Herbert B. CALLEN (1920 - 1993) was a distinguished theoretical physicist best known for his works in thermodynamics, statistical physics and statistical mechanics. He belong to the thermodinamicists, whose works contributed substantially to the statistical thermodynamics and many-particle physics.
Herbert B. Callen, was professor of physics at Randal Morgan Laboratory of Physics, Department of Physics, University of Pennsylvania, Philadelphia.
Dr. Herbert B. Callen was a member of the physics faculty since 1948 and was former chair of the Faculty Senate in 1970-71.
Herbert Callen studied physics at Temple University. He took his Ph.D. at Department of Physics, Massachusetts Institute of Technology (1948). Callen was advised in graduate school by Laszlo Tisza.

In the early 1950s, Dr. Callen made a series of important research on nonequilibrium thermodynamics and the theory the response properties of condensed-matter systems, in particular the understanding of electron transport and conductivity, noise and fluctuation. In his remarkable works he established the so-called fluctuation-dissipation theorem helped explain how electricity flows. In 1951 Professor Herbert B. Callen published an article (with T. Welton) entitled: "Irreversibility and Generalized Noise", Phys. Rev., 83 (1951) pp.34-40. This paper became CITATION CLASSIC.

Herbert Callen contributed a lot in the quantum theory of magnetism. He published many important works, e.g.:

H. B. Callen, Green Function Theory of Ferromagnetism. Phys.Rev. v. 130, 890 (1963).

H. B. Callen and S. Shtrikman, A Probability Density Common to Molecular Field and Collective Excitation Theories of Ferromagnetism. Solid State Comm. v.3, 5 (1965).

In particular, in his paper 1963, he invented an interpolating scheme for decoupling of the infinite chain of the double-time thermodynamic Green functions. His scheme gives a generalization of well-known Tyablikov approximation. In fact, the Tyablikov approximation does not take into account the inelastic quasiparticle's scattering processes.
One should also mention that within the Tyablikov approximation the exact commutation relations
[S^{+}_{i},S^{-}_{j}]_{-} = 2S^{z}_{i} \delta_{ij}
are replaced by approximate relationships of the form
[S^{+}_{i},S^{-}_{j}]_{-} \simeq 2 < S^{z} > \delta_{ij}.
Despite being simple, the Tyablikov approximation is widely used in different problems even at the present time.
Callen proposed a modified version of the Tyablikov approximation, which takes into account some correlation effects. He proposed a linearization of equations-of- motion which is now termed by Callen approximation. Thus, the Callen approximation has an interpolating character. Depending on the choice of the value for the special parameter \alpha, one can obtain both positive and negative corrections to the Tyablikov approximation, or even almost vanishing corrections. The particular case \alpha = 0 corresponds to the Tyablikov approximation.
It is important to stress that the Callen approach is by no means rigorous. Moreover, it has serious drawbacks. However, one can consider this approximation as the first serious attempt to construct an approximating interpolation scheme in the framework of the Green function equations-of-motion method.
See for details:
A.L. Kuzemsky,
Statistical Mechanics and the Physics of Many-Particle Model Systems.
Physics of Particles and Nuclei, (2009), V.40, P.949-997.

A Guggenheim Fellow and fellow of the National Academy of Sciences, he received the Elliott Cresson Medal of the Franklin Institute in 1984 in recognition of lifelong contributions including work on the Manhattan Project during World War II.
He was also active on the national and international scene, as national chairman of the American Professors for Peace in the Middle East and as a visiting professor at Hebrew University, the Weizmann Institute and the University of Recife in Brazil.

Dr. Herbert B. Callen, emeritus professor of physics died in May 1993 at the age of 73. A member of the physics faculty since 1948, Dr. Callen had taken early retirement in 1982 on discovering that he had Alzheimer’s disease. He died at the home of his daughter, Jill Bressler, where he was being cared for by the family including his wife of 48 years, Sara Callen.
Dr. Callen is survived by his wife, Sara, an 1977 alumna of the School of Social Work; his daughter, Jill Bressler, a son, Jed, a brother, Earl, and five grandchildren.

Herbert B. Callen is the author of the book "Thermodynamics and an Introduction to Thermostatistics", which is the most frequently cited thermodynamic reference in physics research literature.

There are a few places where the biography of Herbert B. CALLEN can be found.

Wikipedia electronic Encyclopedia(http://en.wikipedia.org/) , an article H. B. CALLEN .