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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 dT c =dP 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.

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