RT Journal Article SR Electronic T1 Proof of the elusive high-temperature incommensurate phase in CuO by spherical neutron polarimetry JF Science Advances JO Sci Adv FD American Association for the Advancement of Science SP eaay7661 DO 10.1126/sciadv.aay7661 VO 6 IS 7 A1 Qureshi, Navid A1 Ressouche, Eric A1 Mukhin, Alexander A1 Gospodinov, Marin A1 Skumryev, Vassil YR 2020 UL http://advances.sciencemag.org/content/6/7/eaay7661.abstract AB CuO is the only known binary multiferroic compound, and due to its high transition temperature into the multiferroic state, it has been extensively studied. In comparison to other prototype multiferroics, the nature and even the existence of the high-temperature incommensurate paraelectric phase (AF3) were strongly debated—both experimentally and theoretically—since it is stable for only a few tenths of a kelvin just below the Néel temperature. Until now, there is no proof by neutron diffraction techniques owing to its very small ordered Cu magnetic moment. Here, we demonstrate the potential of spherical neutron polarimetry, first, in detecting magnetic structure changes, which are not or weakly manifest in the peak intensity and, second, in deducing the spin arrangement of the so far hypothetic AF3 phase. Our findings suggest two coexisting spin density waves emerging from an accidental degeneracy of the respective states implying a delicate energy balance in the spin Hamiltonian.