High Temperature Superconductors

High-temperature superconductors consist of metal-oxide layers, stacked atop each other along the crystalline c-axis [26, 27]. There are currently more than 100 known HTS systems. Critical temperature of different HTS spans from below 20 K for Bi2Sr2CuO6 [28] or (Nd0 925Ce0 075 ) 2CuO4-s [29] to 164 K for HgBa2Ca2Cu3O8+s at high pressure [30]. Critical temperatures and coherence lengths of some HTSs are given in Table 1. For comparison, the highest values of Tc for classical superconductors are 23.2 K for Nb3Ge [50, 51] and 18 K for Nb3Sn [52].

A common feature of all HTSs is CuO2 layers situated in the crystalline ab-plane (there are some exceptions, for example, Ba1-aKaBiO3 with Tc of 32 K [53-56] and YSr2Ru1-bCubO6 with Tc of 60 K [57]). It is generally accepted that the primary sites of superconductivity in HTS crystal structure are these CuO2 layers [27, 58-60], even though there are some other views, too [61]. While CuO2 layers are a good conducting medium, the oxide layers between CuO2 are poor conductors. This results in anisotropic superconducting properties of HTSs. The values

Table 1. Critical temperature (Tc) and in-plane (£ab) and out-of-plane (£c) coherence lengths of some high-temperature superconductors.

Superconductor

Tc (K)

Ref.

£ab (nm)

£c (nm)

Ref.

YBa^^O,

92

[31, 32]

1.2-1.8

0.2-0.3

[43-47]

DyBa2Cu3O7

89

[33]

Bi2Sr2Ca2Cu3O10

110

[34]

0.63-2.9

[35, 36]

Bi2Sr2CaCu2O8

85

[37]

2-4

0.04

[38, 39,

48, 49]

HgBa2CuO4+s

97

[30]

2.1

1.2

[40]

HgBa2CaCu2O6+s

128

[30]

1.7

0.4

[41]

HgBa2Ca2Cu3O8+s

135

[30]

1.5-2

0.19

[40-42]

Tl2Ba2Ca2Cu3Oz

125

[29]

Note: All values given in the table are measured at atmospheric pressure.

Note: All values given in the table are measured at atmospheric pressure.

of £ and A in the crystalline ab-layer are very different from the ones along the crystalline c-axis. For example, £a ~ £b ~ 1.2-1.8 nm and £c ~ 0.2-0.3 nm for YBa2Cu3O7 superconductors (Table 1) [43-47, 62]. Technologically most important HTSs are currently YBa2Cu3O7, Bi2Sr2Ca2Cu3O10, and Bi2Sr2CaCu2O8, with anisotropy increasing in the same order.

The small coherence length of HTSs is a major cause for poor connectivity between the HTS crystals. This severely limits transport of the superconducting current through macroscopic conductors made of HTSs. The grain connectivity is still a major obstacle for wider introduction of HTSs in practical applications. Complexities of grain connectivity are beyond the scope of this chapter; detailed reviews can be found elsewhere [63]. The problem of grain connectivity was most successfully solved for Bi2Sr2Ca2Cu3O10. It is produced in the form of kilometer-long wires [30], which can be used for electric cables, transformers, electrical motors, and generators. A weak point of this superconductor is strong suppression of its intracrystalline Jc by magnetic field. In contrast to this, Jc of YBa2Cu3O7 crystals is much less sensitive to magnetic field; however, the grain connectivity is a serious problem. Current research efforts are aimed at developing thick films of YBa2Cu3O7 with good grain connectivity over a large length. It is predicted that they will replace Bi2Sr2Ca2Cu3O10-based wires in about 10 years.

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