Novel Jeff=1/2 Mott state induced by relativistic spin-orbit coupling in Sr2IrO4

Metal-insulator transitions in pyrochlore oxides

Pyrochlore oxides with chemical formula ofA2B2O7exhibit a diverse range of electronic properties as a representative family of quantum materials. These properties mostly stem from strong electron correlations at the transition metalBsite and typical geometrical frustration effects on the pyrochlore lattice. Furthermore, the coupling between the magnetic moments of the rare-earthAsite and the conduction electrons at theBsite, along with the relativistic spin-orbit coupling particularly affecting the 4d/5delectrons at theBsite, gives rise to the topological characteristics of the correlated electrons. This review paper focuses on the metal-insulator transitions in pyrochlore oxides as evidence of the strong electron correlation, which is highlighted as a rich source of intriguing charge dynamics coupled with frustrated spin-orbital entangled magnetism.

Magnetic properties and electronic structure ofJeff=12square lattice quantum magnet Bi2ErO4Cl

Two-dimensional (2D) rare-earth-based square lattice (SL) quantum magnets provide a pathway to achieve distinctive ground states characterized by unusual excitations. We investigate the magnetic, heat capacity, structural, and electronic properties of a magnetic system Bi2ErO4Cl. This compound features a structurally ideal 2D SL composed of Er3+rare-earth magnetic ions. The single-phase polycrystalline sample was synthesized using hydrothermal, followed by a vacuum-sealed tube technique. The analysis of heat capacity and magnetic data indicates that the Er3+ion adopts aJeff=12state at low temperatures. Fitting the Curie-Weiss (CW) law to the low-temperature magnetic susceptibility data reveals a CW temperature of approximately -2.1 K, suggesting antiferromagnetic (AFM) interactions between the Er3+moments. Our first-principles calculations validate a 2D spin model relevant to the titled Er compound. The presence of AFM interaction between the Er3+ions is further confirmed using total energy calculations (DFT+U), aligning with the experimental results. The heat capacity measurements reveal the presence of magnetic long-range order belowTN= 0.47 K. The magnetic heat capacity data followsT1.8power law dependence belowTN.

A primer on Kitaev model: basic aspects, material realization, and recent experiments

This elementary review article is aimed to the beginning graduate students interested to know basic aspects of Kitaev model. We begin with a very lucid introduction of Kitaev model and present its exact solution, Hilbert space structure, fractionalization, spin-spin correlation function and topological degeneracy in an elementary way. We then discuss the recent proposal of realizing Kitaev interaction in certain materials. Finally we present some recent experiments done on these materials, mainly magnetization, susceptibility, specific heat and thermal Hall effect to elucidate the recent status of material realization of coveted Kitaev spin-liquid phase. We end with a brief discussion on other theoretical works on Kitaev model from different many-body aspects.

Liquid-like spin dynamics in a hybrid Heisenberg-Ising antiferromagnet

Two-dimensional (2D) materials and their heterostructures enable unconventional electronic properties and functionalities not accessible in their bulk counterparts. This approach is now being extended to magnetic materials to engineer their spin structures and magnetic fields produced by them. However, spin dynamics of 2D magnetic heterostructures remain largely unexplored. Here, we demonstrate that heterointerfacing Heisenberg square-lattice antiferromagnet (AF) Sr2IrO4 with its bilayer variant Ising AF Sr3Ir2O7 in a superlattice leads to liquid-like spin dynamics in the former, characterized by slow recovery of the AF order after its transient suppression by an optical pump, and complete absence of spin waves except in an immediate vicinity of the ordering wavevector. Instead, the spin excitation spectra are dominated by isotropic continua, which in previous works have been interpreted as fractional spin excitations, or spinons, that extends to unprecedentedly low energies. Thus, our results provide a pathway to frustrated magnetism in square lattices by heterointerfacing two distinct types of AFs.

Shot Noise in a Metal Close to the Mott Transition

SrIrO3 is a metallic complex oxide with unusual electronic and magnetic properties believed to originate from electron correlations due to its proximity to the Mott metal-insulator transition. However, the nature of its electronic state and the mechanism of metallic conduction remain poorly understood. We demonstrate that the shot noise produced by nanoscale SrIrO3 junctions is strongly suppressed, inconsistent with diffusive quasiparticle transport. Analysis of thermal effects and scaling with the junction length reveals that conduction is mediated by collective hopping of electrons almost localized by correlations. Our results provide insight into the non-Fermi liquid state close to the Mott transition and advance shot noise measurements as a powerful technique for the study of quantum materials.

Room-Temperature Possible Current-Induced Transition in Ca2RuO4 Thin Films Grown Through Intercalation-Like Cation Diffusion in the A2BO4 Ruddlesden-Popper Structure

Cation deficiency tuning is a central issue in thin-film epitaxy of functional metal oxides, as it is typically more difficult than anion deficiency tuning, as anions can be readily supplied from gas sources. Here, highly effective internal deficiency compensation of Ru cations is demonstrated for Ca2RuO4 epitaxial films based on diffusive transfer of metal cations in the A2BO4 Ruddlesden-Popper lattice from solid-phase cation sources. Through detailed structural characterization of Ca2RuO4/LaAlO3 (001) thin films grown with external cation sources by solid-phase epitaxy, the occurrence of intercalation-like, interstitial diffusion of La cations (from the substrates) in the A2BO4 structure is revealed, and that of Ru cations is also suggested. Relying on the interstitial-type diffusion, an optimized Ru deficiency compensation method, which does not induce the formation of Can +1RunO3 n +1 Ruddlesden-Popper impurity phases with higher n, is proposed for Ca2RuO4 epitaxial films. In the Ca2RuO4/LaAlO3 (001) thin films grown with Ru deficiency compensation, record-high resistivity values (102-10-1 Ω cm) and a large (more than 200 K) increase in the temperature range of the nonlinear transport properties are demonstrated by transport measurements, demonstrating the possible advantages of this method in the control of the current-induced quantum phase transition of Ca2RuO4.

Evolution of structural, magnetic and transport properties of 3d-5dbased double perovskites Nd2-xSrxCoIrO6

The evolution of the structural, magnetic and transport properties of the intermediate compounds Nd2-xSrxCoIrO6withx= 0.2, 0.4, 0.6, 1 and 1.5 have been studied to establish important roles of sizes and oxidation states of cations on various phases. The replacement of Nd3+by Sr2+primarily influences the oxidation states of Co (Co2+→Co3+) and Ir (Ir4+→Ir5+) ions to maintain the charge neutrality in the entire system. The Sr dopants give rise to an increasing Co/Ir antisite disorder (ASD) to accommodate the variation of charge state and ionic radius of Co and Ir. The nature of magnetic interaction induced by Sr changes from being a ferrimagnetic (FIM) to a more dominant antiferromagnetic. The suppression of the second magnetic transition below 30 K in samples forx>0.2 is entirely due to dilution of the Nd-Nd magnetic interaction. The combined effects of ASD and mixed oxidation state of Co and Ir ions generate various types of magnetic exchange pathways and create competitive magnetic interactions to stabilize a particular magnetic ground state. In the middle compound NdSrCoIrO6, a Griffith like phase in the temperature region 65-150 K and exchange bias field of 658 Oe at 2.3 K under a cooling field of 50 kOe has been observed. The compounds show an insulating kind of behaviour, and with hole doping the value of room temperature resistivity drastically decreases. The nature of conduction is found to follow three dimensional Mott's variable range hopping process.

Flexomagnetoelectric Effect in Sr_{2}IrO_{4} Thin Films

Symmetry engineering is explicitly effective to manipulate and even create phases and orderings in strongly correlated materials. Flexural stress is universally practical to break the space-inversion or time-reversal symmetry. Here, by introducing strain gradient in a centrosymmetric antiferromagnet Sr_{2}IrO_{4}, the space-inversion symmetry is broken accompanying a nonequivalent O p-Ir d orbital hybridization along the z axis. Thus, an emergent polar phase and out-of-plane magnetic moment have been simultaneously observed in these asymmetric Sr_{2}IrO_{4} thin films, which both are absent in its ground state. Furthermore, upon the application of a magnetic field, such polarization can be controlled by modifying the occupied d orbitals through spin-orbit interaction, giving rise to a flexomagnetoelectric effect. This Letter provides a general strategy to artificially design multiple symmetries and ferroic orderings in strongly correlated systems.

Spin-orbit entanglement driven by the Jahn-Teller effect

Spin-orbit entanglement in 4d and 5d transition metal systems can enhance electronic correlations, leading to nontrivial ground states and the emergence of exotic excitations. There is also an interest to investigate spin-orbit entanglement in 3d compounds, though this is challenging due to their smaller spin-orbit coupling. Here we demonstrate that the Jahn-Teller effect in Mn3+ reduces the energy gap between high- and low- spin-orbital states that lead to enhanced spin-orbit entanglement. Our results show a rare example of synergistic effects of Jahn-Teller and spin-orbit interactions and provide a way to entangle different degrees of freedom in d-metal oxides, which may allow paths to explore the interplay of orbital, lattice and spins in 3d correlated systems.

A complex magnetic study: evidence of spin-glass transition below long-range magnetic ordering and its correlation with renormalization of phonon modes

In this paper, we report a complex magnetic behavior arising due to the interplay of three active magnetic cations (Nd/Sm, Co and Ir), forming 3d-5d-4f magnetic sublattices. The B-site ordered double perovskites Nd2CoIrO6and Sm2CoIrO6were successfully prepared by conventional solid-state method. Detailed structural analysis revealed that both samples crystallized in monoclinic structure with P21/n (No.-14) space group. X-ray photoelectron spectroscopy analysis confirms the presence of multiple oxidation states of Ir (i.e. Ir4+and Ir5+). Both samples show a paramagnetic-ferrimagnetic (TFiM) transition around 100 K, additionally, a low temperature transition is observed at around 10 K in the SCIO sample. This FM-like behavior of the samples is attributed to the antiparallel alignment of the Co2+and Ir4+spins, resulting in FiM ordering. The ac susceptibility analysis confirms a spin glass type transition just below the long-range ordering temperature in NCIO sample, The obtained characteristic flipping time of a single spin from both the law (Power law and Vogel-Fulcher law) and nonzero Vogel-Fulcher temperature (T0≃ 89.1 K) further verified the existence of cluster spin-glass behavior below the ordering temperature. The temperature evolution of phonon modes (up to 4 K) suggests that the phonon mode above the magnetic ordering temperature is mainly governed by the lattice degrees of freedom; notable renormalization of the mode frequency below the ordering temperature is due to the coupling of lattice with the underlying magnetic degrees of freedom.

Origin of Magnetism in a Supposedly Nonmagnetic Osmium Oxide

A supposedly nonmagnetic 5d^{1} double perovskite oxide is investigated by a combination of spectroscopic and theoretical methods, namely, resonant inelastic x-ray scattering, x-ray absorption spectroscopy, magnetic circular dichroism, and multiplet ligand-field calculations. We found that the large spin-orbit coupling admixes the 5d t_{2g} and e_{g} orbitals, covalency raises the 5d population well above the nominal value, and the local symmetry is lower than O_{h}. The obtained electronic interactions account for the finite magnetic moment of Os in this compound and, in general, of 5d^{1} ions. Our results provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong electron-lattice coupling.

Evidence for electron-hole crystals in a Mott insulator

The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron-hole crystals in a doped Mott insulator, namely, α-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru-Ru bonds, respectively. Moreover, a gate-induced transition of electron-hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron-hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials.

Disordered magnetic ground state in a quasi-1-Dd4columnar iridate Sr3LiIrO6

Spin-orbit coupling offers a large variety of novel and extraordinary magnetic and electronic properties in otherwise 'ordinary pool' of heavy ion oxides. Here we present a detailed study on an apparently isolated hexagonal 2Hspin-chaind4iridate Sr3LiIrO6with geometric frustration. Our structural studies reveal Li-Ir chemical order with desired stoichiometry in this compound, while x-ray absorption together with x-ray photoemission spectroscopic characterizations establish pure 5+ valence of Ir. We have established a magnetic ground state with finite Ir5+magnetic moments in this compound, contrary to the anticipated nonmagneticJeff= 0 state, through combined dc susceptibility,7Li nuclear magnetic resonance (NMR), muon spin relaxation (µSR) andab-initioelectronic structure studies. These investigations together with ac magnetic susceptibility and specific heat measurements reveal that despite having noticeable antiferromagnetic correlation among the Ir5+local moments, this system does not magnetically order down to at least 0.05 K, possibly due to geometrical exchange frustration, arising from the comparable nearest- and next-nearest-neighbor interchain Ir-O-O-Ir superexchange interaction strengths with opposite signs. However, the zero-fieldµSR analysis shows emergence of a considerable proportion of spin-freezing on top of a spin-fluctuating dynamic magnetic background down to the lowest measured temperature of 1.7 K, possibly due to some inhomogeneity and/or the much stronger intra-column Ir-Ir magnetic exchange interaction strength relative to the inter-column Ir-Ir ones. The linear temperature dependence of the magnetic specific heat (Cm) in both zero and applied magnetic fields, plus the power-law behavior of the NMR spin-lattice relaxation rate suggest a gapless spinon density of states in this charge gapped disordered magnetic ground state of Sr3LiIrO6.

Light-induced insulator-metal transition in Sr2IrO4 reveals the nature of the insulating ground state

Sr2IrO4 has attracted considerable attention due to its structural and electronic similarities to La2CuO4, the parent compound of high-Tc superconducting cuprates. It was proposed as a strong spin-orbit-coupled Jeff = 1/2 Mott insulator, but the Mott nature of its insulating ground state has not been conclusively established. Here, we use ultrafast laser pulses to realize an insulator-metal transition in Sr2IrO4 and probe the resulting dynamics using time- and angle-resolved photoemission spectroscopy. We observe a gap closure and the formation of weakly renormalized electronic bands in the gap region. Comparing these observations to the expected temperature and doping evolution of Mott gaps and Hubbard bands provides clear evidence that the insulating state does not originate from Mott correlations. We instead propose a correlated band insulator picture, where antiferromagnetic correlations play a key role in the gap opening. More broadly, our results demonstrate that energy-momentum-resolved nonequilibrium dynamics can be used to clarify the nature of equilibrium states in correlated materials.

Re/Ir@Os-doping induced insulator-to-metal transition in Mott-insulator Ca2FeOsO6: octahedral distortion effects

Regardless of several investigations to elucidate the ground state of the strongly correlated electron systems in doped Mott-insulators (MIs), the origin of the doping-induced insulator-to-metal transition (IMT) remains a crucial and debatable subject in solid-state physics. Herein, we explore the consequences of Re/Ir-doping at the Os-site (Re/Ir@Os), on the physical properties of the MI ferrimagnetic (FiM) Ca2FeOsO6 double perovskite oxides using density functional theory calculations. The doped structures' solidity is analyzed by computing the defect formation energies in terms of the dopant-rich situation, which confirms their growth credibility at ambient conditions along with mechanical and dynamical stabilities. Various FiM spin-ordering is taken into account in the doped structures to analyze the magnetic ground state, which is FiM-I/FiM-II in the Re/Ir@Os-doped system. Remarkably, an IMT is predicted in the Re/Ir@Os-doped structures, which is due to the admixture of the partially occupied 5d orbitals of these ions. The calculated partial spin magnetic moments (ms) of +4.12, -1.58, -0.75 and +0.88μB on the Fe, Os, Re, and Ir ions, endorse the +3, +5, +5, and +4 states having electronic configurations of t32g↑t02g↓e2g↑e0g↓, t32g↑t02g↓e0g↑e0g↓, t22g↑t02g↓e0g↑e0g↓, t32g↑t22g↓e0g↑e0g↓, respectively. The "+" and "-" signs on the ions ms values, lead the systems into various FiM magnetic ordering. Moreover, the estimated Curie temperature (TC) using the Heisenberg model in the pristine structure is 334 K, which is close to the experimentally observed value of 320 K along with a colossal uniaxial magneto crystalline anisotropy energy constant (K) of 2.95 × 107 erg cm-3 having the easy magnetic axis of the ac-plane ([101]). It is established that TC/K reduces and enhances to 298 K/1.33 × 107 erg cm-3 and 365 K/4.71 × 107 erg cm-3 for the Re@Os and Ir@Os-doped motif due to an increase and decrease in the octahedral distortions compared to that of the pristine system, respectively.

Exotic Magnetism in Perovskite KOsO_{3}

A new perovskite KOsO_{3} has been stabilized under high-pressure and high-temperature conditions. It is cubic at 500 K (Pm-3m) and undergoes subsequent phase transitions to tetragonal at 320 K (P4/mmm) and rhombohedral (R-3m) at 230 K as shown from refining synchrotron x-ray powder diffraction (SXRD) data. The larger orbital overlap integral and the extended wave function of 5d electrons in the perovskite KOsO_{3} allow to explore physics from the regime where Mott and Hund's rule couplings dominate to the state where the multiple interactions are on equal footing. We demonstrate an exotic magnetic ordering phase found by neutron powder diffraction along with physical properties via a suite of measurements including magnetic and transport properties, differential scanning calorimetry, and specific heat, which provide comprehensive information for a system at the crossover from localized to itinerant electronic behavior.

Probing the Magnetic Ground State of Ba2YIrO6: Impact of Nonmagnetic Dopants and Spin-Orbit Coupling

Strong spin-orbit coupling (SOC) in iridates has long been predicted to lead to exotic electronic and magnetic ground states. Ba2YIrO6 (BYIO) has attracted particular attention due to the expectation of a Jeff = 0 state for Ir5+ ions under the jj-coupling scheme. However, controversies surround the actual realization of this state, as finite magnetic moments are consistently observed experimentally. We present a multi-physics study of this system by progressively introducing nonmagnetic Sb5+ ions in place of Ir5+ (Ba2YIr​1-ySb​yO​6, BYISO). Despite similar charge and ionic radii, Sb​5+ doping appears highly inhomogeneous, coexisting with a fraction of nearly pure BYIO regions, as confirmed by X-ray diffraction (XRD). This aligns with observations in related compounds. While inhomogeneity creates uncertainty, the doped majority phases offer valuable insights. It is relevant that the inclusion of even small amounts of Sb​5+ (10-20%) leads to a rise in magnetization. This strengthens our previous suggestion that magnetic Ir ions form dynamic singlets in BYIO, resulting in a near-nonmagnetic background. The observed moment enhancement with nonmagnetic doping supports the breakdown of these singlets. Furthermore, the magnetization steadily increases with an increasing Sb​5+ content, contradicting the anticipated approach towards the Jeff = 0 state with increased SOC due to reduced hopping between Ir​5+ ions. This reinforces the presence of individual Ir​5+ moments. Overall, our findings suggest that Ba​2YIrO​6 might not possess sufficiently strong SOC to be solely described within the jj-coupling picture, paving the way for further investigation.

Recent progress on topological semimetal IrO2: electronic structures, synthesis, and transport properties

5dtransition metal oxides, such as iridates, have attracted significant interest in condensed matter physics throughout the past decade owing to their fascinating physical properties that arise from intrinsically strong spin-orbit coupling (SOC) and its interplay with other interactions of comparable energy scales. Among the rich family of iridates, iridium dioxide (IrO2), a simple binary compound long known as a promising catalyst for water splitting, has recently been demonstrated to possess novel topological states and exotic transport properties. The strong SOC and the nonsymmorphic symmetry that IrO2possesses introduce symmetry-protected Dirac nodal lines (DNLs) within its band structure as well as a large spin Hall effect in the transport. Here, we review recent advances pertaining to the study of this unique SOC oxide, with an emphasis on the understanding of the topological electronic structures, syntheses of high crystalline quality nanostructures, and experimental measurements of its fundamental transport properties. In particular, the theoretical origin of the presence of the fourfold degenerate DNLs in band structure and its implications in the angle-resolved photoemission spectroscopy measurement and in the spin Hall effect are discussed. We further introduce a variety of synthesis techniques to achieve IrO2nanostructures, such as epitaxial thin films and single crystalline nanowires, with the goal of understanding the roles that each key parameter plays in the growth process. Finally, we review the electrical, spin, and thermal transport studies. The transport properties under variable temperatures and magnetic fields reveal themselves to be uniquely sensitive and modifiable by strain, dimensionality (bulk, thin film, nanowire), quantum confinement, film texture, and disorder. The sensitivity, stemming from the competing energy scales of SOC, disorder, and other interactions, enables the creation of a variety of intriguing quantum states of matter.

Spin-Orbit Excitons in a Correlated Metal: Raman Scattering Study of Sr_{2}RhO_{4}

Using Raman spectroscopy to study the correlated 4d-electron metal Sr_{2}RhO_{4}, we observe pronounced excitations at 220 meV and 240 meV with A_{1g} and B_{1g} symmetries, respectively. We identify them as transitions between the spin-orbit multiplets of the Rh ions, in close analogy to the spin-orbit excitons in the Mott insulators Sr_{2}IrO_{4} and α-RuCl_{3}. This observation provides direct evidence for the unquenched spin-orbit coupling in Sr_{2}RhO_{4}. A quantitative analysis of the data reveals that the tetragonal crystal field Δ in Sr_{2}RhO_{4} has a sign opposite to that in insulating Sr_{2}IrO_{4}, which enhances the planar xy orbital character of the effective J=1/2 wave function. This supports a metallic ground state, and suggests that c-axis compression of Sr_{2}RhO_{4} may transform it into a quasi-two-dimensional antiferromagnetic insulator.

Pinalites: Optical Properties and Quantum Magnetism of Heteroanionic A3MO5X2 Compounds

Heteroanionic compounds, which contain two or more types of anions, have emerged as a promising class of materials with diverse properties and functionalities. In this paper, I review the experimental findings on Ca3ReO5Cl2 and related compounds that exhibit remarkable pleochroism and novel quantum magnetism. I discuss how the heteroanionic coordination affects the optical and magnetic properties by modulating the d-orbital states of the transition metal ions. Subsequently, I compare these materials with other heteroanionic and monoanionic compounds and highlight the potential of A3MO5X2 materials for future exploration of materials and phenomena.

Continuous strain tuning of oxygen evolution catalysts with anisotropic thermal expansion

Compressive strain, downshifting the d-band center of transition metal oxides, is an effective way to accelerate the sluggish kinetics of oxygen evolution reaction (OER) for water electrolysis. Here, we find that anisotropic thermal expansion can produce compressive strains of the IrO6 octahedron in Sr2IrO4 catalyst, thus downshifting its d-band center. Different from the previous strategies to create constant strains in the crystals, the thermal-triggered compressive strains can be real-timely tuned by varying temperature. As a result of the thermal strain accelerating OER kinetics, the Sr2IrO4 exhibits the nonlinear lnjo - T-1 (jo, exchange current density; T, absolute temperature) Arrhenius relationship, resulting from the thermally induced low-barrier electron transfer in the presence of thermal compressive strains. Our results verify that the thermal field can be utilized to manipulate the electronic states of Sr2IrO4 via thermal compressive strains downshifting the d-band center, significantly accelerating the OER kinetics, beyond the traditional thermal diffusion effects.

Emergence of the isotropic Kitaev honeycomb latticeα-RuCl3and its magnetic properties

We present a comprehensive investigation of the crystal and magnetic structures of the van der Waals antiferromagnetα-RuCl3using single crystal x-ray and neutron diffraction. The crystal structure at room temperature is a monoclinic (C2/m). However, with decreasing temperature, a remarkable first-order structural phase transition is observed, leading to the emergence of a rhombohedral (R3-) structure characterized by three-fold rotational symmetry forming an isotropic honeycomb lattice. On further cooling, a zigzag-type antiferromagnetic order develops belowTN=6∼6.6K. The critical exponent of the magnetic order parameter was determined to beβ=0.11(1), which is close to the two-dimensional Ising model. Additionally, the angular dependence of the magnetic critical field of the zigzag antiferromagnetic order for the polarized ferromagnetic phase reveals a six-fold rotational symmetry within theab-plane. These findingsreflect the symmetry associated with the Ising-like bond-dependent Kitaev spin interactions and underscore the universality of the Kitaev interaction-dominated antiferromagnetic system.

Quantum spin nematic phase in a square-lattice iridate

Spin nematic is a magnetic analogue of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid1,2. Particularly intriguing is a valence-bond spin nematic3-5, in which spins are quantum entangled to form a multipolar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here we establish a spin nematic phase in the square-lattice iridate Sr2IrO4, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet in the strong spin-orbit coupling limit6-9. Upon cooling, the transition into the spin nematic phase at TC ≈ 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic phase below TN ≈ 230 K and becomes directly observable through its interference with the antiferromagnetic order in resonant X-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic X-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a many-body quantum entanglement in the antiferromagnetic state10,11. Taken together, our results reveal a quantum order underlying the Néel antiferromagnet that is widely believed to be intimately connected to the mechanism of high-temperature superconductivity12,13.

Structural dimerization and charge-orbital ordering in a ferromagnetic semiconductor LiV2S4 monolayer

With the rise of two-dimensional (2D) materials, unique properties that are completely distinct from bulk counterparts continue to emerge at low-dimensional scales, presenting numerous opportunities and challenges. It also provides a new perspective for the study of transition metal systems. Here, based on density functional theory (DFT), the physical properties of 2D monolayer LiV2S4 have been studied. Remarkable changes have been observed, i.e., vanadium dimerization, ferromagnetism, charge distribution and metal-insulator transition (MIT). It is argued that the electronic instability leads to the V dimerization, which further lifts the degeneracy of charge distribution and stabilizes the charge and spin ordering state.

Electronic Structures of Kitaev Magnet Candidates RuCl3 and RuI3

Layered honeycomb magnets with strong atomic spin-orbit coupling at transition metal sites have been intensively studied for the search of Kitaev magnetism and the resulting non-Abelian braiding statistics. α-RuCl3 has been the most promising candidate, and there have been several reports on the realization of sibling compounds α-RuBr3 and α-RuI3 with the same crystal structure. Here, we investigate correlated electronic structures of α-RuCl3 and α-RuI3 by employing first-principles dynamical mean-field theory. Our result provides a valuable insight into the discrepancy between experimental and theoretical reports on transport properties of α-RuI3, and suggests a potential realization of correlated flat bands with strong spin-orbit coupling and a quantum spin-Hall insulating phase in α-RuI3.

Epitaxy and transport properties of alkali-earth palladate thin films

Topological insulators and semimetals are an interesting class of materials for new electronic and optical applications owing to their characteristic electromagnetic responses originating from the spin-orbit coupled band structures. However, topological electronic structures are rare in oxide materials despite their chemical stability being preferable for applications. In this study, given the theoretical prediction of Dirac bands in CaPd3O4, we investigate the fabrication and transport properties of SrPd3O4 and CaPd3O4 thin films as candidates of oxide Dirac semimetals. We have found that these materials are epitaxially grown on MgO (100) substrate under limited growth conditions by pulsed laser deposition. The transport properties show a weak temperature dependence, suggestive of narrow-gap properties, although unintentionally doped holes hinder us from revealing the presence of the Dirac band. Our study establishes the basic thermodynamics of thin-film fabrication of these materials and will lead to interesting properties characteristic of topological band structure by modulating the electronic structure by, for example, chemical substitutions or pressure.

Coexistence of spin liquid state and magnetic correlations in 3d-5dbased triangular-lattice antiferromagnet Sr3CuIr2O9

Here, we report detailed lattice structure, magnetization (dc and ac) and specific heat measurements on a 3d-5dbased new triple-perovskite material Sr3CuIr2O9. The Sr/Cu forms a layered structure of triangular-lattice while the Ir forms Ir2O9dimers which lie in chain as well as simultaneously makes layered triangular-lattice with neighboring atoms. Due to random site-sharing with Sr2+, the Cu2+(3d9, spin-1/2) forms a diluted magnetic lattice, thus giving a disordered in-plane exchange interaction. Opposed to conventionalJeffmodel, the Ir5+(5d4,Jeff= 0) is believed to be magnetic here which participates both in-chain and in-plane magnetic interactions. This complex lattice structure driven competing exchange interaction leads the ground state to a gapless quantum-spin-liquid state which coexists with (weak) ferromagnetic spin correlations. While underling the importance of spin state (spin-1/2), we believe that the combined effect of lattice structure, geometric frustration, spin-orbit coupling and spin state has given rise this interesting ground state in this material.

Multiorbital Nature of Doped Sr_{2}IrO_{4}

The low-energy j_{eff}=1/2 band of Sr_{2}IrO_{4} bears stark resemblances with the x^{2}-y^{2} band of La_{2}CuO_{4}, and yet no superconductivity has been found so far by doping Sr_{2}IrO_{4}. Behind such a behavior could be inherent failures of the j_{eff}=1/2 picture, in particular when electrons or holes are introduced in the IrO_{2} planes. In view of this, here we reanalyze the j_{eff}=1/2 scenario. By using the local-density approximation plus dynamical mean-field theory approach, we show that the form of the effective j_{eff}=1/2 state is surprisingly stable upon doping. This supports the j_{eff}=1/2 picture. We show that, nevertheless, Sr_{2}IrO_{4} remains in essence a multiorbital system: The hybridization with the j_{eff}=3/2 orbitals sizably reduces the Mott gap by enhancing orbital degeneracy, and part of the holes go into the j_{eff}=3/2 channels. These effects cannot be reproduced by a simple effective screened Coulomb repulsion. In the optical conductivity spectra, multiorbital processes involving the j_{eff}=3/2 states contribute both to the Drude peak and to relatively low-energy features.

Observation of Mott instability at the valence transition of f-electron system

Mott physics plays a critical role in materials with strong electronic correlations. Mott insulator-to-metal transition can be driven by chemical doping, external pressure, temperature and gate voltage, which is often seen in transition metal oxides with 3d electrons near the Fermi energy (e.g. cuprate superconductor). In 4f-electron systems, however, the insulator-to-metal transition is mostly driven by Kondo hybridization and the Mott physics has rarely been explored in experiments. Here, by combining the angle-resolved photoemission spectroscopy and strongly correlated band structure calculations, we show that an unusual Mott instability exists in YbInCu₄ accompanying its mysterious first-order valence transition. This contrasts with the prevalent Kondo picture and demonstrates that YbInCu₄ is a unique platform to explore the Mott physics in Kondo lattice systems. Our work provides important insight for the understanding and manipulation of correlated quantum phenomena in the f-electron system.

Emergent quantum phenomena in atomically engineered iridate heterostructures

Over the last few years, researches in iridates have developed into an exciting field with the discovery of numerous emergent phenomena, interesting physics, and intriguing functionalities. Among the studies, iridate-based artificial structures play a crucial role owing to their extreme flexibility and tunability in lattice symmetry, chemical composition, and crystal dimensionality. In this article, we present an overview of the recent progress regarding iridate-based artificial structures. We first explicitly introduce several essential concepts in iridates. Then, we illustrate important findings on representative SrIrO3/SrTiO3 superlattices, heterostructures comprised of SrIrO3 and magnetic oxides, and their response to external electric-field stimuli. Finally, we comment on existing problems and promising future directions in this exciting field.

In-gap states and strain-tuned band convergence in layered structure trivalent iridate K0.75Na0.25IrO2

Iridium oxides (iridates) provide a good platform to study the delicate interplay between spin-orbit coupling (SOC) interactions, electron correlation effects, Hund's coupling and lattice degrees of freedom. An overwhelming number of investigations primarily focus on tetravalent (Ir⁴⁺, 5d⁵) and pentavalent (Ir⁵⁺, 5d⁴) iridates, and far less attention has been paid to iridates with other valence states. Here, we pay our attention to a less-explored trivalent (Ir³⁺, 5d⁶) iridate, K_0.75Na_0.25IrO₂, crystallizing in a triangular lattice with edge-sharing IrO₆ octahedra and alkali metal ion intercalated [IrO₂]^- layers, offering a good platform to explore the interplay between different degrees of freedom. We theoretically determine the preferred occupied positions of the alkali metal ions from energetic viewpoints and reproduce the experimentally observed semiconducting behavior and nonmagnetic (NM) properties of K_0.75Na_0.25IrO₂. The SOC interactions play a critical role in the band dispersion, resulting in NM J_eff = 0 states. More intriguingly, our electronic structure not only uncovers the presence of intrinsic in-gap states and nearly free electron character for the conduction band minimum, but also explains the abnormally low activation energy in K_0.75Na_0.25IrO₂. Particularly, the band edge can be effectively modulated by mechanical strain, and the in-gap states feature enhanced band-convergence characteristics by 6% compressive strain, which will greatly enhance the electrical conductivity of K_0.75Na_0.25IrO₂. The present work sheds new light on the unconventional electronic structures of trivalent iridates, indicating their promising application as a nanoelectronic and thermoelectric material, which will attract extensive interest and stimulate experimental works to further understand the unprecedented electronic structures and exploit potential applications of the triangular trivalent iridate.

Non-Centrosymmetric Sr2IrO4 Obtained Under High Pressure

Sr₂IrO₄ with strong spin-orbit coupling and Hubbard repulsion (U) hosts Mott insulating states. The similar crystal structure and magnetic and electronic properties, particularly the d-wave gap observed in Sr₂IrO₄ enhanced the analogies to the cuprate high-T_c superconductor, La₂CuO₄. The incomplete analogy was due to the lack of broken inversion symmetry phases observed in Sr₂IrO₄. Here, under high-pressure and high-temperature conditions, we report a noncentrosymmetric Sr₂IrO₄. The crystal structure and its noncentrosymmetric character were determined by single-crystal X-ray diffraction and high-resolution scanning transmission electron microscopy. The magnetic characterization confirms the Ir⁴⁺ with S = 1/2 at low temperature in Sr₂IrO₄ with magnetic ordering occurring at around 86 K, where a larger moment is observed than the ambient pressure Sr₂IrO₄. Moreover, the resistivity measurement shows three-dimensional Mott variable-range hopping (VRH) existed in the system. This noncentrosymmetric Sr₂IrO₄ phase appears to be a unique material that offers a further understanding of high-T_c superconductivity.

A combined study on structural, magnetic and specific heat on double perovskite iridates Ln2CoIrO6[Ln = Pr, Nd]

We report rich magnetic behavior for Co-Ir based double perovskites consisting of different rare earth cations Pr and Nd: Pr₂CoIrO₆(PCIO) and Nd₂CoIrO₆(NCIO). Both oxides show an antisite disorder of 10% and a ferrimagnetic transition,TFiMaround 96 K and 98 K respectively. The long range magnetic ordering is arising from the canted antiferromagnetic ordering between the Co²⁺and Ir⁴⁺ions. A prominent peak around 27 K in magnetization data of NCIO indicates that the total moment of Nd ion is antiferromagnetically coupled to the Co-Ir sublattice. The long range order of the Nd sublattice is corroborated by the evidence of an anomaly in specific heat at very low temperature. The compounds exhibit a maximum change of magnetic entropy of 0.57 (0.48) J kg.K^-1atTFiMin a magnetic field of 5 T. The strong spin-orbit coupling in 5dstates of Ir and cation disorder lead to the Mott insulating phase as found from the analysis of temperature dependent resistivity. These unique behaviors suggest an interesting interplay between localized Pr/Nd-4f, itinerant Co-3dand Ir-5delectrons.

Enhanced Electron Correlation and Significantly Suppressed Thermal Conductivity in Dirac Nodal-Line Metal Nanowires by Chemical Doping

Enhancing electron correlation in a weakly interacting topological system has great potential to promote correlated topological states of matter with extraordinary quantum properties. Here, the enhancement of electron correlation in a prototypical topological metal, namely iridium dioxide (IrO₂ ), via doping with 3d transition metal vanadium is demonstrated. Single-crystalline vanadium-doped IrO₂ nanowires are synthesized through chemical vapor deposition where the nanowire yield and morphology are improved by creating rough surfaces on substrates. Vanadium doping leads to a dramatic decrease in Raman intensity without notable peak broadening, signifying the enhancement of electron correlation. The enhanced electron correlation is further evidenced by transport studies where the electrical resistivity is greatly increased and follows an unusual T $\sqrt T $ dependence on the temperature (T). The lattice thermal conductivity is suppressed by an order of magnitude via doping even at room temperature where phonon-impurity scattering becomes less important. Density functional theory calculations suggest that the remarkable reduction of thermal conductivity arises from the complex phonon dispersion and reduced energy gap between phonon branches, which greatly enhances phase space for phonon-phonon Umklapp scattering. This work demonstrates a unique system combining 3d and 5d transition metals in isostructural materials to enrich the system with various types of interactions.

Reconciling Monolayer and Bilayer J_{eff}=1/2 Square Lattices in Hybrid Oxide Superlattice

The number of atomic layers confined in a two-dimensional structure is crucial for the electronic and magnetic properties. Single-layer and bilayer J_{eff}=1/2 square lattices are well-known examples where the presence of the extra layer turns the XY anisotropy to the c-axis anisotropy. We report on experimental realization of a hybrid SrIrO_{3}/SrTiO_{3} superlattice that integrates monolayer and bilayer square lattices in one layered structure. By synchrotron x-ray diffraction, resonant x-ray magnetic scattering, magnetization, and resistivity measurements, we found that the hybrid superlattice exhibits properties that are distinct from both the single-layer and bilayer systems and cannot be explained by a simple addition of them. In particular, the entire hybrid superlattice orders simultaneously through a single antiferromagnetic transition at temperatures similar to the bilayer system but with all the J_{eff}=1/2 moments mainly pointing in the ab plane similar to the single-layer system. The results show that bringing monolayer and bilayer with orthogonal properties in proximity to each other in a hybrid superlattice structure is a powerful way to stabilize a unique state not obtainable in a uniform structure.

Fluorite-related iridate Pr3IrO7: crystal growth, structure, magnetism, thermodynamic, and optical properties

Spin-orbit coupling in heavy 5dmetal oxides, in particular, iridates have received tremendous interest in recent years due to the realization of exotic electronic and magnetic phases. Here, we report the synthesis, structural, magnetic, thermodynamic, and optical properties of the ternary iridate Pr₃IrO₇. Single crystals of Pr₃IrO₇have been grown by the KF flux method. Structural analysis shows that Pr₃IrO₇crystallizes in an orthorhombic phase withCmcmsymmetry. The electron energy loss spectroscopy study indicates that Pr is in a 3+ valence state, which implies a 5+ oxidation state of Ir. Magnetization data measured at high and low magnetic fields do not exhibit any bifurcation betweenM_ZFCandM_FC, however, a weak hump inM(T) is observed atT∗∼10.4 K. The specific heat data reveal two maxima at ∼253 and ∼4.8 K. The optical conductivityσ1(ω)spectrum shows 24 infrared-active phonon modes and reveals an insulating behavior with an optical gapΔOPof size ∼500 meV. During cooling down, the temperature-dependent reflectivity spectrum reveals eight extra phonon modes below the structural phase transition (∼253 K). An anomaly is observed at aroundT∗in the temperature evolution of infrared-active mode frequencies suggesting the presence of significant spin-phonon coupling in the system.

Honeycomb lattice iridate on the verge of Mott-collapse

A new honeycomb lattice iridate (La,Na)IrO₃(≈LaNaIr₂O₆) is successfully synthesized from the spin-orbit coupled Mott insulator Na₂IrO₃by replacing the interlayer Na⁺ions with La³⁺ions. (La,Na)IrO₃shows a finite Sommerfeld term in heat capacity and a -lnTdependence of resistivity, indicating a realization of a metallic state driven by a Mott collapse. Furthermore, crystal structure analysis reveals the formation of Ir zig-zag chains with metal-metal bonding, increasing kinetic energy resulting in the Mott collapse. This observation would be due to a Mott collapse induced in aJ_eff= 1/2 spin-orbit coupling Mott insulator with an Ir honeycomb lattice by topochemical control of the ionic configuration.

Electronic and Magnetic Properties of Spin-Orbit-Entangled Honeycomb Lattice Iridates MIrO3 (M = Cd, Zn, and Mg)

By means of density functional theory calculations with the inclusion of spin-orbit coupling, we present a comprehensive investigation of the structural, electronic, and magnetic properties of the novel series of ilmenite-type honeycomb lattice iridates MIrO₃ (M = Cd, Zn, and Mg), the potential candidates for realizing the quantum spin liquid. Our findings are as follows: (i) the structural relaxations could not properly capture the abnormal thin two-dimensional honeycomb IrO₆ layers in CdIrO₃, making the experimentally proposed crystal structure questionable. Furthermore, the calculations within the experimental structure could not correctly determine the magnetic ground state; however, the results within the optimized one rectify this scenario and provide a precise and reasonable description of its electronic and magnetic properties, which is in good agreement with the experimental observations and that of Zn and Mg analogues. In this regard, we hope that our report will inspire additional studies on this issue and eventually resolve the crystal structure of CdIrO₃. (ii) We identified that the magnetic ground state of this series of iridates MIrO₃ is the zigzag antiferromagnetic ordering, where ferromagnetic zigzag chains are coupling antiferromagnetically across the bridging bonds within a hexagon. (iii) Though it is widely assumed that all the iridates can be well described based on the spin-orbit-assisted J_eff = 1/2 Mott insulating state model, detailed analysis of electronic band structures indicates that the formation of quasimolecular orbitals (QMOs) within a hexagon plays a non-negligible role in appropriately depicting the electronic and magnetic properties. Finally, (iv) we found that all the antiferromagnetic patterns are insulating with finite band gaps. Clarifying the effect of magnetic ordering on the electronic structures is important because it reminds us of potential erroneous identification/prediction of the ground state. The results suggest that precisely determining the magnetic ground state and adopting it in the simulations are imperative for faithfully rendering the electronic properties of a compound. Our results underline the importance of structural factor, spin-orbit coupling, correlation correction, the formation of the QMOs within the hexagon, as well as magnetic ordering in elucidating the electronic structure of a series of ilmenite-type honeycomb lattice iridates MIrO₃.

Synthesis and structural link to the electronic and magneto-transport properties of spin-orbit Mott insulator Sr2IrO4

We investigate the effect of sample preparation conditions on the link between the structural and physical properties of polycrystalline spin-orbit Mott insulator, Sr₂IrO₄. The samples were prepared in two batches. With the first batch prepared as per the commonly adopted procedure in literature and the second batch prepared adopting the same procedure as the first batch but with an additional annealing in vacuum. Interestingly, our results show that without change in the value of the Curie temperature (T_C), there occurs increase in the value of magnetization, resistivity, magneto-resistance (MR) and an increase in temperature range of stabilization of the canted antiferromagnetic structure. The temperature behaviour of the difference in the irreversible magnetization between the samples is in line with the difference in the Ir-O-Ir in-plane bond angle. At low temperatures, the conduction mechanism in the first batch of the sample is mainly governed by disorder while in the case of the other sample it is of Arrhenius type. The magneto-transport results have shown its strong link with the disorder and structural results. Although the nature and mechanism of the disorder needs to be investigated further, the present results throw light on the role of disorder and its connectivity between the structure and physical properties to understand its complex behaviours.

Controllable Emergent Spatial Spin Modulation in Sr_{2}IrO_{4} by In Situ Shear Strain

Symmetric anisotropic interaction can be ferromagnetic and antiferromagnetic at the same time but for different crystallographic axes. We show that the competition of anisotropic interactions of orthogonal irreducible representations can be a general route to obtain new exotic magnetic states. We demonstrate it here by observing the emergence of a continuously tunable 12-layer spatial spin modulation when distorting the square-lattice planes in the quasi-two-dimensional antiferromagnetic Sr_{2}IrO_{4} under in situ shear strain. This translation-symmetry-breaking phase is a result of an unusual strain-activated anisotropic interaction which is at the fourth order and competing with the inherent quadratic anisotropic interaction. Such a mechanism of competing anisotropy is distinct from that among the ferromagnetic, antiferromagnetic, and/or the Dzyaloshinskii-Moriya interactions, and it could be widely applicable and highly controllable in low-dimensional magnets.

Nanoscale Antiferromagnetic Domain Imaging using Full-Field Resonant X-ray Magnetic Diffraction Microscopy

The physical properties of magnetic materials frequently depend not only on the microscopic spin and electronic structures, but also on the structures of mesoscopic length scales that emerge, for instance, from domain formations, or chemical and/or electronic phase separations. However, experimental access to such mesoscopic structures is currently limited, especially for antiferromagnets with net zero magnetization. Here, full-field microscopy and resonant magnetic X-ray diffraction are combined to visualize antiferromagnetic (AF) domains of the spin-orbit Mott insulator Sr₂ IrO₄ with area over ≈0.1 mm² and with spatial resolution as high as ≈150 nm. With the unprecedented wide field of views and high spatial resolution, an intertwining of two AF domains on a length comparable to the measured average AF domain wall width of 545 nm is revealed. This mesoscopic structure comprises a substantial portion of the sample surface, and thus can result in a macroscopic response unexpected from its microscopic magnetic structure. In particular, the symmetry analysis presented in this work shows that the inversion symmetry, which is preserved by the microscopic AF order, becomes ill-defined at the mesoscopic length scale. This result underscores the importance of this novel technique for a thorough understanding of the physical properties of antiferromagnets.

Enhanced magnetism and persistent insulating state in Mn doped Sr2IrO4

The influences of Mn substitution at the Ir site of Sr₂IrO₄are investigated via a comprehensive study of the variation of structural parameters, the transport and magnetic properties of the Sr₂Ir_1-xMn_xO₄samples. The incorporation of Mn leads to an increase of the in-plane Ir-O-Ir bond angle, while it is not sufficient to drive the Mott-insulating state to a metallic state. Interestingly, we find a coexistence of Ir⁴⁺-O^2--Ir⁴⁺super-exchange interaction and Mn³⁺-O^2--Mn⁴⁺double exchange interaction inx⩾ 0.06 samples. The Mn³⁺-O^2--Mn⁴⁺ferromagnetic domains are isolated by the Ir⁴⁺-O^2--Ir⁴⁺antiferromagnetic areas, leading to a severely localized electronic and magnetic states. The electron hopping between the localized states dominates the conductivity of the Sr₂Ir_1-xMn_xO₄samples.

Direct Observation of Strong Anomalous Hall Effect and Proximity-Induced Ferromagnetic State in SrIrO3

The 5d iridium-based transition metal oxides have gained broad interest because of their strong spin-orbit coupling, which favors new or exotic quantum electronic states. On the other hand, they rarely exhibit more mainstream orders like ferromagnetism due to generally weak electron-electron correlation strength. Here, a proximity-induced ferromagnetic (FM) state with T_C  ≈ 100 K and strong magnetocrystalline anisotropy is shown in a SrIrO₃ (SIO) heterostructure via interfacial charge transfer by using a ferromagnetic insulator in contact with SIO. Electrical transport allows to selectively probe the FM state of the SIO layer and the direct observation of a strong, intrinsic, and positive anomalous Hall effect (AHE). For T ≤ 20 K, the AHE displays unusually large coercive and saturation field, a fingerprint of a strong pseudospin-lattice coupling. A Hall angle, σ_xy ^AHE /σ_{xx ,} larger by an order of magnitude than in typical 3d metals and an FM net moment of about 0.1 μ_B /Ir, is reported. This emphasizes how efficiently the nontrivial topological band properties of SIO can be manipulated by structural modifications and the exchange interaction with 3d TMOs.

First demonstration of tuning between the Kitaev and Ising limits in a honeycomb lattice

Recent observations of novel spin-orbit coupled states have generated interest in 4d/5d transition metal systems. A prime example is the [Formula: see text] state in iridate materials and α-RuCl₃ that drives Kitaev interactions. Here, by tuning the competition between spin-orbit interaction (λ_SOC) and trigonal crystal field (Δ_T), we restructure the spin-orbital wave functions into a previously unobserved [Formula: see text] state that drives Ising interactions. This is done via a topochemical reaction that converts Li₂RhO₃ to Ag₃LiRh₂O₆. Using perturbation theory, we present an explicit expression for the [Formula: see text] state in the limit Δ_T ≫ λ_SOC realized in Ag₃LiRh₂O₆, different from the conventional [Formula: see text] state in the limit λ_SOC ≫ Δ_T realized in Li₂RhO₃. The change of ground state is followed by a marked change of magnetism from a 6 K spin-glass in Li₂RhO₃ to a 94 K antiferromagnet in Ag₃LiRh₂O₆.

Antiferromagnetic excitonic insulator state in Sr3Ir2O7

Excitonic insulators are usually considered to form via the condensation of a soft charge mode of bound electron-hole pairs. This, however, presumes that the soft exciton is of spin-singlet character. Early theoretical considerations have also predicted a very distinct scenario, in which the condensation of magnetic excitons results in an antiferromagnetic excitonic insulator state. Here we report resonant inelastic x-ray scattering (RIXS) measurements of Sr₃Ir₂O₇. By isolating the longitudinal component of the spectra, we identify a magnetic mode that is well-defined at the magnetic and structural Brillouin zone centers, but which merges with the electronic continuum in between these high symmetry points and which decays upon heating concurrent with a decrease in the material’s resistivity. We show that a bilayer Hubbard model, in which electron-hole pairs are bound by exchange interactions, consistently explains all the electronic and magnetic properties of Sr₃Ir₂O₇ indicating that this material is a realization of the long-predicted antiferromagnetic excitonic insulator phase.

Antiferromagnetic excitonic insulators are a distinct form of excitonic insulator, in which electrons and holes are bound by magnetic exchange rather than Coulomb attraction. Here, Mazzone et al. show, using X-ray scattering, that Sr3Ir2O7 realizes this particular state.

Large-gap insulating dimer ground state in monolayer IrTe2

Monolayers of two-dimensional van der Waals materials exhibit novel electronic phases distinct from their bulk due to the symmetry breaking and reduced screening in the absence of the interlayer coupling. In this work, we combine angle-resolved photoemission spectroscopy and scanning tunneling microscopy/spectroscopy to demonstrate the emergence of a unique insulating 2 × 1 dimer ground state in monolayer 1T-IrTe₂ that has a large band gap in contrast to the metallic bilayer-to-bulk forms of this material. First-principles calculations reveal that phonon and charge instabilities as well as local bond formation collectively enhance and stabilize a charge-ordered ground state. Our findings provide important insights into the subtle balance of interactions having similar energy scales that occurs in the absence of strong interlayer coupling, which offers new opportunities to engineer the properties of 2D monolayers.

Spin-glass transition in the spin–orbit-entangled Jeff = 0 Mott insulating double-perovskite ruthenate

We have successfully synthesized new Ru⁴⁺ double perovskite oxides SrLaInRuO₆ and SrLaGaRuO₆, which are expected to be a spin–orbit coupled J_eff = 0 Mott insulating ground state. Their magnetic susceptibility is much significant than that expected for a single Ru⁴⁺ ion for which exchange coupling with other ions is negligible. Their isothermal magnetization process suggests that there are about 20 percent isolated spins. These origins would be the Ru³⁺/Ru⁵⁺ magnetic defects, while the regular Ru⁴⁺ sites remain nonmagnetic. Moreover, SrLaGaRuO₆ shows a spin-glass-like magnetic transition at low temperatures, probably caused by isolated spins. The observed spin-glass can be interpreted by the analogy of a dilute magnetic alloy, which can be seen as a precursor to the mobile J_eff = 1 exciton as a dispersive mode as predicted.

Interfacial charge transfer and persistent metallicity of ultrathin SrIrO3/SrRuO3 heterostructures

Interface quantum materials have yielded a plethora of previously unknown phenomena, including unconventional superconductivity, topological phases, and possible Majorana fermions. Typically, such states are detected at the interface between two insulating constituents by electrical transport, but whether either material is conducting, transport techniques become insensitive to interfacial properties. To overcome these limitations, we use angle-resolved photoemission spectroscopy and molecular beam epitaxy to reveal the electronic structure, charge transfer, doping profile, and carrier effective masses in a layer-by-layer fashion for the interface between the Dirac nodal-line semimetal SrIrO₃ and the correlated metallic Weyl ferromagnet SrRuO₃. We find that electrons are transferred from the SrIrO₃ to SrRuO₃, with an estimated screening length of λ = 3.2 ± 0.1 Å. In addition, we find that metallicity is preserved even down to a single SrIrO₃ layer, where the dimensionality-driven metal-insulator transition typically observed in SrIrO₃ is avoided because of strong hybridization of the Ir and Ru t_2g states.