A researcher whose joint theory showed that Hall conductivity can be tied to an integer associated with electronic wavefunctions. The result connects an electrical measurement with topology: changing the invariant requires leaving the conditions under which the occupied states remain separated by an energy gap. This helps explain the stability of quantized transport.[ref]
Quantized Hall Conductance in a Two-Dimensional Periodic Potential by D. J. Thouless and others. pp. 405–407: occupied states, gaps and quantized conductance
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