Origin of charge density wave formation in insulators from a high resolution photoemission study of BaIrO3

Iridate Li8IrO6: An Antiferromagnetic Insulator

A polycrystalline iridate Li₈IrO₆ material was prepared via heating Li₂O and IrO₂ starting materials in a sealed quartz tube at 650 °C for 48 h. The structure was determined from Rietveld refinement of room-temperature powder neutron diffraction data. Li₈IrO₆ adopts the nonpolar space group R3̅ with Li atoms occupying the tetrahedral and octahedral sites, which is supported by the electron diffraction and solid-state ⁷Li NMR. This results in a crystal structure consisting of LiO₄ tetrahedral layers alternating with mixed IrO₆ and LiO₆ octahedral layers along the crystallographic c-axis. The +4 oxidation state of Ir⁴⁺ was confirmed by near-edge X-ray absorption spectroscopy. An in situ synchrotron X-ray diffraction study of Li₈IrO₆ indicates that the sample is stable up to 1000 °C and exhibits no structural transitions. Magnetic measurements suggest long-range antiferromagnetic ordering with a Néel temperature (T_N) of 4 K, which is corroborated by heat capacity measurements. The localized effective moment μ_eff (Ir) = 1.73 μ_B and insulating character indicate that Li₈IrO₆ is a correlated insulator. First-principles calculations support the nonpolar crystal structure and reveal the insulating behavior both in paramagnetic and antiferromagnetic states.

Revelation of Mott insulating state in layered honeycomb lattice Li2RuO3

We investigate the role of electron correlation in the electronic structure of honeycomb lattice Li₂RuO₃using photoemission spectroscopy and band structure calculations. Monoclinic Li₂RuO₃having Ru network as honeycomb lattice undergoes magneto-structural transition atT_c∼ 540 K from high temperature phaseC2/mto low temperature dimerized phaseP2₁/m. Room temperature valence band photoemission spectra reveal an insulating ground state with no intensity at Fermi level (E_F). Ru 4dband extracted from high and low photon energy valence band photoemission spectra reveal that the surface and bulk electronic structures are very similar in this system. Band structure calculations using generalized gradient approximation leads to metallic ground state while screened hybrid (YS-PBE0) functional reveals opening up of a gap in almost degenerated_zx/d_yzorbitals, whereasd_xyorbital is already gapped. Ru 3dcore level spectra with prominent unscreened feature provides direct evidence of strong electron correlation among Ru 4delectrons which is also manifested by |E-E_F|²dependence of spectral density of states in the vicinity ofE_Fin the high-resolution spectra, establishing Li₂RuO₃as Mott insulator.

Electronic band structure of iridates

In this review, an attempt has been made to compare the electronic structures of various 5d iridates (iridium oxides), with an effort to note the common features and differences. Both experimental studies, especially angle-resolved photoemission spectroscopy (ARPES) results, and first-principles band structure calculations have been discussed. This brings to focus the fact that the electronic structures and magnetic properties of the high-Z 5d transition iridates depend on the intricate interplay of strong electron correlation, strong (relativistic) spin-orbit coupling, lattice distortion, and the dimensionality of the system. For example, in the thin film limit, SrIrO₃ exhibits a metal-insulator transition that corresponds to the dimensionality crossover, with the band structure resembling that of bulk Sr₂IrO₄.

Evolution of local structure and superconductivity in CaFe2As2

We investigate the evolution of the local structural parameters and their implication in unconventional superconductivity of 122 class of materials employing extended x-ray absorption fine structure studies. The spectral functions near the FeK- and AsK-absorption edges of CaFe₂As₂and its superconducting composition, CaFe_1.9Co_0.1As₂(T_c= 12 K) exhibit evidence of enhancement of Fe contributions near the Fermi level with Co substitution, which becomes more prominent at low temperatures indicating enhanced role of Fe in the electronic properties with doping. As-Fe and Fe-Fe bondlengths derived from the experimental data reveal evolution with temperature across the magneto-structural transition in the parent compound. The evolution of these parameters in Co-doped superconducting composition is similar to its parent compound although no magneto-structural transition is observed in this system. These results reveal an evidence of doping induced evolution to the proximity to critical behavior and/or strong nematic fluctuations which might be important for superconductivity in this system.

Observation of pseudogap in MgB2

We investigate the electronic structure of a specially prepared highly dense conventional high temperature superconductor, MgB₂, employing high resolution photoemission spectroscopy. The spectral evolution close to the Fermi energy is commensurate to BCS descriptions as expected. However, the spectra in the wider energy range reveal the emergence of a pseudogap much above the superconducting transition temperature indicating an apparent departure from the BCS scenario. The energy scale of the pseudogap is comparable to the energy of the [Formula: see text] phonon mode responsible for superconductivity in MgB₂ and the pseudogap can be attributed to the effect of electron-phonon coupling on the electronic structure. These results reveal a scenario of the emergence of the superconducting gap within an electron-phonon coupling induced pseudogap and have significant implications in the study of high temperature superconductors.

Coulomb correlations in 4d and 5d oxides from first principles-or how spin-orbit materials choose their effective orbital degeneracies

The interplay of spin-orbit coupling and Coulomb correlations has become a hot topic in condensed matter theory and is especially important in 4d and 5d transition metal oxides, like iridates or rhodates. Here, we review recent advances in dynamical mean-field theory (DMFT)-based electronic structure calculations for treating such compounds, introducing all necessary implementation details. We also discuss the evaluation of Hubbard interactions in spin-orbit materials. As an example, we perform DMFT calculations on insulating strontium iridate (Sr₂IrO₄) and its 4d metallic counterpart, strontium rhodate (Sr₂RhO₄). While a Mott-insulating state is obtained for Sr₂IrO₄ in its paramagnetic phase, the spectral properties and Fermi surfaces obtained for Sr₂RhO₄ show excellent agreement with available experimental data. Finally, we discuss the electronic structure of these two compounds by introducing the notion of effective spin-orbital degeneracy as the key quantity that determines the correlation strength. We stress that effective spin-orbital degeneracy introduces an additional axis into the conventional picture of a phase diagram based on filling and on the ratio of interactions to bandwidth, analogous to the degeneracy-controlled Mott transition in d¹ perovskites.

Mott insulator-to-metal transition in yttrium-doped CaIrO₃

We report on the study of insulator-to-metal transition in post-perovskite compound CaIrO3. It is discovered that a gradual chemical substitution of calcium by yttrium leads to the onset of strong metallic behavior in this compound. This observation is in stark contrast to BaIrO3, which preserves its Mott insulating behavior despite excess of the charge carriers due to yttrium doping. Magnetic measurements reveal that both compounds tend to exhibit magnetic character irrespective of the chemical substitution of Ca or Ba. We analyze these unusual observations in light of recent researches that suggest that CaIrO3 does not necessarily possess j = 1/2 ground state due to structural distortion. The insulator-to-metal transition in CaIrO3 will spur new researches to explore more exotic ground state, including superconductivity, in post-perovskite Mott insulators.

Hybrid functionals applied to perovskites

After being used for years in the chemistry community to describe molecular properties, hybrid functionals have been increasingly and successfully employed for a wide range of solid state problems which are not accurately accessible by standard density functional theory. In particular, the upsurge of interest in transition metal perovskite-based compounds, motivated by their technological relevance and functional ductility, has incentivized the use of hybrid functionals for realistic applications, as hybrid functionals appear to be capable of capturing the complex correlated physics of this class of oxide material, characterized by a subtle coupling between several competing interactions (lattice, orbital, spin). Here we present a map of recent applications of hybrid functionals to perovskites, aiming to cover an ample spectra of cases, including the 'classical' 3d compounds (manganites, titanates, nickelates, ferrites, etc.), less conventional examples from the the 4d (technetiates) and 5d (iridates) series, and the (non-transition metal) sp perovskite BaBiO3. We focus our attention on the technical aspects of the hybrid functional formalism, such as the role of the mixing and (for range-separated hybrids) screening parameters, and on an extended array of physical phenomena: pressure- and doping-induced insulator-to-metal and structural phase transitions, multiferroism, surface and interface effects, charge ordering and localization effects, and spin-orbit coupling.

Strong magnetic instability in correlated metallic Bi2Ir2O7

We report an experimental/theoretical study of single-crystal Bi(2)Ir(2)O(7) that possesses a metallic state with strongly exchange-enhanced paramagnetism. The ground state of Bi(2)Ir(2)O(7) is characterized by the following features: (1) a divergent low-temperature magnetic susceptibility that indicates no long-range order down to 50 mK; (2) strongly field-dependent coefficients of the low-temperature T and T(3) terms of the specific heat; (3) a conspicuously large Wilson ratio R(W) ≈ 53.5; and (4) unusual temperature and field dependences of the Hall resistivity that abruptly change below 80 K, without any clear correlation with the magnetic behavior. All these unconventional properties suggest the existence of an exotic ground state in Bi(2)Ir(2)O(7).

Revelation of the role of impurities and conduction electron density in the high resolution photoemission study of ferromagnetic hexaborides

We investigate the temperature evolution of the electronic structure of ferromagnetic CaB6 using ultrahigh resolution photoemission spectroscopy; the electronic structure of paramagnetic LaB6 is used as a reference. High resolution spectra of CaB6 reveal a finite density of states at the Fermi level E(F) at all the temperatures and evidence of impurity induced localized features in the vicinity of E(F), which are absent in the spectra of LaB6. Analysis of the high resolution spectra suggests that disorder in the B sublattice inducing partial localization in the mobile electrons and low electron density at E(F) is important to achieve ferromagnetism in these systems.

Critical test for Altshuler-Aronov theory: evolution of the density of states singularity in double perovskite Sr2FeMoO6 with controlled disorder

With high-resolution photoemission spectroscopy measurements, the density of states (DOS) near the Fermi level (E(F)) of double perovskite Sr(2)FeMoO(6) having different degrees of Fe/Mo antisite disorder has been investigated with varying temperature. The DOS near E(F) showed a systematic depletion with increasing degree of disorder, and recovered with increasing temperature. Altshuler-Aronov (AA) theory of disordered metals well explains the dependences of the experimental results. Scaling analysis of the spectra provides experimental indication for the functional form of the AA DOS singularity.