IN MEMORIAM

# Richard Alan Ferrell

##### This memorial obituary was originally published in *Physics Today* (2006) here, written by Joshep Sucher, Douglas Scalapino and Richard Prange.

Richard Alan Ferrell, Professor Emeritus of Physics at the University of Maryland, College Park, died on November 14, 2005 after a four-year struggle with multiple myeloma. He was 79. During his long career, Ferrell made major contributions to theory, especially condensed matter and statistical physics.

Ferrell was born in Santa Ana, California in 1926. After service in the Navy, he attended the California Institute of Technology, receiving BA and MA degrees. His 1952 doctorate at Princeton was under Arthur Wightman. Post-doctoral work with Werner Heisenberg followed. In 1953 John Toll invited him to the nascent Maryland physics department where he became full professor in 1959.

Ferrell was a theorist of unusual breadth and interests, from QED to nuclear physics to condensed matter. The number of experimental results his theories explained and predicted, sometimes to be verified far into the future, is astonishing. His earliest work computed the fine structure of positronium, then recently discovered. He did pioneering calculations of positron annihilation in solids and his 1957 Reviews of Modern Physics article remains a standard reference. The positron is dynamically screened in a metal: the problem requires ideas equivalent to the Lindhardt dielectric function, work then unknown to Americans. Student John Quinn worked on electron self-energies in metals using this methodology. The zero of the dielectric function gives the plasmon. With Ed Stern, an experimental colleague, Ferrell found, at first controversially but soon experimentally verified, that a 20Å oxide coating on a thin film of Al lowered the surface plasmon energy and explained the energy loss spectrum of electrons passing through the film. The plasmon corresponds to the giant dipole resonance in nuclei and his student Stavros Fallieros studied that problem. Among several other distinguished students was Peter Fulde, whose thesis addressed superconductors with a strong spin exchange field. This `Ferrell-Fulde’ phase was recently observed in CeCoIn 5.

A collaboration with experimentalists Rolfe Glover III and Michael Tinkham resulted in the famous FGT sum rule. It asserts that the finite frequency response which is lost at the superconducting transition, reappears as the zero frequency `superconductivity’ delta-function. This is still very important for recent high-T C superconductivity analyses.

In 1966, Ferrell teamed with junior visitors from Europe, Nora Menyhard, Peter Szepfalusy, Hartwig Schmidt and Franz Schwabl. They produced important ideas which marked the birth of the field of statistical physics now called dynamical scaling theory for critical phenomena. These papers have been referred to as the work of “Ferrell and the United Nations.” The basic insight was that not only static quantities, like susceptibilities, but dynamical quantities such as the thermal conductivity, diverge at the phase transition and obey scaling laws. They predicted that on approaching the superfluid transition temperature of helium, from above the thermal conductivity would diverge, while from below, the second-sound damping would diverge. The experimental verification soon followed.

Much of Ferrell’s subsequent research stemmed from these insights. He came to feel that to confront theory and experiment most stringently, one should focus on quantities with weak singularities. One such is the shear viscosity. Experimentalists, many from the Maryland-DC area, worked for a quarter century on this problem in interaction with Ferrell. These include Jan Sengers, Herschel Burstyn, Robert Gammon, Sandra Greer, Michael Moldover, and Robert Berg. Theory collaborators include Douglas Scalapino, Allan Bray and especially Jay Bhattacharjee.

An example: The frequency of a vibrating wire viscometer can match the decay rate of fluctuations very close to fluid criticality. In 1980 Ferrell predicted an associated viscoelastic effect. It took 15 years for experimental verification, of necessity in the space shuttle. Among other things, an exponent of 0.0690±0.0006 was found. Ferrell and Bhattacharjee then did a three loop calculation, the only one for a dynamic exponent, which yielded a value of 0.0679±0.0007. This, Ferrell’s final paper, appeared less than a year before his death. He, however, remained sharp and active until the very end.

Many tales are told about Ferrell’s adventurous trips with his family, about the famous Vermont ski house that he designed and built himself, (with help from sometimes unsuspecting students and colleagues), about his passion for nature and beautification of campus and community. He made a point of learning the language of a country whenever he made an extended visit: in 1967-8 he visited both France and Russia, and so in Paris he simultaneously learned both French and Russian.

Richard Ferrell was a glory to his family, to his friends, to the University and community, to his profession, and indeed, to the world at large.

https://doi.org/10.1063/PT.4.2323

Ferrell served as Trustee at Aspen Center for Physics from 1968-1972.

## Positions Held

Trustee, 1968-1972