Spin-wave theory for dimerized ferromagnetic chains

We describe a Peierls dimerization which occurs in ferromagnetic spin chains at finite temperature, within the modified spin-wave theory. Usual spin-wave theory is modified by introducing a Lagrange multiplier which enforces a nonmagnetic state at finite temperature. It is shown that this method gives results in excellent agreement with the density–matrix renormalization group applied to transfer matrices for dimerized ferromagnetic chains. We study bond correlation functions (Si Si+1) and explore the characteristic features of dimerization in the specific heat.