Obituary of Albert Overhauser (1925-2011)

Albert Warner Overhauser, the Stuart Distinguished Professor Emeritus of Physics at Purdue University passed away on December 10, 2011 in West Lafayette, Indiana, felled by a sudden cardiac arrest. Only the day before he had as customary manned his departmental office of thirty eight years, retirement notwithstanding. One of the last of a great generation of American physicists to command a broad range of interests, over the span of his long career Overhauser contributed a wealth of highly innovative and fundamental ideas that have made and continue to make a significant impact on science and technology.

Overhauser was born in San Diego, California, on August 17, 1925, the son of a US Navy aviation technician. A precocious musical talent, having seen a career in the music business nipped in the bud by familial diktat, he entered The University of California, Berkeley, where in 1948 he received his Magna Cum Laude Bachelor of Science in Physics and in Mathematics, not without a forming interlude as a radar technical specialist in the US Navy Reserve during the Pacific war years. While his young walk across the new Golden Gate Bridge on opening day had stoked early interests in civil engineering, once finished mending circuits and descended from his last mast at sea, it were Berkeley’s “most wonderful professors” of the time who lit the passion for Physics that was to remain with him for the rest of his life. That condensed matter Physics was to be his main playground was however entirely due to serendipity, for Overhauser’s original aspirations to earn a Ph.D. in nuclear Physics at Berkeley were dashed when his advisor, Giancarlo Wick decided to leave the university as a consequence of the notorious loyalty oath fiasco. It was thus that he became one of the earliest students of Charles Kittel’s who was at the time in the process of moving to Berkeley from Bell Laboratories. During one of his visits Kittel assigned to Overhauser his thesis subject: a thorough study of the possible spin relaxation mechanisms in metals. Astonishingly the task was successfully completed upon Kittel’s return a few months later and Overhauser earned his Ph.D. in 1951, merely three years after receiving his Bachelor’s degree.

From 1951 to 1953 Overhauser’s academic career continued as a research associate at the University of Illinois, Urbana-Champaign, and then as a faculty member at Cornell University where he remained until 1958. It was then that Overhauser was recruited to be a key scientist and eventually the director of the Physical Sciences Laboratory of the Ford Motor Company. At Ford Overhauser alternated research on modern theoretical topics with successful forays in optimization and industrial design. Finally in 1973 he left Ford for the more academic and quaint setting of Purdue University. This was however not to be his last academic career step for, to his surprise, he was casually informed one year later that he had in fact earned his tenure at Purdue and thus named the Stuart Distinguished Professor of Physics.

Overhauser original thesis work was only the beginning of a series of fundamental and uniquely creative contributions to Physics that have characterized his career; many of his publications, all characterized by his signature clarity and simplicity, are classics.

Perhaps his most famous and important discovery is that of the effect that bears his name: this stroke of genius occurred to him in 1953 while he was still a young research associate at Urbana, at the time and quite appropriately a hotbed of magnetic resonance. It was while considering the problem of a spin system out of equilibrium that Overhauser devised the astonishing paradigm that underlies the Overhauser effect. His idea was to polarize the nuclear spins by the indirect way of pumping and saturating the conduction electrons spin resonance. The effect was spectacular in that the achieved nuclear polarization was thousands of times larger than one could have naively expected from the strength of the Fermi nuclear coupling constant. Exemplifying a notable feature of his style, the theory was based on a physically and mathematically simple model. This was to be only the first example of the subtlety and incisiveness of his Occam razor. In spite of some initial skepticism the dynamical Overhauser effect (DOE) was quickly experimentally established by Charles P. Slichter and his student Richard E. Norberg also at Urbana. More recently the mechanism of the DOE has been shown by Ionel Solomon at the Centre National de la Recherche Scientifique, Paris, to work also between the spins of two nuclear species in ordinary liquids. This effect takes the name of nuclear Overhauser effect (NOE). As it turns out both DOE and NOE have proven invaluable mechanisms to develop ever more powerful nuclear magnetic resonance techniques which are currently used to imaging and determining the structure of proteins and in general biological macromolecules in solution. Countless books have been written on the subject.

Overhauser’s most important contribution while at Cornell was the development, together with his student Bertram G. Dick, of the “shell model” for the dynamics of bound electrons, a veritable theoretical breakthrough which allowed a tenfold improvement in the parameterization and the quantitative understanding of lattice dynamics in semiconductors. The theory proved successful in a vast variety of non metallic systems and is still employed today in conjunction with modern ab initio and molecular dynamics techniques, the original paper representing a massively cited reference.

While at Ford, Overhauser contributed in 1960 some of the pioneering work in the many-body theory of the electron gas by introducing the modern concepts of spin- and charge-density-waves (SDW and CDW) which represent spontaneously broken symmetry states in an otherwise uniform and isotropic interacting electron system. Both were immediately observed in Cr, and later in many other materials. That the ground states of some of the alkali metals would one day reveal a CDW type of ground state is a theme to which Overhauser often returned later in his career. In the same period he also produced what is referred to as the Hartree-Fock (HF) instability theorem which shows how the popular Slater determinant of plane waves, while being a solution of the HF equations, is not a stable ground state in this approximation.

Finally a fundamental and extremely imaginative contribution was the successful conception and realization of a unique experiment, one in which the intriguing interplay of both quantum mechanics and gravitation is vividly probed. This elegant and technically challenging work, commonly referred to as the COW experiment, was carried out in collaboration with Roberto Colella (Purdue) and Samuel A. Werner (then also at Ford). It consists in the observation of the effect of a tiny change in the gravitational potential of a neutron beam on its self interference pattern, an effect that in modern terminology can be described in terms of a gravitationally induced Berry phase. The feat was achieved by machining a unique neutron interferometer out of a Si single crystal. Overhauser not only contributed to the theoretical conception but also actively took part in the experimental realization to the point that even felt strips of his home pool table made their way into the apparatus.

For the variety, relevance and creativity of his contributions Overhauser was bestowed membership in the National Academy of Science as well as in many others, he was presented with numerous honorary degrees, and was a winner of a number of prestigious awards including the 1975 Oliver E. Buckley Prize of the American Physical Society, the 2009 Russell Varian Prize by the European Magnetic Resonance Congress and the 1994 National Medal of Science.

Alongside his research work Overhauser was widely known as a superb teacher and advisor. His lectures were always followed by enthralled students and colleagues. In particular anecdotes abound about his legendary condensed matter and statistical mechanics lectures, notes and creative homework sets. Besides all throughout his long career Overhauser was a most valuable colleague and advisor to countless students. At Purdue his expert advice on quantum mechanics was sought by a crowd of science and engineering students and faculty, his door being always welcomingly open to all as it was just the day before his passing.

While many of us lost a most distinguished colleague, a teacher and a friend, the intellectual reverberations and practical relevance of Overhauser’s fertile mind still shine.