Constructive Conflict in the Superconductor

Photo: Waves in ceramic yttrium and neodymium barium cuprate

Whether a material conducts electricity without losses is not least a question of the right temperature. In future, it may be possible to make a more reliable prediction for high-temperature superconductors. These materials lose their resistance if they are cooled with liquid nitrogen, which is relatively easy to handle. An international team, in which physicists of the Max Planck Institute for Solid State Research in Stuttgart played a crucial role, has now discovered that this form of superconductivity competes with charge density waves, i.e. with a periodically fluctuating distribution of the charges.

Since the physicists did not previously take account of this competition in their models, their calculations of the transition temperature, where superconductivity sets in, remained inaccurate. In further work, the researchers at the Stuttgart Max Planck Institute have gained insights into how superconducting materials interact with magnetic ones. They observed that the electronic properties affect crystal vibrations to a greater extent than was to be expected. This effect could help to control material properties such as superconductivity or thermoelectricity.

If electricity from high-power offshore wind farms is to be distributed to consumers in Germany in future, quite a bit of energy will be lost in the long power lines. Superconducting cables could prevent this if cooling them does not consume more energy than they help to save. Bernhard Keimer and his colleagues at the Max Planck Institute for Solid State Research in Stuttgart want to identify materials that deserve the name high-temperature superconductor both in practical terms and also in terms of our usual perception of temperature.

According to one of their discoveries we can probably consider ourselves lucky that high-temperature superconductivity - a property which remains promising despite its present disadvantages - exists at all. “It is obviously down to a fortunate coincidence,” says Keimer.

The researchers discovered that the superconductivity in one type of copper oxide ceramic competes with a state in which a charge density wave forms. Physicists have known about such charge density waves for decades from two-dimensional materials such as the niobium selenides, for example. Here, the conduction electrons do not distribute uniformly across the crystal like in a metal. On the contrary, they form a regular pattern of regions in which they concentrate to a greater or lesser extent.

“We did not expect the charge density waves in the superconducting cuprates, because they destroy the superconductivity,” says Keimer. Instead of concentrating at regular intervals to a greater or lesser extent, the electrons in superconductors join up to form Cooper pairs which can slip through a crystal with zero resistance. Accordingly, the researchers observed the charge patterns only above the transition temperature, the temperature at which the material becomes superconducting.

The regions where charge density waves formed initially expanded as the researchers cooled the material down to the transition temperature, however. As soon as they reached the transition temperature at minus 213 degrees Celsius, the charge density waves suddenly disappeared and superconductivity prevailed. “Superconductivity only just prevailed in this competition,” explains Keimer. “If the advantages were distributed slightly differently, there would possibly be no superconductivity at all.”

The team of researchers tracked down the charge density waves by scanning yttrium and neodymium barium cuprates of the composition (Y,Nd)Ba2Cu3O6+x with the aid of resonant X-ray diffraction. This provided them with exclusive information on the electrons which found it hard to decide whether they wanted to form a wave or look for a partner so that, together, they could slip more easily through their crystal. The physicists in Bernhard Keimer’s group are now going to perform these measurements on other high-temperature superconductors as well. They want to find out whether all these materials are in electronic competition.

In addition, the researchers want to take account of the conflict between the two electronic states in their theoretical model of superconductivity. “We can already compute the transition temperature of a material quite well with this model, but still end up slightly too high,” says Keimer. “The competition with the charge density wave explains this discrepancy so that our predictions should become more accurate in the future.”

Charge density waves possibly also explain an observation which his team made recently in a different project. A high-temperature superconductor was also instrumental here. It also was composed of yttrium, barium and copper oxide and is described by the formula YBa2Cu3O7, or YBCO for short. The researchers now combined this ceramic with a magnetic material comprising lanthanum, calcium and manganese oxide, which obeys the formula La2/3Ca1/3MnO3 (or LCMO). They stacked up the two substances to form a superlattice, a sandwich of layers only a few nanometres thick, and they had a clear aim in doing this.; Source: Max-Planck-Institut für Festkörperforschung