Aperiodic and correlated disorder in XY chains: exact results

We study thermodynamic properties, surface magnetization, specific heat and susceptibility of XY quantum chains with coupling constants following arbi trary substitution sequences. Generalizing an exact renormalization group ( RG) transformation, originally formulated for Ising quantum chains, we obta in exact relevance criteria of Harris-Luck-type for this class of models. F or two-letter substitution rules, a detailed classification is given of seq uences leading to irrelevant, marginal or relevant aperiodic modulations. W e find that the relevance of the same aperiodic sequence of couplings in ge neral will be different for XY and Ising quantum chains. By our method, con tinuously varying critical exponents may be calculated exactly for arbitrar y (two-letter) substitution rules with marginal aperiodicity. A number of e xamples are given, including the period-doubling, three-folding and preciou s mean chains. We also discuss extensions of the renormalization approach t o a special class of long-range correlated random chains, generated by rand om substitutions.