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Register a new userLong range magnetic ordering in Na$_2$IrO$_3$
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We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb lattice magnet Na$_2$IrO$_3$, a strong candidate for a realization of a gapless spin-liquid. Using resonant x-ray magnetic scattering at the Ir L$_3$-edge, we find 3D long range antiferromagnetic order below T$_N$=13.3 K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the {it a}-axis. Combining the experimental data with first principles calculations, we propose that the most likely spin structure is a novel zig-zag structure.
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The magnetic structure of honeycomb iridate Na$_2$IrO$_3$ is of paramount importance to its exotic properties. The magnetic order is established experimentally to be zigzag antiferromagnetic. However, the previous assignment of ordered moment to the $bm{a}$-axis is tentative. We examine the magnetic structure of Na$_{2}$IrO$_{3}$ using first-principles methods. Our calculations reveal that total energy is minimized when the zigzag antiferromagnetic order is magnetized along $bm{g}approxbm{a}+bm{c}$. Such a magnetic configuration is explained by adding anisotropic interactions to the nearest-neighbor Kitaev-Heisenberg model. Spin-wave spectrum is also calculated, where the calculated spin gap of $10.4$ meV can in principle be measured by future inelastic neutron scattering experiments. Finally we emphasize that our proposal is consistent with all known experimental evidence, including the most relevant resonant x-ray magnetic scattering measurements [X. Liu emph{et al.} {Phys. Rev. B} textbf{83}, 220403(R) (2011)].
The honeycomb lattice iridate Na$_2$IrO$_3$ shows frustrated magnetism and can potentially display Kitaev-like exchange interactions. Recently, it was shown that the electronic properties of the surface of crystalline Na$_2$IrO$_3$ can be tuned by Ar plasma treatment in a controlled manner leading to various phases of matter ranging from a fully gapped to a metallic surface, where the possibility of a charge-density wave (CDW) like transition is also expected. Here, through direct imaging with an atomic force microscope (AFM) in air, we show that the surface of crystalline Na$_2$IrO$_3$ evolves rapidly as elemental Na effuses out of the interleave planes to the surface and undergoes sublimation thereby disappearing from the surface gradually over time. Using conductive AFM we recorded a series of topographs and surface current maps simultaneously and found that the modification of the surface leads to change in the electronic properties in a dynamic fashion until the whole system reaches a dynamic equilibrium. These observations are important in the context of the exotic electronic and magnetic properties that the surface of Na$_2$IrO$_3$ displays.
Kitaevs honeycomb spin-liquid model and its proposed realization in materials such as $alpha$-RuCl$_3$, Li$_2$IrO$_3$ and Na$_2$IrO$_3$ continue to present open questions about how the dynamics of a spin-liquid are modified in the presence of non-Kitaev interactions as well as the presence of inhomogeneities. Here we use $^{23}$Na nuclear magnetic resonance to probe both static and dynamical magnetic properties in single crystal Na$_2$IrO$_3$. We find that the NMR shift follows the bulk susceptibility above 30 K but deviates from it below; moreover below $T_N$ the spectra show a broad distribution of internal magnetic fields. Both of these results provide evidence for inequivalent magnetic sites at low temperature, suggesting inhomogeneities are important for the magnetism. The spin-lattice relaxation rate is isotropic and diverges at $T_N$, suggesting that the Kitaev cubic axes may control the critical quantum spin fluctuations. In the ordered state, we observe gapless excitations, which may arise from site substitution, emergent defects from milder disorder, or possibly be associated with nearby quantum paramagnetic states distinct from the Kitaev spin liquid.
The Kitaev model of spin-1/2 on a honeycomb lattice supports degenerate topological ground states and may be useful in topological quantum computation. Na$_{2}$IrO$_{3}$ with honeycomb lattice of Ir ions have been extensively studied as candidates for the realization of the this model, due to the effective $J_{text{eff}}=1/2$ low-energy excitations produced by spin-orbit and crystal-field effect. As the eventual realization of Kitaev model has remained evasive, it is highly desirable and challenging to tune the candidate materials toward such end. It is well known external pressure often leads to dramatic changes to the geometric and electronic structure of materials. In this work, the high pressure phase diagram of Na$_{2}$IrO$_{3}$ is examined by first-principles calculations. It is found that Na$_{2}$IrO$_{3}$ undergoes a sequence of structural and magnetic phase transitions, from the magnetically ordered phase with space group $C2/m$ to two bond-ordered non-magnetic phases. The low-energy excitations in these high-pressure phases can be well described by the $J_{text{eff}}=1/2$ states.
Spin orbit assisted Mott insulators such as sodium iridate (Na$_2$IrO$_3$) have been an important subject of study in the recent years. In these materials, the interplay of electronic correlations, spin-orbit coupling, crystal field effects and a honeycomb arrangement of ions bring exciting ground states, predicted in the frame of the Kitaev model. The insulating character of Na$_2$IrO$_3$ has hampered its integration to an electronic device, desirable for applications, such as the manipulation of quasiparticles interesting for topological quantum computing. Here we show through electronic transport measurements supported by Angle Resolved Photoemission Spectroscopy (ARPES) experiments, that electronic transport in Na$_2$IrO$_3$ is ruled by variable range hopping and it is strongly dependent on the magnetic ordering transition known for bulk Na$_2$IrO$_3$, as well as on external electric fields. Electronic transport measurements allow us to deduce a value for the localization length and the density of states in our Na$_2$IrO$_3$ thin crystals devices, offering an alternative approach to study insulating layered materials.
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