How thin can a film of lead be and still retain its ability to conduct electricity? Previous experiments have shown that films with just five layers of lead atoms become superconducting when they are cooled below about 6 K. Now Chih-Kang Shih and co-workers at the University of Texas at Austin have shown that just two layers of lead atoms — which contain just a single channel of quantum-well states — can act as a superconductor (Science doi: 10.1126/science.1170775; 2009).
Shih and co-workers found that two layers of lead atoms on a silicon substrate can actually form two different types of lead film. As these scanning tunnelling micrographs show, type-I films have a 1×1 atomic structure and the same lattice parameter as bulk lead (left, scale bar is 10 nm), whereas type-II films have a √3×√3 structure and a similar lattice constant to the silicon substrate (right). The films also have different transition temperatures — 4.9 K for type-I and 3.65 K for type-II — which the Texas team attribute to the type-II film experiencing tensile strain owing to its interaction with the substrate. Films containing three layers of lead atoms are not thermodynamically stable, whereas those containing four layers have a transition temperature of 6.7 K.
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Rodgers, P. Flat out. Nature Nanotech 4, 346 (2009). https://doi.org/10.1038/nnano.2009.123
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DOI: https://doi.org/10.1038/nnano.2009.123