Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B33: Cooperative Phenomena Theory 
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Sponsoring Units: GMAG Chair: ChiaRen Hu, Texas A&M University Room: E143 

B33.00001: ABSTRACT WITHDRAWN 
Monday, March 15, 2010 11:27AM  11:39AM 
B33.00002: Infinite randomness and ``quantum'' Griffiths effects in a classical system: the randomly layered Heisenberg magnet Priyanka Mohan, Rajesh Narayanan, Thomas Vojta We investigate the phase transition in a threedimensional classical Heisenberg magnet with planar defects, i.e., disorder perfectly correlated in two dimensions. By applying a strongdisorder renormalization group, we show that the critical point has exotic infiniterandomness character. It is accompanied by strong powerlaw Griffiths singularities. We compute various thermodynamic observables paying particular attention to finitesize effects relevant for an experimental verification of our theory. We also study the critical dynamics within a Langevin equation approach and find it extremely slow. At the critical point, the autocorrelation function decays only logarithmically with time while it follows a nonuniversal powerlaw in the Griffiths phase. [Preview Abstract] 
Monday, March 15, 2010 11:39AM  11:51AM 
B33.00003: Quantum Fidelity and Fidelity Susceptibility of the Spatially Anisotropic Triangular Heisenberg Model Mischa Thesberg, Erik S. Sorensen The phase diagram of the spin1/2 antiferromagnetic Heisenberg model on a triangular lattice is studied. Anisotropy is introduced through the diagonal exchange constant J' differing from the intrachain exchange constant J. Previous work on this model has suggested a competition between two (three if secondnearest neighbour interactions are considered) ground state orderings. We use numerical exact diagonalization techniques to investigate the proposed phases by studying the quantum fidelity and fidelity susceptibility as a function of the anisotropy. [Preview Abstract] 
Monday, March 15, 2010 11:51AM  12:03PM 
B33.00004: Static vacancies in a 2D Heisenberg model with nearest and secondnearest exchange Chen Fang, Bao Xu, Jiangping Hu, Wuming Liu, Steven Kivelson We studied the effect of vacancies on the magnetic ordering of the frustrated $J_1$$J_2$ Heisenberg model on a square lattice. Treating the dopants as randomly distributed 'empty sites', it is found that the system indeed undergoes a quantum phase transition into another phase at a certain doping concentration. The change in the local magnetic structure (e.g., the length of the local moments) around the dopants is also investigated and related features are proposed to be probed by experiments. A few connections between our results and ironbased superconductors will be discussed. [Preview Abstract] 
Monday, March 15, 2010 12:03PM  12:15PM 
B33.00005: Two 2D models of color ice as conceptual generalizations of spin ice ChiaRen Hu Spin ice is a novel condensedmatter system with magneticmonopolelike elementary excitations. It is also the first example of fractionalization  a spin (i.e., a magnetic dipole) is split into two oppositely signed magnetic monopoles  in three dimensions. It is realized with Ising spins on a pyrochlore lattice, i.e., cornersharing tetrahedra, obeying the ice rule  two in and two out  with respect to each tetrahedron, forming the highlydegenerate ground states. Here we propose two twodimensional models of ``color ice'', with color (redgreenblue) ``tripoles'' located on the vertices of (i) corner sharing triangles, and (ii) edge sharing hexagons, obeying generalized ice rules to keep each triangle or hexagon color neutral in the highly degenerate ground states. Excitations in this system are color charges (monopoles), fractionalizing a color tripole, created in color neutral combinations. So far, these color charges can only move in 1D. Their interactions are 2DCoulomb like, and Abelian. A color tripole in these models can be shown to be equivalent to two 2D Coulomb dipoles of different kinds in mutually perpendicular orientations. Extension to 3D is still being attempted. [Preview Abstract] 
Monday, March 15, 2010 12:15PM  12:27PM 
B33.00006: Critical Temperatures of Finite Samples at Finite Observation Times O. Vedmedenko, N. Mikuszeit, T. Stapelfeldt, R. Wieser, M. Potthoff, A. Lichtenstein, R. Wiesendanger The analytical form of correlation function G(r), which is an essential ingredient of any theory of phase transitions is known for infinite systems at infinite observation times. At the forefront of the nanoscience revolution important experimental developments involve ever smaller length and time scales. For such nanosystems there is up to now no clear understanding of crossover phenomena like the crossover from a paramagnetic state at high temperatures via the thermo activated switching [Science 317, 1537 (2007),Phys. Rev. Lett. 103, 127202 (2009)] to ferro or antiferromagnetic order at low $T$ and how they manifest themselves in the correlation function. This is related to the problem that a Curie or N\'eel temperature of nanoobjects cannot be defined unambiguously. Here, a general expression for G(r) covering all sample sizes, all observation times and the entire temperature range from zero to infinity is derived. We demonstrate that the Curie temperature does not simply decrease with decreasing sample size but rather splits in finite samples for finite observation times. This new result does not violate scaling invariance, recovers all known laws as limiting cases, and provides a new algorithm by which critical singularities can be predicted or measured. [Preview Abstract] 
Monday, March 15, 2010 12:27PM  12:39PM 
B33.00007: Quench dynamics of interacting Fermi systems: competition between pairing and magnetization Mehrtash Babadi, David Pekker, Rajdeep Sensarma, Antoine Georges, Eugene Demler We study the linearized dynamics of a twocomponent ultra cold Fermi gas which is rapidly quenched to either the repulsive or the attractive side of a Feshbach resonance. On the attractive side, we investigate pairing instability towards BCS and FFLOlike states as a function of temperature and population imbalance. On the repulsive side, we investigate the competition between pairing into Feshbach molecules and the Stoner instability. In all cases, we evaluate the growth rate of unstable modes and predict the typical lengthscale of textures to be formed. [Preview Abstract] 
Monday, March 15, 2010 12:39PM  12:51PM 
B33.00008: Magnetic frustration and tuning of magnetic order by the chemical order parameter in selected alloys Sergii Khmelevskyi, Peter Mohn Chemical disorder on the frustrated lattice often leads to the formation of the spin glass state in magnetic alloys. However, if the system is above percolation threshold or there are strong longdistant interactions between magnetic atoms than the formation of some complex ordered state may occurs in disordered alloys. By changing a longrange atomic order parameter one may tune the magnetic order in such kind of the alloys. Here, we present our abintio investigation of three selected alloys: VAu$_{4}$, Ni$_{3}$Mn and AuFe spin glass, where the degree of the chemical order is greatly affect the magnetic properties leading to very peculiar experimental behavior. Using magnetic force theorem, CPA and LSGF method we study longdistant exchange interactions and local environment effects. We have find, in particular, that contrary to the long standing believe the dominating interactions in VAu$_{4}$ are antiferromagnetic and source of the frustration in AuFe system is not geometrical in origin. [Preview Abstract] 
Monday, March 15, 2010 12:51PM  1:03PM 
B33.00009: Spin density distribution in systems with frustrated triangular magnetic structure Pavel Lukashev, Renat Sabirianov We present results of abinitio calculations for the noncollinear spin density (SD) distribution in the systems with frustrated triangular magnetic structure (Mnbased antiperovskites, Mn$_{3}$AN (A=Ga, Zn)) in the ground state and under external mechanical strain. We show that SD in the (111)plane of the unit cell (in particular, in the atomic sphere around Mn atom) is distinctly nonuniform, i.e. both direction and magnitude of SD strongly depend on the distance from Mn site within atomic sphere. We show that the evolution of SD under external mechanical stress exhibits more diverse features than expected in the rigid spin model. In particular, under applied strain we observe the appearance of spin ``domains'' in the (111)plane of the unit cell in which SD rotates in opposite directions. The rotation of SD reverses if tensile strain changes to compressive, but the shape of the ``domains'' stays somewhat stable. We have shown that the change in SD distribution under strain depends on the interplay of exchange interactions governing the rotation of SD in the localized high SD region and the structure of SD in the highly inhomogeneous vortex structure in the interstitial region. [Preview Abstract] 
Monday, March 15, 2010 1:03PM  1:15PM 
B33.00010: Nonlinear spin wave theory results for the frustrated S = 1/2 Heisenberg antiferromagnet on a bodycentered cubic and simple cubic lattice Trinanjan Datta, Kingshuk Majumdar Using nonlinear spin wave theory at zero temperature we compute the sublattice magnetization and the ground state energy of the quantum spin1/2 Heisenberg antiferromagnet on a bodycentered cubic (BCC) lattice and a simple cubic (SC) lattice with competing first and second neighbor exchange interactions (J1 and J2). For both the BCC and the SC lattice we find the existence of a two sublattice Neel phase (AF) for small J2 and a collinear phase at large J2 (CAF). For the BCC lattice it is found that up to quartic corrections the first order phase transition, predicted from linear spin wave theory, between the AF and the CAF survives and occurs at the critical transition point, J2/J1 = 0.67. For the SC lattice linear spin wave theory predicts an intermediate paramagnetic phase. However, with the inclusion of the nonlinear corrections the intermediate paramagnetic phase disappears. The first order phase transition from the AFphase to the CAFphase occurs at the critical transition point, J2/J1 = 0.28. [Preview Abstract] 
Monday, March 15, 2010 1:15PM  1:27PM 
B33.00011: Role of correlated electron spin dynamics on the finitetemperature magnetization behavior of an itinerant ferromagnet Sudhakar Pandey, Avinash Singh Finite temperature magnetization behavior of an itinerant ferromagnet is investigated in terms of electronmagnon interaction induced thermal evolution of electronic spectral weight transfer across the Fermi level. This mechanism distinguises our approach from the conventional approaches which are based on the mapping of an itinerant ferromagnet into an equivalent localized spin (Heisenberg) model. Apart from being in accordance with the MerminWagner theorem and accounting for the characteristic low temperature Bloch law ($T^{3/2}$) faloff of magnetization, this approach also provides an alternative scheme for a quantitative estimation of Curie temperature ($T_c$). Results for $T_c$ without including the contribution of longwavelength magnon modes, as obtained for a facecenteredcubic lattice, are found in good agreement with the results of dynamical mean field theory. Longwavelength modes are found to suppress the ferromagnetism. [Preview Abstract] 
Monday, March 15, 2010 1:27PM  1:39PM 
B33.00012: Anomalous Pinning Fields in Helical Magnets D. Belitz, T.R. Kirkpatrick The spinorbit interaction strength $g_{\textrm{so}}$ in helical magnets determines both the pitch wave number $q$ and the critical field $H_{\textrm{c1}}$ where the helix aligns with an external magnetic field. Within a standard LandauGinzburgWilson (LGW) theory, a determination of $g_{\textrm{so}}$ in MnSi and FeGe from these two observables yields values that differ by a factor of $20$. This discrepancy is remedied by considering the fermionic theory underlying the LGW theory, and in particular the effects of screening on the effective electronelectron interaction that results from an exchange of helimagnons [1]. \\[4pt] $[1]$ T.R. Kirkpatrick and D. Belitz, Phys. Rev. B, in press. [Preview Abstract] 
Monday, March 15, 2010 1:39PM  1:51PM 
B33.00013: Goldstone mode in the conical phase of helical magnets Yan Sang, D. Belitz, T.R. Kirkpatrick We investigate theoretically the nature of the Goldstone mode in the conical phase of helical magnets such as MnSi. A DzyaloshinskyMoriya term in the action leads to helical order in the ground state, characterized by a pitch vector ${\vec q}$ [1]. The Goldstone mode in the helical phase, the helimagnon, is known to have an anisotropic dispersion relation of the form $\Omega^2 \propto k_z^2 + k_{\perp} ^4/q^2$, analogous to smectic or cholesteric liquid crystals [2]. In the presence of a homogeneous external magnetic field $H$ the helix is superimposed by a homogeneous magnetization, which leads to a conical phase [3]. The Goldstone mode in the latter is found to be a modified helimagnon, with a dispersion relation of the structure $\Omega^2 \propto \Omega_0^2 + H^2 k_{\perp}^2$. The additional term $\propto H^2 k_{\perp}^2$ is a result of the magnetic field breaking the rotational symmetry. In addition, there are remnants of ferromagnetic magnons with masses $\propto H^2$. \\ $[1]$ P. Bak and M.H. Jensen, J. Phys. C 13, L881 (1980). \\ $[2]$ D. Belitz, T.R. Kirkpatrick, and A. Rosch, Phys. Rev. B 73, 054431 (2006). \\ $[3]$ Y. Ishikawa, G. Shirane, J.A. Tarvin, and M. Kohgi, Phys. Rev. B 16, 4956 (1977). [Preview Abstract] 
Monday, March 15, 2010 1:51PM  2:03PM 
B33.00014: Abinitio electronic structure calculations of periodic systems in the presence of arbitrary magnetic fields Alfredo Correa, Giulia Galli, Wei Cai Ab initio electronic structure calculations in the presence of magnetic fields have been mainly performed for isolated systems, or, in the case of periodic systems, by adopting perturbative approaches. Building on a recent formulation of electronic structure calculations in the presence of magnetic fields [1,2], we will discuss calculations for periodic systems under arbitrary conditions, which include arbitrary (finite) magnetic field, arbitrary periodic cell shapes, and magnetic field spatial variations. Preliminary results based on a planewave numerical approach and local approximations to Density Functional Theory will be presented.[1] W.Cai, G.Galli, Phys. Rev. Lett. 92, 186402 (2004).[2] E. Lee, W. Cai, G. Galli, J. Comput. Phys. 226, 1310 (2007) [Preview Abstract] 
Monday, March 15, 2010 2:03PM  2:15PM 
B33.00015: Anisotropy of IronSeries Permanent Magnets Ralph Skomski, G. C. Hadjipanayis, D. J. Sellmyer Elementstrategic considerations have sparked renewed interest in rareearthfree permanent magnets, but the prediction of the magnetocrystalline anisotropy from the atomic structure is still in its infancy, and there are no rules predicting the $d$ anisotropy as function of the atomic structure. We have obtained tightbinding estimates for a variety of clusters and etxtended structures of different symmetry and $d$band filling. A expected, the anisotropy strongly oscillates as a function of the $d$band filling. Our calculations indicate that nearly filled $d$ bands tend to yield anisotropy parallel to the pair axis. Sites with trigonal symmetry support bigger anisotropies than cubic and tetragonal environments, but this is a crystal field effect similar to that in BaFe$_{12}$O$_{19}$ rather than a bandstructure effect. Shape anisotropy is important in alnico type nanostructured permanent magnets. We find a maximum of the energy product as a function of packing fraction, namely a maximum value of $\mu_{o}M_{s}^{2}$/12 realized at a volume fraction of 2/3. For Fe$_{65}$Co$_{35}$, this yields an upper limit of 390 kJ/m$^{3}$ [49 MGOe]. [Preview Abstract] 
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