Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session V45: Spin Structure and Phase Transitions: Theory |
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Sponsoring Units: GMAG Chair: Donald J. Priour, Jr., University of Maryland Room: Baltimore Convention Center 348 |
Thursday, March 16, 2006 11:15AM - 11:27AM |
V45.00001: Disorder and ferromagnetism in a graphene sheet Nuno M.R. Peres, Francisco Guinea, Antonio H.C. Neto We study the presence of ferromagnetism in the phase diagram of the two-dimensional honeycomb lattice close to half-filling (graphene) as a function of the strength of the Coulomb interaction and doping. We show that exchange interactions between Dirac fermions can stabilize a ferromagnetic phase at low doping when the coupling is sufficiently large. In clean systems the zero temperature phase diagram shows both first order and second order transition lines and two distinct ferromagnetic phases: one phase with only one type of carriers (either electrons or holes) and another with two types of carriers (electrons and holes). Using the coherent potential approximation we argue that disorder further stabilizes the ferromagnetic phase. [Preview Abstract] |
Thursday, March 16, 2006 11:27AM - 11:39AM |
V45.00002: The exchange instability in a graphene bilayer Johan Nilsson, Antonio Castro Neto, Nuno Peres, Francisco Guinea We study the problem of the formation of electron-hole pockets in a graphene bilayer(two layers of graphite). By means of a variational calculation in a simple model we show that pockets can be formed due to the combined effects of the Coulomb interaction and a coherent nearest neighbor hopping within the planes and between the layers. The variational calculation shows that the non-interacting ground-state is unstable and that a small net ferromagnetism can be obtained. Upon doping the system away from half filling the instability goes away above a critical value of the doping. We also discuss how modifications to the model such as the inclusion of screening and disorder will affect the instability. [Preview Abstract] |
Thursday, March 16, 2006 11:39AM - 11:51AM |
V45.00003: Spin Liquid States on the Triangular Lattice: A Projective Symmetry Group Analysis of Schwinger Boson States Fa Wang, Ashvin Vishwanath Recent numerical and experimental results on quantum spin systems on the triangular lattice have revealed several unusual phenomena. Motivated by these developments, we apply the Projective Symmetry Group Analysis, previously used to classify spin liquid states obtained within a fermionic spin representation, to Schwinger bosons mean field states on the triangular lattice. We find several distinct $Z_2$ spin liquid states, but restricting attention to states with nonvanishing nearest neighbor amplitudes we find only two candidates. The first (zero-flux state) is the well known state introduced by Sachdev, which on condensation of spinons leads to the 120 degree ordered state. The other solution which we call the Pi-flux state has not previously been discussed. It is found to be stable against order up to a much larger value of the quantum parameter than the zero-flux state, which makes it an attractive candidate spin liquid state. When spinon condensation does occur, the ordering wavevector is at the Brillouin zone edge centers, different from the 120 degree state. While the zero-flux state is more stable with just nearest neighbor exchange, we find that the introduction of either next-neighbor antiferromagnetic exchange or four spin ring exchange tends to favor the Pi-flux state. [Preview Abstract] |
Thursday, March 16, 2006 11:51AM - 12:03PM |
V45.00004: Spin current and spiral spinons in the Heisenberg spin chain Yupeng Wang The twisted boundary magnetic field effects on the one dimensional antiferromagnetic spin chains with spin half and spin one are studied via the density matrix renormalization group method. A spin voltage controlled by the twisted boundary fields as well as the spin current are observed for the spin half chain. The correlation functions reveal the spiral nature of spin structure for both ground state and the spinon excitations for the spin half case. For the spin one chain, a kink is generated in the ground state and the midgap state is induced by small boundary fields. [Preview Abstract] |
Thursday, March 16, 2006 12:03PM - 12:15PM |
V45.00005: Partial order in MnSi? John Hopkinson, Hae-Young Kee Motivated by a recent neutron scattering study of MnSi, we study the ferromagnetic Heisenberg model with Dzyaloshinskii-Moriya interactions on the 3D corner-shared triangle lattice, the magnetic sublattice of MnSi. For classical spins we find evidence of a partially ordered state: degenerate states form a sphere with a fixed radius in momentum space. Further, for a metallic system, we propose an assignment of the Moriya vectors following an RKKY derivation of Fert and Levy. We calculate the structure factor within the mean field approximation and test this result against classical monte carlo calculations. We discuss the connection of our results to phenomenological Landau-Ginzburg treatments of the Dzyaloshinskii-Moriya coupling on this lattice. [Preview Abstract] |
Thursday, March 16, 2006 12:15PM - 12:27PM |
V45.00006: No dominance of RKKY interaction in 4f-magnets German Samolyuk, Vladimir Antropov It is a common belief that in the rare earth materials the dominating exchange mechanism is the magnetic coupling between localized 4f–moments mediated by nearly free conduction electrons resulting in so-called indirect coupling or RKKY interaction. In addition, several experimental proofs have been published in the past. We reanalyzed the validity of this statement from a point of view of realistic atomic and solid state electronic structure. The experimental proofs have been reanalyzed as well. In general, we found that no theoretical validation of RKKY dominance in 4f-magnets exist and all experimental ‘evidences’ cannot rule out other mechanisms. We claim that the direct interatomic exchange between conduction electrons must be always taken into account. [Preview Abstract] |
Thursday, March 16, 2006 12:27PM - 12:39PM |
V45.00007: New classification of magnets Vladimir Antropov Standard methods classify magnets using their magnetic long range order at T=0 (ferromagnets, antiferromagnets and so on). Some classifications can be created inside those groups as well. In addition, there is a very attractive and widely used qualitative ‘localized-itinerant’ picture, which, however, does not have any real criteria behind. I would like to introduce a rather universal and possible quantitative classification using the idea of magnetic short range order, which we recently found theoretically and which has been for years observed experimentally in many materials. [Preview Abstract] |
Thursday, March 16, 2006 12:39PM - 12:51PM |
V45.00008: Exact Diagonalization Study of the Quantum Antiferromagnet Cs$_2$CuCl$_4$ Ookie Ma, J. B. Marston, V. F. Mitrovi{\'c}, M.-A. Vachon We exactly diagonalize a model of the quantum antiferromagnet Cs$_2$CuCl$_4$. The quasi two-dimensional material is one of only two strong candidate systems that may exhibit a spin-liquid phase\footnote{R. Coldea, D. A. Tennant, A. M. Tsvelik and Z. Tylczynski, Phys. Rev. Lett. {\bf 86}, 1335 (2001).}. To compare with NMR experiments we calculate the total magnetization for clusters of up to 24 sites as a function of temperature and the applied magnetic field. The spin-1/2 degrees of freedom reside on an anisotropic triangular lattice; in addition to the usual nearest-neighbor Heisenberg exchange, interactions of the Dzyaloshinskii-Moriya type must also be included\footnote{K. Kodama, S. Miyahara, M. Takigawa, M. Horvatic, C. Berthier, F. Mila, H. Kageyama and Y. Ueda, J. Phys.: Condens. Matter {\bf 17} L61 (2005).}. We investigate the potential importance of additional higher-order terms\footnote{T. Yildirim, A. B. Harris, Amnon Aharony and O. Entin-Wohlman, Phys. Rev. B {\bf 52}, 10239 (1995).}. [Preview Abstract] |
Thursday, March 16, 2006 12:51PM - 1:03PM |
V45.00009: An \textit{ab initio} study of the giant magnetocaloric effect in MnAs Hyejung Kim, Yia-Chung Chang, Sahraoui Chaieb MnAs is one of the materials which show a giant magnetocaloric effect. Electronic and magnetic properties of MnAs in NiAs structure and hypothetical zinc-blende structure are studied using a full potential linear augmented-Slater-type-orbital (LASTO) method within the local spin density approximation. Total energies and magnetic moments as a function of volume as well as band structures are in agreement with previous calculations employing different methods such as full potential linearized augmented plane wave method and a plane-wave pseudopotential implementation. Exchange coupling parameters are determined with a frozen magnon approach. Using the exchange coupling parameters obtained, a mean-field theory is applied to calculate the quantities of interest such as the magnetic entropy and free energy as a function of temperature and the critical temperature, to evaluate the giant magnetocaloric effect in MnAs. [Preview Abstract] |
Thursday, March 16, 2006 1:03PM - 1:15PM |
V45.00010: Spatially Anisotropic d=3 Ising, XY Magnetic and Percolation Systems: Exact Renormalization-Group Solutions of Hierarchical Models Aykut Erba\c{s}, Asli Tuncer, Burcu Y\"ucesoy, A. Nihat Berker Hierarchical lattices that constitute spatially anisotropic systems are introduced. These lattices provide exact solutions for hierarchical models and, simultaneously, approximate solutions for uniaxially or fully anisotropic $d=3$ physical models. The global phase diagrams, with $d=2$ and $d=1$ to $d=3 $ crossovers, are obtained for Ising, XY magnetic models and percolation systems, including crossovers from algebraic order to true long-range order.$^1$ Our approach has been applied to spatially anisotropic $d=3$ tJ models of electronic conduction.$^2$\\ 1. A. Erba\c{s}, A. Tuncer, B. Y\"ucesoy, and A.N. Berker, Phys. Rev. E {\bf 72}, 026129 (2005).\\ 2. M. Hinczewski and A.N. Berker, cond-mat/0504741. [Preview Abstract] |
Thursday, March 16, 2006 1:15PM - 1:27PM |
V45.00011: Simulation of magnetization configurations with constricting magnetization boundary conditions in 2 and 3-D soft-hard magnetic structures Jong Han, Hemachander Subramanian A soft magnet embedded with hard-magnetic spheres/cylinders magnetized to constrict the soft magnet’s magnetization creates a system with multiple minima and related hysterisis effects. Due to many distinct magnetic structures in the soft-magnetic region at fixed magnetization on hard-magnets, frustration develops even in simple hard-magnetic arrays coupled via ferromagnetic exchange interaction. This presentation reports the simulation of such systems. We observed multiple minimum energy configurations and transitions between them in an externally applied magnetic field and/or change in hard magnets’ magnetization. Hysteresis effects due to the above changes are observed. Electron transport through these systems can generate interesting magneto-resistance effects. [Preview Abstract] |
Thursday, March 16, 2006 1:27PM - 1:39PM |
V45.00012: Topological transition in a two-dimensional model of liquid crystal Ana Isabel Fari\~nas-Sanchez, Ricardo Paredes V, Bertrand Berche Simulations of nematic-isotropic transition of liquid crystals in two dimensions are performed using an O(2) vector model characterized by nonlinear nearest neighbor spin interaction governed by the fourth Legendre polynomial $P_4$. The system is studied through standard finite-size scaling and conformal rescaling of density profiles of correlation functions. A topological transition between a paramagnetic phase at high temperature and a critical phase at low temperature is observed. The low temperature limit is discussed in the spin wave approximation and confirms the numerical results. [Preview Abstract] |
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