Unravelling the coexistence of insulating and metallic-like excitations in SrIrO₃

The challenge

Quantum materials are systems in which the collective behaviour of electrons gives rise to phenomena beyond the reach of conventional theories. In 3d transition-metal oxides, strong electron-electron correlations can drive insulating or superconducting ground states. For 5d oxides such as iridates, these correlations are expected to be weaker due to more extended orbitals. However, strong spin–orbit coupling (SOC) in iridium significantly alters the electronic landscape, intertwining spin, orbital, and charge degrees of freedom. 

Among 5d oxides, the Ruddlesden–Popper iridates (e.g., Sr₂IrO₄and Sr₃Ir₂O₇) are correlated insulators with antiferromagnetic order. The series’ end member, SrIrO₃, behaves differently: it is a semimetal with no long-range magnetic order and has long been considered weakly correlated. Despite its metallicity, SrIrO₃ exhibits unusual electronic properties, including enhanced quasiparticle mass and non-Fermi-liquid behaviour. Electron scattering is so intense that electrical resistivity approaches the Planckian limit — the theoretical maximum rate at which electrons can dissipate energy. These features mark SrIrO₃ as part of a broader class of “strange metals.” 

Studying SrIrO₃ is challenging because its bulk perovskite form is difficult to grow as single crystals. An alternative approach is to grow the material in the form of a thin film or a heterostructure. In this case, when confined to fewer than three unit cells in thickness, it undergoes a transition to an insulating antiferromagnetic state (Figure 1b).

The experiment

To reveal the electronic properties of SrIrO₃, artificial heterostructures were constructed by stacking layers of SrIrO₃ and SrTiO₃ with atomic precision (Figure 1a). By varying the number of SrIrO₃ layers (m), it was possible to tune the system from a near-insulating state to one closely resembling the bulk semimetallic limit.

Fig. 1: RIXS investigation of iridate/titanate heterostructures. (a) Representation of (SrIrO₃)_m(SrTiO₃)₁ superlattices grown on a (001)-oriented SrTiO₃ single-crystal substrate. (b) Phase diagram of the (SrIrO₃)_m(SrTiO₃)₁ system, showing the bandwidth-controlled metal–insulator transition that occurs as the confinement increases from m = 3 to m = 2. (c) Intensity map of Ir L₃-edge RIXS spectra for the m = 4 sample at T = 20 K, plotted as a function of energy transfer and momentum transfer Q. Two distinct excitations are observed: a low-energy paramagnon and an SOE at intermediate energies.

Resonant inelastic X-ray scattering (RIXS) measurements were performed at the ID20 beamline. The incident X-rays were tuned to the Ir L₃-edge, and the energy lost upon scattering was measured. This energy loss directly reflects the spectrum of elementary excitations, making the technique particularly powerful in materials lacking long-range order.

Across all heterostructures, two robust collective excitations were observed (Figure 1c): a low-energy paramagnon and a higher-energy spin-orbit exciton (SOE). The paramagnon corresponds to propagating magnetic waves of pseudospins (hybrid spin-orbital moments), while the SOE corresponds to the propagation of a hole across SOC-split energy levels. Both modes are heavily damped, with linewidths comparable to their energies. The paramagnon is reminiscent of an antiferromagnetic insulator (Figure 2a), while the SOE shows an upward dispersion away from the zone centre and a single-site periodicity (Figure 2b), fingerprint of a non-magnetic metal. Interestingly, these seemingly contradictory excitations coexist in the same semimetallic system.

Fig. 2: Dispersion of paramagnon and SOE modes. (a) Momentum dependence of the paramagnon energy ω_m(Q), for all four (SrIrO₃)_m(SrTiO₃)₁ samples. The dashed purple line represents the dispersion of a square-lattice Heisenberg antiferromagnet with J = 100 meV, while the solid black line indicates the finite-size gap arising from ultrashort-range magnetic correlations. (b) Momentum dependence of the SOE energy, ω_o(Q), for all four samples. The purple line shows the SOE dispersion in Sr₂IrO₄ [from Kim et al., Phys. Rev. Lett. 108, 177003 (2012); https://doi.org/10.1103/PhysRevLett.108.177003] for comparison.

The paramagnon dispersion is compatible with a nearest-neighbour magnetic model, with ultrashort-range correlations producing a finite-size gap and broadening of the linewidths. This critical damping arises from metallic charge fluctuations in the non-Fermi-liquid phase, which also governs the upward dispersion of the SOE. The SOE dispersion in SrIrO₃ differs substantially from that of antiferromagnetic insulators such as Sr₂IrO₄, where the dispersion is governed by the underlying magnetic order. 

These observations demonstrate that the coexistence of paramagnon and SOE modes is an intrinsic property of semimetallic SrIrO₃. The findings illuminate the interplay between strong pseudospin and charge fluctuations in high-SOC materials and have implications for correlated “strange” metals. Understanding such interactions could also inform the design of spintronics devices in which both charge and spin are manipulated.

Principal publication
Coexistence of Insulator-like Paramagnon and Metallic Spin-Orbit Exciton Modes in SrIrO₃, E. Paris et al., Phys. Rev. Lett. 135, 186506 (2025); https://doi.org/10.1103/96fl-5zlh