Thermodynamics of the Spin-1/2 Antiferromagnetic 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 {\em two} functions describing the elementary excitations. Using this approach the magnetic susceptibility $\chi$ and specific heat C versus temperature T are calculated to high accuracy for $5\times10^{-25}\leq T/J\leq 5$. The $\chi(T)$ data agree very well at low T with the asymptotically exact theoretical low-T prediction of S. Lukyanov, Nucl. Phys. B {\bf 522}, 533 (1998). The unknown coefficients of the second and third lowest-order logarithmic correction terms in Lukyanov's theory for C(T) are estimated from the C(T) data.