Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session P2: Critical Spin Liquids in Strongly Correlated Systems |
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Sponsoring Units: DCMP Chair: T. Senthil, Massachusetts Institute of Technology Room: Spirit of Pittsburgh Ballrom BC |
Wednesday, March 18, 2009 8:00AM - 8:36AM |
P2.00001: Quantum spin liquid in the spin-1/2 triangular antiferromagnet EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$ Invited Speaker: EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$ (Et=C$_{2}$H$_{5}$-, Me=CH$_ {3}$-, dmit=C$_{3}$S$_{5})$ is one of molecular conductors derived from an anion radical of the Pd(dmit)$_{2}$ molecule and closed-shell monocations (Et$_{x}$Me$_{4-x}$Z)$^{+ }$(Z=N, P, As, Sb; $x$=0, 1, 2) [1]. A common feature of these Pd(dmit)$_{2}$ salts is a conducting anion layer where the Pd(dmit)$_{2}$ anions form a dimer unit [Pd(dmit)$_{2}$]$_{2}^{-} $. Electronic structure around the conduction band can be described by a simple tight-binding calculation based on the dimer unit. The conduction band is half-filled and two-dimensional. At ambient pressure, all the Pd (dmit)$_{2}$ salts behave as Mott insulators where one spin is localized on each dimer. Interdimer transfer integrals indicate that the dimers form a quasi (isosceles) triangular lattice. Interdimer transfer integrals can be tuned by the choice of the cation, which deeply affects the electronic state. The EtMe$_{3}$Sb salt has a nearly regular-triangular lattice. The EtMe$_{3}$Sb cations are located between conduction layers and exhibit orientational disorder. The temperature dependence of the magnetic susceptibility is described in terms of the Pad\'e approximant expression based on the high temperature series expansion of $\chi$ of the antiferromagnetic spin-1/2 Heisenberg model on the triangular lattice with an exchange interaction $J$=220-250 K. The $^{13}$C-NMR measurements show no indication of either spin ordering/freezing or an appreciable spin gap down to 1.37 K, which is lower than 1{\%} of $J$ [2]. The specific heat measurements indicate gapless spin excitation. These results strongly suggest that the ground state of the EtMe$_{3}$Sb salt is a gapless spin liquid state. On the other hand, the Et$_{2}$Me$_{2}$Sb salt, which has also a nearly regular-triangular lattice, shows a first-order transition toward a charge separation state (2Dimer$^{-} \quad \to $ Dimer$^{0}$ + Dimer$^{2-})$ at 70 K [3]. \\[0pt] References\\[0pt] 1) R. Kato, \textit{Chem. Rev}., \textbf{104}, 5319 (2004).\\[0pt] 2) T. Itou et al., \textit{Phys. Rev. B}, \textbf{77}, 104413 (2008).\\[0pt] 3) M. Tamura and R. Kato, \textit{Chem. Phys. Lett}., \textbf {387}, 448 (2004). [Preview Abstract] |
Wednesday, March 18, 2009 8:36AM - 9:12AM |
P2.00002: Spin Liquid State of in the $S=1/2$ Hyper-kagome Antiferromagnet Na$_{4}$Ir$_{3}$O$_{8}$ Invited Speaker: A new Ir oxide, Na$_{4}$Ir$_{3}$O$_{8}$, with a cation-ordered (Ir and Na) spinel structure, was discovered [1]. This compound is an $S$=1/2 Mott insulator with d$^{5}$ (low spin state) Ir4+. As a result of the ordering of Ir and Na within spinel B-sites, magnetic Ir$^{4+}$ ions form a three-dimensional network of corner shared triangles, called hyper-kagome lattice, which provides us with a novel playground for the physics of frustration in $S$=1/2 hyper-kagome A new Ir oxide, Na$_{4}$Ir$_{3}$O$_{8}$, with a cation-ordered (Ir and Na) spinel structure, was discovered [1]. This compound is an $S = 1/2$ Mott insulator with d$^{5}$ (low spin state) Ir$^{4+}$. As a result of the ordering of Ir and Na within spinel B-sites, magnetic Ir$^{4+}$ ions form a three-dimensional network of corner shared triangles, called hyper-kagome lattice, which provides us with a novel playground for the physics of frustration in $S$=1/2 hyper-kagome antiferromagnet. It may be interesting to infer that hyper-kagome lattice has a chirality. The result of magnetization measurements indicates the presence of strong antiferromagnetic coupling (Curie-Weiss temperature $\theta_{\rm CW} \sim -650$ K) between $S = 1/2$ spins. Nevertheless, we find no evidence for long range magnetic ordering in this $S = 1/2$ hyper-kagome antiferro-magnet at least down to 2 K, apparently due to the presence of geometrical frustration. The absence of long range ordering was firmly established by the persistence of $^{23}$Na NMR lines down to 2 K without intensity change [2]. These results strongly suggest that the ground state of this system is a three dimensional $S = 1/2$ spin liquid. Unusual spin excitations of this $S = 1/2$ hyper kagome system will be discussed, based on the specific heat and the NMR data at low temperatures. \\[4pt] [1] Y. Okamoto, M. Nohara, H. Aruga-Katori, and H. Takagi, Phys. Rev. Lett., 99, 137207 (2007). \\[0pt] [2] S. Fujiyama, K. Kanoda, Y. Okamoto, and H. Takagi, in preparation. [Preview Abstract] |
Wednesday, March 18, 2009 9:12AM - 9:48AM |
P2.00003: Critical ``metal''-like phases of frustrated spins and bosons in two dimensions Invited Speaker: I will review recent theoretical progress in understanding quantum phases of 2D correlated boson or spin systems which exhibit ``Bose-metal'' type phases with gapless excitations residing on surfaces in momentum space. A spin liquid phase with a Fermi surface of spinons is one example of interest, being potentially relevant to the organic spin liquid materials $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ and EtMe$_3$Sb[Pd(dmit)$_2$]$_2$. I will discuss frustrated spin and boson models with ring exchanges that may stabilize such phases, in particular in the vicinity of the Mott transition as is the case in the organic materials. [Preview Abstract] |
Wednesday, March 18, 2009 9:48AM - 10:24AM |
P2.00004: Gapless spin liquids on the three dimensional hyper-kagome lattice of Na$_4$Ir$_3$O$_8$ Invited Speaker: Recent experiments indicate that Na$_4$Ir$_3$O$_8$, a material in which s=1/2 iridium local moments form a three dimensional network of corner-sharing triangles, may have a quantum spin liquid ground state with gapless spinon excitations. Using a combination of various theoretical approaches, we propose a quantum spin liquid with spinon Fermi surfaces as a favorable candidate for the ground state of the Heisenberg model on the hyper-kagome lattice of Na$_4$Ir$_3$O$_8$. We also present a theory of the bandwidth-controlled metal-insulator transition that may occur as a pressure-tuned transition in this material. We discuss our predictions in relation to the current and future experiments. [Preview Abstract] |
Wednesday, March 18, 2009 10:24AM - 11:00AM |
P2.00005: Numerical Evidence of Gapless Spin Liquids on Ladders Invited Speaker: I will present numerical evidence of strong-coupling phases for quasi-one-dimensional systems as ladder descendants of candidate models for 2D Bose metal and spin liquid states which possess surfaces of gapless excitations. I will discuss the phase diagrams for two concrete models (square lattice boson and triangular lattice spin models with ring-exchange) based on controlled numerical approaches (DMRG and ED), where such strong-coupling phases are realized in a wide regime of parameters. A close comparison between numerical results and slave-particle descriptions will allow us to characterize these phases in detail and identify signatures reflecting the parent 2D states. [Preview Abstract] |
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