Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session L38: Phase Transitions in Model Magnets |
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Sponsoring Units: DCMP Chair: Markus Huecker, Brookhaven National Lab Room: LACC 513 |
Tuesday, March 22, 2005 2:30PM - 2:42PM |
L38.00001: Magnetic Field Induced Shifts of the Spin Rotation Phase Transition of GdFe$_3$(BO$_3$)$_4$ Fei Yen, Bernd Lorenz, A.N. Vasiliev, L.N. Bezmaternykh, C.W. Chu GdFe$_3$(BO$_3$)$_4$ exhibits a structural phase transition at 156 K, antiferromagnetic order of the Fe moments at 36 K followed by a spin reorientation transition at T$_{SR}$ = 9 K. At the lower transition the dielectric constant of GdFe$_3$(BO$_3$)$_4$ shows a distinct peak indicating an interesting coupling between the magnetic order and the dielectric properties. We study thoroughly this lower temperature phase transition through dielectric, magnetic and thermodynamic measurements under the application of external magnetic fields up to 1 Tesla. The spin reorientation transition is shown to split into two phase transitions under external magnetic fields. The dielectric constant at low temperature changes with the applied field and a magneto- dielectric effect of up to 1\% is observed at 8 K and 0.7 Tesla. [Preview Abstract] |
Tuesday, March 22, 2005 2:42PM - 2:54PM |
L38.00002: Neutron Scattering Studies of the Random Field Domain State in CsCo$_{0.83}$Mg$_{0.17}$Br$_3$ John-Paul Castellan, B.D. Gaulin, W.J.L. Buyers, J. van Duijn We have extended previous neutron scattering measurements on CsCo$_{0.83}$Mg$_{0.17}$Br$_3$[1], a dilute stacked triangular lattice Ising antiferromagnet. Pure CsCoBr$_3$ exhibits three magnetic phase transitions[2]. At $T_{n1}$=28.3 K a 3 sublattice Neel state forms, in which two sublattices are ordered up and down, while the third remains disordered. This disordered sublattice itself orders below $T_{n2} \sim 16 K$, forming ferrimagnetic sheets which stack antiferromagnetically. The non-magnetic dopants induce a random field domain state at $T_{n1}$ in CsCo$_{0.83}$Mg$_{0.17}$Br$_3$, as the magnetic vacancies couple to the disordered sublattice as a random field. We resent new high resolution results showing the evolution of this random field state in both zero magnetic field and for the case where a magnetic field is applied along the Ising moment direction, the c-axis. [1] J. van Duijn et al. Phys. Rev. Lett. 92, 077202 (2004) [2] M. Mao et al. Phys. Rev. B 66, 184432 (2002) [Preview Abstract] |
Tuesday, March 22, 2005 2:54PM - 3:06PM |
L38.00003: Field-Induced ordering in NiCl2-4SC(NH2)2 V.S. Zapf, D. Zocco, M. Jaime, A. Lacerda, A. Paduan-Filho The compound NiCl$_2\cdot$4SC(NH$_2$)$_2$ (DTN) is a potential candidate for Bose Einstein Condensation (BEC) of spins. The S = 1 Ni spin triplet ground state is split by the atomic anisotropy into a lower S = 0 state and a higher S$ = \pm 1$ state. Applied magnetic fields parallel to the tetragonal axis reduce the splitting until a level crossing occurs at H = 2 T. Between H = 2 T and H = 12 T, field-induced long range order has been observed below T = 1 K. In other similar compounds, this long range order has been interpreted in terms of a BEC of triplet spins. Unlike many previous BEC candidates, DTN exhibits a strong single axis anisotropy with rotational symmetry in the ab plane, which allows for the conservation of magnons. We will present the first measurements of specific heat and the magnetocaloric effect of single crystalline DTN in high fields, and investigate the high field phase diagram, long range order, and possible BEC of magnons. This work was supported by the NSF through the National High Magnetic Field Laboratory, the State of Florida and the Department of Energy. [Preview Abstract] |
Tuesday, March 22, 2005 3:06PM - 3:18PM |
L38.00004: Magnetic Transition in Antiferromagnetic Spin-$\frac{1}{2}$ Chains with Staggered Long-Range Interactions Nicolas Laflorencie, Ian Affleck, Mona Berciu Antiferromagnetic spin-$\frac{1}{2}$ chains with non-frustrated long-range couplings are studied using the powerful Quantum Monte Carlo algorithm based on a Stochastic Series Expansion of the partition function [1]. The case of power-law decaying interaction $J(r)=-(-1)^r r^{-\alpha}$ is investigated for the general one-dimensionnal XXZ Hamiltonian $$ {\mathcal{H}}=\sum_{i,j}J(|i-j|)\left(S_{i}^{x}S_{j}^{x}+S_{i}^{y}S_{j}^{y} +\Delta S_{i}^{z}S_{j}^{z}\right).$$ Very large scale numerical results obtained on systems up to $L=8000$ spins are compared and discussed through bosonization and spin-waves predictions [2] for the onset of antiferromagnetic ordering in the ground-state in function of $\beta$.\\ $[1]$ O. F. Sylju{\aa}sen and A. W. Sandvik, Phys. Rev. E {\bf 66}, 046701 (2002).\\ $[2]$ E. Yusuf, A. Joshi, and Kun Yang, Phys. Rev. B {\bf 69}, 144412 (2004). [Preview Abstract] |
Tuesday, March 22, 2005 3:18PM - 3:30PM |
L38.00005: Spin Peierls transitions within the 1/4-filled Peierls extended Hubbard model R.P. Hardikar, R. Torsten Clay We investigate the Peierls transition within the one-dimensional Peierls extended Hubbard model at 1/4 filling. We treat both electron-electron and bond-coupled electron-phonon interactions (finite frequency) exactly using the Stochastic Series Expansion Quantum Monte Carlo method. As previously found for the 1/2-filled band, Peierls distortion for finite-frequency phonons only occurs for electron-phonon coupling above a critical value. Unlike the 1/2-filled band, at 1/4 filling the Peierls transition must be preceded at a higher temperature by another structural or charge order transition. We compare the resulting Peierls transition for two possible high temperature states, either (a) dimerization or (b) $4k_{F}$ ..1010.. charge order at high temperature. [Preview Abstract] |
Tuesday, March 22, 2005 3:30PM - 3:42PM |
L38.00006: Criticality in 2-D frustrated quantum triangular antiferromagnets Jason Alicea, Michael Hermele, Olexei I. Motrunich, Matthew P. A. Fisher We revisit 2-D frustrated quantum magnetism from a new perspective, with the aim of exploring new critical points and exotic critical phases. We study easy-plane s = 1 and s = 1/2 triangular antiferromagnets using a dual vortex approach, fermionizing the vortices with a Chern-Simons field. This enables us to formulate a low-energy QED3 critical theory with emergent SU(2) and SU(4) flavor symmetry in the s = 1 and s = 1/2 cases, respectively. We conjecture that the SU(2) theory describes a multicritical point separating the quantum paramagnet and two magnetically-ordered states, while the SU(4) theory describes a new stable gapless spin-liquid phase. [Preview Abstract] |
Tuesday, March 22, 2005 3:42PM - 3:54PM |
L38.00007: Quantum Griffiths effects in itinerant Heisenberg magnets Thomas Vojta, Joerg Schmalian We study the influence of quenched disorder on quantum phase transitions in itinerant magnets with Heisenberg spin symmetry, paying particular attention to rare disorder fluctuations. In contrast to the Ising case where overdamping suppresses the tunneling of rare regions, the XY- and Heisenberg system displays strong power-law quantum Griffiths singularities in the vicinity of the quantum critical point. We discuss these phenomena based on general scaling arguments, and we illustrate them by an explicit calculation for O(N) spin symmetry in the large-N limit. We also discuss broad implications for the classification of quantum phase transitions in the presence of quenched disorder. [Preview Abstract] |
Tuesday, March 22, 2005 3:54PM - 4:06PM |
L38.00008: On Spin 1/2 Excitations and Quantum Criticality in Two Dimensional $O(3)$ Antiferromagnets Zaira Nazario, David Santiago We utilize the $2+1$ $O(3)$ nonlinear sigma model for antiferromagnets to study the suggestion that there are corrections to quantum criticality due to low energy degrees of freedom intrinsic to the quantum critical point. The N\`eel ordered ground state, besides the gapless Goldstone excitations, has gapped skyrmion and antiskyrmion topological configurations. These are responsible for the system being disordered at all finite temperatures, as they gain energy by becoming arbitrarily large and thus lead to finite correlation length no matter how few of them are present. We map the skyrmions and antiskyrmions to $SU(2)$ spin $1/2$ objects and further show that they superpose in exactly the same way as spin $1/2$ objects. Therefore the N\`eel ground state has gapped spin $1/2$ excitations, i.e. spinons. This conclusion is not due to a Hopf term and it is independent of whether the microscopic spins are integral or half integral. We write an effective low energy field theory that correctly takes into account the spinon and Goldstone excitations, and their interactions. [Preview Abstract] |
Tuesday, March 22, 2005 4:06PM - 4:18PM |
L38.00009: On Fractionalized Quantum Criticality in Two Dimensional $O(3)$ Antiferromagnets David Santiago, Zaira Nazario We consider a recently propose field theory which inculdes magnetizationa and gapped skyrmion and antiskyrmion excitations in 2 +1 dimensional antiferromagnets. The skyrmions and antiskyrmions as spin 1/2 objects, i.e. spinons.From this field theory we show how the spinon fluctuations change the renormalization of the coupling constant, thus changing the critical coupling at which N\`eel order is lost. We also show that spinon fluctuations will lead to corrections to critical exponents as they renormalize the magnetization propagators beyond the usual renormalizations due to order parameter fluctuations. Since the spinon gap is inversely proportional to the coupling constant, and the renormalized inverse coupling constant, or spin stiffness, vanishes at the quantum critical point, the onset of paramagnetism is identified with spinon gap collapse. Because of this we conclude that essentially free skyrmions and antiskyrmions are the low energy degrees of freedom intrinsic to the quantum critical point as there are no Goldstone eigenstates at criticality due to lack of N\`eel order. [Preview Abstract] |
Tuesday, March 22, 2005 4:18PM - 4:30PM |
L38.00010: U(1) spin liquids and valence bond solids in a large-N three-dimensional Heisenberg model Jean-Sebastien Bernier, Ying-Jer Kao, Yong Baek Kim We study possible quantum ground states of the Sp($N$) generalized Heisenberg model on a cubic lattice with nearest-neighbor and next-nearest-neighbor exchange interactions. The phase diagram is obtained in the large-$N$ limit and fluctuation effects are considered via appropriate gauge theories. We find three U(1) spin liquid phases with different short-range magnetic correlations. These phases are characterized by deconfined gapped spinons, gapped monopoles, and gapless ``photons.'' As $N$ becomes smaller, a confinement transition from these phases to valence bond solids (VBS) may occur. This transition is studied by using duality and analyzing the resulting theory of monopoles coupled to a non-compact dual gauge field; the condensation of the monopoles leads to VBS phases. We determine the resulting VBS phases emerging from two of the three spin liquid states. On the other hand, the spin liquid state near $J_1 \approx J_2$ appears to be more stable against monopole condensation and could be a promising candidate for a spin liquid state in real systems. [Preview Abstract] |
Tuesday, March 22, 2005 4:30PM - 4:42PM |
L38.00011: Evolution of the single-hole spectral function across a quantum phase transition in the anisotropic-triangular-lattice antiferromagnet So Takei, Chung-Hou Chung, Yong Baek Kim We study the evolution of the single-hole spectral function when the ground state of the anisotropic-triangular-lattice antiferromagnet changes from the incommensurate magnetically-ordered phase to the spin-liquid state. In order to describe both of the ground states on equal footing, we use the large-$N$ approach where the transition between these two phases can be obtained by controlling the quantum fluctuations via an `effective' spin magnitude. Adding a hole into these ground states is described by a $t$-$J$ type model in the slave-fermion representation. Implications of our results to possible future ARPES experiments on insulating frustrated magnets, especially Cs$_2$CuCl$_4$, are discussed. [Preview Abstract] |
Tuesday, March 22, 2005 4:42PM - 4:54PM |
L38.00012: Ferromagnetic Instability in Disordered Systems: A Hartree-Fock Approach Xiao Yang, Chetan Nayak It was realized two decades ago that two dimensional diffusive Fermi liquid is unstable against arbitrarily weak electronic interactions. Recently, using the nonlinear sigma model developed by Finkelstein, several authors showed the instability gives rise to a ferromagnetic state. In this work, we consider electrons moving in a random potential with the following interaction: $-J\vec{S(x)}\cdot\vec{S(x')}$. We calculate the electron self energy and find that in two dimensions, the total energy is always minimized by ferromagnetic phase, while in three dimensions, ferromagnetism occurs only if $J$ exceeds a critical value proportional to the conductivity. Although the model and the calculation method are apparently different from the ones used before, the results are in qualitative agreement, which shows the robustness of the ferromagnetic instability in interacting disordered systems. [Preview Abstract] |
Tuesday, March 22, 2005 4:54PM - 5:06PM |
L38.00013: Superfluid--Solid Quantum Phase Transitions and Landau-Ginzburg-Wilson Paradigm A.B. Kuklov, N.V. Prokof'ev, B.V. Svistunov We study superfluid (SF)--solid zero-temperature transitions in 2d lattice boson/spin models by Worm-Algorithm Monte Carlo simulations. The SF -- Valence Bond Solid (VBS) transition was recently argued to be generically of II order in violation of the Ginzburg-Landau- Wilson (GLW) paradigm [1]. We simulate the J-current model on lattices up to 64x64x64, and observe that SF- columnar VBS and SF-checkerboard solid transitions are typically weak I-order ones and in small systems they may be confused with the continuous or high-symmetry points [2]. Thus, in the simulated model, the SF-VBS transition proceeds in agreement with the GLW paradigm. We explain this by dominance of standard particle and hole excitations, as opposed to fractionalized (spinon) excitations [1]. We developed a technique based on tunneling events (instantons) in the insulating phase which reveals charges of the revelant long-wave modes. While in 1d systems spinons are clearly seen in tunneling events, in two spatial dimensions tunneling is solely controlled by particles and holes in our system. This work is supported by NSF grant ITR-405460001 and PSC-CUNY- 665560035. [1] T. Senthil, A. Vishwanath, L. Balents, S. Sachdev, and M.P.A. Fisher, Science {\bf 303}, 1490 (2004); [2] A.B. Kuklov, N.V. Prokof'ev, B.V. Svistunov, condmat/0406061; PRL, to be published. [Preview Abstract] |
Tuesday, March 22, 2005 5:06PM - 5:18PM |
L38.00014: Broken Symmetries and Gapless Excitations of SU(N) Antiferromagnets Investigated With Variational Wavefunctions Arun Paramekanti, Brad Marston We use Gutzwiller-projected wavefunctions to investigate variationally the phase diagrams of SU(N) quantum antiferromagnets in the self-conjugate representation. The method is first tested against the known phase diagram of a one-dimensional SU(4) bilinear-biquadratic spin chain which has a quantum-critical point separating a dimerized phase from a phase with spontaneously broken charge-conjugation symmetry\footnote{I. Affleck {\it et al.}, Nucl. Phys. B {\bf 366}, 467 (1991).}. In the case of two-dimensional SU(N) antiferromagnets, recent analytical\footnote{M. Hermele {\it et al.}, \urllink{cond-mat/0404751}{http://arxiv.org/abs/cond-mat/0404751}.} and numerical\footnote{F. F. Assaad, \urllink{cond-mat/0406074}{http://arxiv.org/abs/cond-mat/0406074}.} work suggests the existence of a gapless spin-liquid phase with no broken symmetries. Such a phase would be consistent with a recent generalization of the Lieb-Schultz-Mattis theorem\footnote{M. B. Hastings, Phys. Rev. B{\bf 69}, 104431 (2004); \urllink{cond-mat/0411094}{http://arxiv.org/abs/cond-mat/0411094}.} to more than one spatial dimension. We examine the stability of the $\pi$-flux phase against tendencies to spin-order, crystallize into various valence-bond solids, or break charge-conjugation symmetry. [Preview Abstract] |
Tuesday, March 22, 2005 5:18PM - 5:30PM |
L38.00015: Current Carrying Ground State in a Bi-layer Model Sylvain Capponi, Congjun Wu, Shou-Cheng Zhang Strongly interacting systems have been conjectured to spontaneously develop current carrying ground states under certain conditions. We conclusively demonstrate the existence of a commensurate staggered interlayer current phase in a bi-layer model by using the recently discovered quantum Monte-Carlo algorithm without the sign problem. A pseudospin SU (2) algebra and the corresponding anisotropic spin-1 Heisenberg model are constructed to show the competition among the staggered interlayer current, rung singlet and charge density wave phases. [Preview Abstract] |
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L38.00016: Form of the exact partition function for the generalized Ising model T.R.S. Prasanna The problem of N interacting spins on a lattice is equivalent to one of N identical clusters linked in a specific manner. The energy of any configuration can be expressed in terms of the energy levels of this cluster. A new expression is obtained for the probability of occurrence of any configuration. A closed form expression is obtained for the exact partition function per spin in terms of the energy levels of this cluster and the degeneracies are functions of temperature. This form represents an alternate and equivalent (sum over energy levels) framework to determine the partition function. The partition functions of all Ising-like models have a common form. This raises a new possibility that the partition function may be determined as a sum of finite number of terms, which may not sum to a single term expression. Seven functions need to be determined to describe the exact partition function of the 3D Ising model. The key to understanding phase transitions and critical phenomena lies in the temperature dependence of degeneracies. It is necessary to develop new techniques to determine the partition function that account for this temperature dependence. [Preview Abstract] |
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L38.00017: Valence bond orders in spin-1/2 antiferromagnets in three dimensions Olexei Motrunich, T. Senthil We discuss possible valence bond orders in spin-1/2 quantum antiferromagnets on a 3D cubic lattice and expose their relation to a possible fractionalized Coulomb spin liquid state in the vicinity of the collinear Neel order. The analysis is an extension to (3+1)D of techniques previously developed in two spatial dimensions. In three dimensions, the identification of such confining phases with broken translational symmetry is formulated as a problem of monopole condensation patterns for monopoles hopping on the dual lattice and accumulating nontrivial Berry phases specific for the spin-1/2 system. Columnar and "box" valence bond states appear as natural candidates. [Preview Abstract] |
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