Recent studies have shown that logarithmic divergence of entanglement entropy as function of size of a subsystem is a signature of criticality in quantum models. We demonstrate that the ground state entanglement entropy of $ n$ sites for ferromagnetic Heisenberg spin-1/2 chain of the length $L$ in a sector with fixed magnetization $y$ per site grows as ${ 1/2}\log_{ 2} \frac{ n(L-n)}{ L}C(y)$, where $C(y)=2\pi e({ 1/4}-y^{ 2})$
Categories: quant-ph cond-mat.stat-mech