  We investigate the low-energy properties of antiferromagnetic quantum XXZ
spin chains with couplings following two-letter aperiodic sequences, by an
adaptation of the Ma-Dasgupta-Hu renormalization-group method. For a given
aperiodic sequence, we argue that in the easy-plane anisotropy regime,
intermediate between the XX and Heisenberg limits, the general scaling form of
the thermodynamic properties is essentially given by the exactly known XX
behavior, providing a classification of the effects of aperiodicity on XXZ
chains. As representative illustrations, we present analytical and numerical
results for the low-temperature thermodynamics and the ground-state
correlations for couplings following the Fibonacci quasiperiodic structure and
a binary Rudin-Shapiro sequence, whose geometrical fluctuations are similar to
those induced by randomness.
