High temperature superconductivity more conventional than expected

New measurements refute previous explanatory models

Ellipsometer © MPI for Solid State Physics
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The high-temperature superconductivity was discovered only seventeen years ago and has since found numerous applications. But the mechanism underlying it is still unclear. Researchers at the Max Planck Institute for Solid State Research in Stuttgart have now shown that, contrary to the predictions of some currently dominant theoretical models, high-temperature superconductivity does not rely on an unconventional electron-pairing mechanism.

This was achieved with a high-precision measuring technique, the infrared ellipsometry with the help of synchrotron radiation, which was developed by the scientists in the course of many years of work. The findings made with the synchrotron radiation source (ANKA) at the Karlsruhe Research Center suggest that the high-temperature superconductivity - in modified form - can be explained by the existing Bardeen-Cooper-Schrieffer theory of superconductivity (BCS theory) (Science, April 30, 2004).

Critical temperature crucial

Normal metals have a finite electrical resistance. If a current flows through it, that is associated with considerable losses. Valuable energy is thereby converted into waste heat, which often causes considerable technical problems. However, some metals become superconductors below a so-called transition temperature. These conduct electricity without resistance, ie without losses. Such superconductors have been known for almost a hundred years, but their transition temperature is only a few degrees above the absolute zero of minus 273 degrees Celsius. Since their cooling is very complex, conventional superconductors find only a few applications, for example in coils for very strong magnetic fields.

The discovery of high-temperature superconductivity in a copper oxide compound by JG Bednorz and KA Müller in 1986 caused quite a stir. Meanwhile, the record of the transition temperature in this class of materials is at least -139 ° C. These temperatures can be achieved by cooling with liquid nitrogen (-195 ° C) inexpensively and without much technical effort. However, despite the successes in the production and technical development of these materials, the mechanism underlying the phenomenon of high-temperature superconductivity remains unclear.

Electron pairs despite repulsion

This is all the more astonishing, since for the conventional superconductors since 1957 a very detailed and successful theory exists. According to the BCS (Bardeen-Cooper-Schrieffer) theory, two free electrons of a metal below the critical temperature form so-called Cooper pairs. These pairs of electrons have completely new quantum mechanical properties and enable lossless current transport. The main problem with the formation of Cooper pairs is that electrons have a negative electrical charge and therefore repel strongly. So that Cooper pairs can form, so an attractive force is required, which counteracts the electrostatic repulsion. In conventional superconductors, this force is based on a coordinated distortion of the positively charged nuclei, the so-called phonons. These reduce the electrostatic repulsion or even lift it up. display

In the case of high-temperature superconductors, however, it has largely been proved that the attracting effect due to the phonons is far too weak to explain the extremely high transition temperature of these superconductors. For this reason, a number of alternative models for explaining high-temperature superconductivity have been developed in recent years. For example, some conventional models assume that the stronger interaction is mediated, for example, by the spin excitations of the electrons.

In addition, there are also several unconventional models, which differ fundamentally from the BCS-like models. They assume a very strong interaction of the electrons, which already has serious effects above the critical temperature, ie in the normal state. The crux of these models is the assumption that the electrons can avoid the detrimental interaction by forming Cooper pairs.

Light as measuring aid

Whether the high-temperature superconductivity is actually based on such an unconventional mating mechanism is sought in particular by optical investigations of these materials. Ellipsometry is a suitable method for measuring not only the change in the intensity of the incident and reflected light beam, but also its phase shift. However, to be able to determine the change in the kinetic energy of the charge carriers, ellipsometric measurements down to the far-infrared range are required. Precise ellipsometric measurements on the comparatively small samples of high-temperature choppers until recently were not possible in this spectral range. Only in the last few years has the research group led by Christian Bernhard at the Max Planck Institute for Solid State Research succeeded in developing a precise infrared ellipsometer using a synchrotron beam source, which is also used in the field of solid state research Far infrared radiant and very intense radiation supplies.

Mating yet conventional

The measurements with the newly created synchrotron radiation source ANKA showed that there is no such unconventional pairing mechanism of the electrons in the high-temperature superconductivity. The ellipsometric infrared spectra are in clear contradiction to the predictions of unconventional models of high-temperature superconductivity. Rather, the measurements prove that the kinetic energy of the charge carriers in the superconducting state is not lowered abnormally.

On the contrary, the scientists even observed a slight increase in kinetic energy as predicted by classical BCS theory. Responsible for this is the binding of the electrons to Cooper pairs, which limits the mobility of the individual electrons. In the context of BCS theory, however, this disadvantage is more than made up for by the attractive interaction.

(MPG, 05.05.2004 - NPO)