STRUCTURAL PHASE-TRANSITIONS IN LA2-X(BA,SR)XCUO4-Y AND RELATED PHYSICAL-PROPERTIES

A Landau free energy for the La2-x(Ba,Sr)xCuO4-y family is constructed , including the coupling energy between the order parameter and strain tensor. Using group theory, it is demonstrated that the coupling ener gy can be expressed as a sum of terms linear in the strain and quadrat ic in the order parameter and should contain a coupling term bilinear in the two order-parameter components. The structural phase transition s (SPT's) in La2-x(Ba,Sr)xCuO4-y are discussed on this basis. It is sh own that several SPT's may occur in this system under different condit ions. The observations of the influences of applied pressure, doping c oncentration x, and oxygen deficiency y on the SPT temperature are int erpreted quantitatively. Relations connecting spontaneous strain, soft -mode vibrational frequencies, heat capacity, and elastic constants of the lower-temperature phases are derived. The mean-field results of t he elastic properties and the results of the scaling analysis are also given. The results of ultrasonic measurements on a La1.88Sr0.12CuO4 s ingle crystal are reported. The present and other relevant experimenta l observations are compared with the obtained theoretical results. The possible effects of structural phase transitions or structural instab ilities on the superconductivity of the system are discussed.