Resonant X-ray Diffraction Study of the Strongly Spin-Orbit-Coupled Mott InsulatorCaIrO3

Abstract

We performed resonant x-ray diffraction experiments at the L absorption edges for the post-perovskite-type compound CaIrO(3) with a (t(2g))^{5} electronic configuration. By observing the magnetic signals, we could clearly see that the magnetic structure was a striped ordering with an antiferromagnetic moment along the c axis and that the wave function of a t(2g) hole is strongly spin-orbit entangled, the J(eff)=1/2 state. The observed spin arrangement is consistent with theoretical work predicting a unique superexchange interaction in the J(eff)=1/2 state and points to the universal importance of the spin-orbit coupling in Ir oxides, independent of the octahedral connectivity and lattice topology. We also propose that nonmagnetic resonant scattering is a powerful tool for unraveling an orbital state even in a metallic iridate.

Synthesis, Crystal Structure, and Physical Properties of the Perovskite Iridates

“…These two compounds have been known since 1960s [22,23] and received significant attention from geologists since 2004 as an analogy of MgSiO 3 , the main constituent mineral of the Earth's lower mantle [16,24]. More recently, they have emerged as important correlated 5d-electron systems with strong SOC [14,25]; the strong local distortion in pPv CaIrO 3 makes it a model system to investigate the interplay of non-cubic crystal field splitting and SOC [15], while the orthorhombic Pv CaIrO 3 might be considered as an intriguing semimetal with symmetry-protected Dirac points [26].…”

“…By adopting a tenfold flux and a relatively low soaking temperature of 836 and 950°C, respectively, Sugahara et al [28] and Hirai et al [29] obtained tiny single crystals for the purpose of crystal-structure refinements. On the other hand, Ohgushi et al [14] seems to grow sizable pPv CaIrO 3 single crystals for anisotropic magnetic property measurements by employing a higher flux molar ratio (16:1) and a higher soaking temperature of 1200°C. However, our attempts by using the latter approach ended up with Ca 2 IrO 4 rather than the pPv CaIrO 3 .…”

“…Density functional calculations by Subedi [33] demonstrated that the inclusion of SOC can split the t 2g bands into fully filled J eff = 3/2 bands and half-filled J eff = 1/2 bands, as shown schematic in Figure 1(c), and that both SOC and moderate U are required to reconcile the experimentally observed Mott insulating behaviour. By performing the resonant X-ray diffraction at the L absorption edges of pPv CaIrO 3 single crystals, Ohgushi et al [14] determined its magnetic structure as a stripe-type antiferromagnetic order, i.e. the Ir moments are aligned parallel along the a axis and antiparallel along the c axis with a canted ferromagnetic component along the b axis.…”

Nature of the Insulating Ground State of the5dPostperovskiteCaIrO3

“…Therefore, previous experimental and theoretical studies of CaIrO 3 have not reached a consensus on the presence of the j eff = 1/2 ground state, but concluded in the same way that CaIrO 3 belongs to a Mott-Hubbard insulator. Despite such a contradiction for the nature of the ground state, it is well established [12][13][14][15] that the insulating ground state of CaIrO 3 exhibits the stripe-type magnetic order with a strong antiferromagnetic (AFM) coupling along the c axis and a weak ferromagnetic one along the a axis (hereafter designated as the AFM structure).In this Letter, we investigate the nature of the ground state of CaIrO 3 by using comprehensive DFT calculations with local, semilocal, and hybrid exchange-correlation functionals as well as by including dynamical mean field theory (DMFT). We find that the t 2g states are significantly split by a compression of IrO 6 octahedra along the c axis and particularly two t 2g states (designated as t S1 2g and t S2 2g ) have dominant d yz and d zx characters with large band dispersions, indicating onedimensional (1D) electronic states along the c axis.…”

“…In 3d transition-metal oxides (TMOs), the localized 3d orbitals are responsible for the strong on-site Coulomb repulsion (U), leading to a Mott-Hubbard insulator where U splits a half-filled band into lower and upper Hubbard bands. Surprisingly, despite weaker U in 5d TMOs due to the very delocalized 5d orbitals, a series of Ir oxides such as Sr 2 IrO 4 [3][4][5][6][7], Na 2 IrO 3 [8][9][10], and CaIrO 3 [11][12][13][14][15] including Ir 4+ ions with five valence electrons exhibit an insulating ground state. For this unusual insulating behavior of the 5d iridates, it was proposed that spin-orbit coupling (SOC) splits the Ir t 2g states into completely filled j eff = 3/2 bands and a narrow half-filled j eff = 1/2 band at the Fermi level (E F ), and the latter band is further split into two Hubbard subbands by moderate Coulomb repulsion [3][4][5].…”