Magnetic and Charge Correlations in La2-x-yNdySrxCuO4: Raman Scattering Study

Two aspects in connection with the magnetic properties of La2- x-yNdySrxCuO4 single crystals were discussed in some detail. One of them was related to long wavelength magnetic excitations in x = 0, 0.01, and 0.03 La2-xSrxCuO4 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 Dzyaloshinskii-Moriya 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. We proposed for the field induced magnetic order a state with a net WF moment in the CuO_2 plane and analyzed the field induced modes in the context of in-plane magnetic anisotropy. For these single magnon excitations we mapped out the Raman selection rules in magnetic fields and we also found that their temperature dependent spectral weight (in the presence of a constant external magnetic field) was peaked at the N'eel temperature. The second aspect was related to phononic and magnetic Raman scattering in La2-x-yNdySrxCuO4 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. It is possible that the discovery of weak charge modulations in the hole doped 2D CuO_2 planes characteristic of high T_c materials is just a matter of time. The problem of doped Mott-Hubbard insulators seems however to be one in which numerous possible ground states are allowed and the supremacy of any one of them could require really fine tuning of microscopic parameters. In this respect, even if such a charge density modulation were observed, the question whether it helps understanding the mechanism of superconductivity or not would still need to be answered.