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High-temperature superconductor:

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Unsolved problems in physics: What is the responsible mechanism that causes certain materials to exhibit superconductivity at temperatures above 50 kelvin?

High-temperature superconductors (abbreviated high T_c or HTS) are a family of superconducting ceramic materials largely containing copper-oxide planes as a common structural feature. For this reason, the term was (before 2008) often used interchangeably with cuprate superconductors. "High" temperature in this context just means above 30 K, which was thought (1960-1980) to be the highest possible T_c and was well above the 1973 record of 23 K.

High-T_c superconductivity was discovered in 1986; until then it was thought that BCS theory ruled out superconductivity at temperatures above 30 K. The experimental discovery of the first high-T_c superconductor by Karl Müller and Johannes Bednorz was immediately recognized by the Nobel Prize in Physics in 1987.

High-temperature superconductivity allows some materials to support superconductivity at temperatures above the boiling point of liquid nitrogen (77 K or −196 °C). Indeed, they offer the highest transition temperatures of all superconductors. The ability to use relatively inexpensive and easily handled liquid nitrogen as a coolant has increased the range of practical applications of superconductivity.

The best known HTS are BSCCO and YBCO.

The critical magnetic field that destroys superconductivity tends to be higher for materials with a high T_c and in magnet applications this may be more valuable than the high T_c itself. Some cuprates have an upper critical field around 100 tesla.

Although normal compounds in the normal superconducting state share many characteristics with each other, there is as of 2008^[update] no widely accepted theory to explain their properties. The search for a theoretical understanding of high-temperature superconductivity is an important unsolved problem in physics, and it continues to be a topic of intense experimental and theoretical research, with over 100,000 published papers on the subject.^[1] Cuprate superconductors (and other unconventional superconductors) differ in many important ways from conventional superconductors, such as elemental mercury or lead, which are adequately explained by the BCS theory. There has been much debate as to high-temperature superconductivity coexisting with magnetic ordering in several ruthenocuprates and other exotic superconductors, and the search continues for other families of materials. In February 2008, researchers at the Tokyo Institute of Technology announced that the quaternary compound LaOFeAs (an oxypnictide), when doped with F for O, is a new non-cuprate high-temperature superconductor^[2]. and already hints at an upper critical field around 64 T.

1 Copper-oxide planes in cuprates
2 General phase diagram
3 History and progress
4 Examples
5 Process
6 Ongoing research
7 See also
8 References
9 External links

[edit] Copper-oxide planes in cuprates

The cuprates are quasi-two-dimensional materials which consist of layers of copper-oxide planes separated by other materials. The cuprate superconductors adopt a perovskite structure. It seems that most of the properties are determined by electrons moving within the copper-oxide planes. The remaining components play structural roles and provide screening and doping environments. 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 45° 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 or other 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 determines the Fermi surface and low-energy electronic properties. As such, in the ionization state Cu⁺⁺O₂⁻⁻, the copper-oxide plane is a Mott insulator with long-range antiferromagnetic order of spins at small enough temperatures. A vital feature of cuprates is their ability to accommodate chemical substitutions; i.e., atoms that (i) replace one of the atoms of the original without disrupting the short-range lattice order and (ii) have a 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 "environmental" 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 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] General phase diagram

All known high-T_c superconductors are Type-II superconductors. In contrast to Type-I superconductors, which expel all magnetic fields due to the Meissner Effect, Type-II superconductors allow magnetic fields to penetrate their interior in quantized units of flux, creating "holes" or "tubes" of normal metallic regions in the superconducting bulk. Consequently, high-T_c superconductors can sustain much higher magnetic fields.

H-T diagrams of Type-I and Type-II SC are here > ^[3] <.

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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 be seen in the doping-temperature diagram.

Diagrams of electron and hole doping cuprates can be found here > ^[4] <.

[edit] History and progress

April 1986 - 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,^[5] for which they won the Nobel Prize in Physics the following year. Their discovery of the first high-temperature superconductor, La Ba CuO, with a transition temperature of 35 K, generated great excitement.

LSCO (La_2-xSr_xCuO₂) discovered the same year.

January 1987 - YBCO was discovered to have a T_c of 90 K by M. K. Wu et al.^[6]

1988 - BSCCO discovered with T_c up to 107 K, and TBCCO (T=thallium) discovered to have T_c of 125 K.

As of 2006^[update], the highest-temperature superconductor (at ambient pressure) is mercury thallium barium calcium copper oxide (Hg₁₂Tl₃Ba₃₀Ca₃₀Cu₄₅O₁₂₅), at 138 K is held by a cuprate-perovskite material,^[7] , possibly 164 K under high pressure^[8].

[October 15, 2008] Superconductors.org reports synthesis of the first 200K superconductor in conjunction with the discovery of a new superconductor system. The 200K material is believed to have a B212/1212C intergrowth structure, where B=11 and C=copper chain. This structure has the chemical formula Sn₆Ba₄Ca₂Cu₁₀O_y. The general formula for this new family of superconductors is Sn_xBa₄Ca₂Cu_x+4O_y. Within this new family, unit cells with 3 to 6 atoms of tin (x) have been found to superconduct, with 6 atoms of tin producing a new record high Tc near 201K.^[9].

Recently, other unconventional superconductors, not based on cuprate structure, have been discovered.^[10] Some have unusually high values of the critical temperature, T_c, and hence they are sometimes also called high-temperature superconductors.

After more than twenty years of intensive research the origin of high-temperature superconductivity is still not clear, but it seems that instead of electron-phonon attraction mechanisms, as in conventional superconductivity, one is dealing with genuine electronic mechanisms (e.g. by antiferromagnetic correlations), and instead of s-wave pairing, d-waves are substantial.

One goal of all this research is room-temperature superconductivity^[11].

[edit] Examples

Examples of high-T_c cuprate superconductors include La_1.85Ba_0.15CuO₄, and YBCO (Yttrium-Barium-Copper-Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen.

[edit] Process

Perovskites are made by mixing oxides in stoichiometric quantities and then heating in a furnace at high temperatures in a concentrated oxygen atmosphere.

[edit] Ongoing research

A small sample of the high-temperature superconductor BSCCO-2223.

The question of how superconductivity arises in high-temperature superconductors is one of the major unsolved problems of theoretical condensed matter physics as of 2007^[update]. The mechanism that causes the electrons in these crystals to form pairs is not known.

Despite intensive research and many promising leads, an explanation 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.

[edit] See also

[edit] References

^ Buchanan, Mark (2001). "Mind the pseudogap". Nature 409: 8. doi:10.1038/35051238.
^ Iron Exposed as High-Temperature Superconductor: Scientific American
^ http://www-unix.mcs.anl.gov/superconductivity/phase.html Phase diagrams
^ http://www.wmi.badw-muenchen.de/FG538/projects/P4_crystal_growth/index.htm
^ J. G. Bednorz and K. A. Müller (1986). "Possible highT_c superconductivity in the Ba−La−Cu−O system". Z. Physik, B 64 (1): 189–193. doi:10.1007/BF01303701.
^ K. M. Wu et al. (1987). "Superconductivity at 93 K in a new mixed-phase Yb-Ba-Cu-O compound system at ambient pressure". Phys. Rev. Lett. 58: 908. doi:10.1103/PhysRevLett.58.908.
^ P. Dai, B. C. Chakoumakos, G. F. Sun, K. W. Wong, Y. Xin and D. F. Lu (1995). "Synthesis and neutron powder diffraction study of the superconductor HgBa₂Ca₂Cu₃O_8+δ by Tl substitution". Physica C:Superconductivity 243 (3-4): 201–206. doi:10.1016/0921-4534(94)02461-8.
^ 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 HgBa₂Ca_m-1Cu_mO_2m+2+δ (m=1, 2, and 3) under quasihydrostatic pressures". Phys. Rev. B 50 (6): 4260–4263. doi:10.1103/PhysRevB.50.4260.
^ The First 200k Superconductor, http://superconductors.org/200K.htm.
^ Hiroki Takahashi, Kazumi Igawa, Kazunobu Arii, Yoichi Kamihara, Masahiro Hirano, Hideo Hosono (2008). "Superconductivity at 43 K in an iron-based layered compound LaO1-^xF^xFeAs". Nature 453: 376–378. doi:10.1038/nature06972.
^ A. Mourachkine (2004). Room-Temperature Superconductivity. Cambridge International Science Publishing (Cambridge, UK) (also http://xxx.lanl.gov/abs/cond-mat/0606187). doi:1-904602-27-4 ISBN 1-904602-27-4.