ANISOTROPIC STRESS DEPENDENCE OF TC IN YBA2CU3O7-DELTA SINGLE-CRYSTALS DEDUCED FROM THERMAL-EXPANSION, MEASURED WITH A CAPACITIVE QUARTZ-DILATOMETER

High resolution thermal expansion measurements were carried out on twinned and monodomain YBa2Cu3O7-delta single crystals in the temperature range from 15 to 300 K. At the superconducting transition temperature T(c), anisotropic jumps of the thermal expansion coefficient alpha(T) were observed along all three crystal axes. The monodomain crystal exhibits opposite signs of the jumps DELTAalpha in the a and b directions leading to the conclusion that a tetragonal lattice in YBa2Cu3O7-delta favours superconductivity. Utilizing data for the specific heat, the uniaxial stress and strain dependence of T(c) is calculated. The uniaxial stress dependence is also found to be highly anisotropic leading to a small hydrostatic pressure dependence of 0.03 K/kbar for the monodomain sample.