Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session Y9: Focus Session: Honeycomb and Pyrochlore Lattices |
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Sponsoring Units: DMP Chair: Ribhu Kaul, University of Kentucky Room: 006D |
Friday, March 6, 2015 8:00AM - 8:12AM |
Y9.00001: Electronic structure of the harmonic-honeycomb iridates $\alpha$, $\beta$, $\gamma$-Li$_2$IrO$_3$ Roser Valenti, Ying Li, Harald O. Jeschke Using ab-initio density functional theory we investigate the electronic and magnetic properties of the harmonic-honeycomb iridates $\alpha$, $\beta$, $\gamma$-Li$_2$IrO$_3$ with honeycomb, hyperhoneycomb and stripyhoneycomb crystal structures, respectively. We discuss the distinct features of each class of systems in terms of possible Ir-based molecular-orbitals and the implications on the magnetism in these materials. We further relate the electronic structure to proposals of generalized Kitaev-Heisenberg models. [Preview Abstract] |
Friday, March 6, 2015 8:12AM - 8:24AM |
Y9.00002: Magnetic Excitations in $\alpha$-RuCl$_{3}$ Stephen Nagler, Arnab Banerjee, Craig Bridges, Jiaqiang Yan, David Mandrus, Matthew Stone, Adam Aczel, Ling Li, Yuen Yiu, Mark Lumsden, Johannes Knolle, Roderich Moessner, Alan Tennant The layered material $\alpha$-RuCl$_{3}$ is composed of stacks of weakly coupled honeycomb lattices of octahedrally coordinated Ru$^{3+}$ ions. The Ru ion ground state has 5 d electrons in the low spin state, with spin-orbit coupling very strong compared to other terms in the single ion Hamiltonian. The material is therefore an excellent candidate for investigating possible Heisenberg-Kitaev physics. In addition, this compound is very amenable to investigation by neutron scattering to explore the magnetic ground state and excitations in detail. Here we discuss new time-of-flight inelastic neutron scattering data on $\alpha$-RuCl$_{3}$. A high energy excitation near 200 meV is identified as a transition from the single ion J=1/2 ground state to the J=3/2 excited state, yielding a direct measurement of the spin orbit coupling energy. Higher resolution measurements reveal two collective modes at much lower energy scales. The results are compared with the theoretical expectations for excitations in the Heisenberg - Kitaev model on a honeycomb lattice, and show that Kitaev interactions are important. [Preview Abstract] |
Friday, March 6, 2015 8:24AM - 8:36AM |
Y9.00003: Lattice-Tuned Magnetism of Ru$^{4+}$(4d$^{4})$ Ions in Single-Crystals of the Layered Honeycomb Ruthenates: Li$_{2}$RuO$_{3}$ and Na$_{2}$RuO$_{3}$ Jinchen Wang, Jasminka Terzic, Tongfei Qi, Feng Ye, Shujuan Yuan, Saicharan Aswartham, Sergey Streltsov, Daniel Khomskii, Ribhu Kaul, Gang Cao We synthesize and study single crystals of the layered honeycomb lattice Mott insulators Na$_{2}$RuO$_{3}$ and Li$_{2}$RuO$_{3}$ with magnetic Ru$^{4+}$(4d$^{4})$ ions. The newly found Na$_{2}$RuO$_{3}$ features a nearly ideal honeycomb lattice and orders antiferromagnetically at 30 K. Single-crystals of Li$_{2}$RuO$_{3}$ adopt a honeycomb lattice with either C2/m or more distorted P2$_{1}$/m below 300 K, depending on detailed synthesis conditions. We find that Li$_{2}$RuO$_{3}$ in both structures hosts a well-defined magnetic state, in contrast to the singlet ground state found in polycrystalline Li$_{2}$RuO$_{3}$. A phase diagram generated based on our results uncovers a new, direct correlation between the magnetic ground state and basal-plane distortions in the honeycomb ruthenates. [Preview Abstract] |
Friday, March 6, 2015 8:36AM - 9:12AM |
Y9.00004: Majorana metals in spin-orbit entangled quantum matter Invited Speaker: Simon Trebst The largely accidental balance of electronic correlations, spin-orbit entanglement, and crystal field effects of 5d transition metal oxides results in a remarkably broad variety of metallic and insulating states. In this talk, we will discuss the physics of spin-orbit entangled j=1/2 Mott insulators whose microscopic description gives rise to three-dimensional variants of the Kitaev model. The analytical tractability of this model allows to study the fractionalization of these moments into Majorana fermions (and a Z2 gauge field) and their emergent collective behavior. We show that the Majorana fermions generically form metallic states who precise character intimately depends on the underlying lattice structure. Examples range from the well known Dirac semimetal of the two-dimensional Kitaev honeycomb model to three-dimensional metals, in which the gapless modes either form a Fermi line or a Fermi surface akin to a conventional metal [1]. We further discuss our recent finding of a Weyl spin liquid -- a state with topologically protected Weyl nodes in the bulk and associated Fermi arcs on the surface [2]. Finally, we comment on the thermodynamic and transport signatures of these various Majorana metals.\\[4pt] Joint work with M. Hermanns and K. O'Brien.\\[4pt] [1] M. Hermanns and S. Trebst, PRB 89, 235102 (2014).\\[0pt] [2] M. Hermanns, K. O'Brien, and S. Trebst, arXiv:1411.7379 [Preview Abstract] |
Friday, March 6, 2015 9:12AM - 9:24AM |
Y9.00005: Gap Measurement of Na$_{2}$IrO$_{3}$ with a Scanning Tunneling Microscope Armin Ansary, John Nichols, Gang Cao, Kwok-Wai Ng 5d transition metal oxides such as iridates have recently stimulated substantial interest. Many exciting new phases can be found in this class of materials because of the comparable strength between spin-orbit coupling and the Coulomb interaction. In particular, we have studied high quality single crystal Na$_{2}$IrO$_{3}$ with a scanning tunneling microscope (STM). Na$_{2}$IrO$_{3}$ has a layered structure with a honeycomb lattice. The gap is measured to be about 400 meV according to the dI/dV curve, which is consistent with optical measurements. We will show topographic images and discuss the evolution of the density of states and the behavior of the gap from room temperature down to 100 K. [Preview Abstract] |
Friday, March 6, 2015 9:24AM - 9:36AM |
Y9.00006: Spin-orbit correlated magnetic order in honeycomb $\alpha$-RuCl$_3$ Vijay Shankar Venkataraman, Heung-Sik Kim, Hae-Young Kee There has been a lot of recent interest in the combined effects of spin-orbit coupling (SOC) and electronic correlations in transition metal compounds. RuCl$_3$ with layered honeycomb structure was proposed as a candidate material, where SOC boosts the electronic interaction, leading to an insulating phase. However, the role of SOC is not clear in materials with 4d-orbitals, since SOC strength is weaker than 5d-orbital materials. Here we study electronic band structures of honeycomb RuCl$_3$ using ab-initio and tight binding methods, and estimate its SOC strength. We find that SOC in RuCl$_3$ is not strong enough to justify an effective j$_{\mathrm{eff}}=1/2$ single band unlike in the iridates. However, when electronic interactions are introduced, a magnetic order develops, and upper- and lower-Hubbard bands are characterized by j$_{\mathrm{eff}}=1/2$ and $3/2$, respectively. Within a mean field theory with multi-orbital bands, we find that a zig-zag magnetic order is a ground state. Experimental implications are also discussed. [Preview Abstract] |
Friday, March 6, 2015 9:36AM - 9:48AM |
Y9.00007: Structure and magnetic ground states of spin-orbit coupled compound alpha-RuCl3 Arnab Banerjee, Craig Bridges, Jiaqiang Yan, David Mandrus, Matthew Stone, Adam Aczel, Ling Li, Yuen Yiu, Mark Lumsden, Bryan Chakoumakos, Alan Tennant, Stephen Nagler The layered material alpha-RuCl3 is composed of stacks of weakly coupled honeycomb lattices of octahedrally coordinated Ru3$+$ ions.~ The Ru ion ground state has 5 d electrons in the low spin state, with spin-orbit coupling very strong compared to other terms in the single ion Hamiltonian.~ The material is therefore an excellent candidate for investigating possible Heisenberg-Kitaev physics.~ In addition, this compound is very amenable to investigation by neutron scattering to explore the magnetic ground state and excitations in detail.~~In this talk, we discuss the synthesis of phase-pure alpha-RuCl3 and the characterization of the magnetization, susceptibility, and heat-capacity. We also report neutron diffraction on both powder and single crystal alpha-RuCl3, identifying the low temperature magnetic order observed in the material.~ The results, when compared to theoretical calculations, shed light on the relative importance of Kitaev and Heisenberg terms in the Hamiltonian. [Preview Abstract] |
Friday, March 6, 2015 9:48AM - 10:00AM |
Y9.00008: Quantum phase transition and anomalous Hall effect in a pyrochlore Kondo lattice Sarah Grefe, Wenxin Ding, Qimiao Si Motivated by recent experimental evidence for a possible chiral spin liquid phase in the metallic pyrochlore heavy fermion iridates ($Pr_2Ir_2O_7$) [Phys.Rev.Lett, ${\bf 96}$, 087204 (2006), Phys.Rev.Lett ${\bf 98}$, 057203 (2007), Nature ${\bf 463}$, 210 (2010)] as well as quantum critical behavior in this system [Nat. Mater. ${\bf 13}$, 356 (2014)], we study the effect of Kondo coupling on various spin liquid states of the Heisenberg model on pyrochlore lattices, including states exhibiting time-reversal-symmetry-breaking. Using a slave fermion representation for the f-moments which are coupled to conduction electrons, we study the large-N limit to determine the ground state energies of various feasible states and map out the zero-temperature phase diagram. We calculate the anomalous Hall response across the quantum phase transition from the Kondo destroyed phase to the Kondo screened phase. Finally we discuss the implications of our results for the properties of $Pr_2Ir_2O_7$. [Preview Abstract] |
Friday, March 6, 2015 10:00AM - 10:12AM |
Y9.00009: Ab Initio Simulations of the Structure and Energetics of Harmonic Honeycomb Iridates Tess Smidt, Qimin Yan, Jeffrey Neaton Edge-sharing iridates present an exciting opportunity to study the competition of Mott insulator physics and strong spin-orbit coupling. Harmonic honeycomb iridates are a recently discovered homologous series of stoichometrically identical structures that host anisotropic magnetism and exotic spin ordering. We use density functional theory and lattice kinetic Monte Carlo to investigate structural, energetic, and entropic trends in determining the equilibrium ground states of harmonic honeycomb iridates. We predict the formation energies and geometry of as-yet unsynthesized series members and propose why some structures are more prevalent than others. [Preview Abstract] |
Friday, March 6, 2015 10:12AM - 10:24AM |
Y9.00010: Rotated Heisenberg model Fadi Sun, Jinwu Ye, Wu-Ming Liu We show that Rotated Heisenberg (RH) model is a new class of quantum spin models to describe magnetic materials with strong spin-orbit couplings (SOC). We introduce Wilson loops to characterize frustrations and gauge equivalent class. For a special equivalent class, we identify a new spin-orbital entangled commensurate ground state. It supports a novel gapped elementary excitation named as in-commensurate magnons which have two gap minima continuously tuned by the SOC strength. At low temperatures, the in-commensurate magnons lead to dramatic effects in all physical quantities such as density of states, specific heat, magnetization and various spin correlation functions. At high temperatures, the specific heat and transverse spin structure factors depend on the SOC strength explicitly. We argue that one gauge may be realized in current experiments and other gauges may also be realized in near future experiments. Various experimental detections are discussed. [Preview Abstract] |
Friday, March 6, 2015 10:24AM - 10:36AM |
Y9.00011: Collective modes in two- and three-dimensional electron systems with Rashba spin-orbit coupling Saurabh Maiti, Vladimir Zyuzin, Dmitrii Maslov In addition to charge plasmons, a 2D electron system with Rashba-type spin-orbit coupling (SOC) also supports three collective modes in the spin sector: the chiral-spin modes. We study the dispersions of the charge and spin modes and their coupling to each other within a generalized RPA for arbitrarily strong SOC, and both in 2D and 3D systems. We find that the charge plasmons are coupled to only one of the three chiral-spin modes. In 3D, the chiral-spin modes are strongly damped by particle-hole excitations and disappear for weak electron-electron interaction. Landau damping of the chiral-spin modes in 3D is directly related to the fact that, in contrast to 2D, there is no gap for particle-hole excitations between spin-split subbands. The gapless continuum is also responsible for Landau damping of the charge plasmon in 3D - a qualitatively new feature of the SOC system. The in-plane transverse chiral-spin mode shows up as dispersing peak in the optical conductivity at finite wave number which can can be measured in the presence of diffraction grating. We also discuss possible experimental manifestations of chiral-spin modes in semiconductor quantum wells such InGaAs/AlGaAs and 3D giant Rashba materials of the BiTeI family. [Preview Abstract] |
Friday, March 6, 2015 10:36AM - 10:48AM |
Y9.00012: Anderson Impurity in Dirac and Weyl semimetals Jinhua Sun, Dong-Hui Xu, Fu-Chun Zhang, Yi Zhou We utilize variational method to study the Kondo screening of an Anderson impurity in three-dimensional Dirac and Weyl semimetals. We find that the spin correlation between the magnetic impurity and conduction electrons in both the systems are strongly anisotropic due to the spin-orbit coupling, and the spin-spin correlations are of power-law decay in both systems. The differences between Dirac and Weyl semimetals are also inverstigagted. [Preview Abstract] |
Friday, March 6, 2015 10:48AM - 11:00AM |
Y9.00013: Magnetic-field control of topological charge-transport properties in Nd$_{2}$Ir$_{2}$O$_{7}$ Kentaro Ueda, Bohm-Jung Yang, Jun Fujioka, Junichi Shiogai, Atsushi Tsukazaki, Naoto Nagaosa, Yoshinori Tokura Weyl semimetal is a novel quantum phase with topological properties where the linear-dispersive band with a surface Fermi arc state is realized in a three-dimensional bulk. Pyrochlore-type $R_{2}$Ir$_{2}$O$_{7}$ is one of the promising candidates for realization of the Weyl semimetal. In this system, the magnetic ordering pattern is predicted to play a key role for characterizing its electronic band structure. We report the magneto-transport properties in Nd$_{2}$Ir$_{2}$O$_{7}$ single crystals for several magnetic field directions. We reveal that all-in all-out type magnetic domain walls, at which the conductive mode inherent to the surface state of Weyl semimetal may be preserved, are finely controlled by an applied field along [111] direction showing unique hysteresis in resistivity. With applied field along [001] direction, the Nd-4$f$ moment forms 2-in 2-out configuration that turns the insulating state into the anomalous metallic one near the Weyl (semi-)metal phase. A mean-field calculation consistent with our experimental findings suggests that there are a plenty of exotic states in pyrochlore iridates as functions of electron correlation and external magnetic field that tunes Ir-5$d$ spin texture. [Preview Abstract] |
Friday, March 6, 2015 11:00AM - 11:12AM |
Y9.00014: Intrinsic Damping of Collective Spin Modes in a Two-Dimensional Fermi Liquid with Spin-Orbit Interaction Dmitrii Maslov, Saurabh Maiti We address the issue damping of spin collective modes in systems with spin orbit coupling in 2D. We show that these modes exist for arbitrary nature of spin-orbit coupling and are intrinsically damped even in the long wavelength limit. This damping is driven by electron-electron interactions and is unique to spin orbit coupled systems. Its origin is linked to an imperfect cancellation of the self energy and vertex contributions of the interaction. In the Fermi-liquid language, this is an effect arising from residual interaction between quasiparticles. This damping mechanism exists already at T=0 and without impurities and/or phonons. We also discuss the consequences of this damping for the experiment. [Preview Abstract] |
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