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Dynamic spin correlation functions (Sf(t)Sj) for the one-dimensional S = ~ XX model H = -JI:,{SfSf+l + SfSf+1 } are calculated exactly for finite open chains with uptoN= 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 = oo, exponential at 0 < T < oo, and power-law (~ t-112 ) 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 < oo (~ r 3 12 ), but is site dependent at T = oo. In the high-temperature regime (T I J ~ 1) and in the low-temperature regime (T I J « 1 ), crossovers between different decay laws can be observed in (Sf(t)Sj). 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 -:f. j.

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Copyright 1995 The American Physical Society.