Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session A12: Focus Session: Iridates and Ruthenates |
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Sponsoring Units: DMP Chair: Gang Cao, University of Kentucky Room: 007C |
Monday, March 2, 2015 8:00AM - 8:36AM |
A12.00001: X-ray scattering and spectroscopy studies of strongly correlated iridates Invited Speaker: Young-June Kim Due to strong spin-orbit coupling and electronic correlation, iridates host a number of interesting quantum phases of matter. A broad overview of recent x-ray scattering and spectroscopy studies of iridates on honeycomb, pyrochlore, and square lattice will be given. In particular, we will focus on square lattice iridates, which exhibit physical and magnetic properties remarkably similar to those of cuprates, such as quasi-two-dimensional magnetism and very large magnetic exchange. Various studies found unusual metallic phases in Sr$_{2}$IrO$_{4}$ doped with charge carriers [1-3]. We examine the case of partially replacing Ir$^{4+}$ ions with Rh$^{3+}$ ions, which corresponds to a hole-doping. Our magnetic x-ray scattering and x-ray absorption spectroscopy investigation reveal that the suppression of magnetic order and the rise of metallicity is described by a percolation picture [3]. We also obtained magnetic excitation spectra using resonant inelastic x-ray scattering (RIXS), which will be compared with those of cuprates and possible routes to high temperature superconductivity will be discussed [4]. \\[4pt] [1] T. F. Qi, et al. Phys. Rev. B 86, 125105 (2012).\\[0pt] [2] C. Dhital et al., Nat. Comm. 5, 3377 (2014).\\[0pt] [3] J. P. Clancy et al., Phys. Rev. B 89, 054409 (2014).\\[0pt] [4] F. Wang and T. Senthil, Phys. Rev. Lett. 106, 136402 (2011). [Preview Abstract] |
Monday, March 2, 2015 8:36AM - 8:48AM |
A12.00002: High-energy electronic excitations in Sr$_2$IrO$_4$ observed by Raman scattering Jhih-An Yang, Yi-Ping Huang, Michael Hermele, Tongfei Qi, Gang Cao, Dmitry Reznik The interplay between spin-orbit interaction, on-site coulomb correlation, crystal field splitting, and inter-site hopping leads to a novel insulating behavior in Sr$_2$IrO$_4$ as the realization of the $J_{eff} =1/2$ state. We report results of a large-shift Raman scattering investigation of electronic excitations in Sr$_2$IrO$_4$. We found two high-energy excitations at 690 meV and 680 meV with $A_{1g}$ and $B_{1g}$ symmetry respectively. The two peaks have different temperature and Rh-doping dependences. Symmetry analysis of the dd transitions that contribute to Raman signals will also be presented. The observed peaks are consistent with the scenario of excitons associated with inter-site dd transitions without pseudospin-flip. [Preview Abstract] |
Monday, March 2, 2015 8:48AM - 9:00AM |
A12.00003: Evolution of Magnetism in Single-Crystal Ca2Ru1-xIrxO4 (0 $\le $ x $\le $ 0.65) S.J. Yuan, J. Terzic, J.C. Wang, L. Li, T.F. Qi, W.H. Song, S. Aswartham, G. Cao We report structural, magnetic, transport and thermal properties of single-crystal Ca$_{2}$Ru$_{\mathrm{1-x}}$Ir$_{x}$O$_{4}$ (0 $\le $ x $\le $ 0.65). Ca$_{2}$RuO$_{4}$ is a structurally-driven Mott insulator with a metal-insulator transition at $T_{\mathrm{MI}} =$ 357 K, followed by a well-separated antiferromagnetic order at $T_{\mathrm{N}} =$ 110 K. Substituting Ru with Ir enhances the spin-orbit coupling (SOC) and causes further orthorhombic distortions. As a result, a pronounced weak ferromagnetic behavior occurs, which enhances dramatically with increasing Ir concentration. The magnetic ordering temperature $T_{\mathrm{N}}$ increases from 110 K at $x =$ 0 to 215 K at x $=$ 0.65, along with enhanced magnetic anisotropy due to SOC. In addition, with increasing x, the metal-insulator transition $T_{\mathrm{MI}}$ increases initially and vanishes eventually. [Preview Abstract] |
Monday, March 2, 2015 9:00AM - 9:12AM |
A12.00004: Ground State Tuning by Spin-Orbit Coupling and Lattice Degrees of Freedom in Single-Crystal BaIr$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$O$_{3}$ (0 $\le $ x $\le $ 1) K. Butrouna, S.J. Yuan, T.F. Qi, J. Terzic, S. Aswartham, L.E. DeLong, G. Cao BaIrO$_{3}$ is a magnetic insulator driven by spin-orbit coupling (SOC) whereas BaRuO$_{3}$ is a paramagnetic metal. The contrasting ground states provide a unique opportunity to study the role of the SOC and the lattice degrees freedom. Our investigation reveals that substitution of Ru$^{4+}$ (4$d^{4})$ ions for Ir$^{5+}$ (5$d^{5})$ ions in BaIrO$_{3}$ reduces the magnitude of the SOC and the structural distortion. There are two major effects of Ru additions: (1) Light Ru doping (0$\le $ x $\le $ 0.15) prompts a simultaneous, precipitous drop in both the magnetic ordering temperature T$_{\mathrm{C}}$ and the electrical resistivity, which exhibits a crossover behavior from a metallic to an insulating state near T$_{\mathrm{C}}$. (2) Heavier Ru doping (0.41$\le $ x $\le $ 0.9) induces a robust metallic state with a strong spin frustration. [Preview Abstract] |
Monday, March 2, 2015 9:12AM - 9:24AM |
A12.00005: Surface Bilateral Symmetry on Orthorhombic Double-layer Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ Chen Chen, Jiandi Zhang, Rongying Jin, Ward Plummer The double-layered ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ exhibits very interesting properties especially on the surface because of the broken symmetry (G. Li \textit{et. al.}, Scientific Reports 3 2882 (2013)). Compared to the bulk, the surface not only enhances the octahedral rotation but also introduces tilt (not in the bulk). Partial substitution of Mn for Ru reduces the tilt distortion while keeps the rotation angle constant up to $\sim$ 20{\%} doping. Tilt distortion as expected removes one of the glidelines (associated with rotation) and breaks the mirror symmetry along this broken glideline in the LEED pattern, resulting in a ``bilateral symmetry'' as a prefect human has. It is surprising that the surface has lost much of the symmetry present in the bulk. When the tilt is removed by increased Mn doping ($x)$ in Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7\, }$the LEED pattern returns to the expected one with two glide planes. In comparison, the LEED pattern of single layer Ca$_{1.9}$Sr$_{0.1}$RuO$_{4}$ which has tilt still preserves the mirror symmetry. We assign this difference to the different structures in the first and second octahedral layers. [Preview Abstract] |
Monday, March 2, 2015 9:24AM - 9:36AM |
A12.00006: STM study of lattice distortion effect on Sr$_{3}$(Ru$_{1-x}$Mn$_{x}$)$_{2}$O$_{7}$ surfaces Jisun Kim, Zhenyu Diao, Jiandi Zhang, Rongying Jin, E.W. Plummer The Ruddlesden-Popper ruthenates Sr$_{n+1}$Ru$_{n}$O$_{3n+1}$ (n=1 to $\infty$) exhibit a wide range of distinct electronic and magnetic properties due to strong coupling between charge, spin, lattice, and orbital degrees of freedom. For example, substituting Ru with Mn in Sr$_{3}$Ru$_{2}$O$_{7}$ (Sr327) leads to a metal-insulator transition (MIT) at T$_{MIT}$, as well as a magnetic phase transition from paramagnetic at high temperatures to long-range AFM ordering at T$_{M}$. In the parent compound (x=0), RuO$_{6}$ octahedra are rotated by $\sim$8$^{\circ}$ in the bulk and the distortion is enhanced at the surface (12$^{\circ}$) with the addition of tilt ($\sim$5$^{\circ}$). With Mn doping, the rotation of octahedra at the surface does not change but tilt is eliminated. Our recent study of Mn doped Sr327 shows that the surface symmetry and Mn-induced local disturbance observed by scanning tunneling microscope (STM) changes with increasing Mn doping (6 to 16 $\%$), suggesting that the surface electronic properties change with the concentration of Mn doping, driven by the structural change. By studying lower Mn doped Sr327 using STM/STS we delineate the relationship of size of the disturbance induced by Mn to surface metalicity. The result is also compared to 1 $\%$ Ti doped Sr327 case. [Preview Abstract] |
Monday, March 2, 2015 9:36AM - 9:48AM |
A12.00007: Novel magnetic states in insulating $d^4$ oxides with strong spin-orbit coupling Christopher Svoboda, Nandini Trivedi The comparable energy scales in $4d$ and $5d$ transition metal oxides, arising from Coulomb correlations, spin-orbit coupling and bandwidth, can lead to new phases and phenomena. With this motivation we examine an ion with $d^4$ electron configuration in the $t_{2g}$ sector separated from the other states by crystal field splitting. Upon including spin-orbit coupling, the completely filled $j=3/2$ manifold is nonmagnetic but with a nonzero magnetic susceptibility. Upon introducing hopping between two $d^4$ atoms, we find novel entangled ferromagnetism generated by the superexchange interaction in a significant part of the phase diagram [1]. We further present results for the temperature dependent susceptibility calculated using exact diagonalization to illustrate this novel magnetic behavior and the role Hund's coupling plays in producing these phases. We make predictions for resonant X-ray scattering and magnetic measurements in pyrochlore osmates.\\[4pt] [1] O. N. Meetei, W. S. Cole, M. Randeria, and N. Trivedi, arXiv:1311.2823. [Preview Abstract] |
Monday, March 2, 2015 9:48AM - 10:00AM |
A12.00008: Electronic, magnetic and optical properties of Sr$_{n+1}$Ir$_n$O$_{3n+1}$ ($n$=1, 2, and $\infty$) Peitao Liu, Sergii Khmelevskyi, Bongjae Kim, Xing-Qiu Chen, Dianzhong Li, Cesare Franchini We have studied the crossover between metallic/insulating and non-magnetic/magnetic phases in Ruddlesden-Popper series of iridates Sr$_{n+1}$Ir$_n$O$_{3n+1}$ ($n$=1, 2, and $\infty$) by means of density functional theory including an on-site Hubbard $U$ correction and many-body first principles methods. By systematically investigating the evolution of the orbital and spin properties as a function of $U$, spin-orbit coupling (SOC) strength, and $n$ we have constructed detailed phase diagrams of the metal-insulator transition (MIT) which provide clear evidence for the crucial role played by SOC and $U$ in establishing a relativistic Mott-Hubbard insulating state in the $n$=1 and 2 compounds. Optical spectra computed within a model Bethe-Salpeter scheme show the typical double peak structure observed in experiments and capture well the progressive shrinking of the band-gap and the widening of the bandwidth going from $n$=1 to $n$=$\infty$. Finally, we clarify the origin of the canted magnetic ground state of Sr$_2$IrO$_4$ as due to the the synergistic effect of structural distortions (rotation and tetragonal distortion of IrO$_6$ octahedral) and to the competition between exchange and Dzyaloshinskii-Moriya interactions. [Preview Abstract] |
Monday, March 2, 2015 10:00AM - 10:12AM |
A12.00009: Coherent quasiparticles with a small Fermi Surface in lightly doped Sr$_3$Ir$_2$O$_7$ Alberto De la Torre, Siobhan Mckeown Walker, Anna Tamai, Emily Hunter, Alaska Subedi, Timur Kim, Moritz Hoesch, Robin Perry, Antoine Georges, Felix Baumberger We characterize the electron doping evolution of (Sr$_{1-x}$La$_x$)Ir$_2$O$_7$ by means of angle-resolved photoemission. Concomitant with the metal insulator transition around $x\approx 0.05$ we find the emergence of coherent quasiparticle states forming a closed small Fermi surface of volume $3x/2$, where $x$ is the independently measured La concentration. The quasiparticle weight $Z$ remains large along the entire Fermi surface, consistent with the moderate renormalization of the low-energy dispersion and no pseudogap is observed. This indicates a conventional, weakly correlated Fermi liquid state with a momentum independent residue $Z\approx0.5$ in lightly doped Sr$_3$Ir$_2$O$_7$, in stark contrast with underdoped cuprates. [Preview Abstract] |
Monday, March 2, 2015 10:12AM - 10:24AM |
A12.00010: Experimental bandstructure of the $5d$ transition metal oxide $IrO_2$ Jason Kawasaki, Yuefeng Nie, Masaki Uchida, Darrell Schlom, Kyle Shen In the $5d$ iridium oxides the close energy scales of spin-orbit coupling and electron-electron correlations lead to emergent quantum phenomena. Much research has focused on the ternary iridium oxides, e.g. the Ruddlesden-Poppers $A_{n+1}B_{n}O_{3n+1}$, which exhibit behavior from metal to antiferromagnetic insulator ground states, share common features with the cuprates, and may host a number of topological phases. The binary rutile $IrO_2$ is another important $5d$ oxide, which has technological importance for spintronics due to its large spin Hall effect and also applications in catalysis. $IrO_2$ is expected to share similar physics as its perovskite-based cousins; however, due to bond-length distortions of the $IrO_6$ octahedra in the rutile structure, the extent of similarities remains an open question. Here we use angle-resolved photoemission spectroscopy to perform momentum-resolved measurements of the electronic structure of $IrO_2$. $IrO_2$ thin films were grown by molecular beam epitaxy on $TiO_2$ (110) substrates using an Ir e-beam source and distilled ozone. Films were subsequently transferred through ultrahigh vacuum to a connected ARPES system. Combined with first-principles calculations we explore the interplay of spin-orbit coupling and correlations in $IrO_2$. [Preview Abstract] |
Monday, March 2, 2015 10:24AM - 10:36AM |
A12.00011: Magnetic fluctuations induced insulator-to-metal transition in Ca(Ir$_{\mathrm{x}}$Ru$_{\mathrm{1-x}})$O$_{3}$ Deepak Singh, Jagath Gunasekera, Ashutosh Dahal, Leland Harriger, Thomas Heitmann The Fermi liquid theory dictates the metal-insulator transition in a continuous fashion via the divergence of the quasiparticle mass m*. However, the metallic phase near the Mott insulator in the metal-insulator phase diagram based on the Hubbard model is dominated by the fluctuations of spin, charge and orbital correlations; often termed as the anomalous metallic phase. In this presentation, experimental results manifesting the magnetic fluctuations induced insulator-to-metal transition in Ca(Ir$_{\mathrm{x}}$Ru$_{\mathrm{1-x}})$O$_{3}$ will be discussed in the framework of the Hubbard model. For x $=$ 1, the compound CaIrO$_{3}$ is a Mott insulator with antiferromagnetic order below T $\sim$ 110 K. A gradual substitution of Ir by Ru results in the onset of anomalous metallic behavior as a function of the tuning parameter x. At x $=$ 0, the compound CaRuO$_{3}$ is a non-Fermi liquid metal with no apparent magnetic order. While the orthorhombic structural integrity is maintained throughout the group, strong magnetic fluctuations is detected below x $=$ 0.8. The role of magnetic fluctuations in the metallic transition is further confirmed by first principle theoretical calculation. [Preview Abstract] |
Monday, March 2, 2015 10:36AM - 10:48AM |
A12.00012: Magneto-optical transmission of Bi$_{2}$Se$_{3}$ at the fundamental absorption edge Gerard Martinez, Milan Orlita, Benjamin Piot, Marek Potemski, Yew San Hor, G. Strzelecka, A. Hruban Magneto-optical transmission spectroscopy of a series of n-type Bi$_{2}$Se$_{3}$ samples have been performed, around the fundamental absorption edge, up to 32 T. The absorption edge splits in two components related to the two spin polarization of the transmission. This spin splitting increases linearly with the magnetic field up to a critical value B$_{\mathrm{C}}$, depending on the carrier density, beyond which it remains constant. This corresponds to a complete spin polarization of the compound. The consequences of this specific property put some constrains on the parameters of the Hamiltonian describing the system. [Preview Abstract] |
Monday, March 2, 2015 10:48AM - 11:00AM |
A12.00013: Quasi-local critical nature of cooperative paramagnetic fluctuations in CaRuO$_3$ metal Jagath Gunasekera, Leland Harriger, Thomas Heitmann, Deepak Singh Ruthenate perovskites of the form ARuO$_3$, where A is alkaline earth metal, hold strong promises in developing new paradigm of the quantum magnetism. CaRuO$_3$ is of special interest among this group. The unusual combination of the absence of magnetic order and the anomalous non-Fermi liquid properties makes it an archetypal perovskite for the exploration of the quantum magnetism. Recently, we performed detailed electric, magnetic and neutron scattering measurements on high quality polycrystalline sample of CaRuO$_3$. The experimental results suggest that the underlying magnetism is depicted by the quantum mechanical fluctuations of Ru$^{4+}$- spins, confined to field-independent random domains that form a cooperative paramagnetic state at low temperature. The dynamic structure factor, which increases significantly below T $\sim$ 22 K, manifests a linear (E$\backslash$T) scaling, implying the Curie-Weiss type fluctuations with temperature as the most relevant parameter. Moreover, the linear dynamic scaling in conjunction with the divergence of the spin fluctuations mean relaxation time as T $\sim$ 0 K suggest the existence of a quasi-local critical behavior in the system. [Preview Abstract] |
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