# Perovskite iridates

Iridium oxides (iridates) represent a family of materials in which the energy scale of electronic bandwidth, Coulomb interaction, and spin-orbit coupling have comparable magnitudes. The interplay among these interactions provides a promising playground to realize novel phenomena in solids. The Ruddlesden-Popper iridate series Srn+1IrnO3n+1 has attracted particular attention for its versatility in the electronic ground state and dimensionality tuning. For example, the single-layer member Sr2IrO4 (n = 1) is a spin-orbit-coupled Mott insulator whereas the infinite-layer member SrIrO3 (n = ∞) is a correlated semimetal with Dirac nodes near Fermi level protected by lattice symmetry.

Crystal structure of Ruddlesden-Popper iridate series Srn+1IrnO3n+1. The electronic ground state transitions from a Mott insulator to a correlated semimetal as n increases from 1 to infinity.

Fermi surface of monoclinic SrIrO3 mapped by quantum oscillations. Yellow and cyan color designates the hole and electron character of the Fermi pocket, respectively. Figure reference: npj Quantum Mater. 6:92

We have mapped out the Fermi surface of SrIrO3 using magnetic quantum oscillations on high-quality single crystals [npj Quantum Mater. 6:92 (2021)]. The Fermi surface consists of multiple small pockets, all of which with an enhanced quasiparticle effective mass of up to 5 me, evidencing strong electron correlations. We find further evidence of strong correlations including a large Kadowaki-Woods ratio and a linear-in-temperature (T), “strange-metallic” component in low-T resistivity. Despite the exceptional sensitivity of the Fermi-surface geometry to the precise atomic positions, the Dirac crossings remain robust as protected by lattice symmetry. These characteristics make bulk SrIrO3 an attractive material platform to explore the interplay between strong electron correlations and nontrivial band topology, potentially a topological quantum phase transition in the proximity of Mottness. We are currently investigating the possibility of tuning its electronic ground state using novel structural tuning parameters in order to realize novel quantum phenomena therein.

More recently, we conducted an investigation on the transport and thermodynamic properties of Sr2IrO4 over a wide range of bulk electron doping via La substitution. We find two defining characteristics of the low-T electronic ground state of electron-doped Sr2IrO4: 1) the effective carrier density undergoes a dramatic crossover from n ≈ x to n ≈ 1+x at xc ≈ 0.16 and 2) the electronic specific heat γ shows a strong enhancement near xc. These observations indicate the electronic structure of Sr2IrO4 undergoes a transformation with electron doping near xc. Remarkably, these ground-state characteristics of electron-doped Sr2IrO4 are strikingly reminiscent of that found in hole-doped cuprates near the pseudogap point p*, making Sr2IrO4 an ideal system to study the pseudogap phenomenology in the innate absence of superconductivity. We are currently investigating the possible link between quantum criticality and pseudogap in electron-doped Sr2IrO4, which will provide new insights into the nature of pseudogap in doped two-dimensional Mott lattices.

Doping evolution of the effective carrier density nH and electronic specific heat γ in Sr2-xLaxIrO4 in the T = 0 limit. Near x = 0.16, nH undergoses a crossover from n = x to n = 1+x, meanwhile γ shows a strong enhancement. Three regions of distinct electronic states are identified: Mott insulating (MI), pseudogap (PG), and disordered metal (DM). Figure reference: arXiv:2312.00515

## Further reading

1. W. Witczak-Krempa et al., "Correlated quantum phenomena in the strong spin-orbit regime", Annual Review of Condensed Matter Physics 5, 57 (2014)

2. J. G. Rau et al., "Spin-orbit physics giving rise to novel phases in correlated systems: Iridates and related materials", Annual Review of Condensed Matter Physics 7, 195 (2016)

3. J. Bertinshaw et al., "Square lattice iridates", Annual Review of Condensed Matter Physics 10, 315 (2019)