Lattice vibrations important for high-temperature superconductors
The heat-induced rattling and shaking of the atoms in a crystal lattice is said to also contribute to the so-called high-temperature superconductors, so that these substances suddenly lose all electrical resistance when they cool down to a comparatively high temperature. This is reported by the working group led by Jinho Lee from the American Cornell University in Ithaca, New York State, which examined a typical representative of this type: a ceramic made of bismuth, strontium, calcium, copper and oxygen.
It has long been known that so-called phonons – these are quantum mechanical descriptions of lattice vibrations in a solid body – are responsible for two electrons coming together to form so-called Cooper pairs. These pairs can then slide through the ensemble of atomic cores without any friction and thus carry an electric current with them that does not experience any losses. However, these materials have to be cooled to temperatures just above absolute zero in a complex process.
For a good twenty years, however, scientists have known about substances that also have these properties at much higher temperatures. These substances, known as high-temperature superconductors, only have to be cooled with nitrogen, which is much cheaper and easier to handle. So far, however, physicists have not understood why these materials are able to transport electricity without loss at all. For many researchers, the lattice vibrations necessary to explain normal superconductivity were ruled out because the comparatively high temperatures would have to lead to major disturbances. However, only recently did experimenters discover evidence that the magnetic moments of some atoms apparently play an important role in these processes.
Lee's team now contradicts this, as they want to have proven through measurements with a scanning tunneling microscope that the lattice vibrations of the atomic cores contribute at least as much to the development of superconductivity. To prove their thesis, they moved the tip of the microscope over their sample in nanometer steps and used it to measure the smallest changes in the electrical current between their probe and the material. They found that the superconductivity was most pronounced where the lattice vibrations were also particularly evident.
To further test their assumption, the group also replaced the oxygen in their sample with a heavier isotope of the same element. Due to its higher mass, this vibrates less violently. The team was then able to identify the changes in local conductivity that their theory predicted. From this, the Cornell researchers conclude that lattice vibrations also contribute at least to the development of superconductivity in high-temperature superconductors.
An exact understanding of how these substances work would help to search for such materials or even to produce them artificially. These substances hold enormous economic potential because they can be used to carry electricity from a generator to a consumer without losses. According to experts, this would save around ten percent or even more in terms of power plant output.