Low-High Temperature Superconductivity

Low Temperature Superconductivity

LTS stands for “low temperature superconductor,” which typically refers to the Nb-based alloy (most commonly Nb-47wt.%Ti) and A15 (Nb3Sn and Nb3Al) superconductors in use prior to the discovery of “high temperature” oxide superconductors in 1986. “Temperature” here refers to the temperature below which the superconductor must be cooled in order for it to become superconducting. For LTS superconductors that temperature is usually well below 20 K (-253 °C). Nb-47wt.%Ti alloy has become the dominant commercial superconductor because it can be economically manufactured in a ductile form with the prerequisite nano-structure needed for high critical current. Similarly Nb3Sn based strand, although based on a brittle A15 superconducting phase, can be manufactured into strong composites in km lengths and microstructures that promote high critical current densities. These superconductors are often termed “technical superconductors” because of their applicability to engineering tasks. All these conductors require cooling to 4 K (liquid He is the most common coolant). LTS superconductors are prefered to use at LTS superconductor applications, including MRI and NMR devices.

High-temperature superconductivity

High-temperature superconductors (high-Tc or HTS) are materials that behave as superconductors at unusually high temperatures. The first high-Tcsuperconductor was discovered in 1986 by IBM researchers Karl Müller and Johannes Bednorz, who were awarded the 1987 Nobel Prize in Physics “for their important break-through in the discovery of superconductivity in ceramic materials”.

Whereas “ordinary” or metallic superconductors usually have transition temperatures (temperatures below which they superconduct) below 30 K (−243.2 °C), HTS have been observed with transition temperatures as high as 138 K (−135 °C). Until 2008, only certain compounds of copper and oxygen (so-called “cuprates“) were believed to have HTS properties, and the term high-temperature superconductor was used interchangeably with cuprate superconductor for compounds such as bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). However, several iron-based compounds (the iron pnictides) are now known to be superconducting at high temperatures.

Crystal structures of high-temperature ceramic superconductors

The structure of high-Tc copper oxide or cuprate superconductors are often closely related to perovskite structure, and the structure of these compounds has been described as a distorted, oxygen deficient multi-layered perovskite structure. One of the properties of the crystal structure of oxide superconductors is an alternating multi-layer of CuO2 planes with superconductivity taking place between these layers. The more layers of CuO2 the higher Tc. This structure causes a large anisotropy in normal conducting and superconducting properties, since electrical currents are carried by holes induced in the oxygen sites of the CuO2 sheets. The electrical conduction is highly anisotropic, with a much higher conductivity parallel to the CuO2 plane than in the perpendicular direction. Generally, Critical temperatures depend on the chemical compositions, cations substitutions and oxygen content. They can be classified as superstripes; i.e., particular realizations of superlattices at atomic limit made of superconducting atomic layers, wires, dots separated by spacer layers, that gives multiband and multigap superconductivity.

YBaCuO superconductors

The first superconductor found with Tc > 77 K (liquid nitrogen boiling point) is yttrium barium copper oxide (YBa2Cu3O7-x), the proportions of the 3 different metals in the YBa2Cu3O7superconductor are in the mole ratio of 1 to 2 to 3 for yttrium to barium to copper respectively. Thus, this particular superconductor is often referred to as the 123 superconductor.

The unit cell of YBa2Cu3O7 consists of three pseudocubic elementary perovskite unit cells. Each perovskite unit 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 [Ba–Y–Ba] along the c-axis. All corner sites of the unit cell are occupied by Cu, which has two different coordinations, Cu(1) and Cu(2), with respect to oxygen. There are four possible crystallographic sites for oxygen: O(1), O(2), O(3) and O(4). 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: (CuO)(BaO)(CuO2)(Y)(CuO2)(BaO)(CuO). One of the key feature of the unit cell of YBa2Cu3O7-x (YBCO) is the presence of two layers of CuO2. The role of the Y plane is to serve as a spacer between two CuO2 planes. In YBCO, the Cu–O chains are known to play an important role for superconductivity. Tc is maximal near 92 K when x ≈ 0.15 and the structure is orthorhombic. Superconductivity disappears atx ≈ 0.6, where the structural transformation of YBCO occurs from orthorhombic to tetragonal.

Bi-, Tl- and Hg-based high-Tc superconductors

The crystal structure of Bi-, Tl- and Hg-based high-Tc superconductors are very similar. Like YBCO, the perovskite-type feature and the presence of CuO2 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-Tc superconductors have a tetragonal structure.

Bi-based high-Tc superconductors (Bi–Sr–Ca–Cu–O): The Bi–Sr–Ca–Cu–O system has three superconducting phases forming a homologous series as Bi2Sr2Can−1CunO4+2n+x (n = 1, 2 and 3). These three phases are Bi-2201, Bi-2212 and Bi-2223, having transition temperatures of 20, 85 and 110 K, 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 CuO2 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 CuO2 planes; Bi-2201, Bi-2212 and Bi-2223 phases have one, two and three CuO2 planes, respectively. The c axis of these phases increases with the number of CuO2 planes (see table below). 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.

Tl-based high-Tc superconductors (Tl–Ba–Ca–Cu–O): 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 (Tl-2201), there is one CuO2 layer with the stacking sequence (Tl–O) (Tl–O) (Ba–O) (Cu–O) (Ba–O) (Tl–O) (Tl–O). In Tl2Ba2CaCu2O8 (Tl-2212), there are two Cu–O layers with a Ca layer in between. Similar to the Tl2Ba2CuO6structure, Tl–O layers are present outside the Ba–O layers. In Tl2Ba2Ca2Cu3O10 (Tl-2223), there are three CuO2 layers enclosing Ca layers between each of these. In Tl-based superconductors, Tc is found to increase with the increase in CuO2 layers. However, the value of Tc decreases after four CuO2 layers in TlBa2Can-1CunO2n+3, and in the Tl2Ba2Can-1CunO2n+4 compound, it decreases after three CuO2 layers.

Hg-based high-Tc superconductors (Hg–Ba–Ca–Cu–O): The crystal structure of HgBa2CuO4 (Hg-1201), HgBa2CaCu2O6 (Hg-1212) and HgBa2Ca2Cu3O8 (Hg-1223) is similar to that of Tl-1201, Tl-1212 and Tl-1223, with Hg in place of Tl. It is noteworthy that the Tc of the Hg compound (Hg-1201) containing one CuO2 layer is much larger as compared to the one-CuO2-layer compound of thallium (Tl-1201). In the Hg-based superconductor, Tc is also found to increase as the CuO2 layer increases. For Hg-1201, Hg-1212 and Hg-1223, the values of Tc are 94, 128 and the record value at ambient pressure 134 K, respectively, as shown in table below. The observation that the Tc of Hg-1223 increases to 153 K under high pressure indicates that the Tc of this compound is very sensitive to the structure of the compound.

Critical temperature (Tc), crystal structure and lattice constants of some high-Tc superconductors

Formula

Notation

Tc (K)

No. of Cu-O planes in unit cell

Crystal structure

YBa2Cu3O7

123

92

2

Orthorhombic

Bi2Sr2CuO6

Bi-2201

20

1

Tetragonal

Bi2Sr2CaCu2O8

Bi-2212

85

2

Tetragonal

Bi2Sr2Ca2Cu3O6

Bi-2223

110

3

Tetragonal

Tl2Ba2CuO6

Tl-2201

80

1

Tetragonal

Tl2Ba2CaCu2O8

Tl-2212

108

2

Tetragonal

Tl2Ba2Ca2Cu3O10

Tl-2223

125

3

Tetragonal

TlBa2Ca3Cu4O11

Tl-1234

122

4

Tetragonal

HgBa2CuO4

Hg-1201

94

1

Tetragonal

HgBa2CaCu2O6

Hg-1212

128

2

Tetragonal

HgBa2Ca2Cu3O8

Hg-1223

134

3

Tetragonal

Fe-based high-Tc superconductors

Iron-based superconductors contain layers of iron and a pnictogen—such asarsenic or phosphorus—or a chalcogen. This is currently the family with the second highest critical temperature, behind the cuprates. Interest in their superconducting properties began in 2006 with the discovery of superconductivity in LaFePO at 4 K and gained much greater attention in 2008 after the analogous material LaFeAs(O,F) was found to superconduct at up to 43 K under pressure.

Since the original discoveries several families of iron-based superconductors have emerged:

·         LnFeAs(O,F) or LnFeAsO1-x with Tc up to 56 K, referred to as 1111 materials. A fluoride variant of these materials was subsequently found with similar Tc values.

·         (Ba,K)Fe2As2 and related materials with pairs of iron-arsenide layers, referred to as 122 compounds. Tc values range up to 38 K. These materials also superconduct when iron is replaced with cobalt

·         LiFeAs and NaFeAs with Tc up to around 20 K. These materials superconduct close to stoichiometric composition and are referred to as 111 compounds.

·         FeSe with small off-stoichiometry or tellurium doping.

Most undoped iron-based superconductors show a tetragonal-orthorhombic structural phase transition followed at lower temperature by magnetic ordering, similar to the cuprate superconductors. However, they are poor metals rather than Mott insulators and have five bands at the Fermi surface rather than one. The phase diagram emerging as the iron-arsenide layers are doped is remarkably similar, with the superconducting phase close to or overlapping the magnetic phase. Strong evidence that the Tc value varies with the As-Fe-As bond angles has already emerged and shows that the optimal Tc value is obtained with undistorted FeAs4 tetrahedra. The symmetry of the pairing wavefunction is still widely debated, but an extended s-wave scenario is currently favoured.