We determine the motion of a charge (hole or electron) added to the Mott insulating, antiferromagnetic (AF) ground-state of quasi-2D iridates such as Ba 2 IrO 4 or Sr 2 IrO 4 . We show that correlation effects, calculated within the self-consistent Born approximation, render the hole and electron case very different. An added electron forms a spin-polaron, which closely resembles the well-known cuprates, but the situation of a removed electron is far more complex. Many-body 5d 4 configurations form which can be singlet and triplets of total angular momentum J and strongly affect the hole motion between AF sublattices. This not only has important ramifications for the interpretation of (inverse-)photoemission experiments of quasi-2D iridates but also demonstrates that the correlation physics in electron- and hole-doped iridates is fundamentally different.
We study the correlated electronic structure of single-layer iridates based on structurally-undistorted Ba$_2$IrO$_4$. Starting from the first-principles band structure, the interplay between local Coulomb interactions and spin-orbit coupling is investigated by means of rotational-invariant slave-boson mean-field theory. The evolution from a three-band description towards an anisotropic one-band ($J=1/2$) picture is traced. Single-site and cluster self-energies are used to shed light on competing Slater- and Mott-dominated correlation regimes. We reveal a clear asymmetry between electron and h…
Direct experimental investigations of the low-energy electronic structure of the Na$_2$IrO$_3$ iridate insulator are sparse and draw two conflicting pictures. One relies on flat bands and a clear gap, the other involves dispersive states approaching the Fermi level, pointing to surface metallicity. Here, by a combination of angle-resolved photoemission, photoemission electron microscopy, and x-ray absorption, we show that the correct picture is more complex and involves an anomalous band, arising from charge transfer from Na atoms to Ir-derived states. Bulk quasiparticles do exist, but in one …
Evidence for hidden order and its potential correlation with electron-hole asymmetry in iridate superlattices
Anhui Provincial Laboratory of Low-Energy Quantum Materials and Devices, High Magnetic Field Laboratory, HFIPS, Chinese Academy of Sciences（中国科学院合肥物质科学研究院）；Science Island Branch of Graduate School, University of Science and Technology of China（中国科学技术大学）；Wang Zheng School of Microelectronics, Changzhou University（常州大学王正微电子学院）；Pohang Accelerator Laboratory, POSTECH（韩国科学技术院浦项加速器实验室）；College of Science, National University of Defense Technology（国防科技大学理学院）
We point out the emergence of magnetism from the interplay of electron quasi-itinerancy and quantum order by disorder in pyrochlore iridates. Like other Mott insulating iridates, the Ir$^{4+}$ ion in pyrochlore iridates develops an effective ${J=1/2}$ moment from the on-site spin-orbit coupling. We consider the generic symmetry-allowed exchange between these local moments on a pyrochlore lattice and obtain the mean-field phase diagram. Assuming the superexchange is mediated by direct and/or indirect electron hopping via intermediate oxygens, we derive the exchange interactions in the strong co…
Spin correlated electronic state on the surface of a spin-orbit Mott system (Layered Iridates)
二维电子系统的新相是由新的表面或界面修饰的电子结构引起的，在材料科学中引起了极大的兴趣。我们利用光电子能谱表明，近表面的电子结构的体绝缘铱酸盐Sr $_3$ Ir $_2$ O $_7$位于附近的金属-莫特绝缘体过渡表现出弱的金属性表示有限的电子光谱重量在费米能级。表面电子表现出独特的自旋结构，由自旋轨道，库仑相互作用和表面量子磁性的相互作用，不同于拓扑绝缘体状态。我们的研究结果表明，一个新的准二维相互作用电子表面基态的实验实现，打开了大门，奇异的量子纠缠和输运现象的铱基氧化物器件。
We study the prototype 5d pyrochlore iridate Y2Ir2O7 from first principles using the local density approximation and dynamical mean-field theory (LDA+DMFT). We map out the phase diagram in the space of temperature, onsite Coulomb repulsion U, and filling. Consistent with experiments, we find that an all-in-all-out ordered insulating phase is stable for realistic values of U. The trigonal crystal field enhances the magnetic moments of the jeff=1/2 doublet, and strong inter-band correlations are induced by the Coulomb interaction, which indicates that a three-band description is important. We de…
The fractionalization of elementary excitations in quantum spin systems is a central theme in current condensed matter physics. The Kitaev honeycomb spin model provides a prominent example of exotic fractionalized quasiparticles, composed of itinerant Majorana fermions and gapped gauge fluxes. However, identification of the Majorana fermions in a three-dimensional honeycomb lattice remains elusive. Here we report spectroscopic signatures of fractional excitations in the harmonic-honeycomb iridates $β$- and $γ$-Li$_2$IrO$_3$. Using polarization resolved Raman spectroscopy, we find that the dyna…
The electronic ground state in many iridate materials is described by a complex wave-function in which spin and orbital angular momenta are entangled due to relativistic spin-orbit coupling (SOC). Such a localized electronic state carries an effective total angular momentum of $J_{eff}=1/2$. In materials with an edge-sharing octahedral crystal structure, such as the honeycomb iridates Li2IrO3 and Na2IrO3, these $J_{eff}=1/2$ moments are expected to be coupled through a special bond-dependent magnetic interaction, which is a necessary condition for the realization of a Kitaev quantum spin liqui…
Over the past few years bulk pyrochlore iridates of the form $A_2$Ir$_2$O$_7$ (where $A$ is a rare earth element, Ir is iridium, and O is oxygen) have been studied as model systems for investigating the interplay of electronic correlations and strong spin-orbit coupling, particularly with the aim of finding correlation-driven topological phases. In this work, we use cellular dynamical mean field theory (CDMFT) to study effects of electronic correlations beyond Hartree-Fock theory in thin films of pyrochlore irradiates grown along the $[111]$ direction. We focus on the bilayer and trilayer syst…
We study the effect of isoelectronic doping and external pressure in tuning the ground state of the honeycomb iridate Na$_2$IrO$_3$ by combining optical spectroscopy with synchrotron x-ray diffraction measurements on single crystals. The obtained optical conductivity of Na$_2$IrO$_3$ is discussed in terms of a Mott insulating picture versus the formation of quasimolecular orbitals and in terms of Kitaev-interactions. With increasing Li content $x$, (Na$_{1-x}$Li$_x$)$_2$IrO$_3$ moves deeper into the Mott insulating regime and there are indications that up to a doping level of 24\% the compound…
We report equilibrium and nonequilibrium optical measurements on the recently synthesized "harmonic" honeycomb iridate γ-Li$_2$IrO$_3$ (LIO), as well as the layered honeycomb iridate Na$_2$IrO$_3$ (NIO). Using Fourier transform infrared microscopy we performed reflectance measurements on LIO, from which we obtained the optical conductivity below 2 eV. In addition we measured the photoinduced changed in reflectance, ΔR, as a function of time, t, temperature, T, and probe field polarization in both LIO and NIO. In LIO, ΔR(t,T) is anisotropic and comprised of three T dependent components. Two of …
We report a comprehensive resonant inelastic x-ray scattering (RIXS) study of various iridate materials focusing on core-level excitations and transitions between crystal-field split d-levels. The 2p core hole created at the Ir $L_3$ absorption edge has a very short lifetime giving rise to a broad absorption width ($\sim 5$~eV). This absorption linewidth broadening can be overcome by studying the resonant x-ray emission spectroscopy (RXES) map, which is a two-dimensional intensity map of the Ir L$α_2$ emission obtained with high energy-resolution monochromator and analyzer. By limiting the emi…
Analogs of the high-T$_c$ cuprates have been long sought after in transition metal oxides. Due to the strong spin-orbit coupling (SOC), the $5d$ perovskite iridates Sr$_2$IrO$_4$ exhibit a low-energy electronic structure remarkably similar to the cuprates. Whether a superconducting state exists as in the cuprates requires understanding the correlated spin-orbit entangled electronic states. Recent experiments discovered hidden order in the parent and electron doped iridates, some with striking analogies to the cuprates, including Fermi surface pockets, Fermi arcs, and pseudogap. Here, we study …
We report a resonant inelastic x-ray scattering (RIXS) investigation of ultra-thin epitaxial films of Ba$_2$IrO$_4$, and compare their low energy magnetic and spin-orbit excitations to those of their sister compound Sr$_2$IrO$_4$. Due to the 180$^\circ$ Ir-O-Ir bond, the bandwidth of the magnon and spin-orbiton is significantly larger in Ba$_2$IrO$_4$, making it difficult to describe these two types of excitations as separate well-defined quasiparticles. Both types of excitations are found to be quite sensitive to the effect of epitaxial strain. In addition, we find that the d-level inversion …
$J_{eff}$ states in a quasi one dimensional antiferromagnetic spin chain hexagonal Iridates Sr$_3$MIrO$_6$ (M=Mg, Zn, Cd): an $ab-initio$ comparative perspective
Measurement of the internal magnetic field in the correlated iridates Ca$_4$IrO$_6$, Ca$_5$Ir$_3$O$_{12}$, Sr$_3$Ir$_2$O$_7$ and Sr$_2$IrO$_4$
含有铱离子的氧化物显示出一系列的磁性和导电特性，这些特性取决于相互作用之间的微妙平衡，并且至少部分地由晶体结构的细节控制。我们用μ子自旋转动（$μ$ SR）方法研究了四种铱氧化物（Ca $_4$ IrO $_6$，Ca $_5$ Ir $_3$ O $_{12}$，Sr $_3$ Ir $_2$ O $_7$和Sr $_2$ IrO $_4$）的局域场，它们表现出不同的行为.我们对Ca $_4$ IrO $_6$和Ca $_5$ Ir $_3$ O $_{12}$的$μ$ SR数据与常规反铁磁性相一致，其中准静态磁序分别在低于$T_{\rm N}=13.85(6)$ K和7.84（7）K时出现。一个较低的内部磁场观察到钙$_5$铱$_3$ O $_{12}$，相比，钙$_4$铱O $_6$反映了存在的Ir $^{4+}$和Ir $^{5+}$离子，导致在一个更磁稀释的结构。μ子旋进仅在Sr $_3$ Ir $_2$ O $_7$中在有限的温度范围内观察到，而莫特绝缘体Sr $_2$ IrO $_4$显示出更复杂的行为，$μ$ SR信号包含一个低于$T_{\rm N}=230$ I，K的单个良好分辨的旋进信号，该信号在低温下分裂成两个旋进信号，随后自旋在有序状态下重新取向。
The mixed-valent iridate Ba3InIr2O9 has been discussed as a promising candidate for quantum spin-liquid behavior. The compound exhibits Ir$^{4.5+}$ ions in face-sharing IrO6 octahedra forming Ir2O9 dimers with three t2g holes per dimer. Our results establish Ba3InIr2O9 as a cluster Mott insulator. Strong intra-dimer hopping delocalizes the three t2g holes in quasi-molecular dimer states while inter-dimer charge fluctuations are suppressed by Coulomb repulsion. The magnetism of Ba3InIr2O9 emerges from spin-orbit entangled quasi-molecular moments with yet unexplored interactions, opening up a ne…
Electron-hole asymmetry is a fundamental property in solids that can determine the nature of quantum phase transitions and the regime of operation for devices. The observation of electron-hole asymmetry in graphene and recently in the phase diagram of bilayer graphene has spurred interest into whether it stems from disorder or from fundamental interactions such as correlations. Here, we report an effective new way to access electron-hole asymmetry in 2D materials by directly measuring the quasiparticle self-energy in graphene/Boron Nitride field effect devices. As the chemical potential moves …
