Variation of charge dynamics upon antiferromagnetic transitions in the Dirac semimetal EuMnBi2

We have investigated the temperature- and field-variation of electronic state for the Dirac semimetal of EuMnBi2 by means of optical spectroscopy and theoretical calculation. The optical conductivity spectra show a clear Drude peak in the paramagnetic phase, which gradually diminishes in the Mn-3d antiferromagnetic phase with decreasing temperature. Meanwhile, the absorption peaks due to the interband transition grow at low temperatures, resulting in a pseudogap feature with an energy scale of 0.07 eV. The analysis of Drude weight shows that the Drude response is nearly governed by the Dirac electrons at low temperatures. On the contrary, both the antiferromagnetic transition and spin reorientation of Eu-4f moment do not significantly change the spectra except the moderate variation of Drude weight. As a comparison, we have also investigated the charge dynamics for EuZnBi2, which is an analog without the Mn-3d antiferromagnetic ordering. In EuZnBi2, the optical conductivity spectra do not show the pseudogap structure, but show an intense Drude peak at all temperatures. Combined with the results of ab initio calculation, in EuMnBi2, it is likely that the reconstruction of electronic state driven by the Mn-3d antiferromagnetic ordering causes the Dirac semimetallic state with tiny hole pockets, wherein electronic states other than the Dirac band are nearly gapped-out from the Fermi level.