This award supports theoretical research and education on strongly correlated electron materials with a focus on spontaneous electronic pattern formation at the nanoscale.
The PI will develop new ways of studying materials, explicitly including disorder, and using noise, nonequilibrium effects, and mesoscopic geometries in order to elucidate the local electronic patterns. For example, stripes (like other proposed real space orders) may be important for high temperature superconductivity, but they have only been observed in a subset of cuprate superconductors, most notably in cases where the stripes exhibit true long-range order. However, even disordered or slowly fluctuating stripes (invisible to many standard bulk probes) are sufficient for a stripes-based mechanism of high temperature superconductivity. The PI recently mapped the problem of disordered stripes to the random field Ising model. The PI will use simulations on this model to make predictions about how to detect disordered stripes using noise and nonequilibrium effects in, e.g., transport, STM, neutron scattering, and magnetic hysteresis.
This award also supports the PI?s efforts to continue to develop the mentoring program she began for graduate women in the physics program at her home institution, by initiating a program to invite graduate physics alumnae back to campus to discuss career options with current graduate and undergraduate students in physics. The PI will also continue to visit local high schools to discuss her research. This outreach combines interactive hands-on superconductivity demonstrations with education about contemporary condensed matter research. In addition, the proposed work will advance the training of one graduate student and two postdoctoral associates.
NON-TECHNICAL SUMMARY: This award supports theoretical research and education aimed at understanding fundamental questions raised in the study of high temperature superconductors. These are materials that can transport electric current without loss at sufficiently low temperatures. The physical mechanism by which electrons enter this cooperative quantum mechanical state of superconductivity remains a subject of intense research for the high temperature superconductors. These materials exhibit superconductivity at much higher temperatures, but still well below room temperature, than the much better understood superconducting materials that one might encounter in a medical magnetic resonance imaging machine. Understanding the mechanism for superconductivity may lead to the discovery or engineering of materials that exhibit superconductivity at still higher temperatures, with the possibility of enabling economical new technologies for power transmission and new electronic devices. The research will focus on an interesting aspect of the puzzle, the spatially varying patterns of characteristic quantum mechanical properties of electrons that have been observed in experiments on some high temperature superconductors.
This award also supports the PI?s efforts to continue to develop the mentoring program she began for graduate women in the physics program at her home institution, by initiating a program to invite graduate physics alumnae back to campus to discuss career options with current graduate and undergraduate students in physics. The PI will also continue to visit local high schools to discuss her research. This outreach combines interactive hands-on superconductivity demonstrations with education about contemporary condensed matter research. In addition, the proposed work will advance the training of one graduate student and two postdoctoral associates.
The behavior of electrons inside of copper-oxygen based high temperature superconductors has puzzled researchers since their discovery in 1986. The materials are ceramic, and just like your dinner plates, they’re brittle. If you drop them, they crack; if you step on them, they crush. Unlike shiny metals, these materials are a dull black. By all rights they have no business conducting electricity at all, but under the right conditions, they do conduct electricity, and they do it perfectly, without losing energy. This ability to carry electrical current with no loss of energy is why superconductors have tremendous potential to transform how we use and generate energy. Because copper-oxygen (cuprate) based superconductors do not follow the "usual" rules for how materials work, researchers are still trying to understand how and why the materials superconduct. Once we understand that, we may be able to design even better superconductors. Since superconductors do not "leak" energy like the regular metal wires used to carry electricity from our power plants to our homes and buildings, and ultimately to our lights, computers, and other electronic systems, designing better superconductors will go a long way toward solving the energy crisis. In order to understand how the materials work, this NSF Award has explored the connection between superconductivity and the nanoscale lines the electrons form on the surfaces of cuprate superconductors.[1] While some lines (also called stripes) are long and others short, the lines only run along two directions – "vertical" and "horizontal," corresponding to what’s known as the crystalline axes of the superconductor. The radically different behavior of electrons confined to move only along lines (as opposed to electrons which can move anywhere inside the crystal) may help explain why copper-oxygen based superconductors are able to superconduct at much higher temperatures than any other family of superconductors.[2] However, if the electron lines exist only on the surface, they can’t explain why the whole material superconducts (and not just the surface). There are several families of materials which are all cuprate superconductors. In some of these families (such as those based on yttrium and those based on lanthanum), the nanoscale lines of electrons are sufficiently aligned that they have been shown to exist throughout the interior of the materials using measurement techniques such as neutron scattering. In other families of cuprate superconductors (such as those based on bismuth), the lines of electrons are not lined up, and it has not been clear whether the lines of electrons even exist at all in the interior of the material. If the lines do not exist throughout the interior of the material, then they are not the key to the superconductivity in the materials. For this reason, it is vital to invent new ways to detect such lines, even in cases where they may be short, and even if they do not all line up in the same direction. Something similar happens with magnetic rocks found in nature. Typically, such rocks do not behave like magnets when initally mined, since the rocks contain tiny "domains" in which the direction of the magnetism (the north and south poles) point in different directions. Only when the domains are aligned can the material be used as, say, a refrigerator magnet. Taking a cue from the manufacture of commercially produced magnets, a team of researchers led by Prof. Erica Carlson has modified the protocols used to align domains in magnets, designing new protocols suitable for aligning the lines of electrons observed in cuprate superconductors.[3] These new techniques will allow the detection of these nanoscale electron lines, even if the lines are short and even in cases where the material has many domains of such lines pointing in different directions (i.e. the stripes are disordered). These new techniques will help answer the question of whether the nanoscale electronic lines hold the key to superconductivity in these materials. Broader impacts of this award include: (1) Outreach to public high schools using hands-on experience with levitating magnets over cuprate superconductors; (2) Outreach to preschools on magnetism; (3) Mentoring of women graduate students in physics; (4) Consulting for the popular video podcast Dr. Carlson’s Science theater www.sciencetheater.net ; (5) Development of human resources in the form of training of graduate students and postdoctoral scientists. More information about research supported by this award can be found in the links to videos in the image captions, and also at www.physics.purdue.edu/people/faculty/carlson.shtml . [1] Kohsaka et al., "An intrinsic bond-centered electronic glass with unidirectional domains in underdoped cuprates." Science (2007) vol. 315 (5817) pp. 1380-1385 [2] V. J. Emery, S. A. Kivelson, and O. Zachar, "Spin-gap proximity effect mechanism of high-temperature superconductivity." Physical Review B (1997) vol. 56 pp. 6120 [3] E. W. Carlson and K. A. Dahmen, "Using disorder to detect locally ordered electron nematics via hysteresis." Nature Communications, 2011. http://dx.doi.org/10.1038/ncomms1375
