Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session P18: Focus Session: Low D/Frustrated Magnetism - Triangular Lattices |
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Sponsoring Units: GMAG DMP Chair: Bruce Gaulin, McMaster University Room: D172 |
Wednesday, March 23, 2011 8:00AM - 8:36AM |
P18.00001: Neutron Scattering Studies of the S=1/2 Triangular Lattice Magnets NaNiO$_2$ and LiNiO$_2$ Invited Speaker: NaNiO$_2$ and LiNiO$_2$ are isostructural quantum magnets based on a stacked triangular lattice in which magnetism arises from S=1/2 magnetic moments carried by Ni$^{3+}$ ions. Surprisingly, while these compounds are structurally and electronically very similar, the magnetic properties they exhibit are dramatically different. NaNiO$_2$ undergoes a cooperative Jahn-Teller phase transition at 480K and magnetically orders below T$_N$ $\sim$ 23K, adopting a structure which consists of ferromagnetic sheets of S=1/2 moments stacked in an antiferromagnetic fashion. In contrast, LiNiO$_2$ undergoes a spin glass transition at T$_g$ $\sim$ 9K and remains disordered down to the lowest measured temperatures. Understanding the absence of long-range magnetic order in LiNiO$_2$ is a problem which has attracted considerable interest for more than twenty five years. Among many potential explanations, the answer has most notably been attributed to geometric frustration caused by inherent mixing of the Li and Ni sublattices, or orbital degeneracy resulting from the lack of a coherent Jahn-Teller distortion. In this talk I will describe time-of-flight neutron scattering measurements performed on polycrystalline samples of NaNiO$_2$ and LiNiO$_2$ using the wide Angular-Range Chopper Spectrometer (ARCS) at ORNL and the Disk Chopper Spectrometer (DCS) at NIST. These measurements provide a thorough characterization of the excitation spectra for these two compounds, probing the inelastic scattering over energy scales ranging from $\sim$ 0.1 meV to 1.5 eV. In NaNiO$_2$, our measurements reveal two sets of well-defined spin excitations, which we associate with ferromagnetic spin waves mediated by in-plane interactions and antiferromagnetic spin waves mediated by out-of-plane interactions. In LiNiO$_2$, we observe similar, albeit much broader, excitations consistent with short-range two-dimensional magnetic correlations. In the case of NaNiO$_2$, we have developed a simple linear spin wave theory model to describe these excitations and extract the relevant magnetic exchange couplings for this system. [Preview Abstract] |
Wednesday, March 23, 2011 8:36AM - 8:48AM |
P18.00002: Magnetic order and glassiness in distorted triangular lattice materials, Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$NO$_{3}$/(C$_{7}$H$_{15}$COO) Jian Wu, Fletcher Werner, Julia S. Wildeboer, Alexander Seidel, Zohar Nussinov, S.A. Solin We have synthesized two series of distorted triangular lattice materials Cu$_{2(1-x)}$Zn$_{2x}$(OH)$_{3}$M, where M = NO$_{3}^{-}$ or C$_{7}$H$_{15}$COO$^{-}$, by hydrothermal reaction. The powder X-ray diffraction measurements show that the substitution of Zn for Cu leads to a series of isostructural doped compounds [1]. The C$_{7}$H$_{15}$COO$^{-}$ long chain intercalated samples display a series of intense (00\emph{l}) reflections, which signals their enhanced 2D structures with an almost doping-independent interlayer distance 24.2\AA. In the DC magnetic susceptibility data for all NO$_{3}^{-}$ samples, we observe clear evidence of transitions from a paramagnetic to antiferromagnet phase. The onset of long-range order is further proven by the prominent features in specific heat data. However, all the long chain intercalated samples were found to display several spin-glass-like behaviors. A clear bifurcation between the ZFC and FC data was observed at T $<$ 15K. The time evolution of isothermal remnant magnetization M$_{ZFC}$(t) has a linear dependence on $\ln(t)$. No peak features or broad maximum have been discovered in the specific heat measurements. Further analysis of the above results suggest that the long chain intercalated samples are cluster spin glasses at low temperature. \newline[1] J.Wu et al, J. Phys. Condens. Matter. 22, 334211(2010). [Preview Abstract] |
Wednesday, March 23, 2011 8:48AM - 9:00AM |
P18.00003: Spin glassiness and power law scaling in anisotropic triangular spin-1/2 antiferromagnets Alexander Seidel, Jian Wu, Julia S. Wildeboer, Fletcher Werner, Zohar Nussinov, S.A. Solin We discuss the magnetic properties of a class of spin $S=1/2$ antiferromagnetic quasi-triangular lattice materials, both in the clean limit and in the presence of non-magnetic Zn impurities. These systems are long organic chain intercalated derivatives of copper hydroxy nitrate, with a very large c-axis separation of 24\AA. In these compounds, we find that a spin glass phase is universally preceded by two different power law regimes in the temperature dependence of the DC magnetic susceptibility, separated by a sharp crossover. This is seen both in the presence as well as in the absence of non-magnetic Zn impurities, where the power law exponents are surprisingly unperturbed by the compositional disorder. We argue that these findings may be consistent with a picture based on a self-generated spin glass in the clean undoped compound, where frustration is the driving mechanism of the glassiness rather than disorder. While AC measurements and time dependent magnetization follow traditional spin glass paradigms, the power law structure found in the DC susceptibility is argued to deviate in various ways from scenarios expected based on Griffiths type physics, and may call for new explanations. [1] J. Wu et al., J. Phys. Condens. Matter, 22, 334211 (2010). [2] J. Wu et al., arXiv:1007.0442 [Preview Abstract] |
Wednesday, March 23, 2011 9:00AM - 9:12AM |
P18.00004: Competing interactions and continuum excitations in the spin-1 triangular lattice antiferromagnet NiGa$_{2}$S$_{4}$ J. Wen, Y. Nambu, J. Rodriguez, C. Stock, S. Nakatsuji, S. Onoda, Y. Maeno, C. Broholm In some geometrically frustrated magnets, conventional long range order is replaced by static or dynamic short range order. Quasi-2D NiGa$_{2}$S$_{4}$ is the only known S=1 antiferromagnet with an exact triangular lattice. Recent neutron scattering experiment on high quality NiGa$_{2}$S$_{4 }$single crystals revealed short range quasi-2D incommensurate spin correlation with a critical wavevector close to (1/6, 1/6, 0) [1]. Here we report a measurement of the dynamic spin correlation function through a volume of \textbf{Q}-E space for T$<<$J. A gapless spectrum was observed at the incommensurate critical wavevector while a softened but still gapped response was found at (1/3, 1/3, 0). This indicates dominant third nearest neighbor interaction and competing weaker near neighbor interactions. The excitation spectrum takes the form of a bounded continuum throughout the 2D Brillouin zone. The temperature dependence dynamic correlation length shows that short range correlation persist up to $\Theta _{cw}$=-80(2)K. \\[0pt] [1] C.Stock, \textit{et al, }Phys.Rev.Lett.105,037402 (2010) [Preview Abstract] |
Wednesday, March 23, 2011 9:12AM - 9:24AM |
P18.00005: Quantum Antiferromagnet on an Anisotropic Triangular Lattice Sedigh Ghamari, Catherine Kallin, Sung-Sik Lee The effects of quantum fluctuations on the spin 1/2 Heisenberg antiferromagnet on a triangular lattice, with diagonal interchain exchange J' weaker than the intrachain exchange J, are studied. This model is of considerable interest because of its relevance to Cs2CuCl4, where experiments have been interpreted as evidence for a nearby two-dimensional spin liquid and because numerous theoretical studies have proposed that the incommensurate spiral spin density wave order is destroyed by quantum fluctuations well before the one-dimensional limit (J'=0) is reached. [Preview Abstract] |
Wednesday, March 23, 2011 9:24AM - 9:36AM |
P18.00006: ESR as a probe of spinon excitations of the spin-1/2 antiferromagnet Cs$_2$CuCl$_4$ K. Povarov, A. Smirnov, O. Starykh, S. Petrov, A. Shapiro We report dramatic manifestation of the uniform Dzyaloshinskii-Moriya (DM) interaction in the ESR response of quasi-one-dimensional spin-1/2 antiferromagnet Cs$_2$CuCl$_4$. We find the ESR response in the range 10-90 GHz to be strongly sensitive to the relative orientation of the magnetic field and DM axes. Most importantly, we observe splitting of the ESR line into two lines in the paramagnetic phase, upon lowering the temperature from 10 K to 1.3 K. The latter temperature is about twice the ordering temperature $T_N = 0.62$ K, and yet far below the Curie-Weiss temperature 4 K . The splitting occurs when the static magnetic field H is aligned with one of the DM axes of the material and is absent when H is oriented perpendicular to the axes. This novel phenomenon is a consequence of the critical nature of fractionalized spinon excitations of the individual antiferromagnetic chains in the paramagnetic phase. A uniform (along the spin chain direction) DM interaction provides an effective magnetic field, the sign of which is opposite for the right- and left-moving spinons. In the presence of external magnetic field this difference translates into a pair of ESR frequencies, making the experiment a novel probe of spinon excitations. [Preview Abstract] |
Wednesday, March 23, 2011 9:36AM - 9:48AM |
P18.00007: Metallic magnetization plateau on triangular lattice Zhihao Hao, Oleg Starykh The $1/3$ magnetization plateau is well established for spin $1/2$ Heisenberg antiferromagnetic model on triangular lattice. The state is stable against a large range of spatial anisotropies and is observed in the triangular compound Cs$_2$CuBr$_4$. A natural question to ask is whether the plateau state remains stable if the on-site repulsion $U$ is lowered for the underlying Hubbard model. In our work, we studied the one-band Hubbard model on triangular lattice. Through mean-field calculations, it is discovered that an up-up-down spin density wave state with $1/3$ of saturation magnetization is stablized for a range of $U$ and magnetic field $h$. For $4.44t\le U\le 4.55t$, the state is a half metal: the spin up bands remain metallic while the spin down bands are insulating. For $U> 4.55t$, the spin up bands become gapped and the system is an insulator. It is speculated the plateau state remains stable for the entire range of $U\ge 4.44t$. [Preview Abstract] |
Wednesday, March 23, 2011 9:48AM - 10:00AM |
P18.00008: Phase Diagram of a Geometrically-Frustrated Triangular-Lattice Antiferromagnet in a Magnetic Field Randy Fishman The magnetic phase diagram of a geometrically-frustrated triangular-lattice antiferromagnet is evaluated as a function of magnetic field and anisotropy using a trial spin state built from harmonics of a fundamental ordering wavevector. A non-collinear incommensurate state, observed to be chiral and ferroelectric in CuFeO$_2$, appears above a collinear state with 4 sublattices (SLs). A previously unobserved collinear 5-SL phase may be accessible in a magnetic field for a non-stoichiometric compound with excess or deficient oxygen. The apparent absence of multiferroic behavior for predicted chiral, non-collinear 5-SL states poses a challenge to theories of the ferroelectric coupling in CuFeO$_2$. [Preview Abstract] |
Wednesday, March 23, 2011 10:00AM - 10:12AM |
P18.00009: ABSTRACT WITHDRAWN |
Wednesday, March 23, 2011 10:12AM - 10:24AM |
P18.00010: Phases of spatially anisotropic triangular antiferromagnet in high magnetic field Oleg Starykh, Andrey Chubukov We investigate phases of the Heisenberg spin model on a spatially anisotropic triangular lattice as a function of $J'/J <1$ and a magnetic field $H$ ($J$ is the exchange along the horizontal bonds, and $J'$ is the exchange along the diagonal bonds). The anisotropy of $J$'s competes with quantum fluctuations and this competition leads to a rich phase diagram. Immediately below the saturation field $H_s$ we find three phases: three-sublattice commensurate phase, incommensurate co-planar ``fan'' phase, and incommensurate non-coplanar ``cone'' phase. The former two are supersolids while the latter is a superfluid in the terminology of strongly interacting bosons. At a finite boson density ($H < H_s$) and on approach to the fan-cone phase boundary from within the cone phase with ordering momentum $Q$, we observe softening of the ``roton'' minima at momentum $Q'$ different from $-Q$, which one would expect for a direct cone-fan transition. This points on the existence of the intermediate double-spiral state in which boson density exhibits incommensurate modulations with momenta $Q$ and $Q'$. The extrapolation of our results to $H \sim H_s/3$ predicts that $Q'=Q$, and the intermediate state becomes similar to the ``distorted umbrella'' state that emerges out of up-up-down phase. We discuss the implications of our findings for the global phase diagram of the anisotropic triangular Heisenberg antiferromagnet. [Preview Abstract] |
Wednesday, March 23, 2011 10:24AM - 10:36AM |
P18.00011: Theory of SU(2) invariant spin liquids on the triangular lattice with spinful Majorana excitations Rudro Biswas, Liang Fu, Chris Laumann, Subir Sachdev We present a theory of SU(2) invariant spin liquids on the 2D spin 1/2 triangular lattice described by a parton representation of the spin in terms of spin-1 Majorana particles. These spin liquids break time reversal symmetry and generically possess a novel Fermi surface consisting of three lines intersecting at $k=0$ as well as an unconventional dynamic critical exponent $z=3$. We also present calculations for observable quantities and discuss possible connections to recent experiments involving spin 1/2 Heisenberg triangular lattices. [Preview Abstract] |
Wednesday, March 23, 2011 10:36AM - 10:48AM |
P18.00012: Phase Diagram of Classical Heisenberg Antiferromagnets with Four-Spin Interactions on Stacked Triangular Lattice under Magnetic field Shintaro Takayoshi, Masahiro Sato Classical Heisenberg antiferromagnet (HAF) on stacked triangular lattice is a simple and important model of frustrated systems. Although there are some candidate materials for triangular HAF, they are not ideal ones and various kinds of perturbations should be present. While it is well known that the ground state of triangular HAF is 120-degree structure, how perturbations destabilize the structure has not been well studied. In this study, we consider effects of four-spin interactions on magnetic phase diagram of triangular HAF. In fact, some real mechanisms of generating four-spin interactions have been known: higher-order electron hopping processes in Mott insulators, spin-phonon couplings, etc. We complete the magnetic phase diagrams by using Monte Carlo simulation. We will report new phases induced by four-spin interactions. [Preview Abstract] |
Wednesday, March 23, 2011 10:48AM - 11:00AM |
P18.00013: Magnetic ground state and excitation of SrV_${10}$O$_{15}$ Jooseop Lee, Kazuki Iida, Matthew Stone, Masaaki Matsuda, Tomomasa Kazita, Takuro Katsufuji, Seunghun Lee SrV$_{10}$O$_{15}$ has magnetic bilayers composed of triangular lattices with periodical missing, which can be an interesting playground for the study of frustration. In this new type of magnetic structure, using neutron powder diffraction, we find a magnetic ground state below 45K with ordering vector Q=(0 1/2 1), and study detailed spin configuration. Magnetic excitations have also been investigated using a single crystal at ARCS, a time-of-flight neutron chopper spectrometer at SNS. Our results show quite interesting highly dispersive dispersion relations: a gapless Goldstone mode is strongly dispersive along the a and c axis, and is less-strongly dispersive along the b axis. Another mode is dispersionless along the a and b axis, and is strongly dispersive along the c axis. We determine the spin hamiltonian that sheds light in understanding the interplay between orbital, spin, charge, and lattice degrees of freedom in this compound. [Preview Abstract] |
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