Transport properties of a YBa2Cu4O8 crystal under high pressure

Measurements of the temperature dependence of the resistivity rho(a),rho(b), resistance R(c), and thermoelectric power alpha(a), beta(b), alpha(c) for a YBa2Cu4O8 crystal under high pressure verify that the CuO2 sheets are oxidized by pressure-induced electron transfer from the CuO2 sheets to the Cu(1)-O double chains. The anomaly in the CuO2-sheet resistivity rho(a) at T approximate to 170 K, which moves to a higher temperature on oxidation of the CuO2 sheets, is correlated to a change in the character of R(c). Our data also demonstrate unambiguously the contribution to the transport properties from Cu-O chains.