5 (See the article by Wolfgang Ketterle and Alan Jamison, Physics Today, May 2020, page 32.) The Chern number can be thought of as a generalization of the TKNN result: Every material has a particular integer Chern number defined in the absence of any applied field. In fact, the TKNN invariant has allowed the universal constant e 2 / ħ to be measured to more than 12 digits and now forms the basis for the metrological standard of the kilogram. 4 Their topological invariant accounts for a remarkable universality of the quantum Hall effect among different samples and materials (see the article by Joseph Avron, Daniel Osadchy, and Ruedi Seiler, Physics Today, August 2003, page 38). The existence of a topological invariant for electron systems subjected to a magnetic field was first identified by David Thouless, Mahito Kohmoto, Peter Nightingale, and Marcel den Nijs and bears their initials (TKNN).
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