THz spectroscopy of one-dimensional Ising chain compound CoNb$_2$O$_6$ near the quantum critical point*

One-dimensional Ising spin chain is a novel example of an interacting quantum many body system. The Ising chain in a transverse field is a good candidate to study quantum phase transitions because its low dimensionality increases its tendency to exhibit interesting quantum effects. We studied the one-dimensional ferromagnetic Ising chain material CoNb$_2$O$_6$ using far infrared spectroscopy in high magnetic fields up to 17\,T and down to 0.3\,K using a dilution refrigerator. Special attention is paid to the spectral region near the quantum critical point near 5.5\,T.