Advanced First-Principle Modeling of Relativistic Ruddlesden—Popper Strontium Iridates

Peitao Liu,Cesare Franchini

doi:10.3390/app11062527

Abstract

In this review, we provide a survey of the application of advanced first-principle methods on the theoretical modeling and understanding of novel electronic, optical, and magnetic properties of the spin-orbit coupled Ruddlesden–Popper series of iridates Srn+1IrnO3n+1 (n = 1, 2, and ∞). After a brief description of the basic aspects of the adopted methods (noncollinear local spin density approximation plus an on-site Coulomb interaction (LSDA+U), constrained random phase approximation (cRPA), GW, and Bethe–Salpeter equation (BSE)), we present and discuss select results. We show that a detailed phase diagrams of the metal–insulator transition and magnetic phase transition can be constructed by inspecting the evolution of electronic and magnetic properties as a function of Hubbard U, spin–orbit coupling (SOC) strength, and dimensionality n, which provide clear evidence for the crucial role played by SOC and U in establishing a relativistic (Dirac) Mott–Hubbard insulating state in Sr2IrO4 and Sr3Ir2O7. To characterize the ground-state phases, we quantify the most relevant energy scales fully ab initio—crystal field energy, Hubbard U, and SOC constant of three compounds—and discuss the quasiparticle band structures in detail by comparing GW and LSDA+U data. We examine the different magnetic ground states of structurally similar n = 1 and n = 2 compounds and clarify that the origin of the in-plane canted antiferromagnetic (AFM) state of Sr2IrO4 arises from competition between isotropic exchange and Dzyaloshinskii–Moriya (DM) interactions whereas the collinear AFM state of Sr3Ir2O7 is due to strong interlayer magnetic coupling. Finally, we report the dimensionality controlled metal–insulator transition across the series by computing their optical transitions and conductivity spectra at the GW+BSE level from the the quasi two-dimensional insulating n = 1 and 2 phases to the three-dimensional metallic n=∞ phase.

Highlights

In the last decade, Ir-based transition metal oxides have become a rapidly evolving research area and have stimulated intensive interest due to the emergence of novel phases of matter and exotic quantum phenomena arising from the cooperative interplay among the crystalline electric field, spin–orbit coupling (SOC), Coulomb repulsion (U), and different spin–exchange interactions
We review some results on first-principle modeling of this RP family, putting in evidence the capability of advanced electronic structure methods such as noncollinear local spin density approximation plus an on-site Coulomb interaction (LSDA+U), constrained random phase approximation, GW, and the Bethe–Salpter equation (BSE) to properly account for the novel electronic, magnetic, and optical properties of iridates
For systems with localized d or f states, density functional theory (DFT)+U obtained one-electron energies and orbitals are usually much closer to the ground state as compared to DFT and, is a better starting point for subsequent G0W0 calculations [98,99]. This is the case for iridates where we have shown that DFT+U with the U calculated from constrained random phase approximation (cRPA) predicts very similar band structures to that of G0W0 [100]

Summary

Introduction

Ir-based transition metal oxides have become a rapidly evolving research area and have stimulated intensive interest due to the emergence of novel phases of matter and exotic quantum phenomena arising from the cooperative interplay among the crystalline electric field, spin–orbit coupling (SOC), Coulomb repulsion (U), and different spin–exchange interactions (for reviews, see References [1,2,3,4,5,6,7]). Sr2IrO4 exhibits striking structural and magnetic similarities to high-Tc cuprate superconductors such as La2CuO4: these two compounds share the same quasi-two-dimensional layered perovskite structure and Ir and Cu have nominal d5 and d9 configurations, with one effective hole per site [15,21]. This analogy has boosted the search for superconducting states in a new family of compounds, possibly triggered by doping [22,23,24,25] or strain engineering [26]. It has a three-dimensional crystal structure, and it has been reported to exhibit a nonmagnetic correlated state combined with a topological crystalline semimetal character [7,11,34,35,36], The topological state is associated with surface states protected by the lattice symmetry [34,35], a large quasiparticle mass enhancement [11], and an unusual positive magnetoresistance [36]

Methods

Results

Conclusion