Infrared phonon anomaly and magnetic excitations in single-crystal Cu$_{3}$Bi(SeO$_{3})_{2}$O$_{2}$Cl

Infrared reflection as a function of temperature has been measured on the anisotropic single-crystal Cu$_{3}$Bi(SeO$_{3})_{2}$O$_{2}$Cl. The complex dielectric function and optical properties along all three crystal axes of the orthorhombic cell were obtained via Kramers-Kronig analysis and by fits to a Drude-Lorentz model. Below 110 K drastic anomalies in the phonon spectrum (e.g., new modes and splitting of existing modes) are observed along all three crystal axes. Transmission in the terahertz region as a function of temperature has revealed magnetic excitations originating below the ferromagnetic ordering temperature, T$_{c}$=24 K. The origin of the excitations in the magnetic state will be discussed in terms of their polarization and externally-applied magnetic field dependence.