Session V43: Iridate Heterostructures

Magneto-transport properties in All-in-All-out magnetic ordered epitaxial Sm$_{2}$Ir$_{2}$O$_{7}$ and Nd$_{2}$Ir$_{2}$O$_{7}$ films.

Pyrochlore iridates R$_{2}$Ir$_{2}$O$_{7}$ (R$=$rare earth element) have been predicted to exhibit a variety of exotic physical phenomena, such as the Weyl semimetallic state and topologically insulating behavior with all-in-all-out (AIAO) magnetic ordering. Here, we have observed a metal-insulator transition accompanied by the AIAO-type magnetic ordering in both Nd$_{2}$Ir$_{2}$O$_{7}$ and Sm$_{2}$Ir$_{2}$O$_{7}$ films below their respective ordering temperatures. Negative magnetoresistance (MR) is observed below 20 K and this gradually transitions into positive MR upon warming. We speculate that this characteristic negative MR is related to the $f$-$d$ exchange coupling between the electrons at the Ir sites and localized moments at the R sites, which induces magnetic ordering on the R sub-lattice. Another remarkable feature is unconventional domain wall (DW) conductance. AIAO-type magnetic DW shows metallic behavior in Nd$_{2}$Ir$_{2}$O$_{7}$ while Sm$_{2}$Ir$_{2}$O$_{7}$ shows insulating behavior. This AIAO-type metallic (insulating) DW conductance is thought to be closely related to the existence of Weyl-semimetallic state (Mott insulating state) which is consistent with a recent theoretical prediction.   

Wide gap Chern Mott insulating phases achieved by design

Chern insulators are exciting both as a novel electronic phase and for their novel and potentially useful boundary transport properties. Honeycomb lattices occupied by heavy transition metal ions,have been proposed by Okamoto and coworkers as Chern insulators, but finding a concrete example has been challenging due to an assortment of broken symmetry phases that thwart the topological character. Building on accumulated knowledge of the behavior of the $3d$ series, we tune spin-orbit and interaction strength together with strain to design two Chern insulator systems (one with Ru, one with Os) with bandgaps up to 130 meV and Chern numbers ${\cal C}=-1$ and ${\cal C}=2$. We find, in this class, that a trade-off between larger spin-orbit coupling and strong interactions leads to a larger gap, whereas the stronger SOC correlates with the larger magnitude of the Hall conductivity. Symmetry lowering in the course of structural relaxation hampers retaining QAH character, as pointed out previously. Fortunately there is only mild structural symmetry breaking of the bilayer in these robust Chern phases.Recent (111) growth of insulating, magnetic phases in closely related materials with this orientation supports the likelihood that synthesis and exploitation will follow.   

Topological magnon bands and unconventional superconductivity in pyrochlore iridate thin films

We theoretically study the magnetic properties of pyrochlore iridate bilayer and trilayer thin films grown along the $[111]$ direction using a strong coupling approach. We find the ground state magnetic configurations on a mean field level and carry out a spin-wave analysis about them. In the trilayer case the ground state is found to be the all-in/all-out (AIAO) state, whereas the bilayer has a deformed AIAO state. For all parameters of the spin-orbit coupled Hamiltonian we study, the lowest magnon band in the trilayer case has a non-zero Chern number. In the bilayer case we also find a parameter range with non-zero Chern numbers. We calculate the magnon Hall response for both geometries, finding a striking sign change as function of temperature. Using a slave-boson mean-field theory we study the doping of the trilayer system and discover an unconventional time-reversal symmetry broken $d+id$ superconducting state. Our study complements prior work in the weak coupling limit and suggests that the $[111]$ grown thin film pyrochlore iridates are a promising candidate for topological properties and unconventional orders.   

Strong anisotropy and electronic confinement in 1D quantum-stripe superlattices of iridium oxides

One-dimensional (1D) systems offer a platform for studying low-dimensional phenomena associated with the onset of critical quantum phase transitions. Here we present a new approach of synthesizing 1D quantum systems by creating dimensionally-confined stripe-superlattices from \textit{in-plane} oriented 2D layered crystals. We have synthesized 1D IrO$_{2}$ stripes using $a$-axis oriented superlattices of Sr$_{2}$IrO$_{4}$ and the wide bandgap insulator LaSrGaO$_{4}$, both of which contain the K$_{2}$NiF$_{4}$ symmetry. The dimensional confinement of our 1D superlattices is confirmed experimentally. Linearly polarized optical spectroscopy shows anisotropic characteristics and one-dimensional electronic confinement of the $J_{eff}_{\, }=$ 1/2 band. Spin and orbital excitations observed in resonant inelastic x-ray scattering suggest enhanced exchange interactions and deconfined orbital excitations in the 1D IrO$_{2}$ stripes. The observed electronic confinement is consistent with density functional theory calculations. The method of transforming layered materials into 1D striped structures is a viable technique for studying dimensional-crossover phase transitions from two- to one-dimension.   

Tuning intralayer and interlayer couplings in artificial layered structure of perovskite iridate

Layered Ruddlesden-Popper series Srm$+$1IrmO3m$+$1 iridates have attracted great attention recently for their novel Mott insulating state, spin-orbit Heisenberg magnetism, and latent superconductivity. While intense investigation has been devoted to the bulk crystals, their electric and magnetic properties may be mimicked and tailored by confining ultrathin SrIrO3 layers in artificial superlattices, which affords a versatile platform for tuning the competing interactions. In our present work, we have varied both intralayer and interlayer couplings by preparing [(SrIrO3)m, (SrTiO3)n] (m $=$ 1, 2, 3, 4, and $\infty $, while n $=$ 1, 2, 3) superlattices through layer-by-layer epitaxial growth. Such a thorough dimensionality-modulation is absent in the bulk but can provide unique insight into the spin-orbit-entangled Mott physics. The results from a combination of synchrotron x-ray diffraction, spectroscopy, electrical and magnetic measurements reveal the interplay between m and n, and a control over the structural, electronic, and magnetic degrees of freedom.   

Interfacial charge transfer and magnetism in 5$d$-3$d$ oxide heterostructures

The existence of strong spin-orbit coupling has brought the iridates to the forefront of materials research, whereas strong electronic correlation has proven to produce a plethora of novel properties within the manganites. Here, we investigate the physical properties of interfaces between such materials by synthesizing a series of artificial superlattices with 5$d$ paramagnetic metal SrIrO$_{\mathrm{3}}$ and 3$d$ antiferromagnetic insulators AMnO$_{\mathrm{3}}$, where A $=$ Sr or La. Through our investigations by x-ray diffraction, magnetometry, dc-transport, x-ray circular dichroism, and polarized neutron reflectometry measurements, we observe both novel magnetic and transport properties, which drastically differ from those of the constituent materials and are highly sensitive to the degree of dimensional confinement within the superlattices. Here we will present these results and discuss the implications of these intriguing magnetic and electronic properties. This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering Division.   

Manipulation of magnetic exchange in iridate heterostructures

Artificial layering of disparate materials into superlattices is an emerging method that promises unparalleled versatility in accessing ground states unavailable to bulk synthesis. Heterostructures composed of paramagnetic, metallic SrIrO$_3$ interspaced with band insulating SrTiO$_3$ were found to host a canted antiferromagnetic ground state, mirroring the Ruddlesen-Popper series iridates with enhanced tunability. We investigate the magnetic excitation spectrum of these artificial systems using resonant inelastic x-ray scattering and directly infer the interlayer and intralayer magnetic exchange interactions. Understanding how interfacial effects modify magnetic interactions is a vital prerequisite for efforts to controllably target different ground states within complex oxide-heterostructures.   

Controlling transport of the SrIrO3 correlated semimetal by doping with an ionic liquid.

The interplay between Mott and spin orbit physics in 5d oxides may result from the splitting of crystal field states by the strong spin orbit interaction. Among them, SrIrO$_{\mathrm{3}}$ is a correlated semimetal, with a groundstate which has been proposed to be topologically protected by the crystalline symmetry. The strong coupling of the electronic structure to oxygen rotations and its interplay with spin orbit interaction gives rise to anomalously narrow bands. The semimetallic state results from the compensation of electron and hole carriers (pockets) coming from separated regions in momentum space. This explains how epitaxial strain enhances the asymmetry of electron hole mobilities eventually triggering a metal to insulator transition (MIT). An intriguing question is the correlated nature of this MIT, and if as such, it can be controlled by charge density. To address this question we have conducted doping experiments with ionic liquid gating. In this talk we will show that the strain induced MIT can be in fact controlled by doping indicating the role played by electron correlations in the semimetallic state of SrIrO3.   

Exploring interfacial ferromagnetism and modulation of magnetic anisotropy in Iridate-Manganite superlattices

Recently, research on 5d transition metal oxides (TMOs) with pronounced spin-orbit coupling (SOC) has been flourishing due to the emergence of new topological states and potential application in spintronics. Interfaces between 3d and 5d TMOs, where both the Coulomb correlation (U) and SOC are comparably strong, promise emergent properties that differ from those of the bulk constituents. One intriguing example is the SrIrO$_{\mathrm{3}}$/La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{\mathrm{3}}$ superlattice system. In this series of superlattices, we have observed a metal-insulator transition (MIT) by tuning the hole doping ratio ($x)$. Charge transfer from Ir to Mn cations, as measured by x-ray absorption spectroscopy, depends on the density of Mn e$_{\mathrm{g}}$ electrons (\textit{1-x}). The degree of charge transfer determines the transport properties ranging from metal to insulator. The entire series of superlattices is ferromagnetic despite the fact that La$_{\mathrm{1-x}}$Sr$_{\mathrm{x}}$MnO$_{\mathrm{3}}$ is antiferromagnetic for x\textgreater 0.5. More interestingly, we found a systematic evolution of magnetic anisotropy that can be independently modulated by changing the hole doping (x), the thickness of the manganite layer or the thickness of the iridate layer. The evolution of magnetic anisotropy is likely correlated with the symmetry change of oxygen octahedra (BO$_{\mathrm{6}}$, where B $=$ Ir or Mn) at the interface, as revealed by x-ray dichroism and diffraction measurements. Our results demonstrate that the low dimensional spin-orbit entangled 3d-5d interfaces provide a new playground to uncover electronic/magnetic properties unattainable in the bulk.   

Electronic Properties of SrIrO$_{\mathrm{3}}$ Heterostructures

Strongly-correlated oxide electronic materials have sparked significant interest due to their wide-ranging electronic properties, including 2-dimensional electron gasses, superconductivity, and semi-metallic behavior. Strong spin-orbit interactions have been predicted to introduce additional behaviors due to their interplay with electron-electron interactions. Perovskite SrIrO$_{\mathrm{3}}$ exhibits strong spin-orbit coupling from the heavy iridium atoms, and has been shown to exhibit metal-insulator transitions induced by temperature, thickness, and strain. We probe these states with magnetotransport measurements of ultrathin SrIrO$_{\mathrm{3}}$ films epitaxially layered with polar and nonpolar oxides. The SrIrO$_{\mathrm{3}}$ heterostructures show a low-temperature transition to a state with enhanced magnetoconductivity, and their sheet resistances exhibit a strong dependence on the interfacing oxide. Understanding these observations will give further insights into the relationship between spin-orbit and electron correlations at oxide interfaces, and lead to design rules for strong spin-orbit coupled heterostructures.   

Spin-Orbit Electronic and Magnetic States in 5d Oxide Heterostructures

Complex oxides are a class of quantum materials where the d-electrons may self-organize into a variety of intriguing emergent electronic and magnetic phases. Recently, there are growing interests in systems where the strong spin-orbit coupling (SOC) may add a new dimension to the energetic landscape. On one hand, the interplay of SOC with electronic correlation is believed to derive novel quantum phenomena. But SOC scales with atomic number and is rather small for 3d electrons. On the other hand, while hosting strong SOC, 5d electrons exhibit weaker correlation and often stabilize a nonmagnetic ground state. Establishing approaches to design systems that combine the merits of both fundamental interactions has been challenging. Our investigations on various iridate-based heterostructures, such as iridate-manganite interfaces, show possible routes to control and utilize spin-orbit-entangled 5d electronic states by epitaxial layering. The experimental findings and theoretical analysis demonstrate heteroepitaxial designs for harnessing the non-symmorphic semimetallicity in ortho-perovskite iridates as well as tailoring spin-orbit magnetism and magnetic anisotropy when coupled with 3d magnetic oxides.   

Examining the weak localization and weak anti-localization in correlated semimetallic SrIrO$_{\mathrm{3}}$ thin films

We have studied the weak localization (WL) and weak anti-localization (WAL) effects in epitaxial SrIrO$_{\mathrm{3}}$ (SIO) thin films to probe the electron correlation and spin-orbit coupling (SOC). We deposited 2-30 nm SIO thin films on SrTiO$_{\mathrm{3}}$ (001) substrates via off-axis RF magnetron sputtering, with c-axis (pseudo-cubic) growth and atomically smooth surfaces achieved. Resistance of the films shows a moderate decrease with decreasing temperature. Modeling the Hall effect result with the two-carrier model and assuming equal electron- and hole-densities, we extracted a carrier density of \textasciitilde 10$^{\mathrm{20}}$ cm$^{\mathrm{-3}}$ and comparable electron and hole mobility of \textasciitilde 50 cm$^{\mathrm{2}}$/Vs. For films below 5 nm, we observed a slight resistance upturn at low temperature, which can be attributed to WL. The low temperature magnetoconductance (MC) (2-15 K) shows a transition from WAL to WL. By fitting the MC with the Maekawa-Fukuyama model, we extracted the inelastic dephasing time and spin relaxation time. While the inelastic field shows linear temperature dependence, suggesting electron-electron interaction as the phase breaking mechanism, the spin relaxation field exhibits quadratic temperature dependence. We also explore the effect of carrier doping on the spin relaxation time using the electric field effect approach.   

Dimensionality-strain phase diagram of strontium iridates superlattices

Using {\it ab initio} approach, we study the electronic and magnetic behavior of strontium iridates as a function of dimensionality and epitaxial strain by employing a (SrIrO$_3$)$_m$/(SrTiO$_3$) superlattice structure. We quantitatively evaluate the dimensional and strain-dependent change of the interaction parameters $U$ and $J$ using the constraint random phase approximation and construct a comprehensive phase diagram describing the evolution of the electronic and magnetic ground state upon strain and dimensionality. We find that compressive strain and increasing the dimensionality perturb the insulating relativistic Mott $J_{eff}=1/2$ state, a characteristic of the $m=1$ system, and induce two distinct types of insulator-to-metal transition (IMT) that can be explained from the entanglement of $U$ and the bandwidth of the Ir-$t_{2g}$ manifold. The IMTs are associated with distinctive changes of the spin ordering manifested by spin-flop transitions, correlated with the modulation of the interlayer exchange interaction, and with a complete quenching of any spin-ordered state in the $m\rightarrow\infty$ limit. The fundamental origin of these electronic and magnetic transitions will be discussed and compared with the corresponding situation in the Ruddlesden-Popper series.   

Spin-Orbital entangled 2DEG in the $\delta$-doped interface La$_{\delta}$Sr$_2$IrO$_4$: Density-Functional Studies and Transport Results from Boltzmann Equations

The strong spin-orbit coupled iridates are of considerable interest because of the Mottminsulating state,which is produced by the combined effect of a strong spin-orbit coupling (SOC) and Coulomb repulsion. In this work, using density-functional methods, we predict the existence of a spin-orbital entangled two dimensional electron gas (2DEG) in the delta-doped structure, where a single SrO layer is replaced by an LaO layer. In the bulk Sr$_2$IrO$_4$, a strong SOC splits the $t_{2g}$ states into $J_{eff}=1/2$ and $3/2$ states. The Coulomb repulsion further splits the half-filled $J_{eff}=1/2$ bands into a lower and an upper Hubbard band (UHB) producing a Mott insulator. In the $\delta$-doped structure, La dopes electrons into the UHB, and our results show that the doped electrons are strongly localized in one or two Ir layers at the interface, reminiscent of the 2DEG in the well-studied LaAlO$_3$/SrTiO$_3$ interface. The UHB, consisting of spin-orbit entangled states, is partially filled, resulting in a spin-orbital entangled 2DEG. Transport properties of the 2DEG shows many interesting features, which we study by solving the semi-classical Boltzmann transport equation in the presence of the magnetic and electric fields.