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Understanding how electrons behave may lead to room-temperature superconductors.
When Elbio Dagotto is asked to explain the intricacies of strongly correlated electron systems, he grasps for an analogy. "This is very complicated," he says, stating the obvious about what is considered one of the most important problems in condensed matter physics. Correlated electrons have the best minds searching for needles in a haystack of quantum mechanics in the hope of discovering materials that have fantastic new properties such as superconductivity at room temperature and colossal magnetoresistance. In some metals, electrons flow without difficulty, largely because the distances between electrons are small enough to enable the metal to conduct electricity. As a result, the so-called "one electron approximation" in these metals is valid because the total is obtained from the properties of individual electrons. In materials that are strongly correlated, electrons do not serve as freely operating agents. Instead, these negatively charged particles interact with each other, forming collective states that defy the traditional behavior of their singular counterparts. The result, says Dagotto, a theoretical physicist and distinguished scientist at ORNL and the University of Tennessee, is "all kinds of exotic properties." To explain collective behavior, Dagotto cites a popular analogy to illustrate strongly correlated systems in low-temperature superconductors. "Imagine you are part of a gigantic army of people holding hands," he says. "When the army moves forward, you collide with a rock. The fact that you are attached to a large number of people moving forward will keep you in motion. If you were on your own and collided with a rock, you would stay there. But in a collective state you are pushed forward around the rock. Once the current starts moving in a superconductor, stopping the current is extremely difficult because individual collisions do not affect the collective effect."
Dutch physicist Heike Kamerlingh Onnes discovered this extraordinary behavior in 1911 when he observed that mercury, when cooled to -452°F (about 7 degrees above absolute zero), dispelled electrical resistance, rendering the element a superconductor. A theory to explain low-temperature superconductivity was developed in 1957, but the theory does not apply to the two families of high-temperature superconductors discovered in 1986 and recently. A collection of outstanding theoretical and experimental physicists has been drawn to ORNL by the presence of the Department of Energy's Spallation Neutron Source, the supercomputing power available through the Center for Computational Sciences and the potential collaborations with fellow researchers at ORNL and the University of Tennessee. The unique combination of resources makes the Laboratory one of the premier U.S. institutions for research on superconductors and other strongly correlated systems. ORNL is home to groups of theorists that develop computer simulations of model Hamiltonian systems—simpler expressions of complex physical interactions; perform first-principles calculations for real materials; explore magnetism and predict the effect interfaces will have on the properties of materials featuring strongly correlated systems. Another group of researchers studies interfaces and surfaces of strongly correlated electron materials experimentally, synthesizes new crystals and thin films and analyzes these samples using neutron sources such as the SNS and ORNL's High Flux Isotope Reactor, as well as electron and scanning tunnel microscopy.
"Oak Ridge is very strong in correlated electron research," says David Mandrus, who leads experimental work in ORNL's correlated electron materials group. Noted theorists in the field at ORNL include Dagotto and David Singh, a world-renowned expert in band structure, which describes rules governing the range of energy an electron is allowed to have within a solid. For example, ORNL researchers are the first U.S. group and the fourth in the world to publish a paper on the recently discovered second family of high-temperature superconductors, based on iron and arsenic (the first family is based on copper and oxygen). On the experimental side, the group led by Mandrus grows crystals of materials known or suspected to have an aptitude for correlated electron behavior, which can then be tested using instruments at the SNS, HFIR or other ORNL user facilities. Another experimental group closely associated with the Laboratory's nanoscience center and Materials Science and Technology Division is studying the interfaces and surfaces of strongly correlated systems through laser deposition of thin films. These "superlattices" with atomically abrupt interfaces allow scientists to study the influence of strain and local asymmetry, as well as coupling at the nanoscale, on ferroelectric, transport and magnetic properties. Superconductors, the poster child of correlated electron systems, demonstrate the extraordinary nature of these materials. In superconductivity, electric current flows without resistance below a certain temperature, effectively creating the closest thing to perpetual motion found in nature. Other materials demonstrate less well-known properties such as colossal magnetoresistence, which refers to changes in electrical resistance in the presence of magnetic fields. Today's computers store information using very small magnetic regions that, by pointing in one direction or the opposite, define the well-known bits "1" and "0." In the case of some correlated systems, the resistance changes by several orders of magnitude, making them potentially useful for the detection of the small magnetic fields of those bits, as computer processors and memory continue to shrink. "We need a material with a resistance that changes a lot in the presence of a tiny magnetic field," Dagotto says. "Some of these strongly correlated systems possess that property with the caveat that, like superconductivity, these materials are chilled to low temperatures. One goal is to be able to trigger these phenomena at room temperature, a breakthrough that would make them much more useful for practical application." Achieving this goal requires a much better understanding of why correlated electrons behave the way they do, a question at the heart of research by Dagotto and others at ORNL. The field of strongly correlated systems is divided into several paths of attack, with some scientists focused on superconducting materials and others looking at ways to add functionality to semiconductor devices using these new materials, primarily transition metal oxides, in a path of research known as oxide electronics. Other researchers are theoretically or experimentally probing the interfaces between layers of material that exhibit exciting new properties. Still others are growing crystals of materials that have shown promise as superconductors and then are characterizing them using neutron diffraction or electron microscopy. Hans Christen, leader of ORNL's Thin Films and Nanostructures group, is experimenting with microscopic layers of materials that show promise as strongly correlated systems to learn how these films interact with each other to give the superlattice interesting properties. "We start with a well-prepared crystalline surface and then use pulsed laser ablation to create a highly energetic vapor that condenses onto a substrate and grows a thin layer," Christen says. "We repeatedly grow layers of magnetic materials where electron correlations are important." Electron correlations operate on a certain length scale, but Christen and his colleagues can layer the materials they are studying on a shorter length scale to disrupt some interactions and create new ones. They can get self-organized pockets of competing states, such as conducting and insulating phases. This ability to manage correlated electron behavior in which a very small input triggers a big response shows promise for applications such as sensors. Similarly, researchers in the Low-Dimensional Materials Physics group have been patterning complex oxides into extremely narrow "channels" to see the effect of self-organization on current flow. In Mandrus's lab, scientists grow bulk samples of interest in correlated systems research, with a focus on transition metal oxides. The work also includes doping promising new materials with electrons to test their potential as superconductors. ORNL's work with strongly correlated systems represents the forefront of research not only in physics but also in the future of science as a whole. —Larisa Brass
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