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Dynamic spin correlation functions <Six(t)Sxj> for the one-dimensional S = ½ XX model H = -JΣi{SixSxi+l + SyiSyi+1} are calculated exactly for finite open chains with up to N = 10000 spins. Over a certain time range the results are free of finite-size effects and thus represent correlation functions of an infinite chain (bulk regime) or a semi-infinite chain (boundary regime). In the bulk regime, the long-time asymptotic decay as inferred by extrapolation is Gaussian at T = ∞, exponential at 0 < T < ∞, and power-law (~ t-1/2) at T = 0, in agreement with exact results. In the boundary regime, a power-law decay is obtained at all temperatures; the characteristic exponent is universal at T = 0 (~ r-1) and at 0 < T < ∞ (~ r -3/2), but is site dependent at T= ∞. In the high-temperature regime (T/J ≫ 1) and in the low-temperature regime (T/J ≪1), crossovers between different decay laws can be observed in <Sxi(t)Sxj>. Additional crossovers are found between bulk-type and boundary-type decay for i = j near the boundary, and between spacelike and timelike behavior for i ≠ j.

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Copyright 1995 The American Physical Society.