CORNELL ALUMNI NEWS a Preeminent Group of Physicists Discovers How Oddly Matter Behaves When Cooled to Very Low Temperatures

CORNELL ALUMNI NEWS A preeminent group of physicists discovers how oddly matter behaves when cooled to very low temperatures

Strange Doings Near Absolute Zero

By William Steele '54

It started in the early '60s, about the An inauspicious omen, perhaps, but The group has also received a subtler time the low temperature laboratory in a way it symbolized a beginning. Out but perhaps equally satisfying accolade: started to fall into a hole. Cornell had of the hole behind Rockefeller would their graduate students are much in de- just hired David Lee out of the low tem- rise Clark Hall, home for the newly mand. "They have a remarkable reputa- perature physics group at Yale. Al- created Laboratory of Atomic and Solid tion in the scientific community for the though only a lowly $4,500-a-year State Physics (LASSP). In Room H-8 in students they turn out," says Prof. Her- instructor, he became the reigning mon- the basement the new low temperature bert Johnson, director of the Materials arch of low temperature research; he facility Lee was to design would reach Science Center. "When students come inherited a garage-sized brick and con- temperatures 1,000 times colder than out of here they are prepared to set up a crete building behind Rockefeller Hall anything possible in the "Rockefeller laboratory from virtually a bare room. that housed a helium liquifier and other Annex." He would be joined by col- They have seeded a number of univer- apparatus for cooling things to within a leagues John Reppy and Robert Rich- sities throughout the country and the few degrees of absolute zero. ardson, and the trio would build the low world." Excavations for the basement of temperature experimental group into To which Lee adds wryly: "Where Clark Hall were going on just a few feet one that Prof. Neiί Ashcroft calls some of them are now in dire competi- away. One day Lee came to work and "probably the most prestigious in the tion with us!" found a crack in the floor; each day it world." In many places it would be unusual to grew longer and wider, until it extended Ashcroft is director of LASSP and have three professors working in one up the wall and made a gap wide enough their boss, so is admittedly prejudiced; field, but at Cornell it's becoming stan- for him to watch the construction work- but the world has indeed beaten a path dard practice. "One of the things that's ers walking by. The lab was evacuated to the doors of H-8. In March, 1981, been rather nice about the way Cornell "precipitously," Lee recalls, as the floor Richardson, Lee, and Douglas Osher- has developed in physics," Richardson began to cant toward the hole. off, PhD '73, now at Bell Laboratories, says, "is that instead of trying to cover received the Oliver E. Buckley Solid every possible field they tried to In a public lecture in Rockefeller Hall, State Physics Prize from the American strengthen certain areas." Prof Robert Richardson transfers Physical Society for the discovery and The concentration, he says, allows the liquid helium into a transparent subsequent study of the superfluid state group to share ideas and equipment and vacuum flask for an experiment in low of liquid helium-3. In 1976 they had re- makes it easier to attract outside talent; temperature physics. The fog above the ceived the Simon Memorial Prize from there's a visiting professor in low tem- apparatus is from the cold helium gas. the British Institute of Physics for the On the wall behind, small magnets are perature physics almost every year. In projected to illustrate how matter same discovery. In August 1981, Reppy return, the trio spend their share of time aligns magnetically at very low was a co-recipient of the Fritz London on the road. Reppy has taken sabbati- temperatures. See page 20 for the final Memorial Award for important contri- cals at the universities of Manchester phase of this demonstration. butions to the study of liquid helium. and Sussex in England, Richardson has

MAY 1982 spent a year in Finland, while Lee is the arranged to have Strauss' Also Sprach "far eastern expert," with visits to Ja- Zarathustra played in the background as pan and China on his record. he poured liquid helium, raising spectac- Although Ashcroft likes to call them ular clouds of mist. (The lecture, titled "The Three Horsemen of the Apoca- "A World without Disorder: Absolute lypse," all three say, in almost the same Zero," was videotaped and can be words, "We are not a team." viewed at Rockefeller Hall or borrowed "We are individuals who strongly in- for public showings.) teract," says Lee. In teaching, the three professors are "Individual entrepreneurs," says consciously a team. The low tempera- Richardson. ture group generally includes ten or "A loose coalition," says Reppy. twelve graduate students and two or There are certainly no leaders and no three post-docs (again, more than you'd followers. Lee sums up the relationship find in one field in most places). Stu- this way: "For us, the axiom in plane ge- dents are not only allowed, but encour- ometry is wrong; the whole is not equal aged to seek help from any professor, to the sum of its parts. The axiom and to interact among themselves. doesn't take into account the interaction Lee claims that "students tend to between the parts." learn more from each other than from They certainly don't look like a team, their professors, anyway." To foster though Richardson and Lee might be such interaction, Richardson has orga- taken for brothers. Both are lanky, with nized a Thursday night low temperature conventional short hair and unconven- seminar that may be the only such meet- tional twinkles in their eyes. Richard- ing on campus with a negative honorar- son's voice has a kind of Jimmy Stewart ium: the speaker is expected to provide throatiness; Lee's has the stutter. In this beer and crunchies for the assemblage. company Reppy is the misfit: shorter, One of the reasons these students have with longish hair and beard and a lined been so successful, Reppy says, is that outdoorsman's face that faintly suggests they have tended to stay in school longer Profs John Reppy, above, and David he should be carrying a bagpipe. than most. "During a large part of the Lee, and a jokingly mislabeled vat for Described by most as quiet, Reppy in- time we were around," he says, "it's a low temperature experiment. deed prefers the loneliness of long-dis- been a bad job market, so students tance running; he's competed five times wanted to be well-trained to be sure of new understanding of the forces that in the Boston Marathon, and claims to getting a job." With a kind of inverted hold matter together. The Cornell group have run a 4:40 mile about three years modesty, he adds that it's easy to turn has specialized in the study of liquid he- ago (at the age of 48). But he's also a out good students if you have good ma- lium. So far there seems to be no practi- mountain climber, and that's certainly terial coming in, and that Cornell is now cal application for any of this research, an activity that calls for cooperation. He getting the cream because of its good but liquid helium is definitely fascinating seems to have inveigled Lee into some of reputation. stuff. It was the last of the gases in the these activities, at least to the extent of The job market is getting a lot better atmosphere—once thought to be "per- running around campus. Richardson these days. Low temperature physics is, manent gases"—to be liquified, at a sticks to more sedentary hobbies like if you'll pardon the expression, "hot." temperature of about four degrees above gardening and playing piano to his Its most visible spinoff is superconduc- absolute zero. daughter's violin. tivity, the effect that allows certain While most of America is struggling Richardson is often the most visible of metals to carry an electric current with with (and generally rejecting) a change- the three, being both joiner and orga- zero resistance at temperatures near ab- over from the Fahrenheit temperature nizer. He sits on several local boards and solute zero. Engineers are hard at work scale to the metric-style Celsius scale, committees and is often an organizer in on superconducting generators and scientists long ago switched to a scale of national scientific meetings. A confer- transmission lines. Superconducting "absolute temperature" called the Kel- ence on liquid helium held at Cornell last magnets may provide the means to vin scale, which starts at absolute zero summer was largely his creation. Says create a magnetic "bottle" strong (-273° C or -459° F) and goes up. One Johnson: "He interfaces well with peo- enough to contain a hydrogen fusion re- degree Kelvin is equal to one degree Cel- ple ... He can analyze problems that action. sius; on the Kelvin scale water melts at have both scientific and people compo- By about 1995 the Japan National 273K and boils at 373K. nents. If I have a question about some- Railroad hopes to cut the 350 mile ride With Kelvin temperatures the degree thing I usually go to Bob—but he on its famed "bullet train" from three symbol is usually left out, both in writ- doesn't speak for them; they talk it over hours to one by levitating the train a few ing and speaking. Oxygen boils at a together." Reppy and Lee, Johnson inches off the roadbed with supercon- tropical 90K, and hydrogen freezes solid says, are close with their students, but ducting magnets. The promise of more at 14K. Helium, however, remains a liq- stay off boards and committees "by such wonders to come certainly makes it uid down to, theoretically, absolute choice." easier to get funding for low tempera- zero, and this is one of the things that Richardson's lectures are funny, dem- ture research. makes it exciting. onstration-ridden, and somewhat legen- Superconductivity was discovered Absolute zero was once defined as the dary. For a public lecture on low tem- more than seventy years ago, but scien- temperature at which all motion would perature physics he shattered a "glass tists are just as excited about the "pure" cease. Like a lot of other theoretical pre- banana" frozen in liquid nitrogen and research going on today, which promises dictions in low temperature physics, that

CORNELL ALUMNI NEWS LASSP's decision was helped along by discussion in the Solid State Panel of the National Academy of Sciences, which was urging a research effort to reach extremely low temperatures. Con- ventional refrigeration, applied in some unconventional ways, was capable of cooling helium down to around IK. A technique called paramagnetic cooling could at that time reach temperatures as low as 100 milliKelvin (mK) or 100 thousandths of a degree above absolute zero. One of the orderings that takes place as you cool helium to milliKelvin temperatures is that the two isotopes, helium-3 and helium-4, separate like oil and water. If you mix them again, they have to absorb heat to permit the return of disorder. Using this principle, Henry Hall at the University of Manchester had built a "dilution refrigerator" that could cool experiments to around 50K. Enter David Lee, with an idea he had picked up in graduate school. Although helium remains a liquid down to absolute zero at atmospheric pressure, it can be compressed into a solid at pressures of thirty-five atmospheres or more. It's a one turned out to be wrong. While all tunity to study this kind of ordering is curious solid, because the Heisenberg thermal motion ceases, there is always a what attracts scientists to low tempera- uncertainty for each atom is greater than little "zero point energy" left in each ture research. the distances between atoms. atom. It hides in the space provided by They were not, however, so univer- A Russian theorist named Pomeran- the Heisenberg uncertainty principle, sally attracted to it back in the '60s. It chuk had suggested that one of its curi- which says that we can never know ex- took a fortuitous joining of several osities might be that when helium-3 is actly where something is and how fast events to get Cornell's low temperature compressed, it would cool down, just it's moving at the same time. At absolute effort going. the opposite of what any other substance zero, an atom isn't moving much, so its One was the creation by the Advanced would do. motion is known with great certainty; Research Projects Agency of the Depart- Lee thought this effect might offer a that makes the uncertainty in its position ment of Defense of a nationwide "mate- way to reach lower temperatures than large, and within that uncertainty the rials science" program, designed to en- ever before, but it was clear that it would atom is free to vibrate. courage research into strategic materials take a couple of years just to develop the It turns out that this "wobble" is for aerospace and nuclear technology. technology. It was a natural for funding greater with lighter particles. With the The program continues today under the by the Materials Science Center. "It was very light helium atom, it's greater than National Science Foundation. Thirteen a matter of faith that they would find the distances over which the forces at- Materials Science Centers were created new science when they got there," says tracting atoms to one another can work, at American universities; Cornell's was Johnson. "It would have been very dif- and this is what keeps helium from one of the first and today is the largest. ficult for them to get funding else- freezing into a solid. What we should MSCs are funded by block grants, where." About half of the low tempera- find at absolute zero, current theory which may be a dirty word to Democrats ture group's funding still comes through says, is not perfect stasis, but perfect but offers special advantages to scien- MSC, but the rest is now in straight NSF order. In most substances this is reached tists: a local MSC can fund projects on grants. "Now they know they're buying by having all the atoms lock together in its own evaluations, almost on hunches. a winner," Johnson says. a crystal lattice; in helium it seems to It can start something quickly and stick Meanwhile, John Reppy was at Yale, come from having all the atoms move in with it for a long time, even if there are earning notoriety for experiments with unison. no concrete results right away. rotating liquid helium. If you put super- At 2.17K, helium-4 (the most com- Another was the creation of LASSP, fluid helium in a doughnut-shaped tube mon isotope of helium) begins to change which, it's said, gave the people at Cor- and start it rotating, it will go on for- into a "superfluid" that flows as if it nell who were not high energy physicists ever, as long as it's kept cold. Reppy had had no friction, passing through open- a place to hang their hats. Robert been studying these "persistent cur- ings too tiny for most gases. Richardson Sproull '40, the first director of LASSP rents" and was invited to come to Cor- explains this behavior by analogy: mem- and now president of the University of nell to lecture about them. "After the bers of a marching band can move swift- Rochester, recalls that the new labora- lecture," Reppy recalls, "Jim Krum- ly out of a football stadium through a tory made a conscious decision to ex- hansl came up to me and asked if Vά like small gate; a mob, pushing and shoving pand low temperature research, and that to continue those experiments at Cor- at random, takes much longer to get Lee's hiring was an early step in that ef- nell." Professor Krumhansl [PhD '43] through the same opening. The oppor- fort. was the second director of LASSP and

MAY 1982 another prime mover in building the low lute zero, rather than a few thousandths. temperature group. Although the discovery was acciden- Lee and Reppy had met at the Univer- tal, Richardson staunchly defends this sity of Connecticut and moved more or kind of science. A lot of research, he less together into the low temperature points out, begins with theory, followed group at Yale, while Reppy introduced by experiments to check out the theory. Lee to the joys of rock climbing. Both "But if you only did that kind of re- had gotten into physics somewhat by ac- search," he says, "you could never find cident. Reppy had been a math major out anything you didn't already basically who took a summer job in a physics lab. know. There's a whole other class of Lee had discovered he could get out of things where you stumble onto interest- the Army early by going to graduate ing results. To get there, you have to school, had planned to go into comput- say, 'What are matter and nature like in ers, but was told he couldn't do it with- this extreme environment where people out an electrical engineering degree. haven't looked before?' " Richardson, by his own account, got In his view, low temperature physicists into low temperature physics in a rather are still in a data collection stage, far calculating fashion. He had already ahead of the theorists. They are like Liv- given up on chemistry, being color ingstones in Darkest Africa, drawing blind, and on EE, because "transformer maps of uncharted territory. He suggests design equations drove me nuts." In asking very simple questions. For in- physics, though, "my grades were so stance, he notes, Dutch physicist Heike bad I couldn't get into a good graduate Kammerlingh Onnes was merely trying school." He had, for instance, applied to see how Ohm's Law applied at low at Cornell and been rejected. temperatures when he discovered super- "I looked for a field where I could conductivity. "You only discover break- find a good research group at a less com- throughs in restrospect," Richardson petitive school," he says. He ended up in concludes. the highly respected low temperature The discovery of superfluid helium-3 group at Duke, studying under Swiss was a breakthrough because, among physicist Horst Meyer. other things, it allowed theorists to make Just as Reppy was settling into Cor- comparisons with the behavior of elec- nell, the MSC grant came through, and trons in a superconductor. There is a Lee recalls, "We were informed that rule in physics that sounds ridiculous to Horst Meyer had this wonderful grad outsiders and which even physicists seem student, Bob Richardson, and we to take on faith: that bodies made up of thought this would be the ideal opportu- an even number of particles—called nity to get him here." Richardson signed "bosons"—behave differently from A disc of lead 'levitates* at very low on as a post-doctoral research associate. those made up of an odd number of particles—called "fermions." Bosons are temperatures after the metal becomes a "And we all got together," Lee says, superconductor, the conclusion of allowed to become super fluids, and fer- "and sort of started building this labora- Prof. Richardson's demonstration tory." mions aren't. lecture on the opening page of thie About four years later, all the invest- A helium-4 atom is a boson, with two article. Because the lead is cold enough ments began paying off. Lee, Richard- protons and two neutrons in its nucleus to be a superconductor, a persistent son, and Osheroff were still trying to and two electrons in orbit. A helium-3 current flows in it which produces a perfect Pomeranchuk cooling, squeezing atom has one less neutron, so is a fer- magnetic field repelling the field from liquid helium-3 with a hydraulic press mion. It turns out that in a superfluid, a magnet below the disc. Such a current can be sustained indefinitely in that used liquid helium-4 as the working helium-3 atoms pair off, forming a new structure with a total of ten particles. a superconductor. (The Japanese fluid. They were looking for a magnetic National Railway is building a change that was supposed to occur in Because electrons come one at a time, levitating train based upon this solid helium-3 at about 1 mK, and so far theorists also believe electrons pair off in principle.) The bubbles are the boiling no one at Cornell had cooled anything to a superconductor. of liquid nitrogen in an outer vacuum less than 3 mK. Since the discovery, the Cornell group flask that insulates the inner container The night before Thanksgiving in has studied helium-3 extensively. of liquid helium in which the lead disc 1971 Osheroff noticed a peculiar glitch They've found that it actually has three has been cooled. in the pressure curve as the sample different superfluid phases, distin- cooled. "The first paper we published guished by the ways in which the atoms enough helium-3 and you can keep the was wrong," Richardson says. "We form pairs. helium-4 from going to superfluid down thought we had found the magnetic John Reppy has explored the odd to a temperature where the helium-3 and transition." It took about six months things that happen when helium-3 and helium-4 unmix like oil and water. for them to discover that what they had helium-4 are mixed. They mix nicely un- While exploring these changes, Reppy found was the superfΊuid phase of heli- til the temperature is lowered far enough discovered a special crossroads of tem- um-3, at 2.7 mK. Theorists had pre- to send helium-4 into its superfluid state. perature and concentration where the dicted it would come, if at all, at temper- But the temperature at which that hap- mixing/unmixing and the superfluid atures in the microKelvin range—-i.e., a pens depends on how much helium-3 is transition coincide. Again, it was some- few millionths of a degree above abso- in the mixture to get in the way. Add thing the theorists hadn't predicted.

20 CORNELL ALUMNI NEWS same fluid inside a porous glass called Vycor, manufactured by Corning Glass. Vycor is like a very fine sponge, with pores about fifty Angstroms across, or about ten to fifteen times the diameter of an atom. Reppy works at temperatures where the Heisenberg uncertainty of a helium atom is about 100 Angstroms; an atom is never pinned down to any one pore, so what you get, Reppy says, is something like another superfluid gas. As he makes the fluid more and more dilute, the data looks stranger and stranger, presaging a new phase transition discovery. Richardson is branching out into studies of solids. He has noticed that in some experiments heat transfer from solid to liquid is about 1,000 times what it should be, suggesting some new heat conduction process. In solids, heat trav- els in vibrations of the crystal lattice, with a row of atoms vibrating like a plucked guitar string. As with a string, the vibration is controlled by the length and tautness of the chain. Richardson is using the facilities of Cornell's new Sub- micron Facility to make tiny cubes of matter in which the length of the chain will be too short for a vibration to happen at low temperatures. Where will the energy go then? "That's what we want to find out," he says. While all this is going on, the quest for even lower temperatures continues. Richardson says the lowest temperatures reached in low temperature experiments drops by a factor of ten about every ten Richardson takes in a liquid nitrogen the forces that attract and repel atoms. years. Last year, researchers in Germany 'cocktail.y Vapor is from the very cold It becomes even more interesting to used a new type of magnetic cooling to nitrogen which he holds briefly in his study such changes at very low tempera- reach a temperature of about 20 micro- mouth during the public lecture. tures; with thermal agitation removed, Kelvin, or 20 millionth of a degree above the forces are much easier to measure. absolute zero. The laws of thermody- Someone else named the discovery the If phase transitions are the landmarks namics forbid us from ever going all the * Uncritical point," and the upside-down in this early exploration of Darkest Heli- way to the bottom, but by Richardson's Y graph that illustrates it has become a um, then new landmarks may be just estimate the 1990s might see tempera- part of the low-temperature group's un- over the horizon. Lee is trying to make tures only a few billionths of a degree official "logo," along with a graph of the world's third superfluid out of hy- from the ultimate. the helium-3 super fluid transition. Since drogen that has had its molecules pulled Speaking to the American Physical then, similar tricritical points have been apart into single atoms and held that Society, Richardson reminded his audi- found in a number of other substances, way by a powerful magnetic field. This ence that in 1961, Prof. A. Brian Pip- to the point where looking for them has "spin-polarized hydrogen" may be the pard of Cambridge University declared become almost a specialty in itself. first superfluid gas. The trick is to get that there were "no more really funda- Most of us are familiar with the enough of the gas to work with. So far, mental questions" remaining in pure changes from solid to liquid and from Lee says, he can get it "unreliably." physics, and especially in low tempera- liquid to gas. Physicists call these "phase He's in a sort of informal race for the ture physics. transitions," and the same term applies discovery with MIT, the University of In reply, Richardson listed the discov- to the exotic changes from normal fluid British Columbia, and the University of eries of the '70s, concluding: "Our ex- to superfluid or from one sort of mag- Amsterdam. perience with helium-3 leads me to be an netic behavior to another. A physicist Reppy, meanwhile, has been working optimist. There are innumerable things will pounce on a new phase transition with thin films of helium desposited on a to be discovered. There is no certain like a greedy Irishman upon a sleeping glass surface. At low temperatures, the recipe for success in finding the unex- leprechaun. It's a place where atoms re- first layer of atoms sticks on, but the pected; it is a matter of luck. The only main unchanged within themselves, but second layer moves freely on top of the certain prescription for failure is to be so interact with their fellows in new ways, first, making a sort of two-dimensional pessimistic that one abandons the and so it gives us new information about superfluid. Lately he's been making this search."