Session X10: 4d/5d materials I

Stability of the anti-ferromagnetic state in the hole and electron doped Sr2IrO4

The realization of the spin-orbit coupling (SOC) driven j_eff = 1/2 anti-ferromagnetic (AFM) Mott insulating state in Sr₂IrO₄ has generated a huge interest in the field of 5d transition metal oxides. Recently, much work has been done on the electron and hole doped materials, particularly because doping can reveal many properties of the underlying structure. In this context, we have studied a minimal model, an extended (t-t'-t'') Hubbard model on a square lattice (in terms of which the upper and lower Hubbard bands can be described) using the Hartree-Fock mean field theory to understand the quantum many-body instabilities of the AFM state in the doped relativistic Mott insulator Sr₂IrO₄. Our results for the relevant parameters required to describe the material show that the AFM state is stable for a range of both electron and hole doping concentration. Interestingly, a mixed phase of metallic and insulating regions leading to percolative conduction occurs in case of hole doping. Our analysis of the stability of the AFM state in the doped extended Hubbard model of the present work, with respect to the transverse perturbations shows that the sign of the third nearest neighbor interaction plays a crucial role in determining the stability of the AFM state away from half-filling.
  

Density-functional predicted formation of a spin-orbital entangled two dimensional electron gas (2DEG) at the (100) interface between LaAlO3 and the strong spin-orbit coupled Sr2IrO4 due to polar catastrophe

We show that a 2DEG forms at the (100) interface between LaAlO₃ (LAO) and Sr₂IrO₄ (SIO) due to polar catastrophe, analogous to the well-studied LAO/STO (100) interface. While both constituent materials are insulators, LAO being an ordinary band insulator and SIO being a Mott-Hubbard insulator formed due to the large spin-orbit coupling, the interface is metallic and a host for a novel, spin-orbital entangled 2DEG. An interesting feature is that unlike the LAO/STO interface, where the 2DEG spreads into several monolayers, the 2DEG in the present case is predicted to be sharply localized to more-or-less a single monolayer, occupying the upper J_eff = 1/2 Hubbard band with the electron density being half an electron per interface Ir atom. Unlike the complex sub-band structure in LAO/STO, the interfacial Fermi surface is simple, being square-like. Preliminary calculations indicate that the 2DEG may destroy the AFM state at the interface. Recently, experimenters have been able to grow epitaxial SIO structures, leading to the hope that the predicted 2DEG can be studied experimentally in the near future.   

Optical properties and Γ-X crossover of electron states in Sr2IrO4 under strain: A density-functional study

We study the effect of epitaxial strain on the electronic properties of the strong spin-orbit coupled Sr₂IrO₄ using the first principles methods. The optimized structures show a considerable effect of strain in the structural parameters such as the Ir-O bond lengths and the Ir-O-Ir bond angles. These structural modifications lead to important changes in the electronic structure. The band widths of the upper and the lower J_eff = 1/2 Hubbard bands increase with compressive strain, leading to a reduction in the band gap, while the opposite happens for the tensile strain. An interesting observation is the Γ-X crossover of the valence band maximum with the strain. These points are nearly degenerate for the unstrained structure, but under the compressive epitaxial strain, the valence band maximum occurs only at X and Γ for the tensile strain. This strain dependent Γ-X crossover, in turn, suggests the possibility of the tuning of the transport properties in the hole-doped iridates with strain. We discuss the calculated optical absorption coefficients and how they change with the strain, and compare the results with the experimental absorption spectra.   

Electrolyte gating Sr2IrO4 single crystal

The 5d iridates have intriguing electromagnetic properties due to the interplay of spin-orbit interaction and Coulomb interaction of comparable magnitude. Novel electronic phases are predicted and discovered. The compound Sr₂IrO₄, which identified as spin-orbit Mott insulator, has been studied extensively at various doping level primarily in search of superconductivity, due to its crystal structure similarity to La₂CuO₄. We used different electrolytes to dope the cleaved ab plane surface of a Sr₂IrO₄ single crystal. We found a 2D-Mott variable range hoping state upon hole doping accompanied with a positive to negative magnetoresistance transition at low temperature, while minimal gating effect is observed on electron doping. Transport property measurement of Lithium intercalated surface state achieved by gating with electrolyte consist of lithium ions will be reported.
  

Jeff=1/2 Mott insulating state modification by polarization field effect doping of Sr2IrO4 ultrathin films

The 4d and 5d transition metals are commonly characterized by a decreased Hubbard repulsion U which diminishes correlation effects, but simultaneously by an increased spin orbit coupling to create a new type of correlation effects which have been leading to such as spin-orbit coupled Mott insulators, Weyl semimetals, axion insulators and spin liquids. This rich physics allows small perturbations to create large effects in these strongly correlated materials. The Ruddlesden-Popper series of Sr_x+1Ir_xO_3x+1 shows large differences in conductive behavior, where the n=∞ perovskite SrIrO₃ is metallic while the n=1 Sr₂IrO₄ is an insulator due to a spin-orbit coupling band splitting to a J_eff=1/2 state. This state has many similarities to the high T_C cuprate superconductors which show an S=1/2 state, which loses its antiferromagnetism and becomes superconducting upon hole doping. Likewise, under electron doping it is possible to drive Sr₂IrO₄ to a metallic state. Here the suppression of the octahedral rotations will remove the ferromagnetic moment due to canted antiferromagnetism is studied by growing a high-quality bilayer of single-phase Sr₂IrO₄ ultrafilms with the ferroelectric material PbTiO₃ as a novel way to provide electron doping by polarization field-effect doping.   

A novel route to hole-doping epitaxial thin films: Sr$_{2-x}$K$_{x}$IrO$_4$

The layered, two-dimensional antiferromagnetic insulator Sr$_2$IrO$_4$ has been the subject of intense investigation, in large part due to its strong similarity to the prototypical layered parent cuprate La$_2$CuO$_4$. As both electron and hole doping the parent cuprates result in high-temperature superconductivity, carrier doping Sr$_2$IrO$_4$ has likewise been an important goal in the study of layered iridates. While the electron-doped side of Sr$_2$IrO$_4$ (either by surface K doping or La substitution) has been well explored, the hole-doped side of the phase diagram has been less studied. While substituting Rh on the Ir has been shown to result in hole-doping, this also induces significant disorder to the IrO$_2$ planes. Here, we present a new method to synthesize hole-doped Sr$_{2-x}$K$_{x}$IrO$_4$ by a combination of reactive oxide molecular beam epitaxy utilizing K substitution for Sr which induces much less disorder, resulting in well-defined quasiparticle bands and allowing us to observe a clear hole-doped Fermi surface topology with an intrinsic momentum-dependent pseudogap.   

Spin-orbital interplay in Jeff=3/2 Mott insulators

In d¹ Mott insulators, the spin-orbit coupling (SOC) stabilizes J_eff=3/2 quartet of an effective total angular momentum thus allowing for the emergence of multiorbital physics and related spin-orbital frustration. Considering vanadium, molybdenum, and osmium oxides as examples, I discuss how resulting spin-orbital interplay can give rise to a host of novel quantum phases that includes multipolar order, noncollinear spin patterns, and nonmagnetic disordered valence bond states. Finally, I present an example of the honeycomb lattice d¹ compound, such as zirconium trichloride, in which, paradoxically, the strong SOC enhances the symmetry of spin-orbital space to emergent SU(4) symmetric couplings that in turn may lead to a spin-orbital liquid state.   

Phase transformation and hole doping in iridate thin films by controlled oxygen annealing

In an effort to metallize Ir-based perovskites through hole doping, we anneal epitaxially-grown thin films of Sr₂IrO₄ at controlled oxygen pressures. The post-annealed films show a drop in room-temperature resistivity of up to 3 orders of magnitude, and an evolution from insulating to semimetallic behavior. Scanning transmission electron microscopy, x-ray diffraction, and x-ray absorption spectroscopy reveal a structural transformation to a novel pseudocubic phase, increasing with annealing time, pressure, and reduced film thickness. The evolution towards metallicity is attributed to the phase transformation, interstitial oxygens, and Ir vacancies. We discuss the prospects of achieving hole-doped superconductivity in this spin-orbit semimetal.   

Magnetic Properties of Ln2CoIrO6

Magnetic materials containing 5d ions in oxygen cages can show exotic properties due to competing energy scales for spins, orbits, and the crystalline lattice. So far, the A₂BB′O₆ double perovskites have shown a wide range of magnetic and multiferroic properties. We focus on the magnetic properties of A-site magnetic iridates Ln₂CoIrO₆ (Ln = Eu, Ho, Tb), which are unusual in that they have three magnetic elements. The Ln₂CoIrO₆ polycrystals show hysteretic and ferrimagnetic/ferromagnetic behavior, with extremely large magnetic moments on the Ho and Tb compounds. The phase diagrams extend past 60 Tesla. We will also present a magnetostriction technique using a piezoelectric strain gauge that is suited to ultrasensitive measurements on small samples in high magnetic fields on these and other compounds.
  

Magnetic order and spin dynamics of jeff = ½ Ir4+ moments on the fcc lattice in La2BIrO6 (B = Mg, Zn)

We have performed bulk characterization, neutron diffraction, and inelastic neutron scattering experiments to characterize the magnetism in polycrystalline samples of the weakly-distorted fcc double perovskite iridates La₂ZnIrO₆ and La₂MgIrO₆. We find that these materials are spin-orbit-assisted Mott insulators with A-type antiferromagnetic ground states. Furthermore, the powder inelastic neutron scattering data on these systems provides clear evidence for gapped spin-wave excitations with very weak dispersion. The INS results and thermodynamic data on these materials can be reproduced by a conventional Heisenberg-Ising model with significant uniaxial Ising anisotropy and sizeable second-neighbor ferromagnetic interactions, or perhaps more tantalizingly, with a Heisenberg-Kitaev model featuring a dominant antiferromagnetic Kitaev exchange. Our findings highlight how conventional magnetic orders in heavy transition metal oxides may be driven by highly-directional exchange interactions rooted in strong spin-orbit coupling.   

Valence state transition and the magnetic ground state of Ba2-xSrxTbIrO6

With strong spin orbit coupling, 5d transition metal ions in a d⁴ (t_2g⁴e_g⁰) configuration are expected to be non-magnetic with all 4 electrons in J=3/2 shell leaving j=1/2 empty. This unique feature of 5d⁴ ions can be employed to dilute the magnetic sublattice or even switch on/off the magnetism. Here we report the temperature induced electron transfer from Tb to Ir ions in a double perovskite Ba_2-xSr_xTbIrO₆. This thermally driven valence state transition can be employed to tune the electronic configuration of Ir and thus the magnetic order of the double perovskite. The nonmagnetic Ir⁵⁺ (d⁴) ions dilute the Tb³⁺ sublattice, which impedes any long range magnetic order. The electron transfer from Tb³⁺ to Ir⁵⁺ turns the nonmagnetic Ir⁵⁺ (J=0) to magnetic Ir⁴⁺(J=1), which enhances the Tb-O-Ir-O-Tb exchange interactions and favors long range magnetic order at low temperatures.   

Microscopic model of the mixed-valence iridates Ba3MIr2O9 (M = In, Lu)

Mixed-valence iridates Ba₃MIr₂O₉ (M = In, Lu) where unpaired 5d electrons are localized on dimers of Ir atoms have been recently proposed as a source for novel spin liquid candidates[1]. Interestingly, the susceptibility measurements for Ba₃InIr₂O₉ show an anomalous temperature dependence with the Curie-Weiss law holding up to 80 K only. We performed ab-initio density functional based calculations combined with exact diagonalization of finite clusters and find that electronic states of these systems are driven by a temperature-dependent subtle interplay of spin-orbit coupling and Coulomb repulsion at the dimer level [2].

[1] T. Dey, M. Majumder, J. C. Orain, A. Senyshyn, M. Prinz-Zwick, F. Bert, P. Khuntia, N. Büttgen, A. A. Tsirlin, P. Gegenwart, arXiv:1702.08305 (2017)
[2] Ying Li, Alexander A. Tsirlin, Igor I. Mazin, Stephen M. Winter, and Roser Valentí, in preparation   

Electronic structure evolution across the metal-insulator transition in La-doped Ca2RuO4

Ca₂RuO₄ is a prototypical multi-band Mott insulator, with a metal-insulator transition (MIT) at T_MI = 357 K accompanied by a structural phase transition characterized by a compression of the c axis. We present an angle-resolved photoemission spectroscopy (ARPES) study of the electronic structure evolution across the MIT on a series of La-substituted single crystals Ca_2-xLa_xRuO₄ grown through the floating zone technique. We find that La-substitution does not introduce itinerant carriers in the insulating state, but suppresses the MIT to zero temperature for x > 0.1, permitting a detailed study of the correlated metallic state emerging from the Mott insulating ground state. Our data show that metallicity arises at the structural phase transition after a marked redistribution of spectral weight from the xy orbital, which is completely filled in the Mott state, to the out-of-plane xz/yz bands. The metallic phase has a well-defined Fermi surface of heavy quasiparticle states and shows enhanced signatures of spin-orbit coupling. Dynamical mean field theory calculations are in good agreement with our data and confirm that correlations enhance the bare spin-orbit interaction in ruthenates.