TETRAGONAL SYMMETRY FROM SUPERCONDUCTIVITY IN ORTHORHOMBIC YBA2CU3O7

A shear transformation has previously been shown to account for the discontinuities or jumps in the crystallographic thermal expansion coefficients across the normal to superconducting transition temperature. The crystallographic stress dependence of the superconducting transition temperature is shown here to also support a shear transformation; dT(c)/dtau = 4 K/GPa. The shear motion of the lattice restricts the elastic compliances involved in superconductivity to tetragonal symmetry while considering all nine elastic interactions with the orthorhombic super-conducting crystal. The shear transformation has isothermal elastic interactions with the lattice through the orthorhombic shear compliance changes of (DELTAs11 + DELTAs22 - 2DELTAs12). The onset of superconductivity restricts to a higher symmetry the shear motion of the orthorhombic lattice with DELTAs11 = DELTAs22 and DELTAs23 = DELTAs13 across the superconducting phase boundary. Crystallographic second order thermodynamics predicts that isothermally DELTAs11 = -DELTAs12 = 1.7 X 10(-15)/Pa. It is shown that the jumps in the elastic compliance matrix are unique functions of the thermal expansion matrix jumps and the heat capacity jumps for any second order phase transition.