Optical properties of the iron-pnictide analog BaMn${}_2$As${}_2$

We have investigated the infrared and Raman optical properties of BaMn${}_2$As${}_2$ in the $ab$ plane and along the $c$ axis. The most prominent features in the infrared spectra are the $E_u$ and $A_{2u}$ phonon modes, which show clear transverse optical–longitudinal optical splitting from the energy loss function analysis. All the phonon features we observed in infrared and Raman spectra are consistent with the calculated values. Compared to the iron-pnictide analog $A$Fe${}_2$As${}_2$, this compound is much more two dimensional in its electronic properties. For $E\parallel c$ axis, the overall infrared reflectivity is insulatinglike. Within the $ab$ plane, the material exhibits a semiconducting behavior. An energy gap $2\Delta =48$ meV can be clearly identified below room temperature.

Figure 1

Temperature-dependent optical reflectivity of BaMn${}_2$As${}_2$ measured in (a) the $ab$ plane and (b) along the $c$ direction. The inset shows the room-temperature reflectivity in the whole measured range (40–37 000 cm${}^{-1}$).

Figure 2

Imaginary part of the dielectric function $\mathrm{Im}\left(\epsilon \right)$ and $-\mathrm{Im}(1/\epsilon )$ at 300 K for BaMn${}_2$As${}_2$ single crystal with (a), (b) $E\parallel ab$ and (c), (d) $E\parallel c$. The frequencies of transverse and longitudinal optical modes are respectively identified as labeled in each panel (Ref. 18).

Figure 3

$XX$-polarized Raman spectrum of BaMn${}_2$As${}_2$ at 300 K. The peak at 174 cm${}^{-1}$ is assigned as $A_{1g}$ mode.

Figure 4

Calculated atomic displacements and TO/LO frequencies in cm${}^{-1}$ for zone center phonons. Calculated frequencies are shown in brackets.

Figure 5

Temperature development of conductivity in the region of phonon modes of BaMn${}_2$As${}_2$.

Figure 6

$\Delta {\sigma }_1(\omega ,T)={\sigma }_1(\omega ,T)-{\sigma }_1(\omega ,300\mathrm{K})$ at selected temperatures.

Figure 7

$c$-axis conductivity of BaMn${}_2$As${}_2$ as a function of frequency from 40 to 1000 cm${}^{-1}$ measured at selected temperatures.