ELASTIC AND INELASTIC NEUTRON-SCATTERING STUDIES ON THE TETRAGONAL TO ORTHORHOMBIC PHASE-TRANSITION OF LA2-XSRXCUO4+/-DELTA

The phase transition from the tetragonal to the orthorhombic structure in La2-xSrxCuO4+delta has been studied combining elastic and inelastic neutron scattering and specific heat measurements on the same well characterized crystals. This analysis focusses on the comparison between undoped and doped compounds. The proportionality between the orderparameter and the orthorhombic strain, predicted by the Landau theory, can be confirmed in La2-xSrCuO4+delta in a wide concentration and temperature range. We observe different values for the critical exponent beta for doped superconducting and undoped insulating crystals. The temperature dependent study of the soft modes on a metallic superconducting crystal reveals an extremely large temperature range where this mode is strongly anharmonic. The comparison with previously published data shows that the frequencies of the soft modes at low temperatures decrease with doping, i.e. with increasing charge carrier concentration.