صور: البروفيسور عبد الجليل كرار في ذكرى الرحيل

In Memory of Abdel Galil Karrar Modawi
By

Professor Anthony J. Leggett (Physics Nobel Laureate 2003)
http://www.physics.uiuc.edu/people/Leggett/index.htm

It is with great sadness that I have learned of the death of Dr. Abdel Galil Karrar Modawi, who was certainly one of the most remarkable of the students I have had over my 40-year career in physics.

Abdel came to the University of Sussex to work for a D.Phil. (Ph.D.) degree in theoretical physics in, I think, 1975, and shortly thereafter selected me as his research advisor. Like many of my Sussex students, he spent several years trying out problems which in the end did not work out, but around, I think, the summer of 1980 we settled on the project which became the topic of his thesis. At that time the experimental groups in Amsterdam and MIT had just stabilized spin-polarized gaseous atomic hydrogen, and high hopes existed that it could be cooled into the quantum-degenerate regime (a goal which was in fact eventually realized, though only more than fifteen years later).There were similarly hopes that the fermionic isotope, deuterium, could be similarly cooled, and it seemed likely that in that case it would at some temperature undergo the Cooper-pairing phenomenon already known to occur for the electrons in super-conducting ####ls and the atoms in liquid 3-He,and thereby become super-fluid. Should this happen, the resulting system would be unique in at least two respects: it would be the only known dilute Fermi super-fluid, and the only one possessing triple spin degeneracy (the electrons would be totally spin-polarized and thus fall out of the problem, but the deuterium nucleus has spin 1 and thus three Zeeman substates, in contrast to the two occurring in superconductors and in 3-He).Thus I suggested to Abdel that he should try to predict some of the properties of this, at that time, hypothetical system.

Once he got involved in this problem, Abdel tackled it with enormous energy and initiative, completing his Ph.D. in little over a year. Viewed from the vantage point of 2006, his thesis is a remarkable document: he gives a complete treatment of the problem within the standard mean-field approximation, and derives a number of results peculiar to a system with triple spin degeneracy, in particular the vanishing of the so-called Gor'kov-Melik-Barkhudarov correction to the prefactor of the exponential in the BCS expression for the transition temperature, and the inevitable existence of a branch of gapless excitations. Both these results were rediscovered, and published, by others many years later. Even more remarkable is chapter 8 of the thesis, where Abdel proposes, and analyses in detail, a technique to raise the prima facie depressingly low transition temperature of spin-polarized atomic deuterium by exploiting the phenomenon of Feshbach resonance, concluding that "we therefore obtain from [the relevant equation] a transition temperature of the order of 1 mK: quite observable!". As is widely known, it is precisely by tuning through a Feshbach resonance that experimentalists have been able, over the last
couple of years, to obtain evidence for Cooper pairing in spin-polarized degenerate Fermi alkali gases and to study the so-called "BEC-BCS crossover". In all of this work, while I may have made one or two qualitative initial suggestions, my memory is that all the real work was done by Abdel himself, with my role being little more than that of a sounding-board for his ideas.

I was very impressed by the results Abdel had obtained, and when following his successful D.Phil. defence he went home to take up a faculty position at the University of Khartoum we agreed that he would write them up for publication. Unfortunately, as so often happens in this kind of situation, he found he had other more urgent tasks on his mind, and never got around to doing so.(In retrospect I should no doubt have pressed him harder on this; I think one reason I did not do so was that as time went on the prospects for cooling of spin-polarized deuterium into the relevant temperature regime became, for various technical reasons, bleaker and bleaker, so that it looked as if Abdel's results might be only of academic interest).

When twelve years later Abdel left Khartoum, I invited him to come to the University of Illinois (where I had myself moved in 1983) as a postdoctoral research associate, and he eventually spent two years with us in the mid-nineties. I strongly encouraged him to use the opportunity to write up his thesis results, but (being Abdel!) he found many other things to catch his interest, and the paper eventually appeared under our two names in the Journal of Low Temperature Physics in late 1997. By that time the phenomenon of Bose-Einstein condensation had been realized in the bosonic alkali gases, and people had begun to think seriously about the possibility of observing Cooper pairing in a fermionic alkali isotope such as lithium-6. In particular, the idea of using a Feshbach resonance to enhance the transition temperature had by that time been published by others, so we did not emphasize that in our paper but rather concentrated on Abdel's other results, which are as applicable to lithium-6 as to the original candidate, deuterium. (Serendipitously, the lithium-6 nucleus has, like the deuteron, spin 1). Possibly because it was published in a journal not widely read
by the atomic-physics community (at least at that time),our paper did not initially attract much attention, but over the past nine years it has clocked up a respectable number of citations, and some of its results have been replicated by others.
Over the last ten years, since Abdel returned to Khartoum, we had kept in touch by phone and e-mail. He was always anxious to hear the latest news in physics, not just about the spectacular experimental developments in areas related to his thesis but about other active topics such as high-temperature superconductivity, and we often discussed his own ideas about what might be going on. Whatever his personal circumstances, he was unfailingly cheerful and upbeat-so much so, in fact, that it was not until the last few days that I realized how seriously ill he was. During his years in Sussex and then in
Urbana, my family and I got to know him well, not just academically but also personally. I would like to express my sincere condolences to his
family and colleagues; I know he will be deeply missed in many quarters, not just as a physicist but as a warm human being.



Tony Leggett
Physics Dept., University of Illinois at Urbana-Champaign.