Large Spin-Wave Energy Gap in the Bilayer Iridate ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$: Evidence for Enhanced Dipolar Interactions Near the Mott Metal-Insulator Transition

Using resonant inelastic x-ray scattering, we observe in the bilayer iridate ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$, a spin-orbit coupling driven magnetic insulator with a small charge gap, a magnon gap of $\approx 92\mathrm{meV}$ for both acoustic and optical branches. This exceptionally large magnon gap exceeds the total magnon bandwidth of $\approx 70\mathrm{meV}$ and implies a marked departure from the Heisenberg model, in stark contrast to the case of the single-layer iridate ${\mathrm{Sr}}_2{\mathrm{IrO}}_4$. Analyzing the origin of these observations, we find that the giant magnon gap results from bond-directional pseudodipolar interactions that are strongly enhanced near the metal-insulator transition boundary. This suggests that novel magnetism, such as that inspired by the Kitaev model built on the pseudodipolar interactions, may emerge in small charge-gap iridates.

Figure 1

(a) Image and (b)–(d) stack plot of the RIXS data recorded at $T=30\mathrm{K}$ with $q_{ab}$ along high symmetry lines and $q_c$ fixed at $\frac{\pi }{3}$. Brillouin zone of the undistorted tetragonal unit cell (black square) and the magnetic cell (blue diamond) is shown with the notation following the convention for the tetragonal unit cell, as, for instance, in ${\mathrm{La}}_2{\mathrm{CuO}}_4$. (e) RIXS spectra measured at the ordering vector $\mathbf{q}=(\pi ,\pi ,\pi )$ above (red open circle) and below (blue solid circle) $T_{\mathrm{N}}\approx 285\mathrm{K}$. (f) RIXS spectra at four different $q_c$’s with $q_{ab}$ fixed at ($\pi$, $\pi$). The $q_c$’s of 0, $\pi /3$, $2\pi /3$, and $\pi$ correspond approximately to $l=25.65$, 26.5, 27.35, and 28.25, respectively. Spectra are vertically offset for clarity. The red vertical lines mark the approximate peak positions at $q_c=0$ and $\pi$.

Figure 2

Momentum dependence of the (a) position and (b) intensity of the magnon peak extracted by fitting the RIXS spectra (dots with error bar), overlaid with the fit from theory model at $\theta =0.26\pi$ and $\eta =0.24$ (red solid lines). Because the acoustic and optical branches are not resolved in the experiment, the intensities for the two branches are summed in the theoretical calculations with appropriate weights (see Supplemental Material [23] for details) to compare to the experiment.

Figure 3

(a) Ground state magnetic structure of ${\mathrm{Sr}}_3{\mathrm{Ir}}_2{\mathrm{O}}_7$. (b) A magnetic state with nearly the same energy when only Heisenberg couplings are considered.