Session A19: Metal-Insulator Transitions: Iridiates and Heterostructures - Experiment & Theory

Microscopic Evidence for Slater-Type Metal-Insulator Transition in Sr$_{2}$IrO$_{4}$

The interplay between spin-orbit coupling, bandwidth and on-site coulomb repulsion in layered 5d transition metal oxides (TMO) acquired much interest recently. In Sr$_{2}$IrO$_{4}$, the interplay opens a gap near the Fermi energy and stabilizes a J$_{\mathrm{eff}}=$1/2 spin-orbital entangled insulating state at low temperatures. However, whether this metal-insulating transition (MIT) is Mott-type (electronic-correlation driven) or slater-type (magnetic order driven) is still under hot debate. In this presentation, we give, for the first time, the atomic resolved structure of Sr$_{2}$IrO$_{4}$ surface in real space by using scanning tunneling microscopy. Tunneling spectroscopic results illustrate the gap opening of Sr$_{2}$IrO$_{4}$ at low temperatures with the gap size of 250 mV, indicating the metal to insulator transition. More importantly, the pair of peaks around gap in spectra suggests the quasi-particle coherent excitation, implying the Slater-type insulating state. This is further confirmed by temperature dependent measurements and density functional theory calculations.   

Magnetotransport properties of Sr$_2$IrO$_4$ thin films modulated by epitaxial strain

Sr$_{2}$IrO$_{4}$ (SIO) has attracted much attention due to its $J_{\mathrm{eff}}=$1/2 Mott state induced by relativistic spin-orbit coupling [1]. In 3$d$/4$d$ transition metal oxides, exotic phenomena, such as high-$T_{\mathrm{C}}$ superconductivity and colossal magnetoresistance, occur when a Mott insulting state is suppressed by charge carrier doping or band width tuning. Whether the Mott state in SIO can be tuned to new exotic states is an interesting question under active investigation. We have grown epitaxial SIO films on the substrates of SrTiO$_{3}$(STO) and NdGaO$_{3}$ (NGO) using a pulsed laser deposition method and investigated the strain effect on the properties of SIO. The SIO/STO film exhibits a tetragonal structure, while the SIO/NGO film displays a orthorhombic structure due to the NGO's orthorhombic nature. Although both types of films show insulating properties, their magnetic properties appear to be distinct: the SIO/STO film shows negative magnetoresistance (MR) with negligible anisotropy, whereas the SIO/NGO film exhibits positive MR with two-fold anisotropy. Such differences in magnetotransport imply the strong coupling between the lattice, spin and orbital degrees of freedom in SIO.\\[4pt] [1] B.J. Kim \textit{et al}., Phys. Rev. Lett. \textbf{101}, 076402 (08).   

Magneto-transport of filling controlled Mott insulator, Sr$_2$IrO$_4$

Sr$_2$IrO$_4$ (SIO) is shown to be a special Mott insulator with the ground state stabilized by a combination of electron correlation and spin-orbit coupling. [1] Both structurally and electronically, electron doped SIO shows characteristics comparable to hole doped La$_2$CuO$_4$, one of the parent compounds of the high-T$_{\mathrm{c}}$ cuprates. [2] This leads us to a natural question of whether doped SIO can turn into a metal and eventually a superconductor. Sustained efforts of chemically doping SIO, [3] without altering its band structure significantly, have been severely hampered due to solubility limitations. In this work, we perform a combination of chemical and electrostatic doping of SIO, to explore the possibility of achieving a robust metallic state. We show that undoped SIO shows ambipolar characteristics and there is significant gating action even after heavy alloying of SIO with La. In depth magneto-transport measurements such as Hall effect and magnetoresistance also provide us a deeper understanding of electronic structure and transport in this exotic Mott insulator. \\[4pt] [1] B. J. Kim et. al., PRL (2008).\\[0pt] [2] F. Wang and T. Senthil, PRL (2011).\\[0pt] [3] Y. Klein and I. Terasaki, J. Phys. : CM (2008).   

Visualizing the gap closure by Rh dopant in SOC induced Mott insulator Sr$_{2}$IrO$_{4}$ with Scanning Tunneling Microscopy

Sr$_{2}$IrO$_{4}$ is a novel J$_{\mathrm{eff}}=$1/2 Mott insulator with characteristics of 5d electrons. The strong spin orbit coupling (SOC) in the 5d orbitals of iridium plays an important role in the insulating nature of the parent compound, while replacing Ir$^{4+}$ with the isoelectronic Rh$^{4+}$ is able to drive the system to a metallic regime. We use variable temperature Scanning Tunneling Microscope to study both the insulating parent compound and the Rh doped Sr$_{2}$Ir$_{\mathrm{1-x}}$Rh$_{\mathrm{x}}$O$_{4}$, with x$=$0.04, 0.11. By doing differential conductance measurement, we were able to observe an insulating gap both in the parent compound and the low doping areas of the x$=$0.04 and 0.11 samples. We also observed that in the doped samples, local gaps varies largely at the atomic length scale. By correlating the locations of Rh dopant and the size of local gaps, we found that Rh doping will decrease the insulating gap size which is in accordance with the metallic behavior observed by transport measurements.   

Heterostructuring iridate-based spin-orbit Mott materials

Mott materials with strong spin-orbital coupling (SOC) have emerged as a new playground for searching quantum many-body phases with exotic electronic and magnetic properties. Numerous attentions have been paid to 5d transition metal oxides due to the intriguing opportunities to obtaining novel topological insulators, superconductivity, Weyl semimetals, quantum spin liquid, and so on. While realizing these fascinating phenomena would lead to a new generation of electronic and spintronic devices, the rich physics derived from the cooperation of strong correlation and SOC remains to be explored. Here we present our investigation on using epitaxy to control perovskite-based strontium iridates, a prototype of 5d complex oxides, as ultrathin films and heterostructures. We utilize epitaxial stabilization, strain, confinement and interfacial coupling to tune the competing interactions and the multiple degrees of freedom. The combination of these various controls offers a unique pathway to novel phase behaviors and innovative functions. Our experimental findings derived from transport, magnetometry and advanced resonant x-ray spectroscopy, including linear and circular dichroism, will be discussed.   

Probing spatial evolution of local density of states in Sr3Ir2O7

Amongst the iridate families, the Ruddlesden-Popper series (Sr$_{n+1}$Ir$_{n}$O$_{3n+1})$ goes through a transition from insulator to metal with increasing $n$. Within this series the $n=$2 compound Sr$_{3}$Ir$_{2}$O$_{7}$ (Ir327) occupies a unique place, straddling a well-defined insulator ($n=$1) on one side and a metal ($n=$infinity) on the other, placing Ir327 in close proximity to a delicate and interesting transition point. In this study, we probe the spatial evolution of the local density of states (LDOS) of Ir327 by means of scanning tunneling spectroscopy. In the parent Ir327 compound, we find local regions of metallic density of states that exist within an underlying insulating electronic structure. Based on the experimental data, we discuss the mechanism of how this metallic LDOS evolves from the intrinsically gapped electronic structure in terms of spin-orbit and Coulomb interactions.   

Strongly spin-orbit coupled spin-3/2 model for $5d^1$ $AB_2O_4$ spinels

Research on 5d transition metal oxides has been more and more active recently. Unlike in 3d transition metals, the strong spin orbit interaction cannot be treated as a perturbation. The competition between correlation, spin orbit coupling and the kinetic energy of 5d electrons makes the problem nontrivial. We model the $AB_2O_4$ spinel structure with single d electron on atom B as a Hubbard type model. By treating the hopping term perturbatively under large spin orbit coupling we derive an effective spin 3/2 model which is not Heisenberg-like. We further investigate the possible phase diagram of the effective spin 3/2 model.   

Unusual magnetic phases in the strong interaction limit of two-dimensional topological band insulators in transition metal oxides

The expected phenomenology of non-interacting topological band insulators (TBI) is now largely theoretically understood. However, the fate of TBIs in the presence of interactions remains an active area of research with novel, interaction-driven topological states possible, as well as new exotic magnetic states. In this work we study the magnetic phases of an exchange Hamiltonian arising in the strong interaction limit of a Hubbard model on the honeycomb lattice whose non-interacting limit is a two-dimensional TBI recently proposed for the layered heavy transition metal oxide compound, (Li,Na)$_2$IrO$_3$. By a combination of analytical methods and exact diagonalization studies on finite size clusters, we map out the magnetic phase diagram of the model. We find that strong spin-orbit coupling can lead to a phase transition from an antiferromagnetic Ne\'el state to a spiral or stripy ordered state. We also discuss the conditions under which a quantum spin liquid may appear in our model, and we compare our results with the different but related Kitaev-Heisenberg-$J_2$-$J_3$ model which has recently been studied in a similar context.   

Time-reversal symmetry breaking and anomalous Hall effect in heavy fermion metals

Motivated by recent experimental evidence for a possible chiral spin liquid phase in the metallic pyrochlore heavy fermion iridates (Pr$_2$ Ir$_2$ O$_7$) [Phys.Rev.Lett, {\bf 96}, 087204 (2006), Phys.Rev.Lett {\bf 98}, 057203 (2007), Nature {\bf 463}, 210 (2010), Phys.Rev.Lett, {\bf 106}, 217204 (2011)], we study the effect of Kondo coupling on a time-reversal symmetry breaking state of the $J_1-J_2$ model on square lattices. We use a slave fermion representation for the $f$-moments which are coupled to conduction electrons, and study the mean field solution in the large-N limit. We calculate the ground state energies of various feasible states, and map out the mean field phase diagram by energetic consideration. As the probe for time-reversal symmetry breaking, we calculate the anomalous Hall response for the chiral phase. Finally we discuss the implications of our results on the pyrochlore heavy-fermion iridates.   

The low-energy magnetic excitations of a three-band Hubbard model with a strong spin-orbit coupling for 5d transition metal oxide Sr$_2$IrO$_4$

5$d$ transition metal oxides in a layered perovskite structure such as Sr$_2$IrO$_4$ have attracted much attention because of their unique properties caused by a strong relativistic spin-orbit coupling of 5$d$ transition element. Recent experiments on Sr$_2$IrO$_4$ have revealed that the low-energy magnetic excitations can be described by an ``isospin''-1/2 Heisenberg model with an effective exchange interaction as large as $\sim$ 60-100 meV. Motivated by these experiments, we study theoretically the ground state magnetic structure and the low-energy magnetic excitations for Sr$_2$IrO$_4$ using a three-band Hubbard model with the spin-orbit coupling. Our results demonstrate that the low-energy magnetic excitations are well described by an effective antiferromagnetic Heisenberg model composed of a local Kramers doublet. The estimated value of the effective exchange interaction is as large as 79 meV, which is in good quantitative agreement with the experiments.   

First-order metal-insulator transitions in vanadates from first principles

Materials that exhibit first-order metal-insulator transitions, with the accompanying abrupt change in the conductivity, have potential applications as switches in future electronic devices. Identification of materials and exploration of the atomic-scale mechanisms for switching between the two electronic states is a focus of current research. In this work, we search for first-order metal-insulator transitions in transition metal compounds, with a particular focus on d$^1$ and d$^2$ systems, by using first principles calculations to screen for an alternative low-energy state having not only a electronic character opposite to that of the ground state, but a distinct structure and/or magnetic ordering which would permit switching by an applied field or stress. We will present the results of our investigation of the perovskite compounds SrVO$_3$, LaVO$_3$, CaVO$_3$, YVO$_3$, LaTiO$_3$ and related layered phase, including superlattices and Ruddlesden-Popper phases. While the pure compounds do not satisfy the search criteria, the layered phases show promising results.   

Real-time dynamics in electron-lattice coupled system: Numerical study on an extended double-exchange model

We have developed a new theoretical method to study the photo-induced insulator-to-metal (IM) transition in strongly correlated electron systems [PRL \textbf{103}, 266402 ('09); EPL \textbf{94}, 27003 ('11).]. In the manganese oxides, it has been observed that the photo-induced dynamics with several tens of THz in frequency can drive IM transition [Nature Materials \textbf{6}, 643 ('07).]. The excitation energy with several tens of THz in frequency is fairly lower than the insulating energy gap of the electronic state. In this study, we introduce an extended double exchange model where the conduction electron couples with the orbital-ordering field and lattice distortion, and numerically examine the lattice vibration induced IM transition in the electron-lattice coupled system. To simplify the numerical calculation, the electronic states are restricted in the Hilbert space for perfect ferromagnetic states involving the ground state. In the numerical simulation, we find that the low frequency vibration of Jahn-Teller distortion can change the orbital-ordering pattern and trigger the IM transition. A threshold behavior of the lattice-vibration induced IM transition and the electron-hole excitation by continuous forced lattice-vibration are also examined.   

Photo-doped carrier dynamics in Mott insulatoring systems

Electron/hole doping in Mott insulators, for example two-dimensional cuprates, has been well investigated in relation to high-Tc superconductivity. Especially related to photo-doping, many experiments on photo-induced phase transition in strongly correlated systems have been made. In the usual photo-doping setup, the system is excited with fs-laser pulse and generated electron-hole pairs affect properties of materials. Recently, another type of photo-doped experiment with heterostructure has been made, and hole or electron carriers are dynamically injected through the heterostructure. In this theoretical study, we examine photo-doped carrier dynamics in the t-J model with dynamically doped holes. We formulate dynamics of the carriers by non-equilibrium Green functions. We take an initial state of holes and decompose the non-equilibrium Green's function into a series of equilibrium Green's functions by using Wick's theorem. The effect of the initial distribution appears from the higher terms in the series. We treat magnons with the self-consistent Born approximation. The non-equilibrium Green function derived in this way shows double time dependence. We will present physical quantities in transient process, for example, one-particle excitation spectra for holes.   

Tailoring spin-orbit Mott insulators via designed superlattices

The layered perovskite iridates Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ show a transition from a magnetic insulating to a semi-metallic state as the number of layers $n$ is increased. This behaviour is intimately related to the interplay between spin-orbit coupling, electronic correlations and dimensionality. In this talk, we will show that the fabricated superlattices [(SrIrO$_3$)/SrTiO$_3$], provide new insight into this behaviour. Theoretical calculations using density functional and tight binding approaches will be presented to support our results.   

Tuning the conductivity of LaMnO$_{3}$/SrTiO$_{3}$ superlattices by stacking

First-principles density-functional-theory calculations have been applied to study the structure and electronic properties of ultrathin LaMnO$_{3}$/SrTiO$_{3}$ superlattices. We predict that upon the change of stacking, antiferromagnetic LaMnO$_{3}$/SrTiO$_{3}$ superlattices can be tuned from non-conducting insulator to conducting metal. The corresponding microscopic structure change in the superlattices is also analyzed. We find that the metal-insulator transition is accompanied by a corresponding reduction/disappearance of the Jahn-Teller (JT) distortion in the LaMnO$_{3}$ layer. The findings of this work illustrate the role of the JT distortion in the conductivity of transition-metal perovskites, and also suggest a new method for tuning metal-insulator transitions for functional device design.