Coexistence of insulator-like paramagnon and metallic spin-orbit exciton modes in SrIrO$_3$

TL;DR

This study shows that SrIrO$_3$ resides in a spin-orbit-coupled Mott-Slater regime where AF pseudospin correlations and metallic charge fluctuations coexist. Using Ir $L_3$-edge RIXS on SrIrO$_3$/SrTiO$_3$ heterostructures spanning confinement toward bulk, the authors identify a robust gapped paramagnon with AF-like dispersion and a broad spin-orbit exciton (SOE) whose dispersion is shaped by metallic hopping in the $J=3/2$ band. Both excitations persist across the series, indicating they are intrinsic to SrIrO$_3$ rather than interface-driven. The contrasting but coexisting excitations reveal the complex interplay of strong pseudospin and charge fluctuations in a semimetallic perovskite, offering insights into the Mott-Slater physics and non-Fermi-liquid behavior characteristic of SrIrO$_3$.

Abstract

We probe the spectrum of elementary excitations in SrIrO$_3$ by using heterostructured [(SrIrO$_3$)$_m$/(SrTiO$_3$)$_l$] samples to approach the bulk limit. Our resonant inelastic x-ray scattering (RIXS) measurements at the Ir $L_3$-edge reveal a robust low-lying collective magnetic mode with an antiferromagnetic (AF) dispersion similar to the insulators Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, albeit with a large gap and much larger linewidth. At higher energies we find the spin-orbit exciton, also strongly broadened, but with an inverted dispersion and doubled periodicity that are controlled by the charge hopping. These results demonstrate that the AF paramagnon persists, somewhat counterintuitively, far into the metallic regime of the insulator-metal transition driven by the degree of confinement in the heterostructure. We conclude that these two excitations, which are contrasting but coexisting hallmarks of strong AF pseudospin and charge fluctuations in a spin-orbit-coupled Mott-Slater material, are properties intrinsic to the ground state of semimetallic perovskite SrIrO$_3$.

Figures (14)

• Figure 1: RIXS investigation of (SrIrO$_3$)$_m$(SrTiO$_3$)$_l$ heterostructures. (a) Representation of (SrIrO$_3$)$_m$(SrTiO$_3$)$_1$ heterostructures, which were grown on a (001)-oriented SrTiO$_3$ single-crystal substrate. The samples investigated here had $m = 3$, 4, and 7 with $l = 1$ and $m = 10$ with $l = 4$. (b) Phase diagram of the (SrIrO$_3$)$_m$(SrTiO$_3$)$_l$ system, illustrating the bandwidth-controlled MIT Kong2022 that takes place as the confinement is increased from $m = 3$ to 2 Matsuno2015a. (c) Reciprocal space of the pseudo-cubic AF unit cell. (d) Compilation of Ir $L_3$-edge RIXS spectra at all measured ${\vec{Q}}$ points for the $m = 4$ sample at $T = 20$ K. (e) Intensity data of panel (d) shown as a function of energy transfer for every ${\vec{Q}}$ point. We identify a low-energy spin excitation (light red) and a spin-orbit exciton at intermediate energies (light blue).
• Figure 2: RIXS spectra. Ir $L_3$-edge spectra at selected momenta shown for our four samples. Measured intensity data (symbols) presented together with a best fit (black line) composed of elastic and phonon (dashed), paramagnon (light red), spin-orbit exciton (SOE, light blue), and high-energy contributions (dot-dashed), as detailed in Sec. S2 of the SM sm.
• Figure 3: Paramagnon dispersion, linewidth, and intensity. (a) Momentum-dependence of the energy, $\omega_{\rm m} ({\vec{Q}})$, of the collective magnetic mode, extracted by fitting to the DHO model, for all four samples. The dashed black line shows the dispersion of the square-lattice Heisenberg AF for $J = 100$ meV and the solid purple line the finite-size gap, $\Delta$, arising due to the ultrashort correlation length, $\xi/a \approx 1$. (b) Momentum-dependence of the corresponding paramagnon linewidths, $\gamma_{\rm m} ({\vec{Q}})/2$, and (c) of the paramagnon spectral weights, $A ({\vec{Q}})$, normalized to the ${\vec{Q}}$-averaged magnetic weight. The black dashed line in (c) is the result for the square-lattice Heisenberg AF scaled by an arbitrary constant. (d) ${\vec{Q}}$-averaged magnetic weight normalized by the total spectral weight and shown as a function of the SrIrO$_3$ layer thickness, $m$. (e) Paramagnon linewidth, $\gamma_{\rm m} ({\vec{Q}})/2$, averaged over ${\vec{Q}}$ and shown as a function of $m$.
• Figure 4: Spin-orbital exciton. (a) Momentum-dependence of the SOE excitation energy, $\omega_{\rm o} ({\vec{Q}})$, extracted from the DHO fit for all four samples. The purple line shows the SOE dispersion in Sr$_2$IrO$_4$, taken from the data of Ref. Kim2012 as described in Sec. S3 of the SM sm. (b) Momentum-dependence of the corresponding linewidth, $\gamma_{o} ({\vec{Q}})/2$. (c) $\gamma_{\rm o} ({\vec{Q}})/2$ averaged over ${\vec{Q}}$ and shown as a function of $m$.
• Figure 5: $\theta$-$2\theta$ X-ray diffraction (XRD) scans of the $m = 3$, 4, 7, and 10 u.c. samples around the (002) reflection, indexed with respect to the SrTiO$_3$ substrate. The change in structure is indicated by the dashed lines in panel (a) marking the superlattice period, $\Lambda$, and in panel (b) marking the average $c$-axis lattice parameter, $c_{\rm{av}}$. $\Lambda$ and $c_{\rm{av}}$ are shown in panels (c) and (d) respectively.