Thermodynamics of the Spin-1/2Antiferromagnetic Uniform Heisenberg Chain

We present a new application of the traditional thermodynamic Bethe ansatz to the spin-1/2 antiferromagnetic uniform Heisenberg chain and derive exact nonlinear integral equations for just two functions describing the elementary excitations. By using this approach, the magnetic susceptibility $\chi$ and specific heat $C$ versus temperature $T$ are calculated to high accuracy for $5\times {10}^{-25\mathrm{}}\le T/J\le 5$. The $\chi \left(T\right)$ data agree very well at low $T$ with the asymptotically exact theoretical low- $T$ prediction of Lukyanov [Nucl. Phys. B522, 533 (1998)]. The unknown coefficients of the second and third lowest-order logarithmic correction terms in Lukyanov's theory for $C\left(T\right)$ are estimated from the $C\left(T\right)$ data.