Lanthanum has a hexagonal close packed (hcp) crystal structure. State the coordination number of each lanthanum atom.

Lanthanum becomes superconducting below 5 K. Explain, in terms of Bardeen–Cooper–Schrieffer (BCS) theory, how superconductivity occurs.

Outline why superconductivity only occurs at low temperatures.

twelve/12

[1 mark]

«moving» electron attracts «nearby» positive charges/ions/cations

creates «local» regions of increased positive charge

positive charge/field attracts second electron «with opposite spin»

two electrons form a Cooper pair

«all» Cooper pairs «in sample» interact/form «electron» condensate

«electron» condensate/Cooper pairs move/flow «through sample» freely/without resistance

[3 marks]

reduces the band gap to zero
OR
«at high temperatures» thermal motion disrupts the formation of Cooper pairs

[1 mark]