Uniaxial Strain Effects on Cuprate Superconductor YBa2Cu4O8

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The uniaxial strain effects on the cuprate superconductor YBa2Cu4O8 (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_{\text{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_{\text{c}} than the out-of-plane contraction. The \mathrm{d}T_{\text{c}}/\mathrm{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_{\text{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_{\text{c}} increased against the contraction along all the crystal axes, suggesting that carrier doping from CuO chain layers to CuO2 planes occurred for all contraction types. This is a unique characteristic of YBa2Cu4O8, which has double CuO chain layers that function as an important charge reservoir between CuO2 planes.
2012-11-15