Metallic state in La-doped YBa ₂Cu ₃O _y thin films with n-type charge carriers

We report hole and electron doping in La-doped YBa ₂Cu ₃O _y (YBCO) thin films synthesized by the pulsed laser deposition technique and subsequent in situ postannealing in ambient oxygen and vaccum. The n-type samples show a metallic behavior below the Mott limit and a high carrier density of ∼2.8×1021 cm ^-3 at room temperature (T) at the optimally reduced condition. The in-plane resistivity(ρ _ab) of the n-type samples exhibits a quadratic T dependence in the moderate-T range and shows an anomaly at a relatively higher T probably related to pseudogap formation analogous to underdoped Nd _2-xCe _xCuO ₄ (NCCO). Furthermore, ρ _ab(T), T _c, and T with minimum resistivity (T _min) were investigated in both p and n sides. The present results reveal the n-p asymmetry (symmetry) within the metallic-state region in an underdoped cuprate and suggest the potential toward ambipolar superconductivity in a single YBCO system.