Cuprate superconductor


Cuprate superconductors are a family of high-temperature superconducting materials made of layers of copper oxides alternating with layers of other metal oxides, which act as charge reservoirs. At ambient pressure, cuprate superconductors are the highest temperature superconductors known.
File:Cuphasediag.png|thumb|Phase diagram of cuprate superconductors: They can be basically split into electron and hole doped cuprates, as for the basic models describing semiconductors. Both standard cuprate superconductors, YBCO and BSCCO, are notably hole-doped.
Cuprates have a structure close to that of a two-dimensional material. Their superconducting properties are determined by electrons moving within weakly coupled copper-oxide layers. Neighbouring layers contain ions such as lanthanum, barium, strontium, or other atoms that act to stabilize the structures and dope electrons or holes onto the copper-oxide layers. The undoped "parent" or "mother" compounds are Mott insulators with long-range antiferromagnetic order at sufficiently low temperatures. Single band models are generally considered to be enough to describe the electronic properties.
The cuprate superconductors adopt a perovskite structure. The copper-oxide planes are checkerboard lattices with squares of O2− ions with a Cu2+ ion at the centre of each square. The unit cell is rotated by 45° from these squares. Chemical formulae of superconducting materials contain fractional numbers to describe the doping required for superconductivity.
Several families of cuprate superconductors have been identified. They can be categorized by their elements and the number of adjacent copper-oxide layers in each superconducting block. For example, YBCO and BSCCO can be referred to as Y123 and Bi2201/Bi2212/Bi2223 depending on the number of layers in each superconducting block. The superconducting transition temperature peaks at an optimal doping value and an optimal number of layers in each block, typically three.
Possible mechanisms for cuprate superconductivity remain the subject of considerable debate and research. Similarities between the low-temperature state of undoped materials and the superconducting state that emerges upon doping, primarily the x2−y2 orbital state of the Cu2+ ions, suggest that electron–electron interactions are more significant than electron–phonon interactions in cupratesmaking the superconductivity unconventional. Recent work on the Fermi surface has shown that nesting occurs at four points in the antiferromagnetic Brillouin zone where spin waves exist and that the superconducting energy gap is larger at these points. The weak isotope effects observed for most cuprates contrast with conventional superconductors that are well described by BCS theory.

Types

Yttrium–barium cuprate

An yttrium–barium cuprate, YBa2Cu3O7−x, was the first superconductor found above liquid nitrogen boiling point. There are two atoms of Barium for each atom of Yttrium. The proportions of the three different metals in the YBa2Cu3O7 superconductor are in the mole ratio of 1 to 2 to 3 for yttrium to barium to copper, respectively: this particular superconductor has also often been referred to as the 123 superconductor.
The unit cell of YBa2Cu3O7 consists of three perovskite unit cells, which is pseudocubic, nearly orthorhombic. The other superconducting cuprates have another structure: they have a tetragonal cell. Each perovskite cell contains a Y or Ba atom at the center: Ba in the bottom unit cell, Y in the middle one, and Ba in the top unit cell. Thus, Y and Ba are stacked in the sequence along the c-axis. All corner sites of the unit cell are occupied by Cu, which has two different coordinations, Cu and Cu, with respect to oxygen. There are four possible crystallographic sites for oxygen: O, O, O and O. The coordination polyhedra of Y and Ba with respect to oxygen are different. The tripling of the perovskite unit cell leads to nine oxygen atoms, whereas YBa2Cu3O7 has seven oxygen atoms and, therefore, is referred to as an oxygen-deficient perovskite structure. The structure has a stacking of different layers:. One of the key feature of the unit cell of YBa2Cu3O7−x is the presence of two layers of. The role of the Y plane is to serve as a spacer between two planes. In YBCO, the Cu–O chains are known to play an important role for superconductivity. c is maximal near when x ≈ 0.15 and the structure is orthorhombic. Superconductivity disappears at x ≈ 0.6, where the structural transformation of YBCO occurs from orthorhombic to tetragonal.

Other cuprates

The preparation of other cuprates is more difficult than the YBCO preparation. They also have a different crystal structure: they are tetragonal where YBCO is orthorhombic. Problems in these superconductors arise because of the existence of three or more phases having a similar layered structure. Moreover, the crystal structure of other tested cuprate superconductors are very similar. Like YBCO, the perovskite-type feature and the presence of simple copper oxide layers also exist in these superconductors. However, unlike YBCO, Cu–O chains are not present in these superconductors. The YBCO superconductor has an orthorhombic structure, whereas the other high-c superconductors have a tetragonal structure.
There are three main classes of superconducting cuprates: bismuth-based, thallium-based and mercury-based.
The second cuprate by practical importance is currently BSCCO, a compound of Bi–Sr–Ca–Cu–O. The content of bismuth and strontium creates some chemical issues. It has three superconducting phases forming a homologous series as Bi2Sr2Can−1CunO4+2n+''x. These three phases are Bi-2201, Bi-2212 and Bi-2223, having transition temperatures of, and, respectively, where the numbering system represent number of atoms for Bi Sr, Ca and Cu respectively. The two phases have a tetragonal structure which consists of two sheared crystallographic unit cells. The unit cell of these phases has double Bi–O planes which are stacked in a way that the Bi atom of one plane sits below the oxygen atom of the next consecutive plane. The Ca atom forms a layer within the interior of the layers in both Bi-2212 and Bi-2223; there is no Ca layer in the Bi-2201 phase. The three phases differ with each other in the number of cuprate planes; Bi-2201, Bi-2212 and Bi-2223 phases have one, two and three planes, respectively. The c'' axis lattice constants of these phases increases with the number of cuprate planes. The coordination of the Cu atom is different in the three phases. The Cu atom forms an octahedral coordination with respect to oxygen atoms in the 2201 phase, whereas in 2212, the Cu atom is surrounded by five oxygen atoms in a pyramidal arrangement. In the 2223 structure, Cu has two coordinations with respect to oxygen: one Cu atom is bonded with four oxygen atoms in square planar configuration and another Cu atom is coordinated with five oxygen atoms in a pyramidal arrangement.
;Cuprate of Tl–Ba–Ca: The first series of the Tl-based superconductor containing one Tl–O layer has the general formula TlBa2Can−1CunO2n+3, whereas the second series containing two Tl–O layers has a formula of Tl2Ba2Can−1CunO2n+4 with n =1, 2 and 3. In the structure of Tl2Ba2CuO6, there is one layer with the stacking sequence . In Tl2Ba2CaCu2O8, there are two Cu–O layers with a Ca layer in between. Similar to the Tl2Ba2CuO6 structure, Tl–O layers are present outside the Ba–O layers. In Tl2Ba2Ca2Cu3O10, there are three CuO2 layers enclosing Ca layers between each of these. In Tl-based superconductors, c is found to increase with the increase in layers. However, the value of c decreases after four layers in TlBa2Can−1CunO2n+3, and in the Tl2Ba2Can−1CunO2n+4 compound, it decreases after three layers.
;Cuprate of Hg–Ba–Ca: The crystal structure of HgBa2CuO4, HgBa2CaCu2O6 and HgBa2Ca2Cu3O8 is similar to that of Tl-1201, Tl-1212 and Tl-1223, with Hg in place of Tl. It is noteworthy that the c of the Hg compound containing one layer is much larger as compared to the one--layer compound of thallium. In the Hg-based superconductor, c is also found to increase as the layer increases. For Hg-1201, Hg-1212 and Hg-1223, the values of c are 94, 128, and the record value at ambient pressure, respectively, as shown in table below. The observation that the c of Hg-1223 increases to under high pressure indicates that the c of this compound is very sensitive to the structure of the compound.
NameFormulaTemperature Number of planes of CuO in unit cellCrystal structure
Y-123YBa2Cu3O7922Orthorhombic
Bi-2201Bi2Sr2CuO6201Tetragonal
Bi-2212Bi2Sr2CaCu2O8852Tetragonal
Bi-2223Bi2Sr2Ca2Cu3O101103Tetragonal
Tl-2201Tl2Ba2CuO6801Tetragonal
Tl-2212Tl2Ba2CaCu2O81082Tetragonal
Tl-2223Tl2Ba2Ca2Cu3O101253Tetragonal
Tl-1234TlBa2Ca3Cu4O111224Tetragonal
Hg-1201HgBa2CuO4941Tetragonal
Hg-1212HgBa2CaCu2O61282Tetragonal
Hg-1223HgBa2Ca2Cu3O81343Tetragonal