Magnetism in singlet ground states

The spin‐gap phenomenon is a key characteristic of high T_c superconducting cuprates, whose experimental results are summarized and discussed in the context of the slave‐boson mean field theory of the extended t–J model. It is seen that there exists satisfactory agreement between theory and experiments in bilayer cuprates. As regards the apparent absence of the spin‐gap in single layer La_2-xSr_xCuO₄ (LSCO), the possible roles played by disorder have been pointed out in view of the recent neutron scattering experiment on other spin singlet systems, the spin‐Peierls CuGeO₃ and two‐leg ladders. This revised version was published online in July 2006 with corrections to the Cover Date.