A tribute to James L. Smith

Jason Lashley, Peter Riseborough and Anders Rosengren

This issue contains papers presented at the workshop on strongly correlated materials held in honour of James. L. Smith on his 65th birthday. Over 100 physicists congregated in Santa Fe to commemorate this occasion. Dr. Smith has authored over 400 papers and interacted with a similarly large number of scientists, established and aspiring. In addition to his numerous scientific contributions, he is also known for mentoring and giving sound advice to developing scientists. A brief summary of Jim's extensive scientific career, from start to the time of writing, is given below.

James L. Smith's approach to Physics has always been unique. This uniqueness is manifest in the way he chose Wayne State University for his undergraduate education. He made the decision on where to go after having taken the real inside tour through the bowels of the university conducted by a janitor. After graduating from Wayne State in 1965, Mr. Smith then pursued his PhD at Brown University. At Brown, he worked under the guidance of George Seidel on topics such as low-temperature relaxation of coupled nuclear-electron spin systems and the heat capacity of Fe⁺⁺ in MgO. While still at Brown, Jim also took advantage of the opportunity to perform pioneering research together with Philip J. Stiles on MOSFETS 1. Their results are of note since they found the first evidence of many-body effects in the two-dimensional electron gasses, through the electron density dependence and also through the magnetic field-dependence of the quasi-particle masses. It was also through Philip J. Stiles that Jim had his first encounter with superconductivity. Together they investigated the superconducting gap of semiconducting GeTe.

After finishing in 1973, Dr. Smith took up a permanent position at Los Alamos Scientific Laboratory. These were Jim's formative years in which he became interested in the interplay and coexistence of magnetism and superconductivity. Since he was in close proximity to metallurgists, he began talking with people such as Sig Hecker on a regular basis. Together with Hunter Hill and A.L. Giorgi and others he studied the magnetic properties of the actinide materials U, Pu and Np. In 1979, Jim, C.Y. Huang, C.W. ‘Paul’ Chu and others published the results of their first investigations of the interplay of magnetism and superconductivity in a number of ErRhB₄ alloys. That same year he also published his first paper with Berndt Matthias on TiBe₂, which has become well known as a spin-fluctuator. Berndt Matthias must have had a strong influence on Jim's way of thinking. Berndt Matthias had a strong conviction that there was a class of superconductors in which magnetism was responsible for Cooper pairing. The truth of Berndt's conviction was eventually revealed when the family of heavy fermion superconductors was discovered. Through Matthias, Jim got to know Angus Lawson, Zachary Fisk, and Brian Maple.

In 1975, Johansson and Rosengren 2 predicted that Am is a superconductor, which was a prediction in strong conflict with ‘common belief’. Many tried to do the experiment, but the selfheating from radioactivity of the element prevented them from seeing the transition. In 1977, Jim discovered that Am was superconducting at 0.79 K 3. Jim immediately sent a telex about his findings to Johansson, Rosengren's thesis advisor, not knowing that Rosengren was about to defend his PhD thesis in two days. Thanks to Jim's telex Rosengren defended his thesis against the opponent's angry accusations that the respondent was making claims no one could ever verify, by triumphantly taking the telex out of his pocket and reading it out aloud! In retrospect, it is also interesting to observe the note in Jim's paper 3, saying ‘It should be noted that the rare earth analog, Eu, chooses to change valence and adopt an f ⁷ configuration which makes it magnetic. Thus Am is the only element with the rather special non-magnetic f ⁶ configuration’. Rosengren and Johansson 2,4 predicted that Eu, at high pressure, would attain the non-magnetic f ⁶ configuration and thus be a superconductor. Support for this was then obtained by Jim, who showed that the compound EuIr₂, where Eu is believed to be trivalent, does superconduct below 3 K at ambient pressure 5. Only a week ago high-pressure experiments were reported that show that at 80 GPa Eu becomes superconducting at 1.8 K 6. Jim's discovery of superconductivity in Am demonstrated that the interesting behaviour of the light actinide elements terminated at Am. Jim and his coworkers have formulated a set of extremely influential ideas generated during their studies of the actinides, and have found striking ways of visualising these ideas. An excellent example of this is provided by Smith and Ed Kmetko's nearly periodic table of the elements, which resulted from their studies of the interplay between magnetism and bonding 7. Along similar lines, Jim had been introduced to the isostructural phase transition in cerium, and took this up with Joe Thompson, Zachary Fisk, Richard M. Martin, and Jon Lawrence where, by doping, they revealed that the phase diagram contains a second (precociously hidden) critical point 8.

At about this time, Jim had assembled a team of highly-skilled collaborators around him with members that included Zachary Fisk, Jeff Willis, and Greg Stewart. Together with members of this team, Jim made his greatest breakthroughs by creating the field of heavy fermion superconductivity. In 1979, Frank Steglich and his coworkers reported that the superconductivity occurred in a significant volume fraction of carefully prepared samples of CeCu₂Si₂. They showed that the specific heat jump that occurred at the superconducting transition was extremely large, indicating that the quasi-particles that paired in the superconducting state were extremely heavy. This discovery motivated speculation about the nature of the pairing mechanism. In conventional superconducting metals, s-wave pairing is brought about by the exchange of phonons, which produces a retarded attractive interaction. However, if the quasi-particles are sufficiently heavy so that the Fermi energy is no longer large compared to the Debye frequency, the retarded nature of the interaction would be lost, and the instantaneous Coulomb interaction might result in a repulsion. Therefore, a new pairing mechanism might be at work.

Motivated by Steglich's discovery, in 1983, Jim and his group re-examined UBe₁₃ and found that it behaved similarly to CeCu₂Si₂. Polycrystalline and single crystals were made at Los Alamos and were found to superconduct. The specific-heat data below the superconducting transition showed deviations from the exponential temperature dependence predicted by BCS theory of superconductivity assuming an s-wave order parameter, but it was more consistent with a power-law temperature variation. This observation revealed the possibility that the superconducting pairing was exotic, perhaps similar to the p-wave pairing found in superfluid ³He where the order parameter has lower symmetry than the normal fluid (apart from the breaking of gauge symmetry) and where nodes exist in the order parameter. It had been speculated that the existence of nodes in the superconducting order parameter was conducive to the formation of pairs, since they might minimise the effects of the strong instantaneous Coulomb repulsion. It had also been theoretically predicted that non-magnetic impurities suppress non-s-wave superconductivity in a manner similar to the suppression of s-wave superconductivity by magnetic impurities. Since the possibility of exotic pairing remained to be tested, Jim and his coworkers substitutionally doped UBe₁₃ with Th. Their 1984 study showed a non-monotonic decrease in the superconducting transition temperature with increasing Th concentration. More accurate measurements have indicated that the phase boundary actually may have a cusp in it. Two specific-heat jumps were observed that signaled the existence of two distinct superconducting phases. The presence of the second transition was confirmed by sound velocity and ultrasonic attenuation experiments. The existence of two different superconducting phases and their competing order parameters confirmed that the order parameter must have a lower symmetry than the normal state.

Jim's team also discovered heavy-fermion superconductivity in UPt₃, which shows the signatures of spin fluctuations. Initially, they observed a rounded jump in the heat capacity of UPt₃ at the superconducting transition temperature. The cause of this rounding was discovered later when it was found that, like UBe₁₃, the specific heat of UPt₃ has two superconducting jumps and hence, it is also an unconventional superconductor. UPt₃ is specially important since this material first showed the relationship between superconductivity and magnetism and, since it can be prepared in a very clean state, it is also the material in which the superconducting phases have been most extensively studied. The discovery of the heavy-fermion superconductivity in UBe₁₃ and UPt₃ and their multiple superconducting phases by Jim and his coworkers, marked the explosive birth of the field of heavy fermion physics.

When high-T _c superconductivity came along, Jim also entered the fray. In Jim's characteristic way, Max Fowler, Fred Mueller and Jim shook the scientific world by their explosive studies of the Fermi surfaces of the high-T _c superconductors. However, Jim did not forsake the study of his favorite uranium and cerium compounds. Jim is continuing to uncover new and surprising phenomena. Since we all expect that Jim will continue to do the unexpected, it is highly appropriate to note that the story of his scientific career continues.

Jason Lashley

Los Alamos National Laboratory,

Los Alamos, New Mexico, USA

Peter Riseborough

Temple University,

Philadelphia, Pennsylvania, USA

Anders Rosengren

KTH (Royal Institute of Technology),

Department of Theoretical Physics,

AlbaNova University Centre,

Stockholm, Sweden

With Apologies to Gilbert and Sullivan

D.J. Alexander and R.E. Hackenberg

I am the very model of a condensed matter physicist,

I’ve worked with Börje, Anders, Hunter, Zachariasen, and Fisk,

I know all the elements, and scientists historical,

From Faraday to Seaborg, in an order categorical;

I’ve information hazardous and also radiological,

I scan research proposals for hypotheses illogical,

With editing and paper piles my office is a mess,

But I organize when needed–I’ve laid out the CMS!

My home is in the lab, where to milli-Kelvin strived,

Dilution ‘fridges suck the heat and gases liquefy,

Cryogenic resistivities provide a telling hint

Of electrons shackle-bound or running loose, itinerant;

f-electron physics give phenomena quite strange,

So much that periodic tables had to be rearranged.

For weighty fermions, I have a great proclivity,

Correlations giving rise to superconductivity,

Even better when combined with radioactivity,

Decaying while exhibiting Meissner's magnet-levity.

To prepare exotic compounds I will use exacting measure,

Weighing elements high-purity, bean-counters’ LANL treasure;

I can grow a single crystal from a long, amazing list,

Phase diagrams gave me the eyes of a metallurgist.

Plutonium, uranium, and all the other actinides,

I’ve seen them all, in their pure form, and as powdered oxides,

Monoclinic, orthorhombic and simple old hexagonal,

There's fcc and bcc, and even some tetragonal.

I’ll offer my opinions, scientific and political;

I know the facts, and rest assured, they’re true, not hypothetical;

I’m very well acquainted too, with finer points grammatical,

And modern Magazines as well, with titles Philosophical;

I’ll enlighten you with ancient lore and scientific anecdotes

Of Agnew, Edward Teller, and papers Bernd Matthias wrote.

I’ve mentored post-docs, colleagues, and also upper management,

And even this latter has been time and effort wisely spent;

To make a point I’ll stare you down and say “Do you understand?”

When giving sage advice to Jason, Bob, or other younger hands.

By physicists’ lax standards, I’m sartorially dressed,

Blazer pockets hold agendas, fancy pens, and Nicorettes,

A College scarf from Trinity accompanies my overcoat,

An apple and a manuscript are always found within my tote;

With noon-time care my flower garden has been greatly flourishing,

Proving apple cores and coffee grounds are both superbly nourishing.

I’d like to think, and I would hope that you would all agree it's true,

That I’m a fine example of what Brown can do for you;

Though many years at LANL made me partly a metallurgist,

I remain the very model of a condensed matter physicist.

D.J. Alexander

Los Alamos National Laboratory

R.E. Hackenberg

Los Alamos National Laboratory