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J. Phys. Chem. B 2000, 104, 3768-3772
Harvey Scher
Photograph by Shlomo Taitz
Reminiscences† I hesitate to begin; I recall the old adage of Satchel Paige, “Don’t look back, something may be gaining on you.” My reluctance is dissipated by the opportunity this overly generous honor has given me to share. Sharing, in this way, is a means of enhancing the cross section of our singular paths through life by embracing the paths of others. I am eager to recount stories of mutual efforts with wonderful friends and colleagues and the excitement of discovery. My “scientific path” started in academia, took me through the industrial research laboratories of three corporate giants, and returned to academia. I was active in research across a spectrum of fields, styles, and motivations; I benefited from all of them. I was born in New York City on September 26, 1936, on Yom Kippur, a synchronous event I am still trying to justify. My father Max was also born in New York City, now a century ago. Soon afterward my mother Millie Reiss was carried onto Ellis Island as an infant-in-arms. My father’s working career was bound on both ends by driving a taxicab in the city; in the middle he toiled for many years in the “garment center” in Manhattan (where my son now works as a fashion designer). His job was as a “cutter” (of garment parts). Starting with a thick pile of fabrics, he removed the extraneous material to reveal the pattern. Isn’t that what we strive to do in science? When I was four years old, my parents permanently separated. My mother, two sisters and I moved to a part of the Bronx renowned as a hotbed of radicals. Each neighborhood in the Bronx was like a separate village, and this one had the added distinction of being intellectually vibrant and socially progressive (e.g., in interracial relations). A semicontinuous line of park †
Part of the special issue “Harvey Scher Festschrift”.
benches and trees encircled the neighborhood hub area called the “playground”. On warm summer evenings the benches were crowded with people of all ages; each group had its own section. One could walk along and pick up conversations ranging from labor union politics to modern physics, with the usual gossip interspersed. My friends and neighbors were my extended family, and growing up in that environment had an important impact on my intellectual development. During our City College (CCNY) years, the benches became classrooms for us. A separation of two years was considered another generation and each one accepted the responsibility to teach the next one. The talk, often lasting late into the night, was heavy with our new knowledge. The whole of New York City was the frame of this microcosm of eager learners from modest backgrounds. For the cost of a subway ride, one could go to all the museums, magnificent libraries, and public lectures at the universities. Inexpensive student tickets placed us in box seats at Carnegie Hall and the balcony at the NY Ballet at the 55th St. Playhouse. The center of it all for me was CCNY; I was feasting at a veritable smorgasbord of learning. The emphasis then was on teaching, which culminated for the physics/math majors in the course on mathematical physics taught by Professor Lawrence Wills. He was our architect of the structure of physics built with beams of Hamiltonians, Maxwell Equations, Fourier analysis, etc. We marveled at the edifice. I was fortunate in being selected for the Honors program, which enabled two of us to study classical field theory with Professor Wills for a semester; it was formative. Great “chefs” at the smorgasbord like Professors K. D. Irani and Hans Kohn expanded my interests into philosophy and literature. I thought the expansion would never end.
10.1021/jp000856q CCC: $19.00 © 2000 American Chemical Society Published on Web 04/01/2000
J. Phys. Chem. B, Vol. 104, No. 16, 2000 3769 I was young when I graduated (20 years old) and felt a strong need to experience the world beyond just crossing the Hudson River. I wanted to cross the Atlantic, live in Europe and become a writer. This whole course changed precipitously; I decided to marry Marilyn Sheiner and go to graduate school. My cousin, Edward Stupp, was a graduate student in physics at Syracuse University and he spoke to the chairman about me. I quickly received a telegram, started at Syracuse mid-year and married mid-semester (March 1958). For the first summer at SU, I worked in the lab of Henry Levinstein, which was under the stadium grandstands that held the cheers for the gridiron great, Jim Brown, of an earlier season. I grew n-type and p-type silicon crystals; this semiconductor was to figure importantly in the work I later did with Mel Lax who left SU a few years before my arrival to join Bell Telephone Laboratories. I opted to do my thesis work in quantum field theory. That entitled me to have an office in the theoretical physics Quonset hut. It was a fascinating place. At one end was the office of Peter G. Bergmann, who had the only telephone. Bergmann’s presence was sufficient to ensure that SU was one of the planet’s oases of general relativity research. During my stay, there was a convergence in that ramshackle structure of many of the greatest minds in general relativity, e.g., Roger Penrose, Ivor Robinson, and Jurgen Ehlers. Four-index tensors were scrawled like graffiti over all the blackboards as we observed the mathematical pyrotechnics. Peter wanted me to join the fun as his student, but I decided to work on many-boson systems for my thesis with Allen Miller, newly arrived from the University of Illinois. Many-body physics was a wonderful field as well as a more pragmatic one. The latter took on more significance as Marilyn and I had started our familysour daughter Michele was born in April 1962. I was so distracted by the intensity of my next phase that I regretfully never published my thesissa quite respectable piece of work. However, it was an excellent preparation for this next phase, which began with a letter from Fred Keffer at the University of Pittsburgh. He urged me to accept a post-doc position with Theodore Holstein; he called him a physicist’s physicist. When I told my mother who I would be working for, she said, “you mean Teddy.” I was surprised by her casual familiarity. Her good friend, Mrs. Goldberg, used to make lunches in her apartment in the Bronx for a group of public school teachers. Among them was Mrs. Holstein, who bragged, about her son, Teddy, the genius. I was convinced that there was a cabal of NYC housewives that was assiduously nurturing an important part of the scientific resources of the United States. The training of a scientist is basically the guild system, and working closely with Ted was my apprenticeship with a master. I came to Pitt at a particularly propitious moment. Ted had just finished a masterwork on the many-body transport equations of a strong-coupled electron-phonon gas. In the course of this development, he independently derived the famous “Migdal theorem”. He was shocked to find it in the Russian literature. I leaped at the opportunity to make the first unique application of the transport equationssI turned on a strong magnetic field. I poured over the transport theories of normal metals (nonsuperconducting), including the well-known Azbel-Kaner or cyclotron resonance. Our work was the first to predict measurable, dynamic many-body effects due to strong electron-phonon interactions in normal metals. It was my first published paper (1966) and a group in Paris confirmed the predictions in a tour de force experiment in 1970. When I first presented a fairly complete draft of the paper to Ted, he said it was fine but wanted his name removed. I think the issue was the proof of something.
I implored him to keep his name on the paper, and I appeased him by adding a few lengthy appendices. We used to have lunch together in the Western Psychiatric Center (next to the Physics building). One day Ted said abruptly we weren’t coming back here anymore, “I cannot tell the difference between the patients and the staff”. There are so many of these stories. Ted was a rare physicist and personality. Besides sharing his physical insights and impeccable research style with me, he passed on a number of aphorisms. One I always use is Ted’s version of Gresham’s law: Bad papers drive out good ones. I was invited to give a seminar on our work at Bell Laboratories in Murray Hill, NJ. It must have gone well because I was essentially offered a Fellow position during the visit. I joined in the fall of 1965 and worked in the Chemical Physics Department headed by Andy Hutson. It was my first encounter with an industrial research lab, but a very atypical one. It was equivalent to an aggregate of the best university departments in each of the different fields at Bell. The collective effect was heady; I was on constant information overload. Lunch conversation was like a journal club; seminars and preprints were prolific. The collaborations between staff (present and former), visitors (long and short term), former mentors, and consultants formed a dense network around the world. My important collaboration with Mel Lax (I missed him at SU but we intersected at Bell) started with a casual visit to his office. The top of a leaning stack of papers caught my eye. It was a partial draft on hopping transport containing the relation between the frequency components of conductivity and mean square displacement. Subsequently, this relation for me was to prove germinal (its derivation is in appendix one of our first paper). I also continued with strong electron-phonon interactions and developed the theory for intrinsic infrared absorptivity of normal metals. My research at Bell concluded one scientific pursuit and opened another that was to become a seminal one. Both pursuits reached fruition after I left Bell in the early fall of 1967. I applied for five jobs and received offers from allsone academic, two government labs, two industrial research labs. The branching in my path at that junction was quite discernible; I chose Xerox. The year 1967 started with the birth of our son Joel in January. It was also the year of the June ‘67 war in Israelsa portentous event that created deep stirrings in me that I acted upon 26 years later. Xerox Research was in the “knee” of the curve of its growth (in people and prominence) when I arrived. The first Xerox lab was in Webster, NY (outside of Rochester), at the manufacturing sitesa proximity that set a focus for the lab. Most of us, including an old friend Marty Abkowitz (who hailed from the “benches”) and a new one Richard Zallen were all just launching our careers. There was the excitement of beginning. We had to build research programs and shape a new lab that would enhance Xerox’s core technologies. The physics group concentrated on the image forming process of xerography, which involved properties of noncystalline photoconductors. The photoconductors of commercial interest comprised a variety of novel materials and designs, e.g., amorphous semiconductors, polymeric systems, and mixtures of both. Xerox held a patent on a generic mixed system of photoconducting particles in an inert resin. The defense against a legal challenge to that patent involved the need to prove that the electronic transport occurred exclusively through particle contact. I was asked to devise this proof. I teamed up with Dick Zallen and we solved the problem through an innovative use of percolation theory. In the course of providing the solution, we made a surprising discoverysa critical volume fraction for percolation (its value dependent only on the spatial dimension). This discovery stimulated a new
3770 J. Phys. Chem. B, Vol. 104, No. 16, 2000 subfieldscontinuum percolation. It also provided me with a research leitmotif: technology as a rich source of new scientific questions. In an environment such as a corporate lab, where there is the perennial tension between directed and curiositydriven research, an artful interplay between them can give rise to important consequences. That was the case with anomalous dispersion. I was part of the group creating the program in amorphous semiconductors, which would soon emerge as one of the foremost in a burgeoning area of condensed matter physics. I was also struggling with an approach to impurity hopping conduction in doped (crystalline) silicon. I started this work at Bell motivated by pure scientific interest and transferred it into an environment grappling with the relation between the research activities and Xerox’s interests. The connection for me was disorder. Silicon, at low temperature, was the backdrop; the conduction occurred by electron transfer between randomly located impurity atom sites. I intuited that the theory developed to solve this problem could be the basis of an approach to electronic transport in amorphous semiconductors. It turned out that way. The advantage of the model system was the vast knowledge we had of doped silicon and the abundance of excellent data on impurity conduction in n-type Si. It was a testing ground for theoretical ideas. The key measurement routinely used to characterize the photoconductors was the discharge curve. It is the transient current following a pulse of strongly absorbed light. The features of this discharge determine those of the latent image formation in photocopier machines and laser printers. Merle Scharfe was telling me about some unusual aspects of these measurements that he was finding in amorphous As2Se3, the material being used at that time in Xerox machines. The transient current slowly decayed in time (a long tail) with unusual time-scaling behavior, but most bizarrely, the effective mobility changed with the thickness of the sample! Mobility is always considered as an intrinsic property of the material and independent of the size of the sample. What did we really know about transport properties of these amorphous semiconductors? We were still puzzling out the structure. I was very curious about these strange results and besides, I was expected to solve this problemsit was important to the company. What subsequently evolved was not so much a sequence of clear steps but a brew, an artful interplay between a model system and a set of new scientific questions emerging from xerographic imaging. The clarity that electron hopping was going on in n-type Si prompted me one afternoon to ask Elliot Montroll, a consultant from the University of Rochester, whether it might be a good idea to use random walks for this problem. My other motive for asking was to spark his interest so he would not nod off when I turned to write on the blackboard. He referred me to a paper he published with George Weiss on random walks (RW). I read the paper and found it very interesting but not what I was looking for until I arrived at the last part. The usual interest in a RW is Pn(x), the probability of finding the walker at location x after n steps. The authors generalized the latter to P(x,t), i.e., finding the walker at x at time t, where each step could occur at a random time chosen from a distribution ψ(t). There were pages of calculations using a different method on my desk of just this P(x,t). Formally now the connection was made with a continuous time RW (CTRW). A great deal of work remained, e.g., determining the ψ(t) for hopping between the impurity atoms. The work published with Mel Lax (1973) has been heavily cited not only for the particular computations of impurity conduction in doped Si but also for the new theoretical approach
to transport in disordered solids. The method was tested in a model system, and now a basis was set to try to answer the scientific questions posed by the measurements in the less understood amorphous semiconductors and polymeric systems. A new conceptual step had to be taken to deal with the challenges of the transient current I(t) measurements. I was baffled by that mobility problem (i.e., dependence on the sample thickness), so I focused on the scaling behavior of I(t). To obtain this behavior, I reasoned that there could be no time scale intrinsic to the system during the experiment. I used a ψ(t) that had such a long time tail that the mean time of ψ(t) did not exist. That was the step! Everything fell into place. The computation of I(t) not only predicted both the shape and scaling behavior but also accounted for the mobility problem. How? Ordinarily, the bunch of electrons created by the pulse of light moves across the sample with a mean position l proportional to time t, i.e., l ) Vt, where V is simply the velocity (the mobility is V/E, velocity per unit electric field). The first long-tail ψ(t) I used yielded l ∝ xt. If one insisted on the usual relation, one needed an effective V ∝ 1/xt, a time dependent velocity (hence mobility). Thus, any variable that changed the duration of the experiment (e.g., sample thickness) would change the presumed mobility. The problem was the assumption of l ) Vt when it was not applicable. The moment of discovery of l ∝ xt and all its implications came when I was lying on my living room rug in the middle of the night. If I tried that now I would simply fall asleep. The next morning I excitedly rushed to show Merle Scharfe that the mobility problem was solved. He understood it immediately or was being very polite. A bright graduate student of Elliot Montroll’s, Michael Shlesinger, soon after showed up at my office wanting to talk to me about topics for his thesis. I suggested the generalization of l ∝ xt (i.e., l ∝ tR, 0 < R < 1). He did it quickly and it was very helpful. Elliot and I got very busy working out all the details (e.g., the absorbing boundary condition). Meanwhile, a new colleague at the lab, Gus Pfister, repeated the I(t) measurement of amorphous As2Se3 under very stringent conditions, obtained exactly what we predicted for the shape of I(t), and showed it to be invariant over a very wide range of the measurement duration. These same features of what we called anomalous dispersion were showing up in other amorphous semiconductors and organic electronic materials (e.g., molecularly doped polymers). Some of this was incorporated into the big Physical ReView paper (1975) by Elliot and me, which became one of the ten most cited papers in physics of the decade following its publication. Why? Did transient current measurements in amorphous solids take flight? It is rather the generic idea of a lack of a time scale or a broad distribution of event times over some range that shows up in a large variety of phenomena. What did take flight, over the years, was the imagination of scientists such as Mike Shlesinger with his theory of stretched exponential relaxation and he and Yossi Klafter relating the long tail behavior to chaotic systems. Another example is in geohydrolgy, which I’ll touch upon later. Those were halcyon days. We, including others both inside Xerox (Gus Pfister, Fred Schmidlin) and outside (Mike Shlesinger, Yossi Klafter), moved in both the directions of detailed physical mechanisms for the different kinds of materials and of expanding the conceptual framework. What I especially recall was lunch (not a surprise to my friends). Elliot Montroll and I would have lunch together at Joe’s Diner in Webster, NY. There, under a poster of Elvis, we would peruse a wide swath of science. When I was concerned with some new experimental results he would say, “...do not worry, eventually everything is wrong.” Elliot was the freest man I ever met. He led a very
J. Phys. Chem. B, Vol. 104, No. 16, 2000 3771 special lifesboth in science, family (Shirley and he had ten children), and friends (everywhere). The proprietor of Joe’s could never figure us out, especially when we occasionally would be joined by Michael Rice. All of this period started on fragile footing. During the time I was working on the impurity conduction problem, there was a management shift in the lab, which resulted in more stress on work tied closely to Xerox’s technical needs. Apparently, there was an urgency to model the Xerox machine, and guess who was picked to do it. I was working on this assignment, moonlighting on hopping transport, and looking for another job. Sometimes the thrust and parry of the balancing act in corporate research can end by backing into a pothole. I side stepped it, however; not long in coming, a restoration took place. Jack Goldman, the corporate director of research, led the charge and among other changes, he brought in Ted McIrvine to head the physics lab. Ted “hired” me back into his new lab. All of the work on transport in disordered solids and anomalous dispersion came to fruition upon my return. I spent 16 years at Xerox, slightly more than half my post Ph.D. career in the U.S. My research extended to include interaction effects of the charge transfer steps (e.g., Coulomb and exciton interactions). There were fruitful collaborations with Shelly Rackovsky (now a professor at Mt. Sinai School of Medicine in NYC), continuing until today, on a molecular theory of geminate recombination and with Tom Orlowski on laser pulse probes of these interactions. I also had the challenges (to say the least) of being a manager of a group of outstanding scientists. In the midst of these years tragedy struck my family. My dear wife, Marilyn, a larger-than-life woman, died in May 1976. I will always be grateful to our many friends and the Rochester community for their outpouring of love and care that enabled my two young children and myself to deal with our loss. As the Bible exhorts us, choose life. I did, and in January 1978, I married Audrey Kaplan; in December of that year our daughter Dena was born. Over the next few years, there was a sense of ongoing protracted change in the research character of the Webster lab, and scientists started to leave. There was a wave of departures in the summer and fall of 1983, including me. Standard Oil of Ohio (Sohio), with great resources emanating from the Alaskan pipeline, was building an impressive laboratory and invited me to play a role in shaping the research program for this enterprise. I was intrigued by both the opportunity to assemble a first rate research staff and Sohio’s ambitions to considerably expand the base it had in the energy, materials, and chemical industries. At the site of the new lab outside of Cleveland, OH, there were research activities related to all of these industries. I had to contend with this mixture and a different corporate culture for research. A natural connection for me was made with one of Sohio’s new interestsssolar cells using hydrogenated amorphous silicon (a-Si:H). The lab already had in place a number of young, world-class optical scientistss among them Terry Gustafson and Rob Collins. We initiated a collaboration that led to a fundamental study of light-induced defects in a-Si:H; these defects are the main drawback of the solar cell performance of a-Si:H. The major challenge for me, however, was the mainline interests of the company. The need was for a small research staff of highly talented and versatile scientists. I was very fortunate in being able to hire two outstanding young theoretical physicists, Leonid Turkevich and Bill Curtin. The maximum size of the group reached ten, with high quality accretion from other parts of the lab (including some rescue efforts). British
Petroleum (now BP Amoco) became the sole owner of Sohio and continued to support us. However, a larger part of my time had to be devoted to securing this support for our various research programs (some of which competed with those in the large BP lab in Sunbury-on-Thames, U.K.). At times it felt like white-water rafting with the sense of challenge and accomplishment but also with the bumps. From the best work of the group (which was extensive) and from my own contributions to part of it, one can again discern the pattern of the judicious mixing of scientific pursuits and responsiveness to technology. I became interested in an active area of the physics of fractalsssimulations of diffusion-limited aggregation (DLA). I worked with Lee Turkevich in calculating the perimeter probability Π (using CTRW) of the aggregate. We discovered that the strength of the singularities of Π determined the fractal dimension of the aggregate. At the same time we made this scientific contribution (our paper was one the 100 most cited papers in physics in the year following its publication in 1985), a paper of L. Paterson appeared that showed the relation between DLA and limiting cases of viscous fingering. The latter problem is of great interest in petroleum production. I worked, successfully, along these lines with Mike King (a top-drawer former field theorist from SU in our lab) on our “probability approach” to multiphase flow in porous media (in the viscous force dominated case). Via these steps I found myself involved in part of the mainline technology of an oil production company. A lot of the design of floods for additional oil recovery involves the avoidance of viscous fingering. This work developed for me into a very fruitful collaboration with Martin Blunt (an outstanding theorist from the Cavendish at BP Sunbury) on many aspects of multiphase flow in porous media. This latter topic is very important in the context of my present work, and I enjoy a continuing collaboration with Martin, now a professor at Imperial College, London. I remember Elliot once telling me, “everything is either a random walk or an Ising model.” In multiphase flow in porous media there are both; the viscous fingering is akin to a random walk and imbibition, under capillary dominated forces, is akin to an Ising model. Viscous fingering involves a propagating instability; so does the fracturing of a brittle solid. BP owned Carborundum, a ceramics company, which was in need of a lot of scientific underpinning in its development of tough ceramics. Bill Curtin and I wanted to keep a program going in the physics of solids, so there was a natural overlap in the study of the mechanical properties of ceramics. The only thing wrong with this decision was the fact that neither of us knew very much about the subject. We started right off with a model that would teach us. We turned that to an advantage; we published a critical examination of the physical conditions needed for the applicability of spring network models with distributed properties. The program has been a rich exploration of phenomena, e.g., microcrack toughening and nonlinear time-dependent damage accumulation (creep). Bill devoted a lot of attention to fiber-reinforced ceramics and is now regarded as a world expert in the area. I enjoy continuing work with Bill, who is now a professor at Brown University. We are “creeping” along on our earthquake modeling. In the early 90s there was growing restiveness in corporations toward their research divisions; BP was no exception. In the fall of 1992 they offered a downsizing package. I had the additional advantage of also qualifying for early retirement. Many of us accepted the offer. For me it set in motion long dormant plans to immigrate to Israel. I am deeply grateful to
3772 J. Phys. Chem. B, Vol. 104, No. 16, 2000 my wife Audrey; she went way beyond her devotion to me, to indulge this wild idea of mine. I wrote to Joshua Jortner, and next I heard from Itamar Procaccia and Shlomo Alexander (my late colleague) from the Weizmann Institute of Science. It was Itamar’s suggestion for me to join the Department of Environmental Sciences and Energy Research (ESER). It worked and I am thankful to him. I am surrounded by an excellent interdisciplinary group of colleagues (both in ESER and the Israel Hydrological Service (Dany Ronen)). I was able to make an immediate connection to a less explored part of hydrology (the unsaturated zone) using my knowledge of multiphase flow in porous media; the nonwetting fluid oil is exchanged for air. Dany Ronen tutored me in the importance of the capillary fringe (at the top of an aquifer). We have done fieldwork, and Martin Blunt and I have used pore network models to explore the structure (water distribution) of the capillary fringe (CF). The combined results have disturbed conventional ideas about the CF and more generally the interface region (between the saturated and unsaturated zones) and have naturally aroused controversy in a rather conservative field. There is a great deal of work yet to be done in the unsaturated zone, especially measuring chemical distributions and theoretical treatment of water infiltration. Brian Berkowitz introduced me to the long-standing problem in the saturated zone of the dispersion of chemicals (pollutants). The problem is basically the motion of particles in the complex flow field of a highly heterogeneous geological medium. Into the large literature of this topic we introduced a model based on the CTRW formalism. Here the particle’s encountering a wide velocity spread of the flow field causes the broad distribution of event times. Our approach has been applied successfully to simulations (fracture networks), laboratory models, and field observations that have not been accounted for by the prevailing theoretical models. I am continually being exposed to fascinating problems across a spectrum of the earth sciences, which is the backbone of environmental sciences. Not only are environmental problems important, but they also often offer a rich source of new
scientific questions. The Weizmann Institute is a wonderful choice for my return to academia. My family is now divided between Jerusalem and NYC (our children Michele and Joel live in Manhattan). For me the cities are, respectively, my future and past and I’m at home in both. The earliest recollection I have of my interest in science was my deep puzzlement about the nature of time. Someone, probably on the Bronx benches, told me that Einstein the physicist knew all about it. It is perhaps not a coincidence that transport theory in general and the phenomena associated with random time in particular were writ large in my career. Now I realize what “may be gaining on me”. It is time-past. As it accumulates, it heightens my awareness of what I really want to do. I feel the sequence of the future, but curiously the timepast becomes “space-like”sdistant and present events become simultaneous. The love expressed to me by my late parents, late wife Marilyn, present wife Audrey, children, late sister Estelle, and sister Marilyn is constantly with me and enhances my capacity for wholeness. All the sharing moments with good friends (those long branched from my path and those with me for most of the journey, especially Marty Abkowitz) come together in the handshake with each new friend. The devotion of esteemed teachers, the passing of the baton of masterful scientists, and the feedback from young rising colleagues all blend in the rare moment of a good thought. My cup runs over. I deeply thank all of them, especially my parents for their unqualified faith in me. I want to express my deep gratitude to Yossi Klafter and Mike Shlesinger for their efforts for this special issue (a wonderful birthday present) and for their courage in persevering in the funny idea that I deserve this honor. I strongly appreciate the efforts of all the scientific contributors to this issue. They represent the intertwining of my learned colleagues and friends. Finally, I thank the Journal’s Editors who put it all together. Harvey Scher RehoVot, Israel March 2000
