Session B9: Focus session: Complex Bulk Oxides: Iridates

Dipoles on monopoles in spin ice

Close connection of electricity and magnetism is one of the cornerstones of modern physics. This connection plays crucial role both from the fundamental point of view and in practical applications, including recent advance in spintronics and in the study and development of multiferroic materials. A new breakthrough was the recent proposal of Castelnovo, Moessner and Sondhi that in spin ice systems, e.g. in some pyrochlores, one can model the magnetic monopoles -- the objects displaying the properties of isolated magnetic charges. Such monopoles are a hot topic nowadays. Usually one discuss mainly their thermodynamic and magnetic properties. I will show that every \textit{magnetic monopole} in spin ice should have an \textit{electric dipole} attached to it. This can be seen from the results obtained for frustrated Hubbard system [1]. Both the electronic mechanism discussed in [1] and the lattice effects (magnetostriction) lead to the conclusion that for 3in/1out and 3out/1in tetrahedra there should appear an electric dipole directed from the center of tetrahedron to the ``special'' spin. This will lead to electric activity of monopoles, and to possibility to address and influence them not only by magnetic, but also by an electric field. Several consequences of this effect will be discussed. In general, the analogy between electricity and magnetism goes even further than usually assumed: whereas electrons have \textit{electric charge} and spin, i.e. \textit{magnetic dipole}, magnetic monopoles in spin ice have both \textit{magnetic charge} and \textit{electric dipole}. \\[4pt] [1] L.N.Bulaevskii, C.D.Batista, M.V.Mostovoy and D.I.Khomskii, Phys.Rev. B\textbf{78}, 028402 (2008)   

Raman scattering study of the pressure- and field-dependent phases of Sr$_{2}$IrO$_{4}$

Transition metal oxides (TMOs) with a perovskite structure are of interest due to the many fascinating phenomena they exhibit. Among TMOs, iridates with 5d orbitals exhibit an unexpected insulating state due to large spin-orbit coupling. With the extended nature of the 5d orbital giving rise to a strong crystal field, the competition between the onsite Coulomb interaction and spin-orbit coupling--which have comparable energy scales-opens up the possibility for studying novel phases that develop when tuning this competition with using an external perturbation with magnetic field or pressure. In this talk we present temperature, magnetic field and pressure dependent Raman study of Sr$_{2}$IrO$_{4}$, which provides an opportunity Pressure- and field-tuned Raman spectroscopy provides an opportunity to explore the novel phases of this material under extreme conditions.   

Quantum Hall effects in a Weyl semimetal: Possible application in pyrochlore iridates

There has been much interest in pyrochlore iridates A2Ir2O7 where both strong spin-orbital coupling and strong correlation are present. A recent local density approximation calculation [X. Wan, A. M. Turner, A. Vishwanath, and S. Y. Savrasov, Phys. Rev. B 83, 205101 (2011)] suggests that the system is likely in a three-dimensional topological semimetallic phase: a Weyl semimetal. Such a system has zero carrier density and arrives at the quantum limit even in a weak magnetic field. In this talk, I will discuss two quantum effects of this system in a magnetic field: a pressure-induced anomalous Hall effect and a magnetic-field-induced charge density wave at the pinned wave vector connecting Weyl nodes with opposite chiralities. A general formula of the anomalous Hall coefficients in a Weyl semimetal is also given. Both proposed effects can be probed by experiments in the near future and can be used to detect the Weyl semimetal phase.   

Electronic ground state properties of Iridate oxides from x-ray absorption spectroscopy

Element (Ir)- and orbital (5d)-specific L$_{2,3}$ edge x-ray absorption and magnetic circular dichroism measurements are used to probe the nature of the electronic ground state in magnetic insulators BaIrO$_3$ [1] and Sr$_2$IrO$_4$ [2,3]. A spin-only description of the magnetic ground state is directly ruled out by the measurements. Instead, the measurements show spin-orbit entanglement in 5$d$ states resulting in a larger orbital ($L_z$) than spin ($S_z$) contribution to the magnetic moment, even in the presence of strong crystal field and band effects. Measured x-ray absorption cross sections at spin-orbit split L$_{2,3}$ edges impose constraints on the nature of the ground state [3]. Experiments under chemical- (doping) and applied-pressure conditions provide evidence for a delicate interplay between electronic bandwidth and Coulomb interactions leading to the gapped, spin-orbit coupled ground state of these complex oxides. \\[4pt] [1] M. A. Laguna Marco et al., Phys. Rev. Lett. 105, 216407 (2010).\\[0pt] [2] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).\\[0pt] [3] L. C. Chapon and S. W. Lovesey, J. Phys. Condens. Matter 23, 252201 (2011).   

Lattice-driven magnetoresistivity and metal-insulator transition in single-layered iridates

{Sr$_{2}$IrO$_{4}$} exhibits a novel insulating state driven by spin-orbit interactions. Here we report two novel phenomena, namely a large magnetoresistivity that is extremely sensitive to the orientation of magnetic field but exhibits no apparent correlation with the magnetization, and a robust metallic state that is induced by dilute electron {(La$^{3+}$)} or hole (K$^{+}$) doping on Sr$^{2+}$ ions in {Sr$_{2}$IrO$_{4}$}. This study reveals that a strong spin-orbit interaction alters the balance between the competing energies so profoundly that (1) the spin degree of freedom alone is no longer a dominant force; (2) underlying transport properties delicately hinge on the {Ir-O-Ir} bond angle via a strong magnetoelastic coupling; and (3) a highly insulating state in {Sr$_{2}$IrO$_{4}$} is proximate to a metallic state, and the transition is governed by lattice distortions that can be controlled via either magnetic field or chemical doping.   

Time-resolved optical study of magnetism in Sr2IrO4

We report a time-resolved optical pump-probe study of the Jeff=1/2 Mott insulator Sr2IrO4. The temperature dependence of the electronic relaxation rate exhibits clear anomalies at magnetic ordering temperatures of 240K and 100K, which are consistent with the development of bulk decay channels via emission of magnetic excitations. We will then discuss time-resolved second harmonic generation studies and contrast the magnetic properties of the surface and the bulk.   

Unusual ferromagnetism and strong spin-orbit coupling in Post-Perovskite $CaIrO_3 $

Strong spin-orbit coupling (SOC) and strong correlations have been considered essential in understanding the unusual physical properties of the 4d and 5d transition-metal oxides, such as the SOC driven Mott insulating state in $Sr_2 IrO_4$. Recently, an unusual atomic-like orbital moment and strong SOC have been confirmed experimentally in $9R-BaIrO_3$ through analysis of the branching ratio at the Ir $L_{2,3}$ absorption edges as obtained from x-ray absorption and x-ray magnetic circular dichroism (XMCD) measurements. We have applied the same techniques to probe unusual ferromagnetism and SOC in the post-perovskite (pPv) $CaIrO_3$, which is an insulator and exhibits weak ferromagnetism below $T_C \approx 110K$. The branching ratio at the Ir $L_{2,3}$ absorption edges, which is close to unity in pPv $CaIrO_3$, appears to indicate an even stronger spin-orbit interaction in the pPv $CaIrO_3$ than in $9R-BaIrO_3$. However, it has been challenging to model the Ir 5d orbital moment, as probed by the XMCD measurements, due to the understood local octahedral-site distortions.   

Exploring Magnetic Ordering in Sr3Ir2O7

Iridium oxide members of the Ruddlesden-Popper series have generated a great deal of interest recently due to their novel Mott insulating phases within these 5$d$-electron systems. These surprising Mott phases have been proposed to form due to a delicate interplay between spin orbit coupling effects and electronic correlation [1]. Here we present measurements probing the nature of the spin correlations and charge behavior in the bilayer variant of the Ruddlesden-Popper series, Sr$_{3}$Ir$_{2}$O$_{7}$. Our neutron scattering results reveal an antiferromagnetic spin structure and will be discussed in parallel with transport and bulk magnetization measurements detailing the electronic behavior in this material. \\[4pt] [1] B. J. Kim et al., Phys. Rev. Lett. 101, 076402 (2008).   

Doping a Spin-Orbit Mott Insulator: Topological Superconductivity from the Kitaev-Heisenberg Model and Possible Application to (Na$_2$/Li$_2$)IrO$_3$

We study the effects of doping a Mott insulator on the honeycomb lattice where spins interact via direction dependent Kitaev couplings $J_{\rm K}$, and weak antiferromagnetic Heisenberg couplings $J$. This model is known to have a spin liquid ground state and may potentially be realized in correlated insulators with strong spin orbit coupling. The effect of hole doping is studied within a $t$-$J$-$J_{\rm K}$ model, treated using the SU(2) slave boson formulation, which correctly captures the parent spin liquid. We find superconductor ground states with spin triplet pairing that spontaneously break time reversal symmetry. Interestingly, the pairing is qualitatively different at low and high dopings, and undergoes a first order transition with doping. At high dopings, it is smoothly connected to a paired state of electrons propagating with the underlying free particle dispersion. However, at low dopings the dispersion is strongly influenced by the magnetic exchange, and is entirely different from the free particle band structure. Here the superconductivity is fully gapped and topological, analogous to spin polarized electrons with $p_x+ip_y$ pairing. These results may be relevant to honeycomb lattice iridates such as A$_2$IrO$_3$ (A\,=\,Li or Na) on doping.   

Correlated Itinerant Electrons in Pyrochlore Bi$_{2}$Ir$_{2}$O$_{7}$

Strong spin-orbit coupling in the $5d$-based iridates rigorously competes with other relevant energies, and motivates novel insulating states. Therefore, a metallic state does not commonly occur in the iridates, but the unusual balance between different degrees of freedom in the iridates almost guarantee that it will exhibit extraordinary properties when it does occur. Here we report anomalous transport and thermodynamic properties including Hall effect of single-crystal Bi$_{2}$Ir$_{2}$O$_{7}$ along with our electronic structure calculations utilizing a LSDA+U scheme. The results will be discussed along with comparisons drawn with other pyrochlore iridates and related materials.   

Measurement of the Mott insulating gap of Sr$_{2}$IrO$_{4}$ with a scanning tunneling microscope

Recently the 5d transition metal oxide Sr$_{2}$IrO$_{4}$ has become a material of interest. This is due to comparable interaction strengths between crystal field splitting, the Coulomb interaction, and spin-orbit coupling, resulting in a Mott insulating ground state that has a finite resistance even at cryogenic temperatures. In order to fully understand this material it is important to measure the Mott insulating gap. A scanning tunneling microscope is an excellent tool for studying this material for its ability to directly measure this gap. Our preliminary results show this gap measured on single crystals to be $\sim $50 meV which is comparable to the activation energy of this sample. We will discuss our current STM results and compare our results with other optical conductivity data obtained from this material.   

Mapping the fluctuating Ir$^{4+}$ dimers across the phase diagram of Cu(Ir$_{1-x}$Cr$_{x}$)$_{2}$S$_{4}$ (0$\leq$x$\leq$0.6)

Elucidating the role of fluctuations in systems with competing interactions, such as Cu(Ir$_{1-x}$Cr$_{x}$)$_{2}$S$_{4}$, enables comprehensive understanding of their physical properties. While CuIr$_{2}$S$_{4}$ exhibits complex behavior with metal-insulator transition accompanied by charge ordering and Ir$^{4+}$-spin-dimerization at 226~K, CuCr$_{2}$S$_{4}$ displays ferromagnetic metallic behavior (T$_{C}$$\sim$377~K) understood within the double exchange model. Intermediate composition range sees suppression of end-member properties, with broad features observed in susceptibility around 180~K attributed to Cr$^{3+}$ low spin to high-spin crossover [1]. Robust fluctuating Ir$^{4+}$ dimers are detected and their evolution examined across the phase diagram by the X-ray atomic pair distribution function method. Although their long range order is destroyed already by x$\approx$0.05, Ir$^{4+}$ dimers exist locally at low temperature at all compositions studied. We provide detailed account of the Cr-doping and temperature dependence of the local dimers, and estimate characteristic length-scale on which they are observable. Fluctuating dimers disappear on heating, for intermediate compositions at temperatures above 180~K. \\[4pt] [1] R. Endoh et al., Phys. Rev. B $\bf{68}$, 115106 (2003).   

Coupling of Orbital and Magnetic Orders to Colossal Negative Thermal Expansion in Novel Mott Insulators

Ca$_{2}$RuO$_{4}$ is intimately associated with both \textbf{\textit{negative volume thermal expansion (NVTE)}} and \textbf{\textit{negative linear thermal expansion (NLTV)}} when doped by a 3d transition metal ion M for Ru. The NVTE and NLTE observed in this system constitutes a compelling and extraordinary example in that (1) the coefficient of NVTE and NLTE reaches -213 $\times $ 10$^{-6}$ K$^{-1}$ and -148 $\times $ 10$^{-6}$ K$^{-1}$, respectively, constituting \textbf{\textit{colossal negative thermal expansion (NTE)}}; (2) the NTE anomalies closely track the onset temperatures of orbital and magnetic orders, in sharp contrast to classic NTE that shows no relevance to physical properties; (3) the NTE and physical properties can be effectively tuned via varying M and x in Ca$_{2}$Ru$_{1-x}$M$_{x}$O$_{4}$; (4) the NTE occurs near room temperature and extends over a wide temperature interval ranging from 100 K to 350 K. Moreover, NTE and Invar effect commonly exist in these 4d-based ruthenates and 5d-based iridates, e.g. Sr$_{n+1}$Ir$_{n}$O$_{3n+1}$ and BaIrO$_{3}$. These novel NTE materials provide a much-needed paradigm for functional materials with anomalous thermal expansion and electronic characteristics.