Localisation in quasiperiodic chains: a theory based on convergence of local propagators

Quasiperiodic systems serve as fertile ground for studying localisation, due to their propensity already in one dimension to exhibit rich phase diagrams with mobility edges. The deterministic and strongly-correlated nature of the quasiperiodic potential nevertheless offers challenges distinct from disordered systems. Motivated by this, we present a theory of localisation in quasiperiodic chains with nearest-neighbour hoppings, based on the convergence of local propagators; exploiting the fact that the imaginary part of the associated self-energy acts as a probabilistic order parameter for localisation transitions and, importantly, admits a continued-fraction representation. Analysing the convergence of these continued fractions, localisation or its absence can be determined, yielding in turn the critical points and mobility edges. Interestingly, we find anomalous scalings of the order parameter with system size at the critical points, consistent with the fractal character of critical eigenstates. The very nature of the theory implies that it goes far beyond the leading-order self-consistent framework introduced by us recently [Phys. Rev. B 103, L060201 (2021)]. Self-consistent theories at high orders are in fact shown to be conceptually connected to the theory based on continued fractions, and in practice converge to the same result. Results are exemplified by analysing the theory for three families of quasiperiodic models covering a range of behaviour.