Importance of effective dimensionality in manganese pnictides

In this paper we investigate the two manganese pnictides ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ and LaMnAsO, using fully charge self-consistent density functional plus dynamical mean-field theory calculations. These systems have a nominally half-filled $d$ shell, and as a consequence, electronic correlations are strong, placing these compounds at the verge of a metal-insulator transition. Although their crystal structure is composed of similar building blocks, our analysis shows that the two materials exhibit a very different effective dimensionality, LaMnAsO being a quasi-two-dimensional material in contrast to the much more three-dimensional ${\mathrm{BaMn}}_2{\mathrm{As}}_2$. We demonstrate that the experimentally observed differences in the Néel temperature, the band gap, and the optical properties of the manganese compounds under consideration can be traced back to exactly this effective dimensionality. Our calculations show excellent agreement with measured optical spectra.

Figure 1

Crystal and magnetic structure of LaMnAsO (left) and ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (right) drawn with VESTA [41]. The black arrows represent the Mn spins in the antiferromagnetic states of LaMnAsO [23] (C-type: ferromagnetically stacked antiferromagnetic planes) and ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ [28] (G-type: alternating in all directions). We choose a coordinate system where the $x$ and $y$ axes point towards the nearest-neighbor Mn atoms.

Figure 2

DFT+DMFT paramagnetic (top row) and antiferromagnetic (bottom row) spectral functions of ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (red) and LaMnAsO (blue) compared to DFT (gray). The shaded areas correspond to the Mn-projected spectral functions. The Fermi level is set to $\omega =0\mathrm{eV}$.

Figure 3

Spectral function $A(\mathit{k},\omega )$ for the paramagnetic (left) and antiferromagnetic (right) state of LaMnAsO. The thin solid lines show the DFT bands, while the shading shows the DFT+DMFT spectral weight. The Fermi level is set to $\omega =0\mathrm{eV}$.

Figure 4

DFT+DMFT orbital-resolved paramagnetic (top row) and antiferromagnetic (bottom row) spectral functions of the correlated manganese atom for ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (left) and LaMnAsO (right). The Fermi level is set to $\omega =0.0\mathrm{eV}$.

Figure 5

Real-space representation of the maximally localized Wannier orbitals for the $\mathrm{Mn}\text{−}3d$ shell of ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ constructed with wien2wannier [62] and Wannier90 [63] and visualized in VESTA [41]. The $xz$ orbital, which is not shown here, is related to the $yz$ orbital by crystal symmetry. The thin lines connect the central Mn atoms to the four nearest As atoms. The $xy$ and $z^2$ orbitals have significant weight also on the As atoms of the neighboring Mn-As layers. To emphasize this contribution, in the magnifier symbols we show it enlarged both by applying a zoom and selecting a smaller isovalue (by a factor of ten). By contrast, in LaMnAsO, no weight would be seen on the adjacent Mn-As layers at these isovalues.

Figure 6

Real-space Hamiltonian matrix elements $\left|t\right|$ for ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (red) and LaMnAsO (blue) from a Wannier90 construction of the $\mathrm{Mn}\text{−}3d$ orbitals. Shown are all hoppings between Mn atoms separated by the distance $R$. The given multiplicities correspond to the number of neighbors at that distance.

Figure 7

Magnetic moment $m$ versus temperature $T$ from fully charge self-consistent DFT+DMFT for ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (red triangles) and LaMnAsO (blue triangles). Experimental points are taken from Refs. [23, 33, 35] for LaMnAsO (blue crosses) and from Ref. [28] for ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (red crosses). The horizontal dotted lines mark the saturated magnetic moments and the vertical dotted lines the approximate Néel temperatures.

Figure 8

Optical conductivity tensor components ${\sigma }^{zz}\left(\mathrm{\Omega }\right)$ (solid lines) and ${\sigma }^{xx}\left(\mathrm{\Omega }\right)$ (dotted lines) of ${\mathrm{BaMn}}_2{\mathrm{As}}_2$ (red) and LaMnAsO (blue) from fully charge self-consistent DFT+DMFT including uncorrelated bands outside the projective window. The inset shows the low frequency region of the optical conductivity in the same units as the main panel.

Figure 9

Optical conductivity of LaMnAsO calculated with DFT (solid gray line), fully charge self-consistent DFT+DMFT in the correlated window (dotted blue line) and including uncorrelated bands (solid blue line) as well as one-shot DFT+DMFT (dashed blue line); compared to experimental data (black circles) from Ref. [29]. Above $3\mathrm{eV}$ DFT+DMFT (including the outer window) starts to deviate from the experimental data due to the onset of the $\mathrm{La}\text{−}4f$ bands, which are placed much too low in energy by DFT [55].