Far-infrared and high field electron spin resonance study of magnetic and phonon excitations in the two dimensional cuprate Na$_5$RbCu$_4$(AsO$_4$)$_4$Cl$_2$*

Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ is a novel low dimensional magnetic material, where the magnetic properties are determined by layers Cu$_{4}$O$_{4}$ tetramers [S.-J.~Hwu \textit{et al}., J. Am. Chem. Soc.,\textbf{ 124},~12404 (2002)]. There is a second order phase transition at 17K, that likely involves the onset of antiferromagnetic order [J.A. Clayhold \textit{et al}., Phys. Rev. B, \textbf{66}, 052403 (2002)]. We have measured the far-infrared spectra and high field electron spin resonance spectra at low temperatures. A triplet excitation is observed at 12.68 cm$^{-1}$. The magnetic field dependent triplet components with the electron g-factor $g_{b}$=1.44 are split in zero field by 0.75 cm$^{-1}$. Several temperature dependent resonances are also observed. The results are being analyzed in the framework of a four-tetramer model.