Physics & Astronomy

High-T_C superconducting cuprates

"Superconductivity" is the name attached to the physical phenomenon in which the electrical resistivity of a material entirely vanishes below some critical temperature T_C.  Very large electrical currents can flow through a superconducting wire that would instantly vaporize a normal metallic wire due to resistive heating.  Superconductivity was initially discovered in 1911 by H. K. Onnes (Nobel prize in physics in 1913), who observed that the electrical resistivity of Mercury metal drops to zero below a critical temperature of T_C = 4.2 K.  In 1957, J. Bardeen, L. N. Cooper and J. R. Schreiffer (Nobel prize in physics in 1972) developed a highly-successful theory that finally explained this effect.  In 1986 J. G. Bednorz and K. A. Mueller announced the surprising observation of superconductivity in a cuprate ceramic (La₂CuO₄), and at a far warmer critical temperature (T_C = 35 K) than had been seen before.  The discovery of this "high-temperature" superconductor led to an unprecedented frenzy of scientific activity and a Nobel prize for Bednorz and Mueller the very next year.  By that time, several other cuprate superconductors had been discovered, including YBa₂Cu₃O₇, with its remarkable T_C of 90 K.  Today, over 50 superconductors have been discovered in this unusual family of materials.  Due to the potential for revolutionary new technologies, the quest for a room-temperature (300 K) superconductor, which would require no cryogenic cooling, remains one of the grand challenges of modern physics.

Defective superconductors

In collaboration with groups at the University of Tennessee, Argonne National Laboratory, NIST, and the Japan Central Research Institute of Electric Power, we recently employed single-crystal x-ray diffuse scattering and neutron powder diffraction to solve a long-standing mystery surrounding electron-doped cuprate superconductors like Nd_1.85Ce_0.15CuO₄ and Pr_0.88LaCe_0.12CuO₄.  Unlike their more common hole-doped cousins, these materials generally will not superconduct when first prepared.  And stranger still, high-temperature chemical treatments are found to reversibly enable and disable the low-temperature superconducting state, and to reversibly create and destroy small amounts of an impurity phase.

Our scattering experiments revealed that as-grown crystals of Pr_0.88LaCe_0.12CuO₄ contain a modest concentration (< 3%) of copper vacancies (i.e. defects) that appear to disrupt the formation of the superconducting state. During a high-temperature reduction treatment in argon gas, the copper vacancies were shown to migrate through the crystal and aggregate to form small copper-free regions with a distinct crystal structure.  This effectively repairs the defects in the CuO₂ layers throughout the rest of the material, which then readily support the superconducting state. During a subsequent oxidation treatment in air, the copper vacancies are redistributed throughout the crystal, eliminating the impurity phase and disabling superconductivity again.  By establishing the atomistic nature of the high-temperature annealing process, the team demonstrated that electron-doped and hole-doped cuprates have much in common, both requiring clean CuO₂ layers as a prerequisite for superconductivity.  And understanding structural features that enhance or inhibit superconductivity may lead to the development of new materials with superior properties.  Nature Materials 6, 224-229 (2007).  A recent Nature Materials News & Views article reviews this work.

Figures: (left) The ideal T' crystal structure associated with most electron-doped cuprate superconductors.  Superconducting currents flow in the flat copper(blue)-oxygen(red) planes.  (right) A representation of copper-vacancy defects in the crystal lattice.