Generalities and Theoretical Models for YBCO System

Superconductivity was discovered in mercury by the Dutch physicist Heike Kamerlingh Onnes in 1911, at the University of Leiden in the Netherlands, 3 years after he successfully liquefied helium [1–4]. He observed the elimination of electrical resistivity below a temperature of \(-\) 268.15  \( ^{\circ }\text {C}\) (4.2 K) as documented in his experimental notebook (Fig. 1.1). This phenomenon, wherein an electric current can flow without energy loss, indicated a transition from the normal conducting state to the superconducting state at a temperature known as the critical temperature ( \(T_c\) ). A year after this discovery, Onnes observed the same phenomenon in tin and lead, the latter, which is a poor conductor at room temperature, loses its resistance below a temperature of 6 K, and tin becomes superconducting at 3.7 K [5, 6]. For this discovery H. Kamerlingh Onnes awarded the Nobel Prize in 1913 [7], he coined the term “superconductivity” for this new state of electronic properties of matter.