Abstract

The search for novel superconducting materials has always been an important integral part of superconductivity research for science and technology. Prior to 1986, during the low-temperature superconductivity (LTS) era, it has helped raise the superconducting transition temperature (T _c), broaden the material base, unravel the mystery of superconductivity, and demonstrate the viability of superconductivity technology. The development of the Bardeen–Cooper–Schrieffer (BCS) theory on LTS was greatly assisted by the then-available data on a wide range of high-quality compounds. Today's powerful magnetic resonance imaging technology for medical diagnoses and the omnipotent accelerator technology for particle physics research would not have been possible without superconductors, although having a low T _c. In the process, many new non-superconducting compounds were also discovered, as were the many new associated physical phenomena, resulting in the development of new physics and theories. Itinerant ferromagnetism in ZrZn₂ [1], the charge-density waves in the layered transition metal dichalcogenides [2], and the recently discovered Majorona fermion in insulator-Nb structures [3] are just a few of the examples. The discovery of the 30-K high-temperature superconducting (HTSg) Ba-doped La₂CuO₄ compound in 1986 [4] and the subsequent discovery of the first liquid nitrogen superconductor, in the Y-Ba-Cu-O system with T _c above 93 K in 1987 [5], did not end the search for novel superconductors nor did it lessen the importance of such a search. This is borne out by the discovery of many new HTSg compounds in the ensuing three decades [6, 7] leading to the rapid rise of T _c (Figure D4.1); the discoveries of several unconventional superconducting systems [7]; the development of powerful tools to prepare the high-temperature superconductors (HTSrs) and to probe the origin of high-temperature superconductivity (HTSy) [8]; the revelation of the unusual HTSg properties [9]; the proposition of various theoretical models [10]; and the development of HTSg prototype devices [11]. The great resurgence of interest in the scientifically significant and technologically important colossal magneto-resistance compounds [12], as well as the topological insulators and superconductors, which are scientifically intriguing and hold technological potential [13], are yet more examples of the collateral effects of HTSg material study. In spite of this impressive progress, no commonly accepted microscopic theory of HTSy exists and viable commercialization of HTSrs remains beyond our reach.