In Praise of Exact Quantization

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Science  06 Aug 2010:
Vol. 329, Issue 5992, pp. 639-640
DOI: 10.1126/science.1194123

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In science, great value is placed on quantities that are exact. In physics, it is crucial for our understanding of the world around us to know that the values of fundamental physical constants, such as the mass and charge of an electron (or any other elementary particle), remain precisely the same no matter the circumstance. In solid-state physics, where we deal with complicated systems of many particles, such comforting exactness is hard to come by. A piece of iron will have electrical resistance that can vary greatly with temperature, impurity content, and other factors; the same is true of most of its other measurable properties. There are notable exceptions to this rule, such as when large numbers of particles act in a way that leads to emergence of exactness on a macroscopic scale (1, 2). Two celebrated examples of this phenomenon are quantization of magnetic flux in superconductors and quantization of Hall conductance in quantum Hall fluids. On page 659 of this issue, Chen et al. (3) report findings that may soon establish another entry in this short list. The venue here is the surface of dibismuth triselenide (Bi2Se3), which belongs to the recently discovered class of solids called topological insulators (46).