Universal delocalization transition in chiral-symmetric Floquet drives*

Periodically driven (Floquet) systems often exhibit behavior distinct from undriven systems.  Any amount of disorder in one-dimensional undriven systems generically localizes all eigenstates.  In contrast, we show that in topologically non-trivial, non-interacting Floquet loop drives with chiral symmetry, a delocalization transition occurs at as the time t is varied within the driving period (0 < t < T_drive).  We find that the localization length L_loc at all quasienergies diverges with a universal exponent of 2 as t approaches the midpoint of the drive: L_loc ~ (t - T_drive/2)^-2.  We provide numerical evidence for the universality of this exponent by studying a variety of such drives using exact diagonalization, and we also present an analytical argument based on scattering theory.