Uniaxial Strain Effects on Cuprate Superconductor YBa2Cu4O8

The uniaxial strain effects on the cuprate superconductor YBa 2 Cu 4 O 8 (Y-124) were investigated by magnetic measurements and quantitatively compared with those of hydrostatic contraction up to 1.7 GPa. We observed a change in superconducting transition temperature ( T c ) for three situations: out-of-plane (along the c -axis), in-plane (along the c -plane), and hydrostatic contractions. The in-plane contraction caused a larger increase in T c than the out-of-plane contraction. The d T c /d P for hydrostatic contraction was almost consistent with that for the out-of-plane contraction. The magnitude correlation in the pressure derivative of T c is explained by considering a qualitative change in Madelung site potential with a function of Cu–O apical distance. Contrary to the general trend for cuprates, T c increased against the contraction along all the crystal axes, suggesting that carrier doping from CuO chain layers to CuO 2 planes occurred for all contraction types. This is a unique characteristic of YBa 2 Cu 4 O 8 , which has double CuO chain layers that function as an important charge reservoir between CuO 2 planes.