Charge transfer to the local singlet states as a function of Li content in La 2Cu 1− xLi xO 4 and La 1.85Sr 0.15Cu 1− xLi xO 4

Abstract

Magnetic susceptibility (χ) and NMR/NQR measurements were performed on La 2Cu 1− x Li x O 4 0≤ x≤0.5) and La 1.85Sr 0.15Cu 1− x Li x O 4 (0≤ x≤0.425). The narrow 63Cu NQR line at ν Q = 46.3 MHz (1.3 K) is detected in La 2Cu 0.5Li 0.5O 4. This frequency is higher than all 63Cu-NQR frequencies found so far in all the cuprates. Moreover, a further increase of ν Q is observed in the solid solutions La 2Cu 1− x Li x O 4 with decreasing x. Corresponding electric field gradients (EFG's) are considered in terms of ionic and cluster models. The 3d hole density n x2− y2 ≈ 1 and the radial average 〈 r −3〉 ≈ 9.5 a.u. are derived from the EFG's, suggesting the d 9 L state of the formally trivalent copper. The antiferromagnetic ordering is rapidly suppressed as a function of x in the La 2Cu 1− x Li x O 4, and for x ≥ 0.03 we find T N ≈ 0. The evolution of χ vs. x shows a gradual decrease of Cu spin susceptibility and its disappearance in the diamagnetic La 2Cu 0.5Li 0.5O 4, in which χ = −6.7 × 10 −5 cm 3/mole is composed of Van Vleck and core terms. The electronic state is implied to be a local singlet ( 1A 1g) of primarily d 9 L character. The quadrupole signature of the local singlet in the range 46.3 ≤ 63ν Q ≤ 47.0 MHz is observed for x > 0.15 in both systems, La 2Cu 1− x Li x O 4 and La 1.85Sr 0.15Cu 1− x Li x O 4. The differentiation of charge between local singlet and conductivity band states is discussed.