Terahertz excitations in the 1D Ising chain quantum magnet CoNb$_2$O$_6$

The one-dimensional magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of a one-dimensional quantum Ising spin chain. It has been shown to undergo a quantum phase transition in a magnetic field oriented transverse to its ferromagnetically aligned spin chains. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate optically active low energy excitations in CoNb$_2$O$_6$. We take advantage of the polarization sensitivity of this technique to characterize both electric and magnetic dipole active excitations in this compound. A connection is made from the $q=0$ response observed here to the excitations observed by neutron scattering. In addition, we will show preliminary data on the terahertz spectra of this material as it undergoes the magnetic field-tuned quantum phase transition.