Effects of spin vacancies on the correlated spin dynamics in La2Cu1−xZnxO4 from 63Cu nuclear quadrupole resonance relaxation

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Abstract
63Cu nuclear quadrupole resonance (NQR) relaxation measurements in La2CuO4 doped Zn are used in order to investigate the temperature dependence of the in-plane magnetic correlation length ξ2D and the effects associated to spin vacancies in two dimensional quantum Heisenberg antiferromagnets (QHAF). The relaxation rates T1−1 and T2−1 have been related to the static generalized susceptibility χ(q,0) and to the decay rate Γq of the normal excitations. By using scaling arguments for χ(q,0) and Γq, the relaxation rates have been expressed in close form in terms of ξ2D(x,T) and its dependence on temperature and spin doping x thus extracted. The experimental findings are analyzed in light of the renormalized classical (RC) and quantum critical (QC) behaviors predicted for ξ2D by recent theories for S=1/2 HAF in square lattices. It is first shown that in pure La2CuO4, ξ2D is consistent with a RC regime up to about 900 K, with tendency toward the QC regime above. The spin vacancies reduce the Néel temperature according to the law TN(x)≈TN(0)(1–3.5x). From the temperature dependence of 63Cu NQR relaxation rate T1−1, T2−1 and from the composition dependence of TN it is consistently proved that the effect on ξ2D can be accounted for by the modification of the spin stiffness in a simple dilutionlike model, the system still remaining in the RC regime, at least for T⩽900 K.