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Leon N. Cooper, a Nobel-winning physicist who helped unlock the secret of how some materials can convey electricity without resistance, a phenomenon called superconductivity, and who did pioneering work in understanding how memory and the brain work, died on Wednesday at his home in Providence, R.I. He was 94.
His death was confirmed by his daughter Coralie Cooper.
Dr. Cooper, a longtime professor at Brown University, was something of a bon vivant on its campus in Providence, where he could be seen driving around in a sporty 1968 Chevrolet Camaro convertible. After he received the Nobel Prize in Physics in 1972, sharing it with two colleagues, The Rhode Islander, the Sunday magazine of The Providence Journal, named him its Man of the Year and published a profile of him titled “Dr. Supercool and the First Nobel.”
It was said that Sheldon Cooper, the quirky, geeky character played by Jim Parsons on “The Big Bang Theory,” the hit television sitcom about graduate students at the California Institute of Technology, was in part named after Dr. Cooper.
But it was as a postdoctoral researcher at the University of Illinois Urbana-Champaign that Dr. Cooper first made his mark in the field of superconductivity.
Superconductors can create powerful magnets for use in magnetic resonance imaging machines, and in the giant accelerators that smash particles together to study the origins of the universe.
Superconductivity was accidentally discovered in 1911 by the Dutch physicist and Nobel laureate Heike Kamerlingh Onnes when he cooled mercury down to minus 452 degrees Fahrenheit, or about 7 degrees above absolute zero. Physicists were astounded by what they found — it was as if they had discovered a perpetual motion machine. Indeed, a current passing through a superconductive material will theoretically never dissipate.
Some of the greatest physicists of the 20th century, including Albert Einstein, Niels Bohr, Werner Heisenberg, Wolfgang Pauli and Richard Feynman, tried to explain how superconductivity works. They all came up empty.
Dr. Cooper helped crack the code with two colleagues at the University of Illinois: John Bardeen, a professor of engineering and physics, and J. Robert Schrieffer, a graduate student. (The theory they came up with is known as B.C.S., after their initials.)
Dr. Bardeen was already something of a legend in the physics world. In 1947, while working at Bell Labs, he helped invent the transistor, the tiny semiconductor that largely ushered in the computer and electronics age. It earned him a Nobel in 1956.
One of Dr. Bardeen’s principal goals at Illinois was to try to solve the problem of how superconductivity works. He had Dr. Schrieffer start working on pieces of the puzzle but soon decided that Dr. Schrieffer needed more help. He reached out to Dr. Cooper, who at the time, in 1955, was working at the Institute for Advanced Study in Princeton, N.J., and had come highly recommended by his peers.
Years later, Dr. Cooper recalled that this was the first time he had ever heard of superconductivity. If he had been aware of how many eminent scientists had tried and failed to solve the problem, he said, he would probably not have accepted Dr. Bardeen’s offer.
In a chapter of “BCS: 50 Years,” a book published by World Scientific Publishing in 2011, Dr. Cooper, one of the book’s editors, said it had seemed clear at first that superconductivity was caused by interactions between electrons, the tiny, negatively charged particles that circulate in the nuclei of atoms and that are the source of electric current.
In a superconductive state, electrons must somehow be attracting each other to increase the current. But how was that possible, since electrons normally repel each other? And why did superconductivity occur only at low temperatures and only in some metals?
It took more than a year of false starts and dead ends, but Dr. Cooper came up with an explanation.
He surmised that electrons attract positive ions in the lattices of the atoms that make up certain metals. That creates a charge imbalance in the lattices, making one side of them slightly more positive. In superconductivity, that is enough to attract other electrons flowing through the metals toward the lattices. Each flowing electron pairs up with an electron in the atoms, creating bonds. Many of these electron pairs, which became known as Cooper pairs, can exist in the same proximate space, making superconductivity possible.
The Cooper pair bonds are quite weak, so if the temperature of the metal rises, giving the electrons more kinetic energy, the bonds are broken. That is why superconductivity can, in general, exist only at low temperatures and only in metals with the right kind of lattice structure.
Dr. Cooper wrote up his first paper with his preliminary theory and results and published it in Physical Review in September 1956. There were still holes in the explanation, but Dr. Schrieffer came up with a mathematical formula in early 1957 — doing so while riding the subway in New York City, where he was attending a conference.
By chance, Dr. Cooper and Dr. Schrieffer ran into each other shortly afterward in the Champaign, Ill., airport, where Dr. Cooper was also returning from a conference. They compared notes and realized that they had made a critical breakthrough. As Dr. Cooper recalled in 2010 in an article in World Scientific Review, “We were jumping up and down with excitement, oblivious to what others, who were in that airport on that cold winter evening, might have thought.”
After many more months of ironing out the details, Dr. Bardeen, Dr. Cooper and Dr. Schrieffer submitted their paper to Physical Review in July 1957. It was published in December under the simple title “Theory of Superconductivity.”
Subsequent experiments confirmed the so-called B.C.S. theory, leading to the 1972 Nobel Prize. (Dr. Bardeen became the only person in history to receive two Nobel Prizes in physics.)
Dr. Cooper was born Leon N. Kupchik on Feb. 28, 1930, in the Bronx, the elder of two children of Irving and Anna (Zola) Kupchik. (The “N,” though it has been falsely reported to stand for Neil, did not represent a middle name.)
Irving Kupchik was from Belarus, which was then part of the Russian Empire, and had been a Menshevik, part of the moderate opposition to the Bolsheviks. He left the country after the Bolsheviks prevailed in the Russian Revolution in 1917. After settling in the United States, he worked as a typesetter in a printing business. Leon’s mother, who oversaw the home, was Polish.
Anna Kupchik died when Leon was 7, after which he and his younger sister, Lorraine, were placed in foster care for several years. His father later remarried, but before he did he changed the family name to Cooper. The children then went to live with their father and their new stepmother.
Leon showed an early interest in science, though on at least one occasion his enthusiasm got the better of him. When he was about 12 and the family was living in an apartment in the South Bronx, he blew up a small closet while mixing chemicals. Fortunately, no one was hurt.
He graduated from the Bronx High School of Science and from Columbia University, where he earned his undergraduate degree in physics in 1951 and his doctorate in 1954. He was then hired by the Institute for Advanced Study.
After the breakthrough in superconductivity, Dr. Cooper worked for a year as an assistant professor at Ohio State University before joining Brown as a physics professor in 1958. He spent the rest of his career there, reaching emeritus status.
At Brown, he became interested in neuroscience, and in 1973 the university created the interdisciplinary Center for Neural Science, with Dr. Cooper as its director. In his new role, Dr. Cooper went to work attacking one of the central puzzles in the field: how people learn.
In 1949, Donald O. Hebb, a Canadian psychologist, had put forward the theory that learning was based on the coincident firing of electrical signals between neurons in the brain, which strengthened their synaptic connections. His theory of synaptic plasticity — or, more informally, “cells that fire together, wire together” — soon became widely accepted. But there were outstanding questions over memory storage. Most significant of these: How was it that connections between active neurons did not become totally saturated and, as a consequence, lose the ability to store information?
By the 1970s, scientists had recognized that there could be improvements in memory storage if synapses could be weakened, as well as strengthened, by the timing and relative strength of the neuron electrical signals.
Teaming with two doctoral students, Elie Bienenstock and Paul Munro, Dr. Cooper worked out a rigorous analysis of how such weakening (and strengthening) could occur without saturation of the connections. Their theory was that as synapses approached saturated levels of activity, the electrical signals that were driving them would become less effective, and the synaptic connections would revert to less saturated levels. The connections would thus oscillate between being saturated and unsaturated, like a skier gliding between two fences but never hitting either.
The theory was published in January 1982 in the Journal of Neuroscience and came to be known as the B.C.M. theory, after the last initials of its authors.
Mark F. Bear, a professor of neuroscience at the Massachusetts Institute of Technology, who studied under Dr. Cooper at Brown in the late 1980s, called the B.C.M. theory “foundational” in the field of neuroscience.
“The theory made assumptions about how neurons are modified, and those assumptions were testable,” Dr. Bear said in an interview for this obituary in 2022. “Those assumptions were also correct.”
Among the things the theory has been able to explain and predict accurately is how the visual cortex works and how people learn to see.
Dr. Cooper’s first marriage, to Martha Kennedy, ended in divorce. In addition to his daughter Coralie, from that marriage, Dr. Cooper is survived by his wife, Kay Cooper; another daughter, Kathleen Cooper, also from his first marriage; and four grandchildren. His sister, Lorraine, died in 2022.
Despite his “supercool” reputation around the Brown campus, Dr. Cooper was always focused and serious about his work, his daughters said. Interviewed in 2022, they said he told his father when he was young that he wanted to become a physicist because “there is no other way that I can know about everything.”
Dr. Cooper never lost his curiosity and desire to learn. One day, when he was on sabbatical and the family was living in Paris, Kathleen suddenly asked her father, “What is out there in the universe?” As she recalled, he looked at her with a big smile, clearly pleased by the question, and replied, “Baby, we don’t know.”
Ash Wu contributed reporting.
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