Universal non-power-law scaling from a chaotic renormalisation group in the Harper-Hofstadter model

Abstract

Previous studies of incommensurate systems concluded that their critical scaling is sensitively dependent on the irrational, $α$, which determines the incommensuration. Contrary to this belief, in the canonical Harper-Hofstadter model, we show there is universal $α$-independent scaling for almost all $α$. This critical scaling is characterized by non-power-law time-length scaling $t \sim r^{ζ\log \log r}$. We demonstrate this in the superfluid fraction of a Bose gas, and the specific heat of a Fermi gas. This scaling is generic of a broad class of generalized Harper-Hofstadter models. The $α$-independent scaling emerges as the number theoretic properties of almost all irrational numbers are statistically identical (\emph{à la} Gauss-Kuzmin statistics). Consequently, we conjecture similar $α$-independent scaling applies in incommensurate models more generally.