High-temperature superconductivity
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High-temperature superconductors (High Tc) are a family of materials with a common structural feature, relatively well separated copper-oxide planes. They are also called cuprate superconductors. The temperature of superconducting transition that can be achieved in some compounds in this family is highest among all known superconductors. Normal (and superconducting) state properties exhibit many common features between different cuprate compounds; many of these properties can not be understood within the conventional theory of metals. A consistent theory of cuprate compounds does not currently exist; however, the problem has motivated much experimental and theoretical work, and interest in this field is beyond the goal of achieving the room-temperature superconductivity. The experimental discovery of the first High Tc superconductor in 1986 was immediately recognized by the Nobel Prize.
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[edit] Relevant structural features
[edit] Copper-oxide plane
The cuprates are quasi-two-dimensional materials which consist of layers of copper-oxide planes separated by other materials. It seems that most of the properties are determined by electrons moving within the copper-oxide planes. The rest plays structural role and provides screening and doping environment. The copper-oxide plane is a checkerboard lattice with square backbone lattice of oxygens in the O_ _ state and with, say, "black" squares marked by copper atom in the center; Copper is typically in Cu + + state. The unit cell is, e.g., a square rotated by 45deg containing exactly one "black square". The unit cell contains one copper and two oxygen atoms. Obviously, the unit cell is charged by an equivalent of two electronic charges. These charges are "supplied" by the La, Ba, Sr etc, atoms which in cuprate superconductors are always present between the planes. It may be considered as an experimental fact that the chemical potential crosses one of the electronic bands of the copper-oxide plane and nothing else: it is the copper-oxide plane that determine the fermi-surface and low-energy electronic properties. As such, in the ionization state 
, the copper-oxide plane is a Mott insulator with long-range antiferromagnetic order of spins at small enough temperatures. A vital for cuprates is their ability to accommodate chemical substitutions, i.e, atoms that (i) replace one of the atoms of the original lattice without disrupting the short-range lattice order and (ii) have different number of electrons in their outer shells. The excess electrons may enter the copper oxide plane (electron doping) or electrons can be taken away from the copper-oxide plane (hole doping), as a result of such chemical substitution. It is important that chemical substitutions occur in the substance outside the copper-oxide plane. In other words, a unique property of copper-oxide planes and their "environment" atoms in the copper-oxide superconductors is that such doping is possible at all and charge redistribution is effectively screened and is stable. (Materials that allow doping are not very common, but cuprate superconductors are by no means the only ones). Structural formulas of interesting cuprate superconductors typically contain fractional numbers since they are constitute doping modifications of the particular "mother" compound. Concentration of excess electrons or holes (in short, doping) is one of the most important parameters that determine the low-energy properties of the cuprate compounds.
[edit] Weak distortions
Copper - oxide plane in real material are distorted in several ways. This distortions are usually weak but they can play an important role because they break the symmetries of the original (square, plane) lattice.
- buckling
- orthorombic distortion
- pairs of planes
[edit] General phase diagram
Typically the half-filling state is an insulator with antiferromagnetic ordering and it is not superconducting at any temperature. The "interesting" phases are in the metallic state which is achieved at finite electron/hole doping of copper-oxide planes. The common way of doping is by chemical substitution; other methods, such as pressure may also be used. The "geography" of the copper-oxide materials can in the doping-temperature diagram.
[edit] History and Progress
The term high-temperature superconductor was first used to designate the new family of cuprate-perovskite ceramic materials discovered by Johannes Georg Bednorz and Karl Alexander Müller in 1986,[1] for which they won the Nobel Prize in Physics the following year. Their discovery of the first high-temperature superconductor LaBaCuO, with a transition temperature of 35 K, generated much excitement.
Recently, other unconventional superconductors have been discovered. Some of them also have unusually high values of the critical temperature Tc, and hence they are sometimes also called high-temperature superconductors, although the record is still held by a cuprate-perovskite material (Tc=138 K, that is −135 °C), although slightly higher transition temperatures have been achieved under pressure.[2] Nevertheless, it is believed by some researchers that if a room temperature superconductor is ever discovered it will be in a different family of materials.[citation needed]
[edit] Types of High-Temperature Superconductors
Most prominent materials in the high-Tc range are the so-called cuprates, such as La1.85Ba0.15CuO4, YBCO (Yttrium-Barium-Copper-Oxide) and related substances.
All known high-Tc superconductors are so-called Type-II superconductors. A Type-II superconductor allows magnetic field to penerate its interior in the units of flux quanta, creating 'holes' (or tubes) of normal metallic regions in the superconducting bulk. This property makes high-Tc superconductors capable of sustaining much higher magnetic fields.
[edit] Ongoing Research
One of the top unsolved problems in modern physics is the question of how superconductivity arises in these materials, that is, what mechanism causes the electrons in these crystals to form pairs.
Despite much intensive research and many promising leads, an answer to this question has so far eluded scientists. One reason for this is that the materials in question are generally very complex, multi-layered crystals (for example, BSCCO), making theoretical modeling difficult. But with the rapid rate of new, important discoveries in the field, many researchers are optimistic that a complete understanding of the process is possible within the next decade or so.
[edit] References
- ^ J. G. Bednorz and K. A. Müller (1986). "Possible highTc superconductivity in the Ba−La−Cu−O system". Z. Physik, B 64: 189-193. DOI:10.1007/BF01303701.
- ^ L. Gao, Y. Y. Xue, F. Chen, Q. Xiong, R. L. Meng, D. Ramirez, C. W. Chu, J. H. Eggert, and H. K. Mao (1994). "Superconductivity up to 164 K in HgBa2Cam-1CumO2m+2+δ (m=1, 2, and 3) under quasihydrostatic pressures". Phys. Rev. B 50: 4260-4263. DOI:10.1103/PhysRevB.50.4260.
