat for the Onsager solution was logarithmically infinite rather than merely discontinuously finite, and the partition function was non- analytic at the critical temperature. Though a virtuoso calculation, the Onsager solution was based on a very abstract model far from laboratory measurement. Extending it to a physical three dimensions seemed out of the question at the time, but for obvious reasons it became the gold standard for comparison with non-classical examples of critical behavior. Perturbation series expansions by Domb, Sykes, Fisher, and others subsequently became an especially fruitful domain of calculation in which reference to the Onsager solution was crucial to developing adequate models. One might also ask, what was the relation of the Onsager solution to the eventual development of non-perturbative approaches to critical phenomena? Where and when did the Onsager solution take on heuristic significance of the sort it could not claim when it was first published?
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1949 - 1952

Spontaneous magnetization of planar square lattice
In the two-dimensional Ising model the spontaneous magnetization is equivalent to the long-range correlation function [theta] evaluated at infinity. Onsager somewhat cryptically announced the exact formula for [theta] at a conference in Florence in 1949, while Yang employed perturbation theory in an explicit demonstration. The formula implies that the critical exponent [beta] = 1/8, in marked contrast to the [beta] = 1/2 value for all classical theories. One might term this a "practical" result of Onsager's theory, but it bore little relation to contemporary experimental values for magnetic materials.

Primary: L. Onsager, "Discussion remark," Nuovo Cimento, supplement, 6 (1949): 249. C. N. Yang, "The spontaneous magnetization of a two- dimensional Ising model," Phys. Rev. 85 (1952): 808-816. C. H. Chang, "The spontaneous magnetization of a two- dimensional rectangular Ising model," Phys. Rev. 88 (1952): 1422.

Secondary: Brush 1983, 245. Domb 1996, 146. Fisher 1967, 670.
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1949 - 1959

Exact series expansions
For Ising ferromagnets in finite fields the challenge became to find reliable methods of successive approximation. Domb introduced and advocated the use of exact series expansions of the free energy. Provided one carefully demonstrated the regular and rapid convergence of the solutions, it was possible to obtain fairly accurate values for the critical exponents not only in the two- dimensional case, but for physically meaningful three- dimensional cases. With a center of gravity at King's College, Domb, Sykes, Fisher, Rushbrooke, Wakefield, and others exploited these techniques to their fullest extent, and by the mid-1960s their work started to gain the attention of other physicists who had originally come to study critical phenomena from quite different perspectives.
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1950 - 1964

Critical behavior of mixtures and pure fluids
Not only pure fluids exhibit critical behavior between phases. Mixtures of fluids can also exhibit critical behavior between the two components, as Zimm, Gopal, Rice, and Thompson demonstrated with increasing accuracy, measuring coexistence curves quite close to the critical point for CCl4-C7F14 systems.

Primary: B. H. Zimm, "Opalescence of a two-component liquid system near the critical mixing point," J. Phys. Colloid Chem. 54 (1950): 1306-1317. R. Gopal and O. K. Rice, "Shape of the coexistence curve in the perfluoromethylcyclohexane-carbon tetrachloride system," J. Chem. Phys. 23 (1955): 2428-2431. D. R. Thompson and O. K. Rice, "Shape of the coexistence curve in the perfluoromethylcyclohexane-carbon tetrachloride system. II. Measurements accurate to 0.0001°," J. Amer. Chem. Soc. 86 (1964): 3547-3553.

Secondary: Brush 1983, 249. Heller 1967, 759.
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1952 

Spherical model of a ferromagnet
Let the spin values of the Ising ferromagnet take on continuous values, and the partition function becomes a tractable integral. Berlin and Kac constrained the sum of the squares of the spin values to be equal to the number of lattice sites, and obtained an exactly soluble three- dimensional model that exhibited a phase transition in non- zero field. The model did not approximate any known physical systems, but it did later prove useful as a first step in Wilson's application of the renormalization group.

Primary: T. H. Berlin and M. Kac, "The spherical model of a ferromagnet," Phys. Rev. 86 (1952): 821-835.

Secondary: Domb 1996, 179. Hoddeson et al. 1992, 574.
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1952 

Lattice gas model of Yang and Lee
Yang and Lee lend further credibility to the lattice gas model, which they show can reproduce the crucial features of liquid-gas equilibrium.

Primary: C. N. Yang and T. D. Lee, "Statistical theory of equations of state and phase transitions. I. Theory of condensation," Phys. Rev. 87 (1952): 404-409.

C. N. Yang and T. D. Lee, "Statistical theory of equations of state and phase transitions. II. Lattice gas and Ising model," Phys. Rev. 87 (1952): 410-419.

Secondary: Brush 1983, 252. Domb 1996, 201.
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1953 

Sharpness of critical point: Optical density measurements of Lorentzen
When mapping out the curves experimentally for equations of state, it turns out that the shape of the isotherms can be remarkably sensitive to variations in density. Lorentzen arranged a vertical glass tube filled with CO2 at extremely stable temperatures so that slits at each end would define a cylindrical lens whose focal length depended on the fluid density. Near the critical point tiny differences in height could lead to large discrepancies in density. That most previous tests of equations of state did not take this density inhomogeneity into account meant that coexistence curves had been unduly flattened, implying that the critical transition was more gradual than it actually was.

Primary: H. L. Lorentzen, "Studies of critical phenomena in carbon dioxide contained in vertical tubes," Acta Chem. Scand. 7 (1953): 1335-1346.

Secondary: Brush 1983, 249 Heller 1967, 735
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1953 - 1954

Magnetic analogue of critical opalescence
Shoot a beam of neutrons at a magnetic material, and part of the observed scattering will be due to the interactions of the atomic magnetic moments with the moment of the neutron. Near the critical temperature region the scattering cross section increases sharply, the magnetic analogue of critical opalescence. Palevsky & Hughes and Squires first observe the effect in iron.

Primary: H. Palevsky and D. J. Hughes, "Magnetic inelastic scattering of slow neutrons," Phys. Rev. 92 (1953): 202-203. G. L. Squires, "The scattering of slow neutron by ferromagnetic crystals," Proc. Phys. Soc. 67A (1954): 248- 253.

Secondary: Heller 1967, 742.
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1954 - 1956

The Moscow Zero
Landau, Abrikosov, Khalatnikov, Pomeranchuk, and (independently) Fradkin publish papers in which the ultraviolet asymptotic forms of quantum electrodynamics are derived in the "leading-lograrithm" approximation.

Primary: L. D. Landau, A. A. Abrikosov, I. M Khalatnikov, "Asimptoticheskoe vyrazhenie dlia funktsii Grina elektrona v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 95 (1954): 773.

L.D. Landau, A. A. Abrikosov, I.M. Khalatnikov, "Asimptoticheskoe vyrazhenie dlia funktsii Grina fotona v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 95 (1954): 1177.

L.D. Landau, A. A. Abrikosov, I.M. Khalatnikov, "Massa elektrona v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 96 (1954): 261.

L.D. Landau and I.Ia. Pomeranchuk, "O tochechnom vzaimodeistvii v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 102 (1955): 489.

I.Ia. Pomeranchuk, "Ravenstvo nuliu perenormirovannogo zariada v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 103 (1955): 1005.

L.D. Landau, "Quantum Field Theory," in Niels Bohr and the Development of Physics (London, 1955), 52.

L.D. Landau, "Fundamental problems," in M. Fierz and V.F. Weisskopf, Theoretical Physics in the Twentieth Century: A Memorial Volume to Wolfgang Pauli (New York, 1960), 245-248.

E.S. Fradkin, "Ob asimptotike funktsii Grina v kvantovoi elektrodinamike," ZhETF 28 (1955): 750.

Secondary: V. B. Berestetskii, "Nul' zariad i asimptoticheskaia svoboda," Uspekhi fizicheskikh nauk 120 (1976): 439- 454; "Zero-charge and asymptotic freedom," Soviet Physics Uspekhi 19 (1976): 934-943.

D.A. Kirzhnits, "On the 'Moscow-Zero' Problem," in I. A. Batalin, C. J. Isham, and G. A .Vilkovisky, eds., Quantum Field Theory and Quantum Statistics: Essays in Honour of the Sixtieth Birthday of E S Fradkin (Bristol: Adam Hilger, 1987), vol. 1, 349-369.

E.L. Feinberg, "The Importance of (Sometimes) being Conservative," in ibid., vol. 2, 3-14.
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1954 - 1957

Lattice model applied to liquid helium
Kikuchi in Chicago and Matsubara and Matsuda in Kyoto developed the first lattice models which could be applied to liquid helium and gave plausibility arguments for the second-order phase transition long known experimentally.

Primary: R. Kikuchi, "[Lambda] transition of liquid helium," Phys. Rev. 96 (1954): 563-568. T. Matsubara and H. Matsuda, "A lattice model of liquid helium, I," Prog. Theoret. Phys. 16 (1956): 569-582. H. Matsuda and T. Matsubara, "A lattice model of liquid helium, II," Prog. Theoret. Phys. 17 (1957): 19-29. H. Matsuda, "A lattice model of liquid helium, III," Prog. Theoret. Phys. 18 (1957): 357-366.

Secondary: Brush 1983, 194.
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1955 

Functional renormalization-group equations of QED
Bogoliubov and Shirkov derive functional renormalization- group equations of quantum electrodynamics for the general case. The relationship between the work of Stueckelberg and Petermann (1953) and Gell-Mann and Low (1954) is established. Differential group equations are constructed for the first time, and a program for systematically improving the results of ordinary perturbation theory is formulated.

N. Bogoliubov and D. Shirkov, "O renormalizatsionnoi gruppe v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 103 (1955): 203; "Primenenie renormalizatsionnoi gruppy k uluchsheniiu formul teorii vozmushchenii," Dokl. Akad. Nauk SSSR 103 (1955): 391; "Gruppa mul'tiplikativnykh perenormirovok v kvantovoi teorii polia," ZhETF 30 (1956): 77.

N.N. Bogoliubov and D.V. Shirkov, "Voprosy kvantovoi teorii polia," Uspekhi fizicheskikh nauk 55 (1955): 149-214; 57 (1955): 1-92.
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1955 

Temperature Green's functions
Feynman diagrams are an extraordinarily powerful way to calculate Green's functions for individual particles in quantum field theory, but to carry over diagram techniques usefully into statistical physics, one eventually has to be able to introduce non-zero temperatures. Takeo Matsubara suggested one highly effective way to do so by constructing so-called "temperature Green's functions," which do not depend on time t, but rather on a fictitious "imaginary time" [tau] = it. This made it easier to calculate the partition function in close parallel with the techniques one would use to obtain the vacuum expectation values of the S-matrix in quantum field theory.

Primary: T. Matsubara, "A new approach to quantum statistical mechanics," Prog. Theoret. Phys. 14 (1955): 351-378.

Secondary: Hoddeson et al. 1992, 582.
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1955 

Microscopic causality condition for scattering matrix
Bogoliubov introduces an explicit formulation of the microscopic causality condition for the scattering matrix expressed in terms of its variational derivatives.

N.N. Bogoliubov, Izv. Akad. Nauk SSSR Ser. fiz. 19 (1955): 237 [Bull. Acad. Sci. USSR. Phys. Ser. 19 (1955): 215].
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1955 

Functional renormalization-group equations of QED
Bogoliubov and Shirkov derive functional renormalization-group equations of quantum electrodynamics for the general case. The relationship between the work of Stueckelberg and Petermann (1953) and Gell-Mann and Low (1954) is established. Differential group equations are constructed for the first time, and a program for systematically improving the results of ordinary perturbation theory is formulated.

N. Bogoliubov and D. Shirkov, "O renormalizatsionnoi gruppe v kvantovoi elektrodinamike," Dokl. Akad. Nauk SSSR 103 (1955): 203; "Primenenie renormalizatsionnoi gruppy k uluchsheniiu formul teorii vozmushchenii," Dokl. Akad. Nauk SSSR 103 (1955): 391; "Gruppa mul'tiplikativnykh perenormirovok v kvantovoi teorii polia," ZhETF 30 (1956): 77.

N.N. Bogoliubov and D.V. Shirkov, "Voprosy kvantovoi teorii polia," Uspekhi fizicheskikh nauk 55 (1955): 149-214; 57 (1955): 1-92.
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1956 - 1963

R-operations
Bogoliubov, Parasiuk, and Stepanov develop R-operations demonstrating self-consistency of perturbation theory in QFT.

N.N. Bogoliubov, O.S. Parasiuk, "Ueber die Multiplikation der Kausal Funktionen in der Quantentheorie der Felder," Acta Mathematica 97 (1957): 227; "Umnozhenie prichinnykh funktsii pri nesovpadaiushchikh argumentakh," Izv. Akad. Nauk SSSR ser. mat. 20 (1956): 843.

B.M. Stepanov, "Abstraktnaia teoriia R-operatsii," Izv. AN SSSR, ser. mat. 27 (1963): 819.
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Sep 1956 - 1958

Dispersion relations
Causality, Lorentz invariance, spectral conditions, unitarity, and other conditions for the axiomatic foundations of QFT are shown by Bogoliubov using dispersion relations for pion-nucleon scattering. Generalizations of dispersions relations by Bogoliubov, Vladimirov, Logunov, Stepanov, Tavkhelidze.

N.N. Bogoliubov, paper in Seattle (1956).

N.N. Bogoliubov, B.V. Medvedev, and M. K. Polivanov, Voprosy teorii dispersionnykh sootnoshenii (Moscow, 1958).

N.N. Bogoliubov and V.S. Vladimirov, "Ob analiticheskom prodolzhenii obobshchennykh funktsii," Izv. Akad. Nauk SSSR ser. mat. 22 (1958): 15.

V.S. Vladimirov and A.A. Logunov, "Dokazatel'stvo nekotorykh dispersionnykh sootnoshenii v kvantovoi teorii polia," Izv. AN SSSR ser. mat. 23 (1959): 661.

A.A. Logunov and B.M. Stepanov, "Dispersionnye sootnosheniia dlia reaktsii fotorozhdeniia pi-mezonov," Dokl. Akad. Nauk SSSR 110 (1956): 368; 117 (1957): 492.

A.A. Logunov, I.T. Todorov, "On the proof of dispersion relations for inelastic scattering," Nucl. Phys. 10 (1959): 552.

A.A. Logunov and A.N. Tavkhelidze, "Generalised dispersion relations," Nuovo Cim. 10 (1958): 913.
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