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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 Tc 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 Tc and was well above the 1973 record of 23 K.

High-Tc 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-Tc 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 high temperature superconductors are bismuth strontium calcium copper oxide, BSCCO and yttrium barium copper oxide, YBCO.

The critical magnetic field that destroys superconductivity tends to be higher for materials with a high Tc and in magnet applications this may be more valuable than the high Tc 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 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 superconductor2. and already hints at an upper critical field around 64 T.

Contents

Copper-oxide planes in cuprates

The cuprates are typically considered by quasi-two-dimensional materials which consist of layers of copper-oxide planes separated by other material layers. The cuprate superconductors adopt a perovskite structure. It seems that most of the properties are determined by electrons moving within the copper-oxide planes in this structure. The remaining components play structural roles like an anisotropic effect 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 superconductor 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. A mechanism of quasi two dimensional cuprates is almost thought that is strongly related with antiferromagnetic phase in copper-oxide plane. In fermi surface of copper-oxide, there are a nesting point where an electron is slower than other electrons. The slower a velocity of electron is in Fermi surface, the stronger an interaction between electrons is. And these parts always cross the antiferromagnetic briluoin zone where a spin wave exists for antiferromagnet. In low temperature, superconducting phase has a competition with antiferromagnetic phase, so that spin fluctuation(strong electron-electron interaction) makes cooper pairs or spin wave in each condition. In copper-oxide fermi surface, nesting points are on the four sides and a superconducting energy gap of these points is larger than others. In the result, d wave superconducting energy gap is emerged in cuprates when superconducting phase is appeared. 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. Hole doped cuprates have been researched for long time because these samples had high quality for research. But typical electron doped cuprates 'R'2-xCexCuO4-y('R'=Nd,Sm,Pr) has rare-earth constitution, so that the dynamic properties of electron doped cuprates are difficult to measure by uSR experiment because of rare-earth magnetic moments. At present, many methods to measure the superconducting properties are developed. So, the properties of electron doped cuprates are revealed. The different things with hole doped superconductor are happened in uemura relation and pseudogap state. Firstly, uemura relation states that the london penetration depth at 0 temperation is directly related with the critical temperature and this universal relation adjusts to hole doped superconductor well. The london penetration depth is determined by effective mass and density of cooper pair in copper oxide plane and critical temperature is also determined. But electron doped cuprates is quitely deviated from uemura relation for hole doped superconductor. In the result, we can know that the universal relation between the penetration depth and critical temperature is different in electron/hole doped cuprates. The reason for breaking the symmetry in electron/hole doping is not understood yet. Secondary, a pseudogap phase only exists in hole doped cuprates and isotope effect is emerged in the boundary between pseudo gap and superconducting phase. The quasi-two-dimensional Cuprates usually have a weak coupling between CuO2 layers. Each CuO2 planes are considered that are dominant part for superconducting state. Superconducting energy gap usually has a d wave structure as we know already. But, Infinite layer electron doped cuprate SLCO(ILS) has a strong coupling between CuO2 layers. A energy gap is shown by a substantial s wave structure in this system. So, anisotropic s or double s+s or s+d energy gap are a candidate for ILS. The origin of s wave energy gap in this system is not exactly solved, but the strength of coupling between CuO2 layers is considered that dominantly effect on s wave energy gap.

General phase diagram

All known high-Tc 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-Tc 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 <.


History and progress

  • LSCO (La2-xSrxCuO2) discovered the same year.
  • January 1987 - YBCO was discovered to have a Tc of 90 K by M. K. Wu et al.6
  • 1988 - BSCCO discovered with Tc up to 107 K, and TBCCO (T=thallium) discovered to have Tc of 125 K.
  • As of 2006, the highest-temperature superconductor (at ambient pressure) is mercury thallium barium calcium copper oxide (Hg12Tl3Ba30Ca30Cu45O125), at 138 K and is held by a cuprate-perovskite material,7 , possibly 164 K under high pressure8.
  • [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 Sn6Ba4Ca2Cu10Oy. The general formula for this new family of superconductors is SnxBa4Ca2Cux+4Oy. 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.

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 superconductivity11.

Examples

Examples of high-Tc cuprate superconductors include La1.85Ba0.15CuO4, and YBCO (Yttrium-Barium-Copper-Oxide), which is famous as the first material to achieve superconductivity above the boiling point of liquid nitrogen.

Process

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

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 2008. 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.

See also

References

  1. ^ Buchanan, Mark (2001). "Mind the pseudogap". Nature 409: 8. doi:10.1038/35051238. 
  2. ^ Iron Exposed as High-Temperature Superconductor: Scientific American
  3. ^ http://www-unix.mcs.anl.gov/superconductivity/phase.html Phase diagrams
  4. ^ http://www.wmi.badw-muenchen.de/FG538/projects/P4_crystal_growth/index.htm
  5. ^ J. G. Bednorz and K. A. Müller (1986). "Possible highTc superconductivity in the Ba−La−Cu−O system". Z. Physik, B 64 (1): 189–193. doi:10.1007/BF01303701. 
  6. ^ 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. 
  7. ^ 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 HgBa2Ca2Cu3O8+δ by Tl substitution". Physica C:Superconductivity 243 (3-4): 201–206. doi:10.1016/0921-4534(94)02461-8. 
  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 HgBa2Cam-1CumO2m+2+δ (m=1, 2, and 3) under quasihydrostatic pressures". Phys. Rev. B 50 (6): 4260–4263. doi:10.1103/PhysRevB.50.4260. 
  9. ^ The First 200k Superconductor, http://superconductors.org/200K.htm. 
  10. ^ Hiroki Takahashi, Kazumi Igawa, Kazunobu Arii, Yoichi Kamihara, Masahiro Hirano, Hideo Hosono (2008). "Superconductivity at 43 K in an iron-based layered compound LaO1-xFxFeAs". Nature 453: 376–378. doi:10.1038/nature06972. 
  11. ^ A. Mourachkine (2004). Room-Temperature Superconductivity, Cambridge International Science Publishing (Cambridge, UK) (also http://xxx.lanl.gov/abs/cond-mat/0606187). doi:ISBN 1-904602-27-4. 

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