The Ruddlesden-Popper phases Srn+1IrnO3n+1 are discussed to exhibit a semimetal to insulator transition for the series n=∞,2,1 layers of SrIrO3, where the n=1 member is a so-called spin-orbit driven Mott insulator [1-3]. Here the spin-orbit coupling is large enough to split overlapping, mixed bands into a manifold of narrow bands with well-defined effective total angular momentum Jeff. With lowering the dimensionality of the IrO2 plane structure these bands become successively more susceptible to Mott localization since the coupling strength gets enhanced even though the local onsite Coulomb repulsion U is relatively small in 5d as compared to 3d transition metal oxides.
A systematic investigation of the Srn+1IrnO3n+1 series, however, is difficult since the bulk compounds are unstable in ambient pressure for n ≥ 3 and thus extremely challenging to grow as single crystals. An alternative way to investigate the effect of dimensionality on the electronic properties is the fabrication of ultrathin SrIrO3 films with varying thickness.
We have grown high-quality SrIrO3 thin films on SrTiO3 by pulsed laser deposition and investigated their electronic structure by soft x-ray angle resolved photoemission. Indeed, a semimetal-insulator transition is found as function of film thickness as evidenced by the opening of a gap at the chemical potential. We discuss the nature of the observed transition based on density-functional calculations including U (DFT+U), taking the thickness-dependent octahedra rotation and tilting as observed by low-energy electron diffraction into account. While the inclusion of electron correlations and structural distortions is required in order to reproduce the electronic dispersions, DFT+U can reproduce the insulating ground state only if (antiferro)magnetic order is considered. The phase transition appears as a complex, concerted action in which structural distortions, electron correlations, spin-dependent hopping enabled by the entanglement of orbital and spin wave functions as well as magnetic coupling all cooperate in driving the transition upon tuning the dimensionality.
1. S. J. Moon et al., Dimensionality-Controlled Insulator-Metal Transition and Correlated Metallic State in 5d Transition Metal Oxides Srn+1IrnO3n+1 (n=1, 2, and ∞), Phys. Rev. Lett. 101 (2008) 226402.
2. B. Kim et al., Novel Jeff=1/2 Mott State Induced by Relativistic Spin-Orbit Coupling in Sr2IrO4, Phys. Rev. Lett. 101 (2008) 076402
3. B. J. Kim et al., Phase-Sensitive Observation of a Spin-Orbital Mott State in Sr2IrO4, Science 323 (2009), 1329-1332