Infrared optical absorption spectra of CuO single crystals: Fermion-spinon band and dimensional crossover of the antiferromagnetic order

We have obtained mid-infrared optical absorption spectra of the $S=1/2$ quasi one-dimensional CuO using polarized transmission measurement and interpreted the spectra in terms of phonon assisted magnetic excitations. When the electric field is parallel to the main antiferromagnetic direction a $\Delta$ shaped peak is observed with the maximum at $\omega = 0.23$~eV which is attributed to spinons along Cu-O chains. At low temperatures in the antiferromagnetic phase another peak appears at $\omega = 0.16$~eV which is attributed to two-magnon absorption but the spinon peak remains. This behavior is interpreted as due to a dimensional crossover where the low temperature three- dimensional magnetic phase keeps short range characteristics of a one- dimensional magnet.