Superconductor materials have no resistance to the flow of electricity and are thus useful in a vast range of present and future scientific and technical applications, such as high-energy particle research, sensitive electromagnetic instrumentation, magnetic levitation, biomagnetics, production of high magnetic fields, commercial electricity, and computers. The properties of superconducting metal alloys and other compounds are made evident at low temperatures. Physicist Francisco de la Cruz, a Foreign Associated Member of the National Academy of Sciences since 2002, has spent most of his research career studying such properties, especially as they pertain to magnetic vortices in superconductors.
In his Inaugural Article in this issue of PNAS, de la Cruz and colleagues examined the elastic properties of superconducting vortex structures (1). Says de la Cruz, “In a way, it is simpler to use the vortices to study elastic behavior than other elastic systems.” The team introduced external magnetic forces to see how vortex lattice structures respond. “By studying the elastic vortex lattices... we can learn about other elastic systems and elastic matter,” he says (2).
Figure 1.
Francisco de la Cruz. Photograph courtesy of Dr. Gladys Nieva (Low-Temperature Division, Centro Atómico Bariloche).
de la Cruz did not follow a standard path to academia, but this has not stopped him from achieving a successful career in physics. In addition to the usual challenges faced by budding scientists, geographical isolation, war, and political upheaval have all been roadblocks overcome during his studies and research.
Barcelona to Bariloche
de la Cruz was born in Barcelona, Spain, in 1938, in the middle of the Spanish Civil War. “My mother was a schoolteacher. She was forbidden to teach after Franco became the head of the state in Spain,” says de la Cruz, “so I never went to primary school, because I was the only one my mother could teach.” She taught him at home and encouraged logical and mathematical thinking. de la Cruz did not begin formal schooling until he was 12 years old, when he went straight to high school. The following year, his family moved to Salta in northern Argentina, near the Bolivian border, in hopes of providing a better education for him and his sister.
Graduating from high school in 1955, de la Cruz entered the Universidad Nacional de Córdoba (Córdoba, Argentina) in the following year to study civil engineering, which he did not enjoy. In Córdoba, he met Ricardo Broglia, who suggested that they both transfer to the then-new Institute of Physics in Bariloche, Argentina, a national atomic research facility and teaching institution founded in 1955 by the Comision Nacional de Energía Atomica. The teaching institute has since been renamed Instituto Balseiro in honor of its founder, Jose Antonio Balseiro. The prerequisites of the institute comprised two years of math, physics, or engineering, in addition to an entrance examination. de la Cruz and Broglia both passed the examination and transferred to the institute in 1958. de la Cruz obtained his master's and Ph.D. degrees in physics in 1961 and 1968, respectively.
During this time, de la Cruz—known as Paco to his friends—met the two men who would have the most significant influence on his career. The first was Balseiro, a theoretical physicist who had convinced the Argentinean government to establish his physics institute 1,600 km from the country's capital, Buenos Aires. Located in the mountainous Patagonian countryside, Balseiro's goal was to isolate the institute from Argentina's often unstable politics. “It sounds very awkward, but it proved to be a good idea,” de la Cruz says. “I enjoyed studying physics in a place that was very isolated.”
High on Low-Temperature Physics
Balseiro was an active researcher and teacher who taught many physics courses. He was convinced that the institute had to become more active in laboratory research, not just theoretical physics, and looked for an area it could take up. In 1959, Jim M. Daniels suggested that low-temperature physics was “a competitive area of research that could be done without spending too much money,” de la Cruz recalls. Daniels took a few people, including de la Cruz's future wife, Maria Elena Porta, to Vancouver, Canada, for a one-year training period in experimental techniques, while de la Cruz remained at the institute.
Acting on Daniels' suggestion, Balseiro invited John C. Wheatley as a Fulbright fellow to a 1.5-year contract to help establish a low-temperature physics laboratory at the institute. Because the startup costs were much less than those of nuclear reactors or particle accelerators, this was a good way for the institute to enter experimental research. By 1961, Wheatley and his team had liquefied hydrogen and helium and cooled it down to 30 milliKelvins by adiabatic demagnetization, a significant achievement (3).
Wheatley, then a 33-year-old physicist, became de la Cruz's biggest influence as an undergraduate and as a beginning Ph.D. student. “[Wheatley] was strong and difficult, but he impressed me very much,” remembers de la Cruz. “We fought many times because I didn't always agree, but he was an excellent teacher.” Wheatley was also a positive role model in terms of the outlook he brought to science, teaching de la Cruz and other graduate students how to approach science seriously and ask appropriate questions. “When he left, there was only one cryostat shared by six graduate students,” de la Cruz says, “and we communicated with him by short-wave radio asking for directions and advice every Wednesday.”
“Not so many were ready to consider the challenge of making a worldwide-respected low-temperature laboratory in Bariloche.”
de la Cruz and two others decided to build another low-temperature facility. His Ph.D. thesis was on thermal and electric transport properties of pure metals at low temperatures (4). That subject “was not so easy to do, but not impossible, and maybe not a breakthrough, but interesting,” says de la Cruz. His supervisor, Ricardo Platzeck, was a local astronomer and “outstanding experimental physicist” who was kind enough to keep an eye on him, according to de la Cruz.
After graduating from the institute in 1968, de la Cruz obtained a postdoctoral position at Brown University (Providence, RI) working for Manuel Cardona in collaboration with George Seidel. At Brown, de la Cruz had the opportunity to change fields and study the optical properties of semiconductors, in which Cardona was a world leader, or continue with low-temperature physics research. “Since my wife and I were convinced of our return to Bariloche, the decision was clear,” explains de la Cruz. “Working in low-temperature research with [the] Cardona and Seidel groups was an excellent opportunity to increase my expertise in the area to be applied in our return to Bariloche.” He proceeded to study type I superconductors and thermodynamic fluctuations in super-cooled superconductors in Cardona's and Seidel's laboratories.
de la Cruz returned to the institute in 1972, where he has remained since. Returning to Argentina was an important family decision, he says. “We were convinced we had a mission to carry out, helping to sustain the work made by Balseiro,” who had passed away in 1962, at the age of 42. “I came back to Bariloche because I thought my chances of doing something relevant were better in Argentina,” says de la Cruz. Although confident in his own abilities to perform good physics research, he was sure “other physicists could do much better than I in the U.S.” Furthermore, de la Cruz felt that “not so many were ready to consider the challenge of making a worldwide-respected low-temperature laboratory in Bariloche.” This year, de la Cruz and the institute will hold numerous events and celebrations to mark its 50th anniversary, observe its achievements, and honor Balseiro's memory.
Conductor of Superconductivity
The personal and scientific relationships established with Cardona and Seidel at Brown strongly influenced the work that de la Cruz undertook in Bariloche upon his return. He studied and used low-temperature physics to address the goal of understanding superconductors and the behavior of other solid materials.
Superconductors lose electrical resistance at low temperatures. Mercury was the first superconductive material discovered, in 1911; at temperatures close to 4 K, mercury suddenly loses electrical resistance and becomes superconductive. Until 1986, superconductivity was observed only in materials at temperatures <30 K, but that year scientists at IBM Zurich discovered that some ceramics become superconductors above 30 K (5). The discovery of these high-temperature superconductors presented new challenges and opportunities for de la Cruz and his group.
Possessing years of low-temperature physics research and the necessary equipment and skills, de la Cruz was poised to attack this high-temperature field immediately. A collaborator, Ivan Schuller, telephoned him and said that, although several previous high-temperature superconductivity discoveries never panned out, ceramic superconductors posed a promising field that should be investigated quickly. “We had support from an excellent local solid state theory group, very capable people in materials sciences, and a just-begun collaboration with David Bishop from Bell Labs,” says de la Cruz. He asked the materials laboratory if it could prepare these compounds, which it did. “We were among the first to dive in,” he says.
According to de la Cruz, two main problems currently face high-temperature superconductivity. The first is solving the mechanism that produces the high-temperature superconductivity. The second is how to deal with the superconducting vortices created when the magnetic field enters into a superconductor. “When the magnetic field penetrates into a superconductor, it does so in a very fascinating way,” says de la Cruz. “It goes in magnetic vortices, similar to those you can observe in water, but they are created by superconducting currents that do not dissipate.” The number of vortices and distances between them are governed by the applied magnetic field, and changing the magnetic field alters the vortices' properties. In fact, vortices move when all electrical current goes through the superconductors, like those in water move in a running stream. As vortices move, electrical dissipation is induced, and the material loses its most relevant property for applications. How these vortices form and interact is still a matter of research.
Into the Vortex
“The physics of vortices is what we have contributed best,” de la Cruz says of his research team's scientific achievements. He and his team have developed “an intense and productive” collaboration with Bishop's group, continuing today. “Vortices tend to form ordered structures,” explains de la Cruz. “The structures are nearly perfect lattices.” In low-temperature superconductors, the lattices are rigid and stable, behaving like a solid structure. In high-temperature superconductors, however, the lattices melt into a vortex liquid through a first-order thermodynamic transition (6). In this liquid state, the superconductor cannot sustain electrical currents without dissipation, and technical applications require research to overcome this problem (7–9).
Vortices can also interact with each other, which de la Cruz addresses in his Inaugural Article. If vortices overlap enough, they can become entangled, and at that point “it's difficult to define their length,” he says. “We invented a way to measure the effective length of a vortex” (10).
An ideal hexagonal periodic lattice, called an Abrikosov Lattice, expected for interacting vortices is not the observed structure in a real superconducting material. Atomic defects, which are always present in materials, interact with the ideal periodic vortex structure and give rise to a distorted system where periodicity is lost. This nonperiodic but almost ordered elastic configuration of vortices (called hexatic) is often detected in other problems in condensed matter. Thus, vortices provide an excellent model to study how an elastic hexatic structure responds to the presence of a structural rigid perturbation.
Based on this finding, de la Cruz and his collaborators have been able to identify the conditions for matching between commensurate three-dimensional hexatic systems and rigid structures induced at the free surface of the vortex system. In his Inaugural Article, de la Cruz and coauthors investigated the elastic properties of such systems by studying the interaction of three-dimensional Bi2Sr2CaCu2O8 vortex lattices with two-dimensional surface pinning potentials (1). This experimental study has broadened the previously known conception of commensurability between periodic systems.
Life in Argentina
During all of his research studies, de la Cruz seriously considered leaving Instituto Balseiro once, in the 1970s. At that time, “the military government was very strong, and it was a difficult time for many people,” he says. He left in 1975 on a previously scheduled one-year appointment at the Max Planck Institute in Stuttgart, Germany, where his postdoctoral supervisor from Brown University, Manuel Cardona, was now an institute director. At the end of that appointment “the situation in Argentina was very, very bad,” de la Cruz says. The Max Planck Institute offered him a permanent position, but he and his family decided to return to Bariloche. Just in case, the Max Planck Institute kept their offer open for an additional six months, should de la Cruz choose to return based on Argentina's political situation. “I appreciated it very much, since I didn't know what we would face when we returned,” he says.
Having lived through the country's turmoil, today de la Cruz finds his 22 former doctoral students spread across Argentina as well as North America and Europe. “Teaching is maybe what I enjoy most, but also staying in the lab with students,” he says. “I've spent many hours with them and enjoyed it.”
de la Cruz has won several awards throughout his career, including the Téofilo Isnardi Award from the Academia Nacional de Ciencias in 1979 and the Dr. Ricardo Gans Award from the Fundación Gans and Universidad Nacional de La Plata in 1988. He was named a Chevalier of the Academic Palms by the French government in 1986 and is a member of the Argentinean Academia Nacional de Ciencias of the Academia Nacional de Ciencias Exactas, a fellow of the American Physical Society, and a member of the Third World Academy of Sciences. In 2004, he was nominated as a Fellow of the Institute of Physics and received the prestigious Fundación Bunge y Born Prize.
Now 66 years old, de la Cruz has just passed Argentina's mandatory retirement age of 65. “By law I should be retired,” he says, but the law is “pretty relaxed.” He gave up control of the low-temperature physics laboratory a few years ago and is ready to leave the institute to a new generation of scientists, many of whom are his former students. “It's not easy [to leave] after being leader for so long,” he says, “but it was also difficult for them having someone watching.” After he completes a four-month appointment in 2005 in his birthplace, Barcelona, he will be fully retired. He and his wife, Maria Elena, also a physicist, have decided to remain in Bariloche, where their daughter, an interior decorator, lives. Their son is a lawyer in New York City. Unlike de la Cruz's dedication to research, he jokes, “they wanted nothing to do with science.”
This is a Biography of a recently elected member of the National Academy of Sciences to accompany the member's Inaugural Article on page 3898.
References
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