Effect of pseudogap on electronic anisotropy in the strain dependence of the superconducting T-c of underdoped YBa2Cu3Oy

For orthorhombic superconductors we define thermodynamic anisotropy N equivalent to dT(c)/d epsilon(22) - dT(c)/d epsilon(11) as the difference in how superconducting T-c varies with strains epsilon(ii), i = (1, 2), along the in-plane directions. We study the hole doping (p) dependence of N on detwinned single crystals of underdoped YBa2Cu3Oy (YBCO) using the ultrasound technique. While the structural orthorhombicity of YBCO reduces monotonically with decreasing doping over 0.065 < p < 0.16, we find that the thermodynamic anisotropy shows an intriguing enhancement at the intermediate doping level, which is of electronic origin. Our theoretical analysis shows that the enhancement of the electronic anisotropy can be related to the pseudogap potential in the electronic spectrum that itself increases when the Mott insulating state is approached. Our results imply that the pseudogap is controlled by a local energy scale that can be tuned by varying the nearest-neighbor Cu-Cu bond length. Our work opens the possibility to strain engineer the pseudogap potential to enhance the superconducting T-c.