Universal delocalization transition in 1D chiral Floquet topological insulators

Periodically driven systems of non-interacting fermions, also known as Floquet topological insulators, exhibit novel topological characteristics distinct from those of static systems. In the case of 1D Floquet topological insulators with chiral symmetry (class AIII), the dynamical topological nature can be understood through studying the time evolution operator U(t) evaluated close to the midpoint of a driving period, i.e. U(T/2). We investigate the localization properties at this special point, for flat-band, two-step Floquet drives. Specifically, for topologically non-trivial disordered drives, we show that the localization length of eigenstates of U(T/2 - epsilon) diverges as epsilon approaches 0 as a power law with exponent nu = 2, and argue that this property is universal for this class of AIII models.