**1. Introduction**

Various metals exhibit modest electrical resistance owing to normal room temperatures, however, may be turned into superconductors by employing a frozen route towards absolute zero temperature. The very first metal presented in favor of superconductors was mercury that was discovered just after cryogenic refrigerator in 1908, attaining that temperature at which phase shift of helium may occur as liquid form showing 4.2 K = −452 °F. In addition, enveloping more than 60 years, further superconductor's discoveries continued and proved to be high-quality superconductors at such low temperatures. Furthermore, during 1960s, specific niobium alloys were also turned into superconductors, however, at the temperature range 11–24 K. Subsequently, theoretical studies also showed and proved that there is no existence of superconductors above 30 K. Superconductors being non-resistive, are considered fast driving current carriers without voltage or electricity [1–3].

Starting current continuously flows for "geological" periods subject to keeping cold the relevant superconductors. Over a long time, chilling requirements for

extremely low temperatures showed greater effect towards confinement of superconductivity in the territory of literature laboratories [4, 5]. The running expenditure attributed to superconducting current loop is evaluated subject to refrigeration cost i.e., \$7 per liter that is mostly considered as liquid helium purchasing cast accordingly. As far as electromagnets are concerned, their application as a current loop is very important. However, it is more expensive electromagnet that may be made by utilizing copper wires. Further by 1970s, it has become more cost-effective in numerous cases in the form of paying price for freezing a superconductor rather than bearing utility bills for electricity used against resistance [6, 7]. Industrial level setup was evolved so far, in which high-quality superconducting magnets were launched towards versatile applications. The most familiar application is now in hospitals in the form of Magnetic Resonance Imaging (MRI), proving as standarddiagnostic-tool to diagnose dead cells in the human body by scanning it successfully. This is a low-cost running device as compared with the cost of "exploratory surgery" [8, 9].

High-temperature superconductors (HTS) are strongly considered as defined materials behaving as superconductors at high temperatures (> 78 K) showing liquid nitrogen (boiling point), which is considered the simplest cryogenic- coolant [8, 10]. All types of superconductors are currently working nowadays at normal pressure but below ambient temperatures that require more cooling environment. However, most HTS behave such as ceramic type materials while metallic superconductors often work at temperature (< −200 °C), therefore referred to as LTS (low-temperature superconductors) [11, 12]. Furthermore, metals based superconductors are numerously identified as common superconductors owing to fine discovery as well as proper use before the introduction of high-temperature ones. Additionally, ceramic superconductors have also been proved to be suitable for practical application, rather they still show various fabricating issues since only very few examples of employment are on the screen up till now. Owing brittle nature of most ceramics they present behavior while fabricating wires from them for manufacturing superconductors [13, 14]. On the other hand, a major benefit belonging to high-temperature ceramic superconducting materials is their cooling through liquid-nitrogen on the contrary; metallic superconductors often need rare coolants that may be liquid helium [7, 15]. Unfortunately, a more common disadvantage is that no HTS may be refrigerated using dry ice, and none amongst those may work at room temperature as well as pressure. They can only work reasonably below the lowest-temperature measured on Earth's surface. Necessarily, HTS sufficiently requires some cooling system at every cast. Superior high-temperature superconductors belong to only particular class of copper oxides. Another class of HTS is practically classified as iron-based compounds [6, 7, 15]. Magnesium diboride is considered another HTS because of easy manufacturing, however, working conditions under −230 °C (lower than triple point temperature of nitrogen) make it unsuitable concerning cooling with liquid nitrogen (below nitrogen triple-point-temperature). Ideally, liquid-helium can be used to achieve extremely lower temperatures for proper application. Various ceramic superconductors may also depict superconducting behavior owing to second type. The very first HTS has been discovered by Bednorz and Müller in 1986 [11, 12, 16] and obtained Nobel Prize (1987) for the "discovery of superconductivity in ceramic materials". Various high-pressure super-hydride chemical species are often incorporated in the realm of HTS. Indeed, much literature work containing HTS has been found owing to gases with high-pressure, however, unfavorable for synergetic applications. Finally, the latest critical temperature (TC) record holder is identified as carbon nature sulfur hydride, showing leading contribution leaving behind the previous record inherited in lanthanum deca-hydride (about 30 °C) [16–18].

**59**

**Figure 1.**

*from Ref. [25].*

*High Temperature Superconductors*

*DOI: http://dx.doi.org/10.5772/intechopen.96419*

**2. Milestone of high-temperature superconductivity**

relevant to these materials is considered open-ended up-till now.

Kamerlingh Onnes was one of researcher who introduced superconductivity in metal solid (1911). Researchers always struggled to make observations towards superconductivity at high temperatures [10, 19] for achieving goals of evaluating normal room temperature superconductors [20, 21]. Besides, superconductivity has been detected in various metallic compounds such as Nb containing compounds, for example (Nb3Ge, NbTi, and Nb3Sn) at much higher- temperatures as compared with elemental-metals, exceeding −253.2 °C (late 1970s). Moreover, in 1986, IBM research lab (Zurich) provided an opportunity to Bednorz and Müller who were working on superconductivity research route for generating a new class of ceramics (maybe cuprates as well as copper oxides). Bednorz discovered a zero resistance copper oxide at 35.1 K = −238 °C [22]. However, collected results were soon supported by numerous thoughts, notably Paul Chu and Shoji Tanaka at Houston and Tokyo universities one after others [23, 24], all the story illustrated in **Figure 1** and **Table 1**. Very shortly after, Anderson worked at Princeton University and presented a new theoretical concept relating to these materials. The theoretical idea was based upon RVBT (resonating valence-bond theory) [43] however still, full exploring

Above mentioned superconductors may possess identical d-wave pair. The very

first suggestion in favor of high-temperature cuprate superconductors d-wave pair symmetry was offered by Scalettar, Scalapino, and Bickers [44], which was associated with theories presented in 1988 by famous researchers known as Hirschfeld, Doniach, Inui, and Ruckenstein [45], they used spin fluctuation theory. Additionally, Rice, Gros, Zhang, and Poilblan [46], and Kotliar, as well as Liu, identified pairing concept representing usual consequence based on RVBT [47]. On the other hand, d-wave shape attributing cuprate superconductors was observed by many experiments. Further, the involvement of d-wave nodes was observed directly during excitation-spectrum by employing Angle-Resolved Photoemission-Spectroscopy. Half-integer flux observation was indicated through tunneling experiments whereas indirect temperature-dependence related to penetration

*(a) Maximum known Tc of molecular (TMTSF and BEDTF-TTF), iron-based, metallic, and oxide superconductors. Metallic superconductors' Tc increased from 4.2 K (Hg) to 23.2 K (Nb3Ge) from 1911 and 1974. However, after unexpected discovery of superconductivity in MgB2 in 2001, maximum Tc of 39 K was achieved. In 1986, highest Tc of oxides exceeded the boiling point of liquid nitrogen (77 K), after the discovery of high-Tc superconductivity in (La, Ba)2CuO4. (b) The first molecular superconductor was discovered in 1980 where high Tc of 40 K was discovered in Cs3C60 fullerene. From 2006 to 2013, the maximum known Tc of iron-based superconductors gradually increased from around 4 K for LaOFeP to 58 K for SmO0.74F0.26FeAs. Reproduced* 

*Transition Metal Compounds - Synthesis, Properties, and Application*

extremely low temperatures showed greater effect towards confinement of superconductivity in the territory of literature laboratories [4, 5]. The running expenditure attributed to superconducting current loop is evaluated subject to refrigeration cost i.e., \$7 per liter that is mostly considered as liquid helium purchasing cast accordingly. As far as electromagnets are concerned, their application as a current loop is very important. However, it is more expensive electromagnet that may be made by utilizing copper wires. Further by 1970s, it has become more cost-effective in numerous cases in the form of paying price for freezing a superconductor rather than bearing utility bills for electricity used against resistance [6, 7]. Industrial level setup was evolved so far, in which high-quality superconducting magnets were launched towards versatile applications. The most familiar application is now in hospitals in the form of Magnetic Resonance Imaging (MRI), proving as standarddiagnostic-tool to diagnose dead cells in the human body by scanning it successfully. This is a low-cost running device as compared with the cost of "exploratory

High-temperature superconductors (HTS) are strongly considered as defined materials behaving as superconductors at high temperatures (> 78 K) showing liquid nitrogen (boiling point), which is considered the simplest cryogenic- coolant [8, 10]. All types of superconductors are currently working nowadays at normal pressure but below ambient temperatures that require more cooling environment. However, most HTS behave such as ceramic type materials while metallic superconductors often work at temperature (< −200 °C), therefore referred to as LTS (low-temperature superconductors) [11, 12]. Furthermore, metals based superconductors are numerously identified as common superconductors owing to fine discovery as well as proper use before the introduction of high-temperature ones. Additionally, ceramic superconductors have also been proved to be suitable for practical application, rather they still show various fabricating issues since only very few examples of employment are on the screen up till now. Owing brittle nature of most ceramics they present behavior while fabricating wires from them for manufacturing superconductors [13, 14]. On the other hand, a major benefit belonging to high-temperature ceramic superconducting materials is their cooling through liquid-nitrogen on the contrary; metallic superconductors often need rare coolants that may be liquid helium [7, 15]. Unfortunately, a more common disadvantage is that no HTS may be refrigerated using dry ice, and none amongst those may work at room temperature as well as pressure. They can only work reasonably below the lowest-temperature measured on Earth's surface. Necessarily, HTS sufficiently requires some cooling system at every cast. Superior high-temperature superconductors belong to only particular class of copper oxides. Another class of HTS is practically classified as iron-based compounds [6, 7, 15]. Magnesium diboride is considered another HTS because of easy manufacturing, however, working conditions under −230 °C (lower than triple point temperature of nitrogen) make it unsuitable concerning cooling with liquid nitrogen (below nitrogen triple-point-temperature). Ideally, liquid-helium can be used to achieve extremely lower temperatures for proper application. Various ceramic superconductors may also depict superconducting behavior owing to second type. The very first HTS has been discovered by Bednorz and Müller in 1986 [11, 12, 16] and obtained Nobel Prize (1987) for the "discovery of superconductivity in ceramic materials". Various high-pressure super-hydride chemical species are often incorporated in the realm of HTS. Indeed, much literature work containing HTS has been found owing to gases with high-pressure, however, unfavorable for synergetic applications. Finally, the latest critical temperature (TC) record holder is identified as carbon nature sulfur hydride, showing leading contribution leaving behind the previous record inherited

**58**

in lanthanum deca-hydride (about 30 °C) [16–18].

surgery" [8, 9].
