What can be learned from this phenomenon?
http://www.ims.demokritos.gr/people/mpi ... tivity.htmWhen cooled below a critical temperature (which is called Tc), some materials become superconductors. Superconductivity is characterised by: (a) Zero resistance (for low current densities and magnetic fields). i.e. perfect conduction of the electrons and (b) Expulsion of magnetic flux (B=0, perfect diamagnetism). A superconductor is fundamentally different from a highly conducting metal, as the charge carriers are in a “superfluid” state – a large number of particles with the same ground state wavefunction moving collectively. This is impossible for individual electrons (fermions), but can arise for (Cooper) pairs of electrons (like bosons) with equal and opposite momentum and spin close to the Fermi surface. The formation of Cooper pairs (at least in elemental superconductors and binary alloys) is attributed to an attractive electron-phonon (lattice vibration) interaction. The local displacement of cations towards an electron creates a region of positive potential, which attracts the other electron; the motion of the displacement wave (phonon) must match the motion of the electrons through the lattice.
In 1986, a breakthrough discovery was made in the field of superconductivity. Alex Müller and George Bednorz, researchers at the IBM Research Laboratory in Rüschlikon, Switzerland, created a brittle ceramic compound that superconducted at the highest temperature then known: 40 K. What made this discovery so remarkable was that ceramics are normally insulators. They don't conduct electricity well at all. So, researchers had not considered them as possible high-temperature superconductor candidates. Soon after this discovery a large number of mixed cupper oxides are found to be superconductors. It is interesting to mention the YBa2Cu3O7 superconductor. It was the first high Tc superconductor superconducting above the boiling point of N2 (77.3 K in comparison to the boiling point of He 4.2 K) and hence can be used in many applications.
The early superconductors require liquid helium to keep them cool. These are mixed valent, “layered perovskites” containing 2-dimensional CuO2 sheets with other (not necessarily perovskite) layers in between. The simplest example is (doped) La2CuO4. This becomes superconducting when Sr is substituted for La. This has the K2NiF4 structure. Based on perovskite structure. The pure Cu2+ oxides are antiferromagnetic insulators (very strong antiferromagnetic s- superexchange – J/K ~ 1500 K). To give metallic and superconducting behaviour they must be oxidised (hole-doped). This can be achieved by cation substitution or changing oxygen content. All the pure Cu2+ compounds are antiferromagnetic insulators with high Neel temperature (TN). The Cu must be oxidised (hole doped) to achieve superconductivity i.e. by cation substitution or changing the oxygen content. Superconductors are divided into two categories, type-I and type-II. In type-I superconductors the magnetic induction inside the superconductor is zero (B=0) and via a first order transition it goes into normal metallic state. In type-II superconductors the energy of an interface between a normal and a superconducting region is negative. This implies that it is energetically favorable for these materials, when placed in an external magnetic field, to subdivide into alternating normal and superconducting regions. This effect takes place above the so-called first critical field (Hc1). Above this, magnetic field penetrates into these materials as quantized vortex filaments. Every vortex has a normal core that can be approximated by a log thin cylinder, with its axis parallel to the external magnetic field. Inside the cylinder the order parameter (which is a complex number, with amplitude equal to the density of the superconducting electrons) is zero. The radius of the cylinder is of the order of the coherence length (the length where the electrons in a Copper pair are correlated). The direction of the supercurrent circulating around the normal core is such that the magnetic field generated by it, is parallel to the external field. The vortex current circulates within an area of radius of penetration depth. Each vortex carries one magnetic flux quantum. Penetration of vortices into the interior of a superconductor becomes thermodynamically favorable for H>Hc1 and arrange themselves at distances
from each other so that in the cross-section they form a regular triangular lattice (discovered theoretically by Abrikosov 1957). Our research in superconductivity concerns the properties of magnetic vortices in high Tc-superconductors Nb and MgB2. Flux lines or vortices can be regarded as atoms or molecules of the conventional matter. The vortex matter establishes a very interesting system with tunable parameters. The density of the constituent particles (vortices) and their interactions can be changed over several orders of magnitude in a controllable way, simply by varying the external magnetic field. As a result, the vortex matter displays a rich phase diagram that includes several vortex solid, liquid and gaseous phases, the exact nature of which is still unclear. The vortex matter phase diagram and the dynamics of vortices in high-temperature superconductors are being investigated by experimental technique based on microscopic Hall sensors and SQUID mangetomenty. Below presented are representative results of our research in the area of vortex matter properties.