High Frequency Electron Paramagnetic Resonance studies of NiCl$_{2}$-4SC(NH$_{2})_{2}$

High-frequency (50 to 311~GHz) Electron Paramagnetic Resonance (HFEPR) measurements have been performed on a single-crystal sample of the easy-plane $S$~=~1 linear-chain Heisenberg antiferromagnet NiCl$_{2}${\-}4SC(NH$_{2})_{2}$ (DTN). DTN has received much interest due to the possibility that it undergoes a Bose-Einstein condensation of the $S^{z}$~=~1 excitations (magnons) in the vicinity of the field tuned crossing of the $S^{z}$~= 0 and 1 states. Low-temperature frequency-dependent studies provide a direct measure of the zero-field spin gap separating the $S^{z}$ = 0 and 1 states. Lower frequency (92~GHz), temperature dependent studies with $B$//$c$ reveal two resonances either side of the $S^{z}$ = 0 and 1 crossing. Strong shifts and changes in the lineshapes of these resonances as the temperature is raised from 2~to ~15~K can be understood in terms of the gradual population of multi-magnon states. The trends observed from HFEPR experiments can be reproduced via numerical simulations using accepted values for the intra-chain Heisenberg interaction $J$ and the local anisotropy parameter $D$.