Quantum antiferromagnetism in 2 dimensions

We study the spin dynamics of the 2-dimensional spin 1/2 Heisenberg model at finite temperatureT, in particular the frequency region <TJ (the coupling constant). The main conclusion reached is that the spin fluctuations have relaxational character, no propagating spin waves exist for <T. Comparison with experimental results on La₂CuO₄ give good agreement. Similarities and differences to previous theoretical treatments are discussed.