LOCAL CRITICAL-BEHAVIOR AT APERIODIC SURFACE EXTENDED PERTURBATION INTHE ISING QUANTUM CHAIN

The surface critical behaviour of the semi-infinite one-dimensional qu antum Ising model in a transverse field is studied in the presence of an aperiodic surface-extended modulation. The perturbed couplings are distributed according to a generalized Fredholm sequence, leading to a marginal perturbation and varying surface exponents. The surface magn etic exponents are calculated exactly, whereas the expression of the s urface energy density exponent is conjectured from a finite-size scali ng study. The system displays surface order at the bulk critical point , above a critical value of the modulation amplitude. It may be consid ered as a discrete realization of the Hilhorst-van Leeuwen model.