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.

No abstract

No abstract

We have synthesized epitaxial Sr 2 IrO 4 thin-films on various substrates and studied their electronic structures as a function of lattice-strain. Under tensile (compressive) strain, increased (decreased) Ir-O-Ir bond-angle is expected to result in increased (decreased) electronic bandwidth. However, we have observed that the two optical absorption peaks near 0.5 eV and 1.0 eV are shifted to higher (lower) energies under tensile (compressive) strain, indicating that the electronic-correlation energy is also affected by in-plane lattice-strain. The effective tuning of electronic structures under lattice-modification provides an important insight into the physics driven by the coexisting strong spin-orbit coupling and electronic correlation. PACS: 71.70.Ej, 72.80.Sk, 81.15 In this letter, we report on the growth and optical properties of Sr 2 IrO 4 (SIO-214) thin films. The in-plane lattice mismatches between SIO-214 and various oxide substrates can exert both tensile (+) and compressive (-) strains to films, as shown in Fig. 1(a). We find that the electronic structure of SIO-214 films are effectively altered by lattice strain, and we observe 3 shifted optical transitions (absorptions) between the J eff = 1/2 lower Hubbard band (LHB) and the J eff = 1/2 upper Hubbard band (UHB), and between the J eff = 3/2 band and the J eff = 1/2 UHB band. Our observations strongly suggest that not only the electronic bandwidth, but also the magnitude of the effective electronic correlation energy (U eff ), can be manipulated by lattice strain. Our results demonstrate that epitaxial SIO-214 thin films can be used as a model system to study the physics of coexisting strong electron correlation and strong spin-orbit coupling under lattice modification.We have used a custom-built, pulsed laser deposition system equipped with in-situ Table I. The epitaxial growth conditions are found to be the following: an oxygen partial pressure (P O2 ) of 10 mTorr, a substrate temperature of 700 °C, and a laser (KrF excimer, λ = 248 nm) fluence of 1.2 J/cm 2 . Figure 2 shows θ-2θ X-ray diffraction scans of the samples discussed herein. Well-defined 00l-peaks are present due to the films' 00l-orientation along the perpendicular to the substrates. The full widths at half maximum in rocking-curve scans of the 00l peaks are all less than 0.05°, which confirms the high crystallinity of the films. Note that the thin films' 0012-peaks are shifted to low angles as the substrate lattice parameters decrease (from GSO to LAO). This behavior is consistent with the schematic diagrams in Fig. 1(b), since elongated (contracted) out-of-plane lattice parameters are expected as compressive (tensile) in-plane strain is exerted on thin films. 4Figure 3(a) shows X-ray reciprocal space maps, which reveal important information about both the in-plane and the out-of-plane lattice parameters of the SIO-214 thin films near the 332-reflection (103-reflection) of orthorhombic (pseudo-cubic) substrates. The 1118-peaks from the thin films are clearly observed, and are...

Our magnetic, electrical, and thermal measurements on single crystals of the J eff =1/ 2 Mott insulator, Sr 2 IrO 4 , reveal a giant magnetoelectric effect ͑GME͒ arising from a frustrated magnetic/ferroelectric state whose signatures are: ͑1͒ a strongly enhanced electric permittivity that peaks near an observed magnetic anomaly at 100 K, and ͑2͒ a large ͑ϳ100%͒ magnetodielectric shift that occurs near a metamagnetic transition. The GME hinges on a spin-orbit gapping of 5d bands rather than the magnitude and spatial dependence of magnetization, as traditionally accepted.