16 Ë 1 Fundamentals of superconductivity
play an important role to prevent the flux line from motion. The better the flux-pinning
properties, the higher the critical current density. When the magnetic field is sufficient-
ly strong, the flux becomes free from the pinning and can move freely. This magnetic
field is called as the irreversible fieldHirr, and it is one of the important parameters for
superconducting applications. At the irreversible fieldHirr, the critical current density
reduces to zero. The irreversible fieldHirris determined by the mechanism of flux
creep, and can be derived from theE-Jcharacteristics caused by the flux creep. [24]
As the critical temperature is approached, the normal cores are more closely packed
and eventually overlap so that the superconducting state is lost. At this upper critical
fieldHc2, superconductivity is destroyed.
To avoid the motion of vortices and thus ensure zero resistance of a superconduc-
tor, various defects such as granular structure, lattice defects, or artificial defects are
introduced into the superconducting material. Each defect attracts vortices, and these
vortices produce a series of pinning centers in the superconductor. Below the critical
current densityJc, a hard type II superconductor can have zero resistance up to very
high magnetic fields. The high magnetic field is notHc2, but is the irreversible fieldHc3
andHc1<Hc3<Hc2.
HTSCs are limited by the irreversibility fieldHc3, above which magnetic vortices
melt or decouple. Even though BSCCO has a higher upper critical fieldHc2than YBCO,
it has a much lowerHc3(typically smaller by a factor of 100), thus limiting its use for
making high-field magnets. It is for this reason that conductors of YBCO are preferred
to BSCCO.
1.12 Specific characteristics of HTSC
From the London theory, GL theory to BCS theory, the theory of superconductivity has
experienced a long history. Although the BCS theory can explain the characteristic
properties of conventional superconductors, the HTSC materials still have no widely
accepted theory to explain their properties. The HTSCs with their layered structures
have strong anisotropy and giant flux creep effects. It is complicated and difficult to
study HTS. Although the properties of HTSCs are in many respects similar to those
of conventional superconductors, the HTSCs have a series of special characteristics
compared with LTSCs. The critical temperaturesTcare higher by almost one order
of magnitude, and the superconducting energy gaps 2훥are also larger by one order
of magnitude. In addition, the penetration depths휆are also larger for one order of
magnitude, and the superconducting coherence lengths휉are much shorter, thus,
the GL parameters휅are larger (on the order of 100). The superconducting wave
function has d-wave symmetry in HTSCs, but that of LTSCs shows s-wave symmetry.
A series of the differences and problems between HTSCs and LTSCs have posed serious
challenges for theoretical explanation of the physical mechanisms of HTSCs.