Electrical resistance in metals arises because electrons propagating through the solid are scattered due to deviations from perfect translational symmetry. These are produced either by impurities (giving rise to a temperature independent contribution to the resistance) or the phonons (lattice vibrations in a solid - the temperature dependent occupancy of these boson states produces a temperature dependent resistivity p = p0 + AT5 in a metal at low temperature.

In a superconductor below its transition temperature Tc, there is no resistance because these scattering mechanisms are unable to impede the motion of the current carriers. The current is carried in all known classes of superconductor by pairs of electrons known as Cooper pairs. The mechanism by which two negatively charged electrons are bound together is still controversial in "modern" superconducting systems such as the copper oxides or alkali metal fullerides, but well understood in conventional superconductors such as aluminium in terms of the mathematically complex BCS (Bardeen Cooper Schrieffer) theory.

The essential point is that below Tc the binding energy of a pair of electrons causes the opening of a gap in the energy spectrum at Ef (the Fermi energy - the highest occupied level in a solid), separating the pair states from the "normal" single electron states.

The size of a Cooper pair is given by the coherence length which is typically 1000Å (though it can be as small as 30Å in the copper oxides). The space occupied by one pair contains many other pairs, and there is thus a complex interdependence of the occupancy of the pair states. There is then insufficient thermal energy to scatter the pairs, as reversing the direction of travel of one electron in the pair requires the destruction of the pair and many other pairs due to the nature of the many-electron BCS wavefunction. The pairs thus carry current unimpeded.

BCS theory applies directly to superconductors such as Nb3Ge (Tc = 23K) in which the electrons are bound together by their interaction with the vibrations of the underlying lattice: one electron in the pair polarises the lattice by attracting the nuclei towards it, leaving a region of excess positive charge (a potential well) into which a second electron is attracted - the positively charged nuclei thus mediate an attraction between the negatively charged electrons. Only electrons within the vibrational frequency of EF can be paired by this interaction, and so only a small fraction of the electrons become superconducting.

The most obvious experimental signature of superconductivity is the observation of zero D.C. electrical resistance, and the possibility for low loss power transmission and large fast computers is one of the main reasons for technological interest in breakthroughs in superconductivity.

Zero resistance is hard to measure, and the most definitive evidence for superconductivity in fact arises from d.c. magnetic measurements. Persistent currents on the surface of the superconductor make it a perfect diamagnet below Tc i.e. it expels all magnetic flux. The difference between superconducting diamagnetism and that of benzene is both its larger size and more complex history dependence. This is illustrated by the effect of the cooling procedure on the measured superconducting diamagnetism:

This hysteresis of the diamagnetic susceptibllity below Tc is known as the Meissner effect and is definitive proof of superconductivity. The magnetic behaviour of a superconductor below Tc as a function of field is also more complex than that of a simple para- or diamagnet. The magnetisation initially varies linearly with applied field, but its behaviour above a critical field allows the division of superconductors into Type I or the more common Type II.

Type I superconductors lose their superconductivity above a critical field Hc as the field penetrates the material. In type II superconductors, the field penetrates the superconductor partially to form the Abrikosov flux lattice above the lower critical field Hc1. Above Hc1 the diamagnetism decreases with increasing applied field until superconductivity is quenched at the upper critical field Hc2, returning to the normal metallic state.

The value of HC2 is very important as it partially determines the current carrying capacity of the superconductor and its uses e.g. to produce high field superconducting magnets. In the copper oxides, Hc2 is of the order of 40T at liquid helium temperatures.

Copper oxide superconductors

The YBa2Cu307 sample made in this experiment was the first material ever to have a superconducting transition temperature above the boiling point of liquid nitrogen (BCS based predictions had suggested a limit to Tc of about 30-40K). It is important to note that superconductivity only occurs for copper oxidation states either greater or less than, but not equal to, two e.g. La2CuO4 is an antiferromagnetic insulator whereas La1.85Sr0.15CuO4 is a metal and superconductor. Question 6 asks you to account for the insulating nature of the Cu(II) compounds.

The binding mechanism leading to the formation of Cooper pairs in the copper oxides is thought to be related to the extremely strong superexchange interactions between the copper spins and not to result from the electron-vibration interaction which produces the lower Tc's.

The variation of electronic properties with copper oxidation state is well illustrated in the La2-xSrxCuO4 series. For 0>x>0.03, the Neel temperature TN decreases from 250K at x=0 to zero at 0.03, and a transition to a superconducting metal occurs at x = 0.06, with Tc rising to a maximum of 42K at 0.15. Beyond x=0.2, the superconductivity disappears and the compounds are non-superconducting metals. The importance of the copper oxidation state and the chemical environment of the copper cations in controlling the properties is shown by the very wide variation in Tc in the compounds listed in Table I. The influence of applied pressure on superconductivity is also remarkable, and suggests the likely influence of possible chemical substitutions. The highest superconducting transition temperature yet achieved reproducibly is 150K under 23.5 GPa of hydrostatic pressure in HgBa2Ca2Cu3O8+d.

Table I: Transition temperatures in inorganic superconductors

Compound     Tc (K)
PbMo6S8 12.6
SnSe2(Co(C5H5)2)0.33 6.1
K3C60 19.3
Cs3C60 40 (15 kbar applied pressure)
Ba0.6K0.4BiO3 30
Lal.85Sr0.l5CuO4 40
Ndl.85Ce0.l5CuO4 22
YBa2Cu3O7 90
Tl2Ba2Ca2Cu3O10 125
HgBa2Ca2Cu3O8+d 133

© Copyright 1996 Oxford University