Superconductors amaze researchers

High and low temperature superconductors more similar than expected

Scanning tunneling microscope photograph of a sample of a cuprate superconductor Cornell University
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A great surprise came from American scientists when they first observed the behavior of so-called high-temperature superconductors using a new method at the level of single atoms. Because the mechanisms that give these materials their almost resistance-free conductivity, resemble those of the low-temperature superconductors in a startling way. As the researchers report in the current issue of the journal Nature, this contradicts previous assumptions.

Superconductors are usually metallic materials, which can conduct electrical current with almost no electrical resistance if properly cooled. If the temperature falls below the threshold, they almost suddenly lose their resistance and also change their magnetic behavior. While normally the electrical resistance is due to the interaction of electrons with tiny defects in the crystal lattice, in superconductivity, electrons combine to form electron pairs and these no longer give off energy to the lattice - the inhibitory interaction is suppressed, the resistance disappears.

Whereas the high-temperature conductors only have to be cooled down to the temperature of liquid nitrogen, this is not sufficient with "normal" superconductors. They show their particular conductivity only near absolute zero. Scientists at Cornell University in Ithaca and the physicist Séamus Davis investigated the behavior of certain modified copper oxides, the cuprates. These are considered to be high temperature superconductors as they become superconducting at about minus 123 degrees Celsius. They are widely used in industry because, unlike normal superconductors, they can be cooled with liquid nitrogen.

Atomic behavior in the scanning tunneling microscope

The researchers built their analysis on a method developed at Cornell University a decade ago that allows one to measure the vibrations of individual atoms. Now, Davis and colleagues used this method based on scanning tunneling microscopy for the first time to measure a larger sample. As a sample, they used pumice-strontium-calcium-copper oxide, a cuprate that becomes superconducting below -185.15 degrees Celsius. At each position of their survey, the scientists performed several measurements, including one to detect the presence of so-called paired electrons and one to show the presence of vibrations in the crystal lattice of the superconductor.

Vibration instead of magnetic fields?

"Our main expectation was that electron pairing in cuprates would be due to magnetic interactions, " explains Davis. "The goal of our experiment was to find this magnetic 'glue'." Instead, the researchers found that the interaction of electrons with vibrating atoms of the crystal lattice looked different than previously thought. Not magnetic forces, but the interaction with the vibrating atoms of the lattice, the so-called "phonon", seemed to influence the pairing of the electrons. Although this has so far been considered as a possible mechanism for the "normal" low-temperature superconductor, but not for the high-temperature variant. display

High and low temperature superconductors more similar than expected

However, according to the researchers, their observations suggest that both types of superconductors appear to be at least partially due to a similar mechanism. "That was a huge shock, " Davis describes his first reaction. We showed that we can not ignore the electron-phonon interaction. While we can not prove she is involved in pairing yet, we have proven that she can not be excluded. "

In further experiments, the scientists found similar interactions even if they used variants of the previously tested Cuprats. In these, individual atoms of other elements were incorporated into the crystal lattice, thus changing the magnetic fields in the lattice. However, when they used another oxygen isotope to make the cuprate, one with an atomic weight of 18 instead of 16, the result changed very well. This indicates, the researchers say, that not magnetic effects, but the vibrations of the atoms play an important role here. A direct influence of the lattice vibrational energy at the atomic level on the energy of the electron pairing is suggested, the physicists conclude.

(Cornell University, 03.08.2006 - NPO)