1.3 Meissner-Ochsenfeld effects Ë 3In January of 1987, Chu, [11] from the University of Houston, and his associates
Wu and Ashburn, from the University of Alabama at Huntsville, discovered the
ceramic yttrium barium copper oxide (YBa 2 Cu 3 O) superconductor with a critical
temperature above the temperature of liquid nitrogen (77 K). Meanwhile, Zhao et al.,
[12] from the Institute of Physics of the Chinese Academy of Sciences, synthesized the
same YBaCuO HTSCs in February of 1987.
For a normal conductor, the electric current can be visualized as a fluid of electrons
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 phenome-
non of electrical resistance in the normal conductors (non-superconductivity). The
resistance in normal metals decreases continuously with the decrease in temperature
and reaches a constant value, known as the residual resistance that arises due to
the presence of impurities. However, the DC electrical resistance in a superconductor
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 decre-
asing resistance of normal metals. This is an important characteristic parameter of
superconductors, i.e. the critical temperatureTc. Although the theoretical transition
rangeΔTfrom the normal to the superconducting state is very sharp, it sometimes
occurs gradually. For a pure and homogeneous superconductor, the transition range
ΔTcan be in the order of millikelvin.
The conventional superconducting theory, for example, the BCS theory, cannot
fully explain the phenomenon of HTS. The best available theory model of high-
temperature superconductivity is still somehow incomplete.
Superconductivity describes a thermodynamic state below certain critical tempe-
ratureTc. The several parameters that characterize the superconductivity are critical
temperatureTc, energy gapEg, coherence length휁, penetration depth휆L, Ginzburg-
Landau (GL) parameter휅, critical current densityJc, thermodynamic critical fieldHc,
lower critical fieldHc1, upper critical fieldHc2, and so on. Generally, after knowing or
measuring a few parameters, one can derive the other related parameters.
1.3 Meissner-Ochsenfeld effects
The Meissner-Ochsenfeld effect [8] (often abbreviated as the Meissner effect) is one
important signature of superconductivity. Perfect diamagnetism is observed due to
expulsion of the magnetic flux from inside the superconducting material for fields