ELASTIC AND INELASTIC NEUTRON-SCATTERING STUDIES ON THE TETRAGONAL TOORTHORHOMBIC 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 inelasti c neutron scattering and specific heat measurements on the same well c haracterized crystals. This analysis focusses on the comparison betwee n undoped and doped compounds. The proportionality between the orderpa rameter and the orthorhombic strain, predicted by the Landau theory, c an be confirmed in La2-xSrCuO4+delta in a wide concentration and tempe rature range. We observe different values for the critical exponent be ta for doped superconducting and undoped insulating crystals. The temp erature dependent study of the soft modes on a metallic superconductin g 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 decr ease with doping, i.e. with increasing charge carrier concentration.