A metal-insulator transition is when a material changes from being able to conduct electricity into one where to flow of charge is impossible. This normally happens due to the interaction between pairs of electrons in an orbit especially the Coulomb force they both experience splitting the band structure. The transport properties of electrons in lead sulphide (PbS) films which had been doped with manganese were examined.
Normally lead sulphide is a semiconductor but at low temperatures a form of conduction called variable range hopping where the conductivity of the material is related to the temperature as an exponential that approaches normal conductivity at high temperatures but is significantly lower at low ones. In a way this can be seen as an inverse of superconductivity where the resistance increases rapidly below a certain temperature. As the material was cooled even further another change occurred where the transport mechanism shifted again into a different method of charge carrying. When an external magnetic field was applied this transfer didn’t happen leading the researchers to conclude that this sudden shift is caused by the most likely magnetic source that could be affected: the magnetic polarons.
These polarons are believed to be brought in by the doping with manganese (as this doping increases the resistivity significantly) and are then held in the lead sulphide lattice. The interesting coincidence is that the change of transport occurs at about the same temperature that lead becomes a superconductor although there has yet to be any evidence linking this fact (as well as the fact the transition doesn’t occur in undoped lead sulphide); if there was it would suggest an insulator semiconductor transition would be possible. The theory explaining the results is as follows: Electrons at certain locations polarise the manganese present through magnetic interaction which forms the quasi particle the polaron. The movement of the polaron is stifled at low temperatures and as a result a transition in conductivity is observed which a magnetic field can prevent demonstrating the magnetic polarons are essential.