Doping n-type carriers by La substitution for Ba in the YBa2Cu3Oy system

Thus far, there is no cuprate system where both n-type and p-type charge carriers can be doped without changing the crystallographic structure. For studying the electron-hole symmetry in an identical structure, we try to dope n-type carriers to YBa2Cu3Oy system by reducing oxygen content and substituting La3+ ions for Ba2+. Single crystals of La-doped YBa2Cu3Oy are grown by a flux method with Y2O3 crucibles and it is confirmed that La actually substitutes similar to 13% of Ba. The oxygen content y can be varied between 6.21 and 6.95 by annealing the crystals in an atmosphere with controlled oxygen partial pressure. The in-plane resistivity rho(ab) at room temperature was found to increase with decreasing oxygen content y down to 6.32, but interestingly further decrease in y results in a decrease in rho(ab). The most reduced samples with y=6.21 show rho(ab) of similar to 30 m Omega cm at room temperature, which is as much as seven orders of magnitude smaller than the maximum value at y=6.32. Furthermore, both the Hall coefficient and the Seebeck coefficient of the y=6.21 samples are found to be negative at room temperatures. The present results demonstrate that the nondoped Mott-insulating state has been crossed upon reducing y and n-type carriers are successfully doped in this material.