High-temperature superconductivity

As a high- temperature superconductor ( HTSC) materials are referred to, whose superconductivity - unlike conventional superconductors - does not come through the electron -phonon interaction about. Usually it is not the usual way to metallic, but ceramic materials. Although seems assured, that also pair formation (so-called " Cooper pairs " ) of the electrons responsible for superconductivity, but occurs instead of the conventional singlet pairing predominantly d- wave pairing, which suggests unconventional electronic pairing mechanisms. The cause is unknown for more than 25 years.

The name comes from the fact that high-temperature superconductors usually have significantly higher critical temperatures Tc than conventional superconductors, hence the English name and is often derived in the German term used HTc. The temperatures are, however, less than -140 ° C - equivalent to 130 K, which is namely to more than 100 K higher than the temperature range of the conventional superconductors, but is still well below those known in everyday life temperatures.

History

Based on work by Arthur W. Sleight at DuPont, which previously proved superconductivity in ceramic, Johannes Georg Bednorz and Karl Alexander Müller had experimented with perovskite structures since 1983 at the IBM Zurich Research Laboratory. Through exchange of certain atoms, they were able to influence the distance between the copper and oxygen atoms in all levels targeted.

The substance lanthanum -barium- copper oxide ( La1 85Ba0, 15CuO4 ) they eventually discovered in April 1986, superconductivity with a transition temperature of 35 K. This result they published first in the journal for physics, where it does - especially in the U.S. - did not receive the proper attention. Then they presented their studies in March 1987 prior to the big Spring Meeting of the American Physical Society in New York. Now the publication has been included as a sensation and discussed in a to night -reaching special session together with other results. In no time, several research institutes have confirmed the discovery of the world. Already in the autumn of 1987 Bednorz and Müller received for their discovery of the Nobel Prize for physics.

Parallel began an intensive search for other similar substances with even higher critical temperatures. Important milestones were in 1987 the discovery of YBa2Cu3O7 at 92 K and 1988 of Bi2Sr2Ca2Cu3O10 110 K transition temperature, both of which can be held with inexpensive liquid nitrogen in the superconducting state. The record is held since 1994 Hg0, 8Tl0, 2Ba2Ca2Cu3O8 at 138 K.

The technical utilization of high temperature superconductivity was from the outset as a major driving force for further research. Transition temperatures above 77 K in principle allow an inexpensive cooling by the use of liquid nitrogen instead of helium.

Use

Other than indicated by the transition temperatures, high temperature superconductors are often operated at power applications at temperatures well below 77 K. Reason is the interdependence of the possible transport streams, the critical field strength and temperature. With nitrogen -cooled HTSC thus give away a large portion of their potential because the relatively high operating temperature limits field and current. If the same performance as in conventional superconductors, such as niobium - titanium, be reached, the temperature must therefore be correspondingly lowered.

In other application fields, such as metrology, HTSC can actually get along with pure nitrogen cooling it. For SQUIDs with which very small changes in magnetic field can also be measured, it has been practiced for some time. However, increases with increasing temperature, even in principle, the noise of the signal, which is why a superconducting at room temperature material for example, would not receive wide distribution in the electronics in today's opinion. For high-temperature SQUIDs for the high noise over the older helium technique, although also present and undesirable, but is often accepted because of the cost and handling advantages of nitrogen cooling.

However, the main drawback of the high temperature superconductor is ultimately the brittleness of the non-metallic material.

Theory

Currently, the cause of the high transition temperatures is not known. Due to unusual isotope effects, however, can be ruled out that the electron pairing occurs as in conventional superconductivity only by the conventional electron -phonon interaction about. However, the BCS theory is still applicable, as this theory leaves open the type of interaction and ultimately acts as a kind of " molecular field ". Similar to the theory of critical phenomena in second -order phase transitions, but is observed in many sizes deviations from usual power laws.

Instead of the electron -phonon interaction is assumed here to be the cause of superconductivity antiferromagnetic electron-electron correlations due to the special lattice structure of the ceramic superconductor to an attractive interaction between neighboring electrons and thus similar to a pairing as in conventional Cooper pairs of the BCS theory lead. However, these interactions can be personalized with the isotope effects even more difficult to explain. Alternatively, it is also a generalization of the BCS theory by Gorkov ( Glag Theory) or entirely new explanations as Bisolitonen model.

All HTSC with really high transition temperatures show characteristic abnormalities in the electrical properties and thermal conductivities already in the normal state: The electrical resistance increases at low temperatures linearly with temperature and the Wiedemann - Franz law is satisfied also in the middle T area. Normal metals exhibit a potency- dependent temperature behavior of the resistance, and the WF law is not met in the mid- T area. So far there is no theory that can explain these anomalies and superconductivity in cuprates the same time.

Also, has so far shown neither experimentally nor disproved theoretically whether superconductivity at room temperature (20 ° C, about 293 K) is possible. Previous theoretical estimates of " maximum critical temperature " have proved by the discovery of high temperature superconductors to be wrong.