Critical behavior of the quasi-periodic quantum Ising chain

Abstract

The interplay of correlated spatial modulation and symmetry breaking leads to quantum critical phenomena intermediate between those of the clean and randomly disordered cases. By performing a detailed analytic and numerical case study of the quasi-periodically (QP) modulated transverse field Ising chain, we provide evidence for the conjectures of Ref. [Crowley et. al. 2018] regarding the QP-Ising universality class. In the generic case, we confirm that the logarithmic wandering coefficient $w$ governs both the macroscopic critical exponents and the energy-dependent localisation length of the critical excitations. However, for special values of the phase difference $\Delta$ between the exchange and transverse field couplings, the QP-Ising transition has different properties. For $\Delta=0$, a generalised Aubry-Andr\'e duality prevents the finite energy excitations from localising despite the presence of logarithmic wandering. For $\Delta$ such that the fields and couplings are related by a lattice shift, the wandering coefficient $w$ vanishes. Nonetheless, the presence of small couplings leads to non-trivial exponents and localised excitations. Our results add to the rich menagerie of quantum Ising transitions in the presence of spatial modulation.