Magnetic properties of high temperature superconductors: A spin liquid approach

We argue that strongly correlated two dimensional electrons form a spin liquid in some regimes of density and temperature and give the theory of the magnetic properties of this spin liquid using a representation in terms of fermions interacting with a gauge field. We show that this state is characterized by anomalous power law spin correlations and discuss the implications of these correlations for the temperature dependence of NMR relaxation rates 1 T₁ and 1 T₂ and for the uniform susceptibility. We also discuss the transition from the spin liquid to antiferromagnet and the critical behavior of these properties at the transition. We compare these theoretical results with data on high T_c superconductors. Finally, we discuss the formation of a spin gap due to the spin exhange interaction between adjacent layers in bilayer materials.