Electronic structure, magnetic and dielectric properties of the edge-sharing copper-oxide chain compound NaCu$_{2}$O$_{2}$

TL;DR

This study investigates the electronic, magnetic, and dielectric properties of NaCu$_{2}$O$_{2}$, revealing incommensurate magnetic order, orbital-specific valence holes, and the absence of ferroelectricity, providing insights into multiferroic behavior in copper-oxide chains.

Contribution

The paper presents the first detailed experimental analysis of NaCu$_{2}$O$_{2}$'s properties, including a new orbital selection rule for resonant magnetic x-ray scattering.

Findings

Valence holes are on Cu$^{2+}$ sites within chains.

Incommensurate magnetic order below 11.5 K.

No significant ferroelectric polarization observed.

Abstract

We report an experimental study of \nco, a Mott insulator containing chains of edge-sharing CuO$_4$ plaquettes, by polarized x-ray absorption spectroscopy (XAS), resonant magnetic x-ray scattering (RMXS), magnetic susceptibility, and pyroelectric current measurements. The XAS data show that the valence holes reside exclusively on the Cu$^{2+}$ sites within the copper-oxide spin chains and populate a $d$-orbital polarized within the CuO$_4$ plaquettes. The RMXS measurements confirm the presence of incommensurate magnetic order below a N\'eel temperature of $T_N = 11.5$ K, which was previously inferred from neutron powder diffraction and nuclear magnetic resonance data. In conjunction with the magnetic susceptibility and XAS data, they also demonstrate a new "orbital" selection rule for RMXS that is of general relevance for magnetic structure determinations by this technique. Dielectric property measurements reveal the absence of significant ferroelectric polarization below $T_N$, which is in striking contrast to corresponding observations on the isostructural compound \lco. The results are discussed in the context of current theories of multiferroicity.