1.2　Discovery of superconductivity

In 1908，Heike Kamerlingh Onnes found the way to liquefy helium and to reach temperatures as low as 4 K. Superconductivity was discovered first in mercury by Onnes in 19111，in the University of Leiden, Netherlands.The experiment was performed on October 26，1911 with the historic plot showing that the resistance dropped abruptly to zero when the material was cooled below its critical temper ature at 4.2 K7.A type I superconductor is a material that exhibits two features, namely zero electrical resistance and perfect diamagnetism, when it is cooled be low a critical temperature Tc .The feature of the zero electrical resistance is that the electrical resistance of the material suddenly drops to zero as the temperature decreases.The ratio of the resistances between normal state and the superconduct-ing state was measured to exceed 1014.In 1933，Meissner and Ochsenfeld8 found that when a superconducting sphere was cooled below its transition temperature in an applied magnetic field, it excluded the magnetic flux.This perfect diamagnetic phenomenon is called the Meissner-Ochsenfeld effect.

In 1913，the superconductivity of lead was found at 7 K. In 1937，at Kharkov, Shubnikov, Khotkevich, Shepelev and Ryabinin9 experimentally discovered the phenomenon of type II superconductivity in single-crystal, single-phase supercon ducting alloys.In 1941，the superconductivity of niobium nitride was found at 16 K.However, even though thousands of superconductors were discovered for many years, the highest critical temperature was still below 20 K, generally been de fined as low-temperature superconductor（LTSC）.Among LTSCs, only Nb-Ti and Nb3Sn are deemed promise in engineering applications, but rather low operating temperatures（4.2 K, liquid helium）are required.After the discovery of Nb3 Ge（Tc =23.2 K）in 1973，the critical temperature of LTSC didn't increase for more than 10 years.Nearly 30 years later, in 2001，magnesium diboride（MgB2 ）was discovered which has a transition temperature of 39 K.The MgB2 has the highest critical temperature among conventional superconductors.

In 1986，Bednorz and Müller of IBM research lab near Zurich, Switzerland, discovered superconductivity in a lanthanum-based cuprate perovskite material（La2−x Bax CuO4 ）with a transition temperature of 35 K10.

In January of 1987，Chu11from the University of Houston and his associates Wu and Ashburn from the University of Alabama at Huntsville, discovered the ceramic yttrium barium copper oxide（YBa2 Cu3 O）superconductor with a critical temperature above the temperature of liquid nitrogen（77 K）. Meanwhile Zhao, Chen, et al.12from the Institute of Physics of the Chinese Academy of Sciences, synthesized the same YBaCuO high temperature superconductors in February of 1987.

For a normal conductor, the electric current can be visualized as a fluid of elec trons moving through the ionic lattice. The electrons constantly collide with the ions in the lattice, and during each collision some of the dynamic energy carried by the electrons is absorbed by the lattice and converted into heat.This is the phe nomenon of electrical resistance in the normal conductors（non-superconductivity）.The resistance in normal metals decreases continuously with the decrease in tem perature and reaches a constant value, known as the residual resistance that arises due to the presence of impurities.But the dc electrical resistance in a supercon ductors drops abruptly to an immeasurably small value, i.e.almost zero below the critical temperature, Tc .

For a conventional superconductor, the superconducting carriers consist of bound pairs of electrons known as Cooper pairs. This pair is constrained by an attractive force between electrons through the exchange of phonons.The Cooper pair fluid is a superfluid, which means it can flow without energy dissipation.

This zero resistance phenomenon is completely different from the steadily de creasing resistance of normal metals.This is an important characteristic parameter of superconductors, i.e.the critical temperature Tc .Although the theoretical tran sition range ΔT from the normal to the superconducting state is very sharp, it sometimes occurs gradually.For a pure and homogeneous superconductor, transi tion range ΔT can be of the order of mK.

The conventional superconducting theory, for example, BCS theory, cannot fully explain the phenomenon of the high temperature superconductivity. The best available theory model of high-temperature superconductivity is still somehow in complete.

Superconductivity describes a thermodynamic state below a certain critical tem perature Tc . The several parameters that characterize the superconductivity are critical temperature Tc , energy gap Eg , coherence lengthζ，penetration depth λL , Ginzburg-Landau parameterκ，critical current density Jc , thermodynamic critical field Hc , lower critical field Hc1 ，upper critical field Hc2 ，and so on.Generally, after knowing or measuring a few parameters, one can derive the other related parameters.