Bond-operator theory of doped antiferromagnets: From Mott insulators with bond-centered charge order to superconductors with nodal fermions - art. no. 184510

The ground states and excitations of two-dimensional insulating and doped M ott insulators are described by a bond-operator formalism. While the method represents the degrees of freedom of an arbitrary antiferromagnet exactly, it is especially suited to systems in which there is a natural pairing of sites into bonds, as in states with spontaneous or explicit spin-Peierls or der (or bond-centered charge order). In the undoped insulator. as discussed previously, we obtain both paramagnetic and magnetically ordered states. W e describe the evolution of superconducting order in the ground state with increasing doping-at low doping, the superconductivity is weak. can coexist with magnetic order, and there are no gapless spin-1/2 fermionic excitatio ns; at high doping, the magnetic order is absent and we obtain a BCS d-wave superconductor with gapless spin-1/2 nodal fermions. We present the critic al theory describing the onset of these nodal fermionic excitations. We dis cuss the evolution of the spin spectrum and obtain regimes where a spin-1 e xciton contributes a sharp resonance in the dynamic spin susceptibility. We also discuss the experimental consequences of low-energy, dynamically fluc tuating spin-Peierls order in an isotropic CuO2 plane-we compute consequenc es for the damping and dispersion of an optical phonon involving primarily the O ions and compare the results with recent neutron scattering measureme nts of phonon spectra.