New material may reveal inner workings of hi-temp superconductors

September 1, 2010, National Institute of Standards and Technology

Measurements taken* at the National Institute of Standards and Technology may help physicists develop a clearer understanding of high-temperature superconductors, whose behavior remains in many ways mysterious decades after their discovery. A new copper-based compound exhibits properties never before seen in a superconductor and could be a step toward solving part of the mystery.

Copper-based high-temperature are created by taking a nonconducting material called a Mott insulator and either adding or removing some electrons from its . As the quantity of electrons is raised or lowered, the material undergoes a gradual transformation to one that, at certain temperatures, conducts electricity utterly without resistance. Until now, all materials that fit the bill could only be pushed toward either by adding or removing electrons—but not both.

However, the new material tested at the NIST Center for Neutron Research (NCNR) is the first one ever found that exhibits properties of both of these regimes. A team of researchers from Osaka University, the University of Virginia, the Japanese Central Research Institute of Electric Power Industry, Tohoku University and the NIST NCNR used neutron diffraction to explore the novel material, known only by its chemical formula of YLBLCO.

The material can only be made to superconduct by removing electrons. But if are added, it also exhibits some properties only seen in those materials that superconduct with an electron surplus—hinting that scientists may now be able to study the relationship between the two ways of creating superconductors, an opportunity that was unavailable before this "ambipolar" material was found.

The results are described in detail in a "News and Views" article in the August, 2010, issue of , "Doped Mott insulators: Breaking through to the other side."**

Explore further: Physicists discover surprising variation in superconductors

More information: * K. Segawa , M. Kofu, S.-H. Lee, I. Tsukada. H. Hiraka, M. Fujita, S. Chang, K. Yamada and Y. Ando. Zero-Doping State and Electron-Hole Asymmetry in an Ambipolar Cuprate. Nature Physics, August 2010, pp. 579-583, DOI 10.1038/NPHYS1717

** J. Orenstein and A. Vishwanath. Doped Mott insulators: Breaking through to the other side. Nature Physics, V. 6, August, 2010. DOI:10.1038/nphys1751

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3 / 5 (2) Sep 01, 2010
It's not so difficult to understand, what happens there in illustrative way. Try to imagine the atom lattice with atoms surrounded with movable electrons. If the concentration of electrons outside of atoms will be sufficient, the repulsive forces between electrons will arrange them into regular hexagonal lattice between atoms, where electrons cannot move freely and the material will change into Mott's insulator.

If some of atoms will attract the electrons more then others, the electrons will collect and they will form a dense clouds around them. If the concentration of electrons will be sufficient, a superconductive chaotic phase will emerge there. In this way, the ratio of positively and negatively charged atoms can switch the material from insulating to superconductive state. The presence of f-orbitals of neodymium plays a significant role in the described flexibility of material behavior - they're forming rigid handles for atoms in lattice independent to movable electrons position.
3 / 5 (2) Sep 01, 2010
In many other materials the electronegativity of atoms changes significantly, but the position of these atoms changes accordingly to the local distribution of electron pressure. Within such materials no anomalous effects following from absence or abundance of electrons cannot be observed, because distance between atoms is flexible there.

Only materials, where atoms are kept firmly at their positions with orbitals, which aren't affected with distribution of movable electrons can exhibit anomalous effects involving superconductivity, Kondo-effect, Mott's and/or topological insulating state. As a general rule, these materials are forming layered structures, where most of atoms are serving like rigid cage. Because repulsive forces between electrons are very strong at higher density of electrons, only small portion of atoms with movable electrons can exhibit effects of electron condensation around and/or between atoms. This limits the usage of these materials to very pure, ordered states.

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