Uniaxial Strain Effects on Superconducting Transition in Y0.98Ca0.02Ba2Cu4O8

The effects of uniaxial strain and hydrostatic pressure on Y0.98Ca0.02Ba2Cu4O8 [called Y-0.98(Ca-0.02)-124] are investigated via AC magnetic measurements under stress corresponding to a pressure of (at most) 20 GPa. Hydrostatic contraction results in the largest enhancement of the superconducting transition temperature (T-c), whose maximum is 99 K, which is slightly lower than the maximum T-c achieved in YBa2Cu4O8 (called Y-124). The Meissner signal disappears at a hydrostatic pressure of approximately 15 GPa. This pressure region representing the Meissner signal is reduced by the out-of-plane contraction (along the c-axis), whereas it is increased by the in-plane contraction (along the c-plane). In the case of Y-124, both uniaxial contractions result in the disappearance of the Meissner signal at approximately 10 GPa prior to reaching the overdoped region. However, by doping Ca into the Y-site, we could investigate uniaxial strain effects characteristic of the so-called 124 structure without causing the stress-induced structural transition through the underdoped, optimally doped, and overdoped regions. Except for the uniaxial contraction along the b-axis, the other contractions (i.e., hydrostatic contraction, and uniaxial contractions along the aand the c-axes) exhibit consistent compression effects when T-c is normalized by the highest T-c and the stress is normalized by the stress realizing its highest T-c.