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HTS at CERN & LHC Current LeadsContact: Dr. Amalia Ballarino (AT-MEI-SD) |
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Physics of HTSSince the discovery of High Temperature Superconductivity in lanthanum copper oxides, discovery marked by the award of the Nobel prize to Bednorz and Muller in 1987, no generally accepted microscopic theory for the mechanism responsible for the superconductivity in High Temperature Superconductors (HTS) has been formulated. After the lanthanum-barium-copper oxide superconductors (La2-xBaxCuO4), having a critical temperature of about 30 K, other materials belonging to the same cuprate family were found to be superconducting at even higher temperatures: the yttrium-barium-copper-oxide superconductors YBa2Cu3O7, with a critical temperature of 92 K, the bismuth-strontium-calcium-barium oxide superconductors Bi2Sr2Ca2Cu3O10 and Bi2Sr2CaCu2O8, with a critical temperature of 110 K and 85 K respectively and the thallium-barium-calcium oxide superconductors Tl2Ba2Ca2Cu3O10, with a critical temperature of 125 K. All these materials have as common feature a crystal structure that includes layers of copper-oxygen planes, through which the superconducting current flows. In the BCS quantum-mechanical theory of conventional superconductors, the electron flux consists of bound pairs of electrons. The pairing is caused by an attractive force between electrons. The electrons are bound in Cooper pairs by an electron-phonon interaction, i.e. by phonon-mediated pairing. The electron's wave function that describes the pair is spherical, indicating that the chance of finding one carrier in a Cooper pair given the position of the other falls off at the same exponential rate in all directions in space. This pairing is said to have s-wave symmetry. The HTS superconductors display many of the well-known properties of conventional superconductors, such as Josephson tunneling, vortex structure, type II behavior and Meissner effect. However, they also have properties that are unusual for BCS-like materials. Some of these are their high critical temperature, their linear dc resistivity in the normal state and their extremely small coherence length, which is comparable to the grain boundary thickness and therefore makes the weak-link behavior a real problem for transport properties. In addition, HTS are characterized by a large spatial anisotropy, which is due to their layered crystal structure. These layers are composed of Cu-O planes, separated from each other by planes of other oxides and rare earths. It is believed that superconductivity and charge transport are mostly confined in the Cu-O planes, called the ab planes, perpendicular to the c axis. This structural anisotropy translates into anisotropy of most physical properties. While it is admitted that there is some electron pairing mechanism involved in high temperature superconductivity, the nature of the pairing mechanism is not yet understood. It is considered that the pairing interaction may not be phonon-mediated and may not be the same for all HTS superconductors. Lattice vibrations alone are not strong enough to maintain electron pairing at elevated temperatures. Pairing mechanisms of magnetic origin have been proposed, mainly to justify the high critical temperatures of HTS: the magnetic exchange energies are about four times the phonon energies. In this case, the electron pairing would have a wave function with d-wave symmetry. The d-state appears as a four lobes lying in a plane, like a four-leaf clover. One of the most dominant theories that contains d-wave symmetry is the spin wave model. According to this theory, the carrier leaves a magnetic disturbance (a spin wave) in its wake. This wake pulls a second carrier, so that the two forms a Cooper pair. The spin waves are short-lived, so they are often called spin fluctuations. However, some measurements are in apparent contradiction with this picture. For instance, results of Josephson tunneling experiments are an argument for the paired electrons being in a spin-singlet s-state. It was pointed out that Josephson tunneling should not be possible between paired electrons in two different superconductors unless they have the same symmetry. The Josephson tunneling experiment between yttrium-barium-copper-oxide superconductors and a Pb/Sn (an ordinary s-wave superconductor) point contact, would be in favor of paired electrons in a s-state. Also, the temperature-dependent penetration-depth for HTS gives weight to the argument that these materials are s-wave. Phonon mediated pairing would be consistent with the experimentally observed s-state pairing. For the electron pairing mechanism, other quasiparticles such as antiferromagnetic magnons or excitons have also been proposed as pairing intermediates. Alternatively, other mixed mechanisms have been considered, like a phonon-mediated mechanism with some sort of a booster to increase the critical temperature of the superconductor. The MgB2 superconductor, discovered in 2001 by a team of Japanese researchers, does not belong to the family of cuprates. This superconductor, with a critical temperature of 39 K, is not a copper oxide: it is a much simpler compound that seems to represent a whole new superconductor family, more than being simply a Low Temperature Superconductor (LTS) with an unusually high critical temperature. The large isotope effect found in this superconductor confirms the key role of electron-phonon coupling. However, the electronic structure in MgB2 is such that there are two types of electrons at the Fermi level, one of them being much more strongly superconducting than the other. This is in contrast with the theory of phonon mediated superconductivity, which assumes that all the electrons behave in the same manner.
It is certainly true that the HTS field is still young and it evolves rapidly. It is also true that it took 45 years after the discovery of superconductivity in solid mercury at 4.2 K -by Kamerlingh Onnes in 1911- to arrive at a solid understanding of conventional superconductors through the microscopic BCS theory. While some practical HTS superconductors are now being made and some applications are appearing, it is felt that it may still be a long time before the physics of HTS is fully understood and explained in one -or more- theories enjoying consensus among theoretical physicists. Progress in the field has been, up to now, mainly driven by experimental work. Finding a theory would probably help researchers to address some of the problems encountered when working with this new generation of superconductors. |
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