Research For Our Energy Future

Complex Materials

Unusual properties may lead to new superconductors

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Harnessing the power of plants to fuel our future.

A new Long Island Power Authority transmission system uses the first generation of a high-temperature superconductor wire technology first studied at Brookhaven Lab.

The development of superconductors that could be used in real-world applications, particularly power transmission, could transform the U.S. energy landscape.

The energy systems we rely on today were designed at the start of the last century and are based on scientists’ understanding of the properties of basic materials — simple metal wires for conducting electricity, insulators for holding in heat.

In recent years, scientists have discovered an array of unusual and dramatic properties in materials with more complex composition. Certain layered materials that act as insulators at room temperature, for example, can carry electrical current with no resistance when doped with a small amount of impurity and cooled below a certain temperature. Other complex materials convert small amounts of heat into electric currents. Though similar effects had been observed in some simple materials, the effects in complex materials appear to be significantly enhanced.

Today’s scientists are looking for ways to harness these dramatic new properties to create the transformational applications needed to meet the energy challenges of our future. Understanding high-temperature superconductors, for example, and tailoring them for applications such as zero-loss power lines could lead to $120 billion in cost savings per year, with five times the power-carrying capacity for the U.S. electrical grid. Brookhaven Lab, with world-renowned scientific expertise and a suite of high-tech tools for probing the mysteries of complex materials, is eager to meet that challenge.

Brookhaven National Laboratory is home to the Center for Emergent Superconductivity, one of 46 multi-million-dollar Energy Frontier Research Centers (EFRCs) which will seek to understand the underlying nature of superconductivity in complex materials.

The Electron Connection

Understanding the startling properties of complex materials has changed the way scientists think about condensed matter — the regular arrays of atoms making up most of the materials around us.

As is the case with simple materials, a lot of the properties of complex materials depend on the movement and interactions of electrons among atoms. In ordinary metals, despite the huge number of electrons, the interactions are fairly straightforward and can largely be understood by a model in which the electrons are considered to be independent from one another.

But in complex materials, electron interactions extend beyond the immediate neighbors. You tend to get “ordering” phenomena — such as alignments or alternations of individual electron spin directions (magnetic order), or periodically spaced clusters of charges (charge order) — that extend over a large area. And sometimes properties of individual electrons, like spin and charge, act independently of one another.

Scientists must understand the complexity of these interactions if they are to tap into these new materials’ potential for energy-saving applications.

Super Current Carriers

Brookhaven Lab has a world-class research program aimed at understanding high-temperature (high-Tc) superconductors. Unlike ordinary conductors, in which electron interactions build up a significant amount of heat as electrical current moves through, superconductors carry current with no resistance. That means none of the energy is lost. Metallic superconductors, discovered nearly a century ago, operate at temperatures near absolute zero (0 Kelvin or -273 degrees Celsius), requiring costly cooling systems. But the newer complex materials discovered within the past two decades transition to superconductivity at warmer temperatures — hence their designation as high-Tc. Their discovery has sparked the hope of finding or designing materials that perform this current-carrying magic at room temperature.

The key, of course, is to understand how they do it.

Intriguing Stripes and Other Clues

The basic idea behind superconductivity is that electrons, which ordinarily repel one another because they have like charges, pair up and move with fluid-like properties to carry current with no resistance. But what causes the pairing in high- Tc superconductors remains one of the great mysteries of condensed matter physics — and a key quest for Brookhaven researchers.

Brookhaven physicists have probed the interactions of electrons with vibrations in the crystal lattice making up the material — which are known to cause electron pairing in conventional metallic superconductors — as well as how changes in the spin alignments, or magnetic polarities, of adjacent electrons might contribute to electron pairing. Recent studies indicate that electrons pair up at a temperature higher than that at which superconductivity sets in. This may offer more clues to the superconducting mechanism and suggest ways to push the superconducting transition closer to room temperature.

Another particularly interesting finding has been the discovery of a pattern of alternating regions of charge and spin order known as “stripes” in certain superconducting materials. Many theorists have long assumed that such stripes should be incompatible with superconductivity because their static pattern implies that the charges are localized in relatively fixed positions, rather than able to move freely to carry current. Brookhaven’s measurements suggest that stripes — though perhaps in a more fluid, hard-to-detect form — may, in fact, be essential to superconductivity.

Brookhaven scientists are also investigating the role of dimensionality in determining the extreme physical properties of complex materials. Recent studies show that the ability to carry current in the high-temperature superconducting materials, which are made up of many layers, is much greater within the layers than between them. This will have important implications for those manufacturing high-Tc power lines, who will either have to find ways to maintain this dimensionality over very long distances — or discover new superconductors that operate equally well in all directions.

Another area of investigation is the role of variations in the composition of the superconducting materials. Scientists need to understand how to deal with patches of irregularity if such patchiness impedes superconductivity.

Last Modified: November 6, 2009