Copper-based superconductors, and the new iron-based family, operate over a wider temperature range than most superconductors that are only effective near absolute zero.
"Since the new findings are similar to those we discovered in the parent state of copper-based superconductors, this suggests that this may be a common feature of a general mechanism for high-temperature superconductivity in these two completely different types of materials," explains lead researcher JC Séamus Davis, professor of science Physics at Cornell University. The research findings were published in the prestigious scientific journal Science.
Many theorists predicted that iron-based materials would behave similarly to metallic superconductors, where electrons pair up to conduct electricity without resistance without requiring a special arrangement of the atoms in the material. These materials conduct electricity without any resistance only at temperatures very close to absolute zero (minus 270 degrees Celsius).
Cupratic, or copper-based, superconductors, and the new family based on iron, operate in a wider temperature range - up to minus 120 degrees Celsius for the copper family and up to minus 220 degrees Celsius for the iron family, so they are more useful for large-scale operation, for more practical applications such as resistanceless electrical conductors.
Cuprates are copper oxides that contain "impurities" of a variety of other atoms. Iron-based superconductors - first demonstrated only in 2008 - are "contaminated" compounds of iron and arsenic. In some way, the contamination distorts the crystal structure of the material in such a way that allows the electrons to move without any resistance. Understanding this mechanism could open a window to the development of superconductors at higher temperatures, and optimally, at room temperature.
The scientists used a special scanning tunneling microscope (STM) in which a tiny detector moves over a surface at intervals smaller than the diameter of a single atom. By purposefully changing the current between the detector and the surface, the different energy levels of the electrons in the material can be measured using the device and thereby obtain the distribution picture of the electrons in it.
The researchers examined samples of calcium, iron, cobalt and arsenic compounds that become superconductors when the amount of cobalt in them increases. When they tested the special material, they got completely unexpected results. They discovered a nanoscale, stationary array of electrons that span a distance eight times greater between the individual iron atoms, all of which are arranged around one axis of the crystal, reminding them of the way individual particles are organized in a liquid crystal.
Liquid crystals, used in electronic displays, are a type of intermediate state between a liquid and a solid, where the cells are arranged in parallel rows, and in these materials the amount of light passing through them and its direction can be controlled. In solid crystals that make up materials such as superconductors at high temperature, the electrons are not confined to individual atoms, but behave like a liquid, and here, says the lead researcher, the electrons seem to exist in a state similar to a liquid crystal. "You can't use normal condensed-state physics to understand such complex materials," notes the lead researcher.
In addition, the researchers discovered that the free electrons move inside the material in a direction parallel to the direction of the electrons present in liquid crystals. This result indicates that the electrons carrying the current are different from those present in liquid crystals.
The next step in the research will be to examine how these conditions affect the conductivity of the material when it begins to become a superconductor. The findings are surprisingly similar to those the researchers uncovered in coprats. "If we can parallel our findings obtained for the iron-based superconductors with those for the copper-based superconductors, this could help us understand the general mechanism that governs high-temperature conductivity for all materials of this type. This understanding could, as a result, help us develop new materials with improved superconducting properties for energy applications," says the lead researcher.
