  The optical properties of YbMnSb2 have been measured in a broad frequency
range from room temperature down to 7 K. With decreasing temperature, a flat
region develops in the optical conductivity spectra at about 300cm-1, which can
not be described by the well-known Drude-Lorentz model. A frequency-independent
component has to be introduced to model the measured optical conductivity. Our
first-principles calculations show that YbMnSb2 possesses a Dirac nodal line
near the Fermi level. A comparison between the measured optical properties and
calculated electronic band structures suggests that the frequency-independent
optical conductivity component arises from interband transitions near the Dirac
nodal line, thus demonstrating that YbMnSb2 is a Dirac nodal line semimetal.
