Strontium ferrite under pressure: Potential analog to strontium ruthenate

As an alternative platform to unravel some of the perplexing characteristics of strontium ruthenate, we propose to study the isostructural and more correlated material strontium ferrite. Using density functional theory combined with dynamical mean-field theory, we attribute the experimentally observed insulating behavior at zero pressure to strong local electronic correlations generated by Mott and Hund's physics. At high pressure, our simulations reproduce the reported insulator-to-metal transition around 18 GPa. Along with distinctive features of a Hund metal, the resulting metallic state is found to display an electronic structure analogous to that of strontium ruthenate, suggesting that it could exhibit similar low-energy properties.

Figure 1

Comparison of the open $d$-shell orbital character on the band structure of (a) SRO, and (b) SFO under 40 GPa of isotropic pressure. The $d_{xy}, d_{yz/zx}, d_{z^2}$, and $d_{x^2-y^2}$ orbital characters are shown in red, green, blue, and orange, respectively. The horizontal line at zero marks the Fermi energy. The $e_g$ orbitals are unoccupied in SRO while the $d_{x^2-y^2}$ orbital is slightly metallic in SFO at 40 GPa.

Figure 2

Phase diagrams of SFO in the space of the interaction parameters at $T=146$ K for three pressures. Here, the Hund's coupling $J$ and on-site Coulomb repulsion $U$ are expressed in the Slater definition. The red squares distinguish Mott insulating $t_{2g}$ orbitals, whereas the blue triangles correspond to metallic $t_{2g}$ orbitals. The filling of the markers reflects whether both $e_g$ orbitals are partially occupied (solid), only the $d_{x^2-y^2}$ is partially occupied (half solid), or none are (open). The narrow region with green stars corresponds to an orbital-selective Mott phase (OSMP) with $d_{xy}$ and $e_g$ metallic, and $d_{zx/yz}$ Mott insulating. On the right, a selected region is compared for three different pressures: 0, 20, and 40 GPa. It highlights the insulator-to-metal transition observed around 18 GPa [29, 30] for $U\sim 2.5\mathrm{eV}$, where the resulting metallic states have empty $e_g$ orbitals.

Figure 3

Correlated band structure on the left, DOS in the middle, and Fermi surface on the right of (a) 40 GPa SFO in the three-orbital metallic phase and (b) SRO, both at $T=146$ K. The DFT result is represented by the black lines. Clearly, correlations push $e_g$ orbitals away from the Fermi level. The Fermi surfaces are labeled $\alpha ,\beta ,\gamma$ in (b). The parameters for the calculations are on top of the Fermi surfaces. Although calculations for both of the materials rely on the Slater Hamiltonian, SRO's parameters are presented in the Kanamori convention to ease the comparison with previous studies. The corresponding values of SRO in Slater are $U=1.66$ eV and $J=0.56$ eV. See Eqs. (S30) and (S31) of the SM for conversion relations between Slater and Kanamori values.