Doping n-type carriers by La-substitution for Ba in YBa_2Cu_3O_y system

TL;DR

This study demonstrates successful n-type doping in YBa2Cu3Oy by La substitution and oxygen reduction, revealing electron-hole symmetry in the same crystal structure, with significant changes in resistivity and carrier type.

Contribution

It introduces a method to dope n-type carriers in YBa2Cu3Oy without altering its structure, enabling direct comparison of electron and hole doping effects.

Findings

Negative Hall and Seebeck coefficients at low oxygen content

Resistivity decreases dramatically in highly reduced samples

N-type carriers are successfully doped in the material

Abstract

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.