  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
\~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 ~30 mOhm 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 non-doped Mott-insulating state has been crossed upon
reducing y and n-type carriers are successfully doped in this material.
