News Release

Magnetic moments in a crystal mosaic

Peer-Reviewed Publication

Max-Planck-Gesellschaft

In experiments with neutron beams, an international team of scientists discovers important clues to an explanation of high temperature superconductivity.

High temperature superconductors have been known for 15 years and are already being used in a variety of technological applications. However, the mechanism responsible for this phenomenon has thus far remained elusive. A team of physicists led by Prof. Bernhard Keimer, director at the Max-Planck-Institute for Solid State Research, and including researchers at the Centre d'Energie Atomique in France and at the Russian Academy of Sciences, has now shown that an unusual, fluctuating type of magnetic order plays a central role for high temperature superconductivity. Their results are published this week in the online edition of Science magazine (Science EXPRESS, January 24, 2002).

Normal metals such as copper heat up when they carry an electrical current. The transport of electricity is therefore always associated with significant losses, and large technical facilities are necessary to minimize these losses. Some metals, however, become superconducting below a certain temperature (the so-called transition temperature), that is, they begin to conduct electrical currents without any losses. Unfortunately these transition temperatures are just a few degrees above absolute zero (-273°C) in ordinary superconductors. Since cooling materials to these temperatures is difficult and expensive, ordinary superconductors are only found in few practical applications. In 1986, J.G. Bednorz and K.A. Müller discovered a class of copper oxides whose record superconducting transition temperature at ambient pressure is now 134 K (-139°C). For this discovery they were awarded the physics Nobel prize in 1987. Since then, the interest in high temperature superconductivity has remained high, because it is far less expensive to cool materials with liquid nitrogen (77 K) than with liquid helium (4.2 K).

Despite significant progress in the synthesis of materials and the development of applications, an explanation of the mechanism underlying this phenomenon has remained elusive. A theory proposed 50 years ago, and known to provide an excellent description of ordinary superconductors, applies only partially to the high temperature superconductors. According to this theory two free electrons in a metal form a so-called "Cooper pair" below the transition temperature. In quantum mechanics, every Cooper pair can be described as a new particle, a "boson". The theoretical work of Einstein and Bose in the 1920's has shown that at low temperatures a system of bosons condenses into a macroscopically coherent state whose quantum mechanical wave function extends over the entire system. In the condensed state, every boson can therefore move without resistance from one end of the system to the other. In 2001, three physicists received the Nobel prize in physics for their experimental discovery of Bose-Einstein condensation in atomic systems.



Figure: Investigation of a mosaic of crystals of a high temperature superconductor with neutron beams (yellow). Neutrons are elementary particles that generate a magnetic field through their internal rotation ("spin"), similar to a tiny bar magnet. When a neutron beam falls onto a magnetic material, the neutron spin is flipped and the beam is deflected. In experiments with neutron beams, an international team of scientists has discovered evidence of an unusual, fluctuating magnetic order in high temperature superconductors that could be of central importance for an explanation of this phenomenon.

Max Planck Institute of Solid State Research


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The principle at the heart of superconductivity is similar - with an important difference: The bosonic Cooper pairs consisting of two electrons are charged, so that they can carry an electrical cur-rent through the entire material without resistance. Since the two electrons forming a Cooper pair are both negatively charged, they repel each other. An attractive force counteracting this electrical repulsion is thus required for the formation of Cooper pairs. In ordinary superconductors this force is supplied by "phonons", that is, a coordinated motion of the positively charged nuclei in the solid. Phonons reduce or even neutralize the electrical repulsion between the electrons. How-ever, the strength of this type of pairing force is only sufficient for superconductivity at very low temperatures, as observed in ordinary superconductors.

The formation of Cooper pairs in high temperature superconductor requires a stronger pairing force whose origin is still highly controversial after 15 years. It was the quest for this pairing mechanism that motivated the MaxPlanck researchers and their partners to investigate the high temperature superconductor Tl2Ba2CuO6 with neutron beams. Neutron scattering works in analogy to the scattering of light beams that makes objects visible to the human eye. Detailed data about the properties of materials can be gleaned from the scattering of neutrons, with the added advantage that neutrons penetrate deeply into the material and provide information about the en-tire bulk in addition to the surface. Neutrons possess an internal angular momentum, called "spin", and hence a magnetic moment, which makes them behave like tiny bar magnets. The same applies to electrons inside the solid - they also have a spin. When two bar magnets are held next to each other, they attract or repel each other depending on their relative orientation, and this also happens to neutrons and electrons inside the solid. By virtue of this interaction, the neutron spin is flipped and the beam is deflected. This can be measured and analyzed (see figure).

So far, however, neutron scattering experiments on high temperature superconductors have been severely restricted because the requisite large single crystals are very difficult to prepare. The Max-Planck researchers were able to use a trick to circumvent this problem: They aligned several hundred small crystals in a "mosaic" that, as a whole, is almost equivalent to a large single crystal. This made neutron scattering experiments Tl2Ba2CuO6 possible (see figure).

In these experiments the German-French-Russian team discovered compelling clues to an explanation of high temperature superconductivity - a magnetic mechanism for the formation of Cooper pairs. Earlier neutron scattering experiments by other research groups had already provided indications that the electron spins in high temperature superconductors behave in a fundamentally different fashion than those in ordinary superconductors. Whereas they are completely disordered in conventional superconductors, they exhibit an unusual type of order in the high temperature superconductors: In a snapshot of the system, the spin of every second electron is reversed with respect to the first. The magnetic order differs from that seen, for instance, in magnetized iron in other important respects. Whereas in iron all electron spins point permanently in the same direction, the ordering pattern in the high temperature superconductors fluctuates, that is, it appears and disappears over short periods of time. Says Prof. Bernhard Keimer, director at the Max Planck Institute of Solid State Research: "The data acquired from our crystal mosaic make a magnetic mechanism for the formation of Cooper pairs plausible. The origin of the pairing force may be that pairs of electrons can move more easily than single free electrons through a background of fluctuating electron spins - they could thus save magnetic energy. After 15 years of research, our results may put a definite theory of high temperature superconductivity finally within reach."

However, the scientist cautions: "Such an explanation will only be truly persuasive if the signature of fluctuating magnetic order can be found in all high temperature superconductors, especially in the purest materials with the highest transition temperatures." The scientists therefore intend to test the results they gleaned from the "prototypical" high temperature superconductor Tl2Ba2CuO6 on further materials. An important question is, for instance, why some materials become superconducting at 134 K, while in others this occurs at lower temperatures. What is the reason for this difference? Is the same magnetic order or a different type of ordering pattern responsible? A final theory of high temperature superconductivity can only be formulated when these questions have been answered.

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Contact: Prof. Bernhard Keimer
Max Planck Institute for Solid State Research Heisenbergstr. 1
70569 Stuttgart
Phone: +49 - 7 11 - 6 89 - 16 31 and - 16 50
Fax: +49 - 7 11 - 6 89 - 16 32
E-mail: b.keimer@fkf.mpg.de


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