Transport Properties of a YB${\mathrm{a}}_2$C${\mathrm{u}}_4$${\mathrm{O}}_8$ Crystal under High Pressure

Measurements of the temperature dependence of the resistivity ${\rho }_a,{\rho }_b$, resistance $R_c$, and thermoelectric power ${\alpha }_a,{\alpha }_b,{\alpha }_c$ for a YB${\mathrm{a}}_2$C${\mathrm{u}}_4$${\mathrm{O}}_8$ crystal under high pressure verify that the Cu${\mathrm{O}}_2$ sheets are oxidized by pressure-induced electron transfer from the Cu${\mathrm{O}}_2$ sheets to the Cu(1)-O double chains. The anomaly in the Cu${\mathrm{O}}_2$-sheet resistivity ${\rho }_a$ at $T\approx 170\mathrm{K}$, which moves to a higher temperature on oxidation of the Cu${\mathrm{O}}_2$ 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.