Resonant two-magnon Raman scattering in parent compounds of high-Tc_c superconductors.

We propose a theory of two-magnon Raman scattering from the insulating parent compounds of high-T$_c$ superconductors, which contains information not only on magnetism, but also on the electronic properties in these materials. We use spin density wave formalism for the Hubbard model, and study diagrammatically the profile of the two-magnon scattering and its intensity dependence on the incoming photon frequency $\omega_i$ both for $\omega_i \ll U$ and in the resonant regime, in which the energy of the incident photon is close to the gap between conduction and valence bands. In the nonresonant case, we identify the diagrams which contribute to the conventional Loudon-Fleury Hamiltonian. In the resonant regime, where most of the experiments have been done, we find that the dominant contribution to Raman intensity comes from a different diagram, one which allows for a simultaneous vanishing of all three of its denominators (i.e., a triple resonance). We study this diagram in detail and show that the triple resonance, combined with the spin-density-wave dispersion relation for the carriers, explains the unusual features found in the two-magnon profile and in the two-magnon peak intensity dependence on the incoming photon frequency. In particular, our theory predicts a maximum of the two-magnon peak intensity right at the upper edge of the features in the optical data, which has been one of the key experimental puzzles.Comment: Revtex, 12 postscript figures (uuencoded

Magnetic and Charge Correlations in La{2-x-y}Nd_ySr_xCuO_4: Raman Scattering Study

Two aspects in connection with the magnetic properties of La_{2-x-y}Nd_ySr_xCuO_4 single crystals are discussed. The first is related to long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La_{2-x}Sr_xCuO_4 detwinned crystals as a function of doping, temperature and magnetic field. Two magnetic modes were observed within the AF region of the phase diagram. The one at lower energies was identified with the spin-wave gap induced by the antisymmetric DM interaction and its anisotropic properties in magnetic field could be well explained using a canonical form of the spin Hamiltonian. A new finding was a magnetic field induced mode whose dynamics allowed us to discover a spin ordered state outside the AF order which was shown to persist in a 9 T field as high as 100 K above the N\'eel temperature T_N for x = 0.01. For these single magnon excitations we map out the Raman selection rules in magnetic fields and demonstrate that their temperature dependent spectral weight is peaked at the N\'eel temperature. The second aspect is related to phononic and magnetic Raman scattering in La_{2-x-y}Nd_ySr_xCuO_4 with three doping concentrations: x = 1/8, y = 0; x = 1/8, y = 0.4; and x = 0.01, y = 0. We observed that around 1/8 Sr doping and independent of Nd concentration there exists substantial disorder in the tilt pattern of the CuO_6 octahedra in both the orthorhombic and tetragonal phases which persist down to 10 K and are coupled to bond disorder in the cation layers. The weak magnitude of existing charge/spin modulations in the Nd doped structure did not allow us to detect specific Raman signatures on lattice dynamics or two-magnon scattering around 2200 cm-1.Comment: 26 pages, 22 figure