Antiferromagnetic ordering coupled with phonon mode anomalies in rare-earth cuprate NdCu₂O₄, probed by nuclear quadrupole resonance and raman spectroscopy

We found evidence that two successive antiferromagnetic orderings of a rare earth cuprate NdCu₂O₄ (nonstandard monoclinic structure with magnetic Cu(1) and nonmagnetic Cu(2) sites) are strongly coupled with phonon mode anomalies. The nuclear quadrupole resonance frequency of ^63,65Cu on the Cu(2) site shows a step decrease below each of the ordering temperatures T_N1 ∼ 20 K and T_N2 ∼ 10 K, that originates from changes in the symmetry of the charge distribution on the surrounding lattice. In the Raman spectrum, the shift of a phonon mode near 300 cm^-1 displays an anomalous temperature dependence, which suggests a hardening of the lattice below T_N1 and a softening below T _N2. The Cu²⁺ spins on the Cu(1) site order antiferromagnetically below T_N1, and the complex magnetic behavior is ascribed to the variations in the exchange interactions between the Cu(1) spins and in the paramagnetic spin polarization of Nd³⁺ below T _N2.