High Temperature Cuprate Superconductors and Later Discoveries

Abstract

The discovery of superconductivity at 30 K in an oxygen deficient La4Ba1Cu5O5(3−y) compound by Bednorz and Muller in 1986 lifted the despondency under which the superconductivity community was reeling since 1973 when highest T c = 23 K was recorded in sputtered Nb3Ge films and T c went no further. Chu raised T c of this compound to 40 K by applying pressure and Cava to 36 K after substituting Sr at the La-site. A breakthrough came in early 1987 when Wu and Chu announced a record T c = 93 K in Y1Ba2 Cu3 O7. T c thus crossed the 77 K mark first time. Soon Maeda discovered superconductivity at 110 K in another cuprate of the type Bi2Sr2Ca2Cu3O10 (Bi-2223 with 3 CuO2 layers). T c thus increases with the number of CuO2 layers but not beyond three layers. These layered curates have large anisotropy. The superconductivity is strong in the a-b planes (CuO2 layers) and weak along the c-axis. Critical parameters B c2 and J c too are high in the a-b plane and low along c-direction. Both the materials, Bi-2223 and YBCO are produced commercially and used for selected applications. The new improved 2G-YBCO wires are coated multilayered thin film conductors produced by employing sophisticated techniques and getting popularity among the community. Two more cuprates with still higher T c and analogous to Bi-system were discovered. A T c = 125 in Tl2Ca2Ba2Cu3O x (Ti-2223) and 135 K in Hg1Ba2Ca2Cu3O6+δ (Hg-1223) were reported. These materials were, however, not pursued for commercial production because of the toxicity involved. Superconductivity was also found in 2001 in MgB2 at 39 K, easily attainable with the use of a cryo-cooler. Besides, the material is quite cheap. MgB2 has two energy gaps and appears to be a BCS superconductor. It is being produced commercially now. The chapter also reviews all the discoveries that took place in 2008 onwards. Hosano reported superconductivity in iron based oxy-pnictides of the type LaFeAsO(1111) around 25 K. T c in excess of 50 K were reported in Sm and Nd based pnictides. The strategy adopted to enhance T c has been to dope the insulating La2O2 layer suitably whereby a charge, electron/hole is transferred to the Fe2As2 conduction layer. Thus a T c = 38 K was reported in a K-doped (Ba0.6K0.4) Fe2As2 compound. Very strange behavior of the appearance, disappearance and re-appearance of superconductivity with increasing pressure was announced in an iron-chalcogenide, Tl0.6Rb0.4Fe1.67Se2 hinting at the possibility of two different types of superconducting phases appearing at different pressures. A maximum T c = 48 K was obtained in this material at a pressure of 12.4 GPa.