Competing lattice fluctuations and magnetic excitations in CuO

Lattice vibrations as well as magnetic excitations are investigated using Raman scattering to understand the high-T-c multiferroic properties of CuO. We observe a flat, broad spinon continuum extending to 2800 cm(-1), being superimposed by magnon excitations with energies below 1350 cm(-1). This allows us to estimate an exchange coupling constant of J = 108 meV. Three Raman-active phonon modes predicted by lattice dynamical calculations are assigned and analyzed. We find a crossover temperature T* = 140-150 K through which new phonon modes are activated and phonon parameters show appreciable anomalies. This feature is discussed in terms of a competition of polar and nonpolar lattice distortions. Our study suggests the importance of competing lattice fluctuations for multiferroic compounds, which possess an intermediate multiferroic phase.