When cooled to a very low temperature (below -139 °C or 134 K), superconductors conduct electric current without resistance, i.e. without losing energy. This property occurs due to the formation of so-called Cooper pairs. In this state, electrons can flow through the material of the superconductor without encountering resistance or obstacles. In high-temperature superconductors, unlike conventional ones, the superconductivity does not result from electron-phonon interaction and they are not metallic but ceramic materials , . The name „high temperature superconductor“ is based the fact that much higher transition temperatures can be achieved (Up to -23 °C or 250 K).
The history of superconductor research began in 1911, when Heike Kamerlingh Onnes was the first to succeed in cooling mercury down to about 4 K or – 269 °C using liquid helium (LHe), thus enabling research into metals in a previously unknown temperature range. At that time, ideas about how an electrical conduction mechanism might work in general were still very vague and incomplete , , .
During his experiments Onnes’s measuring instruments indicated that below the temperature of 4 K the metal (mercury) conducted the applied electric current without loss, the electrical resistance disappeared. In 1913, it was demonstrated that below a temperature of -266 °C also lead becomes superconducting, that its electrical resistance below that point disappears. In the following years, more and more materials were found that exhibited a similar behavior. If the temperature drops below the so called transition temperature, the respective material becomes a superconductor , , .
The idea of using superconductors for technical applications already existed in 1913. Since then physicists have been searching for other materials with a higher transition temperature, mainly to make the application more practical. The superconductor with the highest transition temperature (as far as currently known) is mercury cuprate becoming superconducting at 134 Kelvin ( -139 °C) and when pressurized at 153 K ( -120 °C). It is therefore considered a so-called high-temperature superconductor. Thanks to much higher transition temperatures, liquid nitrogen can be used instead of liquid helium, which makes the entire process much more economical and which is a great advantage of this kind of superconductor. However, these so-called cuprates are brittle and therefore difficult to process. There is also research in the field of material mixtures of e.g. iron, lanthanum, phosphorus, oxygen and arsenic, but so far only transition temperatures of 56 K ( -217 °C) could be achieved with them .
The question remains as to why superconductors behave the way they do. Basically, the concept of superconductivity is based on the occurrence of a macroscopic, coherent matter wave causing the formation of Cooper-pairs (electron pairs). Established in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer this theory is now known as BCS theory. It states that electrons in a superconductor are coupled into pairs due to interactions between electrons and the crystal lattice at low temperatures. The behavior of high temperature superconductors is only partially described by the theory and not fully understood down to the present day., .
One of the latest discoveries in 2018 were ultrathin layers of carbon also exhibiting superconducting properties. When two graphene layers are superimposed and twisted against each other by an angle of 1.3 degrees, superconducting properties are exhibited when cooled down to a temperature of 1.65 K (-271.5 °C). Scientists hope to be able to study the behavior of the so-called unconventional superconductors (high-temperature superconductors) with the twisted graphene layers in more detail , . Spektrum: Supraleiter, (Link: https://www.spektrum.de/thema/supraleiter/1314677), accessed 06 May 2020  Welt der Physik: Supraleiter, (Link: https://www.weltderphysik.de/gebiet/materie/supraleiter/), accessed 06 May 2020  Buckel, Werner; Kleiner, Reinhold: Supraleitung – Grundlagen und Anwendungen; WILEY-VCH; 2004  Pollmann, Maike: Welt der Physik: Geschichte der Supraleitung, (Link: https://www.weltderphysik.de/gebiet/materie/supraleiter/geschichte/), 07.04.2011; accessed 07 May 2020 0  Onnes, Heike Kamerlingh; Edited by Gavroglu, Kostas; Goudaroulis, Yorgos: Through Measurement to Knowledge: The Selected Papers of Heike Kamerlingh Onnes 1853- 1926; Boston Studies in the Philosophy of Science Volume 124; 1991
 Tipler, Paul; Llewellyn, Ralph: Moderne Physik; Oldenbourg; 2010 Cooper, Leon N.; Feldman, Dmitri: BCS: 50 Years; World Scientific Publishing CO. Ptc. Ltd.; 2011  Löfken, Jan Oliver: Welt der Physik: Supraleiter aus Kohlenstoff, (Link: https://www.weltderphysik.de/gebiet/materie/news/2018/supraleiter-aus-kohlenstoff/), 05.03.2018, accessed 07 May 2020  Cao, Yuan; Fatemi, Valla; Fang, Shiang; et. Al.: Unconventional superconductivity in magic-angle graphene superlattices; Nature 556; 05.03.2018
Further reading: Kempf, Achim: Could the Casimir Effect explain the Energetics of High-Temperature Superconductors?; arXiv 03/2006  Kiehn, R. M.: Are There Three Kinds Of Superconductivity?; International Journal of Modern Physics B, 06/1991  Rao, C. N. R.; Raychaudhuri, A. K.: CRC Handbook of chemistry and physics, CRC Press, 2004  Donglu, Shi: High-Temperature Superconducting Materials Science and Engineering: New Concepts and Technology, Pergamon Press, 1995  Mankowsky, Roman: Supraleitung bei Raumtemperatur; Physik in unserer Zeit Colume 46, 09/2015