Helicoidal magnetic order in a clean copper oxide spin chain compound

摘要

We report susceptibility, specific heat, and neutron diffraction measurementson NaCu$_2$O$_2$, a spin-1/2 chain compound isostructural to LiCu$_2$O$_2$,which has been extensively investigated. Below 13 K, we find a long-rangeordered, incommensurate magnetic helix state with a propagation vector similarto that of LiCu$_2$O$_2$. In contrast to the Li analogue, substitutionaldisorder is negligible in NaCu$_2$O$_2$. We can thus rule out that the helix isinduced by impurities, as was claimed on the basis of prior work onLiCu$_2$O$_2$. A spin Hamiltonian with frustrated longer-range exchangeinteractions provides a good description of both the ordered state and theparamagnetic susceptibility.