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The magnetic susceptibility chi and specific heat C versus temperature T ofthe spin-1/2 antiferromagnetic alternating-exchange (J1 and J2) Heisenbergchain are studied for the entire range 0 \leq alpha \leq 1 of the alternationparameter alpha = J2/J1. For the uniform chain (alpha = 1), detailedcomparisons of the high-accuracy chi(T) and C(T) Bethe ansatz data of Kluemperand Johnston are made with the asymptotically exact low-T field theorypredictions of Lukyanov. QMC simulations and TMRG calculations of chi(alpha,T)are presented. From the low-T TMRG data, the spin gap Delta(alpha)/J1 isextracted for 0.8 \leq alpha \leq 0.995. High accuracy fits to all of the abovenumerical data are obtained. We examine in detail the theoretical predictionsof Bulaevskii for chi(alpha,T) and compare them with our results. Ourexperimental chi(T) and C(T) data for NaV2O5 single crystals are modeled indetail. The chi(T) data above the spin dimerization temperature Tc = 34 K arenot in agreement with the prediction for the uniform Heisenberg chain, but canbe explained if there is a moderate ferromagnetic interchain coupling and/or ifJ changes with T. By fitting the chi(T) data, we obtain Delta(T = 0) = 103(2)K, alternation parameter delta(0) = (1 - alpha)/(1 + alpha) = 0.034(6) andaverage exchange constant J(0) = 640(80) K. The delta(T) and Delta(T) arederived from the data. A spin pseudogap with a large magnitude \approx 0.4Delta(0) is consistently found just above Tc, which decreases with increasingT. Analysis of our C(T) data indicates that at Tc, at least 77% of the entropychange due to the transition at Tc and associated order parameter fluctuationsarise from the lattice and/or charge degrees of freedom and less than 23% fromthe spin degrees of freedom.Comment: 53 two-column REVTeX pages, 50 embedded figures, 7 tables. Revisions  required due to incorrect Eq. (39) in Ref. 51 which gives the low-T  approximation for the specific heat of a S = 1/2 1D system with a spin gap;  no conclusions were changed. Additional minor revisions made. Phys. Rev. B  (in press

physical review. b, condensed matter

Thermodynamics of spin<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>S</mml:mi><mml:mo>=</mml:mo><mml:mn>1</mml:mn><mml:mo>/</mml:mo><mml:mn>2</mml:mn><mml:mn /></mml:math>antiferromagnetic uniform and alternating-exchange Heisenberg chains

Thermodynamics of spinS=1/2antiferromagnetic uniform and alternating-exchange Heisenberg chains