On the possibility of excitonic magnetism in Ir double perovskites

We combine several numerical and semianalytical methods to study the $5d$ double perovskites ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$ and ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$, which were recently proposed to exhibit excitonic magnetism. Starting from the density-functional theory and the constrained random-phase approximation, we construct effective multiband Hubbard models. These are analyzed by means of static and dynamical mean-field theories and strong-coupling expansion. We find both materials to be insulators, but, contrary to the experimental claims, with a large spin gap of several hundreds of meV preventing the formation of an ordered state at low temperature.

Figure 1

The GGA+so spectral density of BYIO projected on the irreducible representations of the cubic symmetry in the space of $pd$ Wannier functions. Top panel: projection on WFs centered on the Ir atom (the $t_{2g}$ functions were further resolved into the $j_{3/2}$ and $j_{1/2}$ states). Bottom panel: projection on WFs centered on the O atoms. The $a_{1g}, e_g$, and $t_{1u}\left(io\right)$ MOs are built from atomic O-$p$ orbitals pointing to the central Ir atom, while the remaining MOs are built from atomic O-$p$ orbitals perpendicular to the Ir-O bond.

Figure 2

The GGA+so band structure of BYIO (black). The bands of the $pd$ model with $t_{2g}$ orbitals are described in the text (red).

Figure 3

The Ir $t_{2g}$ spectral densities for the nonmagnetic GGA+$U$ solutions obtained with $J=0.6$ eV and two values of $U$.

Figure 4

(a) The energies of the lowest atomic multiplets of the $d^4$ charge state in BYIO (black circles) and SYIO (red squares) for $J=0.4$ eV and the $d$ model. (b) The singlet-triplet splitting in the ${\mathrm{IrO}}_6$ cluster ($pd$ model) for $J=1.1$ eV as a function of $U$ and Ir-$d$ occupancy. (c) Horizontal cuts in (b) at $U=0$ and 10 eV.

Figure 5

The one-particle spectral density for the $d$ model of BYIO along high-symmetry directions in the Brillouin zone obtained with DMFT. The side panel shows the $k$-integrated density resolved into $j_{1/2}$ (red) and $j_{3/2}$ (black) orbital contributions. The calculation were performed with $U=1.8$ eV and $J=0.4$ eV at a temperature of 290 K.

Figure 6

Dispersion of a triplet excitation in a singlet background obtained from the $d$ model with $U=1.8$ eV and $J=0.4$ eV.