Low-temperature anomaly at the edge of the Heisenberg spin chains: a boundary conformal field theory approach

Asymptotically exact low temperature expansions for the $s=1\mathrm{/}2$ Heisenberg XXZ chains with boundaries are implemented by using the boundary conformal field theory. It is found that for $1\mathrm{/}2<\mathrm{\Delta }\le 1$, ($\mathrm{\Delta }$, an anisotropic parameter) the boundary terms of the spin susceptibility and the specific heat coefficient show divergent behaviors, as temperature decreases. The degrees of the divergence coincide with those obtained by the Bethe ansatz method at zero temperature. Such low-temperature anomalous behaviors at boundaries are deeply related with finite-temperature corrections of the boundary entropy or the ground state degeneracy, the presence of which yields a very sensitive response of spin excitations.