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

The effects of uniaxial strain and hydrostatic pressure on Y0.98Ca0.02Ba2Cu4O8 [called Y0.98(Ca0.02)-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 (Tc), whose maximum is 99 K, which is slightly lower than the maximum Tc 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 a- and the c-axes) exhibit consistent compression effects when Tc is normalized by the highest Tc and the stress is normalized by the stress realizing its highest Tc.