Hierarchical band clustering and fractal spectra in incommensurate systems

Abstract
An analytic derivation is given of rules exactly describing the hierarchical band clustering in the spectrum of Harper's equation for various incommensurate systems. These rules, involving transformations of the incommensurability parameter phi , are shown to apply exactly for arbitrary coupling strength. The approach also provides energy scale factors and hence the fractal dimension Df(=0.50+or-0.01) of the self-similar spectrum occurring when phi takes a fixed point value phi g (the golden mean).

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