We have combined single crystal neutron and x-ray diffractions to investigate the magnetic and crystal structures of the honeycomb lattice Na2IrO3. The system orders magnetically below 18.1(2) K with Ir 4+ ions forming zigzag spin chains within the layered honeycomb network with an ordered moment of 0.22(1) µB/Ir site. Such a configuration sharply contrasts with the Néel or stripe states proposed in the Kitaev-Heisenberg model. The structure refinement reveals that the Ir atoms form a nearly ideal two-dimensional honeycomb lattice while the IrO6 octahedra experience a trigonal distortion that is critical to the ground state. The results of this study provide much needed experimental insights into the magnetic and crystal structure that are crucial to the understanding of the exotic magnetic order and possible topological characteristics in the 5d-electron-based honeycomb lattice.

We report a single-crystal neutron diffraction study of the layered Sr2IrO4. This work unambiguously determines the magnetic structure of the system and reveals that the spin orientation rigidly tracks the staggered rotation of the IrO6 octahedra in Sr2IrO4. The long-range antiferromagnetic order has a canted spin configuration with an ordered moment of 0.208(3) µB/Ir site within the basal plane; a detailed examination of the spin canting yields 0.202(3) and 0.049(2) µB/site for the a axis and the b axis, respectively. It is intriguing that forbidden nuclear reflections of space group I41/acd are also observed in a wide temperature range from 4 K to 600 K, which suggests a reduced crystal structure symmetry. This neutron-scattering work provides a direct, well-refined experimental characterization of the magnetic and crystal structures that are crucial to the understanding of the unconventional magnetism exhibited in this unusual magnetic insulator.

Evidence of an odd-parity hidden order in a strongly spin-orbit coupled correlated iridate Contents: S1. RA-SHG data for Sin-Pout and Sin-Sout geometries above and below TΩ show that the crystal structure of Sr2IrO4 belongs to the centrosymmetric tetragonal 4/m point group as opposed to the previously accepted 4/mmm point group 4,5 . Given the presence of inversion symmetry, the leading order contribution to SHG is the non-local term of electricquadrupole type, which can be expressed as an effective nonlinear polarization aswhere is the electric-quadrupole susceptibility tensor. By enforcing 4/m point group symmetry, is reduced to having 21 non-zero independent elements 6 :With the four additional constraints from degenerate SHG { = , = , = , = }, the number of non-zero independent tensor elements is further reduced to 17. The rotation of the crystal by an angle φ about the c-axis is carried out mathematically by applying a basis transformation on the reduced tensor from the original (primed) to rotated (unprimed) reference frame usingwhere ( ) is the rotation matrix about the c-axis. Finally, the expression that is used to fit theA is a constant determined by the experimental geometry, ( ) is the intensity of the incident 4 beam and ̂ is the polarization of the incoming or outgoing light, which we select to be either linearly P or S polarized. We note that previous work has already shown that there is no evidence of a surface electric-dipole contribution SHG 3 and that the crystallographic symmetry of the surface remains unchanged across TΩ 7 .ii) Fitting RA-SHG data for T < TThe low temperature RA-SHG data are fit to a coherent sum of the electric-quadrupole term described above and a hidden order induced electric-dipole term. The electric-dipole contribution is expressed as a nonlinear polarization (2 ) ∝ ( ) ( ). By enforcing 2/m magnetic point group symmetry, is reduced to having 14 non-zero independent elements:We only discuss the results using a 2/m magnetic point group here although the same procedure was applied to all of the magnetic point groups we surveyed. The additional constraints from degenerate SHG { = , = , = , = } leaves 10 non-zero independent tensor elements remaining. A basis transformation was then carried out on using ( ) = ′ ′ ′ ′ ′ ′ and the expression used to fit the RA-SHG data at Invariance of the magnetic structure under the elements of 2/m1 is explicitly shown in Figs S2 b-e. We note that the 2/m1 magnetic point group assignment does not rely on the magnitude of the magnetic moments on the two structural sub-lattices being equal, even though experimentally they are found to be so. S4. Loop-current order in cuprates versus iridatesLoop-current ordered phases were initially proposed in a three-band CuO2 model of the copperoxide planes where the charge currents are emergent complex hopping terms between oxygen and copper sites 8,9 . In this section, we examine how this model would be modified for Sr2IrO4.Our intention is not to elaborate on any of the details of loop-curr...

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We synthesize and study single crystals of a new double-perovskite Sr2YIrO6. Despite two strongly unfavorable conditions for magnetic order, namely, pentavalent Ir5+(5d4) ions which are anticipated to have Jeff=0 singlet ground states in the strong spin-orbit coupling (SOC) limit and geometric frustration in a face-centered cubic structure formed by the Ir5+ ions, we observe this iridate to undergo a novel magnetic transition at temperatures below 1.3 K. We provide compelling experimental and theoretical evidence that the origin of magnetism is in an unusual interplay between strong noncubic crystal fields, local exchange interactions, and "intermediate-strength" SOC. Sr2YIrO6 provides a rare example of the failed dominance of SOC in the iridates.

The physics of doped Mott insulators remains controversial after decades of active research, hindered by the interplay among competing orders and fluctuations. It is thus highly desired to distinguish the intrinsic characters of the Mott-metal crossover from those of other origins. Here we investigate the evolution of electronic structure and dynamics of the hole-doped pseudospin-1/2 Mott insulator Sr2IrO4. The effective hole doping is achieved by replacing Ir with Rh atoms, with the chemical potential immediately jumping to or near the top of the lower Hubbard band. The doped iridates exhibit multiple iconic low-energy features previously observed in doped cuprates—pseudogaps, Fermi arcs and marginal-Fermi-liquid-like electronic scattering rates. We suggest these signatures are most likely an integral part of the material's proximity to the Mott state, rather than from many of the most claimed mechanisms, including preformed electron pairing, quantum criticality or density-wave formation.

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The electronic structure of Sr3CuIrO6, a model system for the 5d Ir ion in an octahedral environment, is studied through a combination of resonant inelastic x-ray scattering (RIXS) and theoretical calculations. RIXS spectra at the Ir L3-edge reveal an Ir t2g manifold that is split into three levels, in contrast to the expectations of the strong spin-orbit-coupling limit. Effective Hamiltonian and ab inito quantum chemistry calculations find a strikingly large non-cubic crystal field splitting comparable to the spin-orbit coupling, which results in a strong mixing of the j eff = 1 2 and j eff = 3 2 states and modifies the isotropic wavefunctions on which many theoretical models are based.