Thermodynamics of the low-temperature structural transition in rare-earth-doped La2-xSrxCuO4

The Sr concentration dependence of the structural transition from the ortho rhombic into the tetragonal low-temperature phase in rare-earth-doped (R=Nd ,Eu) La2-x-ySrxRyCuO4 has been studied in detail. Independently of the rare earth concentration the transition temperature is strongly reduced at x si milar to 0.05. We give a qualitative argument that the effect can be attrib uted to the Coulomb repulsion between doped carriers. We find the enthalpy jump at the low-temperature transition to scale transition temperature betw een the high-temperature tetragonal and the orthorhombic phase. This effect can be understood in terms of a Landau expansion.