Orbital occupancies and the putativejeff=12ground state in Ba2IrO4

Abstract

The nature of the electronic ground state of Ba 2 IrO 4 has been addressed using soft x-ray absorption and inelastic scattering techniques in the vicinity of the oxygen K edge. From the polarization and angular dependence of x-ray absorption spectroscopy (XAS) we deduce an approximately equal superposition of xy, yz, and zx Ir 4+ 5d orbitals. By combining the measured orbital occupancies, with the value of the spin-orbit coupling provided by resonant inelastic x-ray scattering (RIXS), we estimate the crystal field splitting associated with the tetragonal distortion of the IrO 6 octahedra to be small, ≈ 50(50) meV. We thus conclude definitively that Ba 2 IrO 4 is a close realization of a spin-orbit Mott insulator with a j eff = 1 2 ground state, thereby overcoming ambiguities in this assignment associated with the interpretation of x-ray resonant scattering experiments.

CaIrO3: A Spin-Orbit Mott Insulator Beyond thejeff=1/

“…3), from which one obtains ζ ¼ 0.52 and Δ ¼ −0.71 eV and the expression for the ground state wave function j0; AEi ¼ ∓0.32jxy; ∓i þ 0.67ðjyz; AEi∓ıjzx; AEiÞ. The value of ζ compares favorably with previous estimates in other iridium oxides [7,12,26,28], and in particular to the recently calculated value of 0.47 eV by Bogdanov et al [19] for the specific case of CaIrO 3 . The sign of Δ is consistent with structural studies reporting a compression of the IrO 6 octahedra [8], while its magnitude implies a minor contribution of the xy orbital to the ground state wave function (only 0.32 2 ≈ 10%), in agreement with ab initio quantum chemistry calculations which predict a splitting of t 2g states in the absence of spin-orbit coupling in the order of 0.63-0.76 [19].…”

“…Features B and C are centered at 0.65 and 1.22 eV, respectively. In order to better understand the nature of these excitations, and to assess the effective tetragonal crystal field splitting and spin-orbit coupling in CaIrO 3 , we adopt a single-ion model [7,14,17,22,26,27]. The weak momentum transfer dependence of these excitations suggests a dominant intrasite character, for which a local model is justified.…”

“…Instead, the role of electronic correlation is enhanced here by spin-orbit coupling through the formation of the so-called j eff ¼ 1=2 ground state. Its realization arises from the interaction of strong spin-orbit coupling (ζ ∼ 0.5 eV) and a cubic crystal field (10Dq ∼ 3 eV), and is perturbed by short-and longranged anisotropies which could cause departures from the j eff ¼ 1=2 ground state [4][5][6][7]. A small, but sizable tetragonal contribution jΔj ∼ 0.01 eV to the cubic crystal field 10Dq was detected in both Sr 2 IrO 4 [6] and Ba 2 IrO 4 [7], with opposite signs, in agreement with recent theoretical calculations [5].…”

Unveiling the origin of the basal-plane antiferromagnetism in the spin–orbit Mott insulator Ba2IrO4: a density functional and model Hamiltonian study

“…The IrO 2 square-lattice planes are separated by two successive BaO layers and the Ir 4+ ions in the neighboring IrO 2 square-lattice planes are spatially separated as far as about 7.3 Å [18] (see figure 1(a)). Recent experiments show that the crystal field splitting associated with the tetragonal distortion of the IrO 6 octahedron is small so that BIO is a close realization of the spin-orbit Mott insulator [32].…”

“…Here we first recall the Hamiltonian employed by the previous studies on the iridium oxides [8,22,35,36]. The Ir 4+ ions yields an effective isospin Kramers-doublet ground states due to the strong octahedral crystal field and SOC [8,20,32]. The Kramers-doublet ground states are the pseudo-spin states |  ñ  and |  ñ  (denoted as S to be distinguished from the usual spin S) which are in the form of [8]:…”