THEORY OF EFFECT OF UNIAXIAL-STRESS ON T(C) IN YBA2CU3O7

Recent measurements of the effect of the uniaxial stress on T(c) can b e understood in terms of the anisotropy dependence of the density of s tates at the Fermi energy. We show that if the Fermi energy of YBa2Cu3 O7 is located very close to the van Hove singularity as suggested by s ome authors, the DOS (E(F)) will decrease as one increases the anisotr opy, and the uniaxial pressure in the a-direction will reduce T(c) whi le the uniaxial pressure in the b-direction will increase T(c). These two effects will almost cancel out when one applies an isotropic hydro static pressure.