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

Abstract

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 $\ensuremath{\Delta}$ shaped peak is observed with the maximum at $\ensuremath{\omega}=0.23\text{ }\text{eV}$ which is attributed to spinons along Cu-O chains. At low temperatures in the antiferromagnetic phase another peak appears at $\ensuremath{\omega}=0.16\text{ }\text{eV}$ 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.