Physics Group - II (3110018)

Type - 1 superconductors:

• The so-called type 1 superconductors have an H–T phase diagram of the kind shown in Fig. in which, with increasing H or T, an abrupt change from a superconducting to a non-super-conducting state occurs.
• Type 1 superconductors are modelled well by BCS theory in which the Cooper electron pairs are coupled by lattice vibrations.
• This correlates with the observation that the best metallic conductors at room temperature, Cu, Ag and Au, are not superconductors, even at low temperatures: they have the smallest lattice vibrations, consistent with their high metallic conductivity, which correlates with the weakness of the electron–phonon interactions required for Cooper pair formation.
• Most elemental superconductors are type 1 and are characterized by small H_c values, which means that they lose their superconductivity in modest magnetic fields.
• In type 1 superconductors, the Cooper pairs have opposed spins, i.e. they couple anti-ferromagnetically, with zero angular momentum and spin: they are said to be s-wave singlet superconductors; an applied magnetic field decouples the Cooper pairs easily so that the spins align preferentially with the field.

Type - 2 superconductors:

• In type 2 superconductors, there is a transition state, the so-called vortex state, or mixed state, between superconducting and metallic (or ‘normal’ state) regimes.
• In this state, magnetic lines of force are able to pass through the material but only in narrow regions called vortices and the lines of force or flux lines bunch together through these vortices.
• A key requirement for certain applications is to pin the vortices so that they cannot undergo lateral displacement; they are then trapped inside the material.
• You may have seen demonstrations in which a superconducting material is suspended beneath, but not touching, a magnet or, alternatively, in which a superconducting pellet is hanging, in mid-air, alongside a magnet.
• These are possible when the superconductor is type 2 and the local T, H conditions are such that the superconductor is in the vortex state.
• Type 2 superconductors, typified by the alloys Nb₃Sn and Nb₃Ge, have much higher H_c values than type 1 superconductors; in these, the Cooper pairs of electrons have parallel spins, forming a triplet state, and cannot be decoupled readily on application of a magnetic field.
• Because of their ability to pass high currents without loss of their superconductivity, they have given rise to a new generation of permanent magnets with critical H_c values.