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Quantum theory to explain high-temp superconducting

Posted: 20 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:high temperature semiconducting? quantum theory? pnictides cuprate?

Superconductors carry electrons with zero resistance by synchronizing their movement through changes in the internal structure of materials. Hence, no physical collisions occur.

The exact character of these changes has been the subject of much speculation, prompting over 100,000 scholarly papers on the subject in the last 20 years. A novel theory developed by university researchers in the United States and China now attempts to explain high-temperature superconductivity using a new class of materials discovered last year called iron pnictides (pronounced NIK-tides).

Researchers will attempt to explain high-temperature superconductivity through a theory of quantum phase changes this week during the American Physical Society in Pittsburgh. The new approach adds to several competing theories that aim to explain high-temperature superconductivity.

The key to high-temperature superconductors, according to the new theory, is their different "quantum phases," which are similar to the difference between solids and liquids, according to researchers from Rice University, Rutgers University, Zhejiang University and the Los Alamos National Laboratory.

Ice and water are two phases of H?O; above the critical melting point the molecules are ordered as solids, but below it they melt in a disordered liquid. Likewise, above its critical melting point, the quantum phase of high-temperature superconductors is antiferromagnetic; below it, they melt into magnetic disorder.

"Our theory addresses the nature of quantum magnetic fluctuations in the framework of quantum criticality, which has potential relevance to a broad range of materials," said professor Qimiao Si, a physicist from Rice University. He performed the work with researchers Jianhui Dai of Zhejiang University and Jian-Xin Zhu of Los Alamos National Laboratory.

Cuprates and pnictides
Quantum criticality defines the transition between phases, possibly explaining superconductivity in iron pnictides, which interleave iron and arsenic in a layered structure similar to cuprates.

Cuprates, another high-temperature superconductor that layers copper and oxygen atoms, are natural insulators because of their oxygen layers. They must be doped to make them conductors.

Pnictides, on the other hand, are a metallic conductor. Despite their differences, however, both are high-temperature superconductors characterized by magnetic spins that are antiferromagnetic, that is, adjacent atoms have opposite spins. Below a critical temperature, quantum magnetic fluctuations change phases, thereby helping to explain superconduction.

"Our theory explains how electron-to-electron interactions can give rise to quantum magnetic fluctuations as the origin of superconductivity," Si claimed.

The newest high-temperature superconducting material is a metal called iron pnictides with alternating layers of iron and arsenic.

For superconductivity to occur, pairs of electrons must careen through metals in synchronized motion. Electrons also must overcome their natural repulsion for each other to couple in this way. In low-temperature superconductors, the Bardeen, Cooper, and Schrieffer (BCS) theory of superconductivity postulates that ionic vibrations in the lattice helps electrons pair up so they can slip through without bumping into its atoms. For high-temperature superconductors, however, the new theory postulates that magnetic interactions related to quantum criticality are responsible for superconductivity.

So far, the research is theoretical, but the scientists have developed a set of specific predictions about how high-temperature superconductors behave as they change phases internally. These predictions are currently being tested by experimental groups around the world.

If the research holds up under laboratory scrutiny, then the new theory could take its place beside the BCS theory as the best explanation for high-temperature superconductors. That, in turn, could prompt a search for new materials with similar quantum phases in hopes of further raising the temperature of superconduction, ideally to room temperature.

Research funding was provided by the National Science Foundation, the Energy Department, the Robert A. Welch Foundation, the National Natural Science Foundation of China and the Education Ministry of China.

- R. Colin Johnson
EE Times





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