room temperature using commercial spectrophotometry (Hitachi, U-3010) and referenced to an aluminum mirror. Additionally, the DC resistivity of the sample has been measured using conventional 4-probe measurements down to 2K.

Figure \[fig:ref\](a) shows the measured reflectivity in the infrared regime. The reflectivity tends to 1 at low frequencies and drops with a broad plasma edge-like feature above 4000 cm$^{-1}$ indicating metallic behavior. Besides the usual Drude response, two distinct features are observed in the reflectivity spectra that becomes particularly prominent at low temperatures. The lowest one is close to 200 cm$^{-1}$ where there is an abnormal increase in reflectivity and the second feature is a rather broad bump around 950 cm$^{-1}$.

Band structure calculations were performed using the linear muffin-tin orbital method [@And75] as implemented in the relativistic PY LMTO computer code. Some details of the implementation can be found in Ref. . Calculations were done assuming collinear AFM ordering of Mn moments which were aligned along the $c$ axis. Completely filled Yb$^{2+}$ 4$f^{14}$ states were treated as semi-core states. SOC was added to the LMTO Hamiltonian in the variational step. All theoretical results presented below were obtained within the local spin-density approximation (LSDA) with the Perdew-Wang parameterization [@PW92] for 