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 Δ shaped peak is observed with the maximum at ω=0.23 eV which is attributed to spinons along Cu-O chains. At low temperatures in the antiferromagnetic phase another peak appears at ω=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.