Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session D4: Focus Session: Itinerant Frustrated Magnets |
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Sponsoring Units: GMAG Chair: Graeme Luke, McMaster University Room: 112/110 |
Monday, March 3, 2014 2:30PM - 2:42PM |
D4.00001: Loop-liquid State in an Ising-spin Kondo Lattice Model on a Kagome Latticeh Yukitoshi Motome, Hiroaki Ishizuka Emergence of paramagnetic states with strong local correlations is one of the characteristic features of geometrically frustrated magnets. One such example is the spin-ice compounds, where all the tetrahedra favor two-in two-out spin configurations, and the ``in" and ``out" spins individually form loop-like structures on the pyrochlore lattice. When such a locally-correlated spin state is coupled to itinerant electrons, recent theoretical studies have shown that it considerably affects the electronic state of coupled electrons. While several studies have been done, possible realization and stability of such a correlated-disorder state remains to be studied. To address these issues in the presence of spin-charge coupling, we here study an Ising-spin Kondo lattice model on a kagome lattice [1]. By using a Monte Carlo simulation, we show that a locally-correlated state with all the triangles being in up-up-down spin configurations spontaneously emerges in this model. We also show that the peculiar state considerably affects the electronic state, giving rise to a resonant peak in the optical conductivity. [1] H. Ishizuka and Y. Motome, PRB 88 081105 (2013). [Preview Abstract] |
Monday, March 3, 2014 2:42PM - 2:54PM |
D4.00002: New Kagom\'{e} Metal Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$--- Quantum Spin-Liquid Candidate? Hua He, Wojciech Miiller, Meigan Aronson While most of the reported Kagom\'{e} systems are semiconductors or insulators, in which the magnetic moments have a highly localized character, here we present a new intermetallic compound, Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$, as a rare example of a Kagom\'{e} metal. The structure of the compound was established by single-crystal X-ray diffraction, and it crystallizes with a hexagonal structure (Sc$_{3}$Ni$_{11}$Si$_{4}$ type) with Mn atoms forming the Kagom\'{e} lattice. The \textit{dc} magnetic susceptibility measurements find a Curie-Weiss moment of $\sim$ 0.51 $\mu_{\mathrm{B}}$/Mn, however, no magnetic order is found for temperatures as low as 1.8 K. Electrical resistivity and heat capacity measurements show that this compound is definitively metallic, with an enhanced specific heat Sommerfeld coefficient below 10K, indicating strong electronic correlations. Intriguingly, these features have revealed Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$ as a possible quantum spin liquid. The role of the geometrically frustrated structure and Mn-ligand hybridization in the magnetism of Sc$_{3}$Mn$_{3}$Al$_{7}$Si$_{5}$ is also discussed. [Preview Abstract] |
Monday, March 3, 2014 2:54PM - 3:06PM |
D4.00003: Noncoplanar magnetism in the Hubbard model on frustrated lattices Sanjeev Kumar, Kanika Pasrija Ferromagnets and staggered antiferromagnets are the most common forms of magnetic orderings that one comes across in models and materials. However, during the last few years non-collinear and non-coplanar magnetic states have been of special interest for condensed matter researchers due to their relevance to a variety of phenomena, such as, ferroelectricity, anomalous Hall effect, etc. A number of theoretical studies have shown that such magnetic states exist in Kondo-lattice model at special band-fillings on various geometrically frustrated lattices. It was recently shown that the Kondo-lattice model on a checkerboard lattice supports non-coplanar magnetic states which lead to a topologically non-trivial band gap in the electronic spectrum (PRL 109, 166405(2012)). We begin by asking if such magnetic ground-states can also be realized in a Hubbard model which, unlike the Kondo-lattice model, does not contain ``pre-formed'' localized magnetic moments. We make use of a mean-field decoupling scheme which allows for non-collinear and non-coplanar magnetic states in the Hubbard model. We show that the triangular lattice and the checkerboard lattice do support non-coplanar magnetic phases similar to the ones found in a Kondo-lattice model. [Preview Abstract] |
Monday, March 3, 2014 3:06PM - 3:18PM |
D4.00004: Tuning a spin-liquid into a correlated metal in Na$_{4-x}$Ir$_3$O$_{8-\delta}$ Yogesh Singh, Ashwini Balodhi Na$_4$Ir$_3$O$_8$ is a candidate material for a 3D quantum spin-liquid. We present a comprehensive study of the structure, magnetic susceptibility, heat capacity, and electrical transport on polycrystalline samples with nominal composition Na$_{4-x}$Ir$_3$O$_8$ ($x \approx -.08~{\rm to}~1$). The structure refinement shows that even though Na vacancies are being introduced the lattice parameters do not change much with $x$. The $x \geq 0$ samples show insulating behavior with strong antiferromagnetic interactions between effective $S = 1/2$ Ir$^{4+}$ moments. For the Na$_{4.08}$Ir$_3$O$_8$ sample, magnetic susceptibility suggests a magnetic transition below $\approx 15 K$. The $x \approx 1$ sample is a paramagnetic (semi)metal with various physical properties suggesting strong electronic correlations. The materials mid-way between the insulating and metallic samples show indication of having both localized and itinerant electrons. The strong antiferromagnetic interactions present in the $x = 0$ material survive in these mixed materials making them candidate spin-liquids in the presence of itinerant electrons. The electrical transport of the doped materials are consistent with the behavior of a semi-metal/semi-conductor with low carrier concentrations. [Preview Abstract] |
Monday, March 3, 2014 3:18PM - 3:30PM |
D4.00005: Anomalous Hall Effect Arising form Noncollinear Antiferromagnetism: Mn3Ir as an Example Hua Chen, Qian Niu, Allan H. MacDonald Ferromagnetic conductors exhibit anomalous contributions to their transverse (Hall) conductivities that cannot be attributed to Lorentz force on electrons from a magnetic field. The anomalous Hall conductivity is often assumed to be proportional to the magnetization, allowing transport measurements to be used in spintronics as a convenient proxy for magnetometry. However, simple symmetry arguments demonstrate that the anomalous Hall effect requires only time-reversal symmetry breaking and spin-orbit coupling, not net magnetization, and we illustrate our ideas by examining a toy model of noncollinear antiferromagnet on a two-dimensional kagome lattice. This is further backed up with a realistic example based on first-principles calculations, predicting that single-crystals of Mn$_3$Ir, a high-temperature antiferromagnet commenly used in spin-valve devices, have large anomalous Hall conductivities. Hua Chen, Qian Niu, and Allan H. MacDonald, arXiv:1309.4041 [Preview Abstract] |
Monday, March 3, 2014 3:30PM - 3:42PM |
D4.00006: Reconsidering the magnetic structure in NiS$_{2-x}$Se$_{x}$ Shinichiro Yano, Despina Louca, Utpal Chatterjee, Duck Young Chung, Daniel E. Bugaris, Mercouri Kanatzidis, Joerg C. Neuefeind, Mikhail Feygenson The Mott metal-insulator transition (MIT) has been at the forefront of condensed matter research for decades. A Mott insulator is associated with antiferromagnetism (AFM) as well as an energy gap. The AFM order parameter can be directly traced by neutron scattering measurements. We focused on the study of MIT on the NiS$_{2-2x}$Se$_{x}$ system. With increase in $x$ where $x$ corresponds to the atomic volume of S that is replaced by Se, the system undergoes an AFM insulator to an AFM metalic transition at $x = 0.43$ at $T = 0$. Although NiS$_{2-2x}$Se$_{x}$ has been previously studied, the magnetic structure is not well understood. We measured the powder neutron diffraction for 4 compositions, $x =$ 0, 0.4, 0.6, and 0.8, as a function of temperature. At T = 2K, we observed a clear composition dependence of the magnetic structure. While NiS$_2$ (x = 0) has two magnetic propagation vectors, M1 = (000) and M2 = (0.5 0.5 0.5), NiS$_{1.6}$Se$_{0.4}$ and NiS$_{1.4}$Se$_{0.6}$ have only one magnetic phase, M1. However, the M1 structure vanishes by NiS$_{1.2}$Se$_{0.8}$. While the two magnetic phases have been previously reported, we determined the magnetic structures by using representation analysis.the magnetic structure and physical properties of this system will be discussed. [Preview Abstract] |
Monday, March 3, 2014 3:42PM - 3:54PM |
D4.00007: Theory of a Competitive Spin Liquid State for Weak Mott Insulators on the Triangular Lattice Ryan V. Mishmash, James R. Garrison, Samuel Bieri, Cenke Xu We propose a novel quantum spin liquid state that can explain many of the intriguing experimental properties of the low-temperature phase of the organic spin liquid candidate materials $\kappa$-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ and EtMe$_{3}$Sb[Pd(dmit)$_{2}$]$_{2}$. This state of paired fermionic spinons preserves all symmetries of the system, and it has a gapless excitation spectrum with quadratic bands that touch at momentum $\vec{k}=0$. This quadratic band touching is protected by symmetries. Using variational Monte Carlo techniques, we show that this state has highly competitive energy in the triangular lattice Heisenberg model supplemented with a realistically large ring-exchange term. [Preview Abstract] |
Monday, March 3, 2014 3:54PM - 4:06PM |
D4.00008: Quantum Oscillations of the Metallic Triangular-Lattice Antiferromagnet PdCrO$_{2}$ Jong Mok Ok, Y.J. Jo, Kyoo Kim, T. shishidou, E.S. Choi, Han Jin Noh, T. Oguchi, B.I. Min, Jun Sung Kim We report the electronic and transport properties of the triangular antiferromagnet PdCrO$_{2}$ at high magnetic fields up to 33 T, using measurements of the de Haas-van Alphen oscillations and the Hall resistivity. The de Haas-van Alphen oscillations below the magnetic ordering temperature T$_{\mathrm{N}}$ reveal several two-dimensional Fermi surfaces of smaller size than those found in nonmagnetic PdCoO$_{2}$, consistent with the band structure calculation. This evidences Fermi surface reconstruction due to the 120$^{\circ}$ helical ordering of the localized Cr spins, suggesting significant coupling of the itinerant electrons to the underlying spin texture. This induces the nonlinear Hall resistivity at low temperatures via the magnetic breakdown in the reconstructed Fermi surface. Furthermore, such a coupling leads to the unconventional anomalous Hall effects near T$_{\mathrm{N}}$ due to the field-induced spin chirality at high magnetic fields. [Preview Abstract] |
Monday, March 3, 2014 4:06PM - 4:18PM |
D4.00009: Importance of anisotropy in the spin-liquid candidate Me$_3$EtSb[Pd(dmit)$_2$]$_2$ Luca F. Tocchio, Anthony Jacko, Harald O. Jeschke, Roser Valenti Organic charge transfer salts based on the molecule Pd(dmit)$_2$ display strong electronic correlations and geometrical frustration, leading to spin liquid, valence bond solid, and superconducting states, amongst other interesting phases. The low energy electronic degrees of freedom of these materials are often described by a single band model; a triangular lattice with a molecular orbital representing a Pd(dmit)$_2$ dimer on each site. We use \textit{ab initio} electronic structure calculations to construct and parametrize low energy effective model Hamiltonians for a class of Me$_{4-n}$ Et$_nX$[Pd(dmit)$_2$]$_2$ ($X$=As, P, N, Sb) salts and investigate how best to model these systems by using variational Monte Carlo (VMC) simulations. Our findings suggest that the prevailing model of these systems as a $t-t'$ triangular lattice is incomplete, and that a fully anisotropic triangular lattice description produces importantly different results, including a significant lowering of the critical $U$ of the spin-liquid phase.[1,2] [1] A.C. Jacko, L. F. Tocchio, H. O. Jeschke, R. Valenti, Phys. Rev. B 88, 155139 (2013). [2] L. Tocchio, H. Feldner, F. Becca, R. Valenti, C. Gros Phys. Rev. B 87, 035143 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:18PM - 4:54PM |
D4.00010: Itinerant spin ice Invited Speaker: Masafumi Udagawa Spin ice is a prototypical frustrated magnet defined on a pyrochlore lattice. The ground state of spin ice is described by a simple rule called ``ice rule": out of four spins on a tetrahedron, two spins point inward, while the other two outward. This simple rule is not sufficient to determine the spin configuration uniquely, but it leaves macroscopic degeneracy in the ground state. Despite the macroscopic degeneracy, however, the ground state is not completely disordered, but it exhibits algebraic spatial correlation, which characterizes this state as ``Coulomb phase'' where various exotic properties, such as monopole excitations and unusual magnetic responses are observed. Given the peculiar spatial correlation, it is interesting to ask what happens if itinerant electrons coexist and interact with spin ice. Indeed, this setting is relevant to several metallic Ir pyrochlore oxides, such as Ln$_2$Ir$_2$O$_7$ (Ln=Pr, Nd), where Ir 5d itinerant electrons interact with Ln 4f localized moments. In these compounds, anomalous transport phenomena have been reported, such as non-monotonic magnetic field dependence of Hall conductivity [1] and low-temperature resistivity upturn [2]. To address these issues, we adopt a spin-ice-type Ising Kondo lattice model on a pyrochlore lattice, and solve this model by applying the cluster dynamical mean-field theory and the perturbation expansion in terms of the spin-electron coupling. As a result, we found that (i) the resistivity shows a minimum at a characteristic temperature below which spin ice correlation sets in [3]. Moreover, (ii) the Hall conductivity shows anisotropic and non-monotonic magnetic field dependence due to the scattering from the spatially extended spin scalar chirality incorporated in spin ice manifold [4]. These results give unified understanding to the thermodynamic and transport properties of Ln$_2$Ir$_2$O$_7$ (Ln=Pr, Nd), and give new insights into the role of geometrical frustration in itinerant systems. This work has been done in collaboration with H. Ishizuka, Y. Motome and R. Moessner. \\[4pt] [1] Y. Machida et al., Phys. Rev. Lett. 98, 057203 (2007).\\[0pt] [2] S. Nakatsuji et al., Phys. Rev. Lett. 96, 087204 (2006).\\[0pt] [3] M. Udagawa, H. Ishizuka and Y. Motome, Phys. Rev. Lett. 108, 066406 (2012).\\[0pt] [4] M. Udagawa and R. Moessner, Phys. Rev. Lett., 111, 036602 (2013). [Preview Abstract] |
Monday, March 3, 2014 4:54PM - 5:06PM |
D4.00011: Many-Variable Variational Monte Carlo Study of Triangular Hubbard Model with Next-Nearest-Neighbor Hopping Ryui Kaneko, Satoshi Morita, Masatoshi Imada Motivated by previous numerical studies on the triangular Hubbard model, we study how next-nearest-neighbor hopping affects the ground states of the model at half filling by using the many-variable variational Monte Carlo method. We consider the fermionic type variational wave functions with the Gutzwiller-Jastrow factor and the projection that restore the lattice point group symmetry. We find that the spin liquid state, sandwiched by the metallic state and the antiferromagnetic insulating states with 120$^{\circ}$ or stripe spin structure, becomes more stable as the negative next-nearest-neighbor hopping increases. By using the total momentum projection scheme, we also find that the spin liquid state is characterized by nearly gapless excitations in extended total momenta. Possible nature of the present spin liquid state is discussed. [Preview Abstract] |
Monday, March 3, 2014 5:06PM - 5:18PM |
D4.00012: Two-peak structure in the K-edge RIXS spectra of a spatially frustrated Heisenberg antiferromagnet Trinanjan Datta, Cheng Luo, Dao-Xin Yao Quantum fluctuations due to spatial anisotropy and strong magnetic frustration lead to the formation of a two-peak structure in the K-edge bimagnon RIXS intensity spectra of a Jx-Jy-J2 Heisenberg model on a square lattice. We compute the RIXS intensity, including up to first order 1/S spin wave expansion correction, using the Bethe-Salpeter equation within the ladder approximation scheme. The two-peak feature occurs in both the antiferromagnetic phase and the collinear antiferromagnetic phase. A knowledge of the peak splitting energy from both magnetically ordered regime can provide experimentalists with an alternative means to measure and study the effects of local microscopic exchange constants. [Preview Abstract] |
Monday, March 3, 2014 5:18PM - 5:30PM |
D4.00013: First-Principles Study on New Spin Liquid Candidate $\kappa$-H$_3$(Cat-EDT-TTF)$_2$ Takao Tsumuraya, Hitoshi Seo, Reizo Kato, Tsuyoshi Miyazaki A new class of molecular conductors based on catechol with ethylenedithiote-tetrathiafulvalene, (H$_2$Cat-EDT-TTF) has been synthesized recently. Among them, $\kappa$-type H$_3$(Cat-EDT-TTF)$_2$ is considered to be a dimer-type Mott insulator at ambient pressure and emerges as a candidate of realizing quantum spin liquid down to lowest temperature.[1] In this crystal, two H$_2$Cat-EDT-TTF molecules share a hydrogen (H) atom, and face-to-face dimers are formed in a anisotropic triangular lattice. Differently from conventional charge transfer salts, this compound does not have insulating layers. Here we study $\kappa$-H$_3$(Cat-EDT-TTF)$_2$ based on first-principles density-functional theory (DFT) calculations. We evaluate inter-dimer hopping integrals by fitting to the DFT bands, and find a quasi-one-dimensional anisotropy in the effective dimer-dimer interactions with frustrated inter-chain couplings. Furthermore, the inter-layer hopping integrals are non-negligible compared to the intra-layer couplings and the Fermi surface shows a warped cylinder, indicating their three-dimensionality of the electronic structure. Lastly, we report sensitivity of the electronic structure depending on the position of the shared H atom. [1] T. Isono et al, Nature Comm. 4 (2013) 1344; Priv. comm. [Preview Abstract] |
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