Controlled switching of intrinsic localized modes in a 1-D antiferromagnet

Nearly steady-state locked intrinsic localized modes (ILMs) in the quasi-1d antiferromagnet (C2H5NH3)2CuCl4 are detected via four-wave mixing emission or the uniform mode absorption. Exploiting the long-time stability of these locked ILMs, repeatable nonlinear switching is observed by varying the sample temperature, and localized modes with various amplitudes are created by modulation of the microwave driver power. This steady-state ILM locking technique could be used to produce energy localization in other atomic lattices.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Lett. v.2 : clarifications of text and figures in response to comment

Rheological and structural studies of liquid decane, hexadecane, and tetracosane under planar elongational flow using nonequilibrium molecular-dynamics simulations

We report for the first time rheological and structural properties of liquid decane, hexadecane, and tetracosane using nonequilibrium molecular-dynamics (NEMD) simulations under planar elongational flow (PEF). The underlying NEMD algorithm employed is the so-called p -SLLOD algorithm [C. Baig, B. J. Edwards, D. J. Keffer, and H. D. Cochran, J. Chem. Phys. 122, 114103 (2005)]. Two elongational viscosities are measured, and they are shown not to be equal to each other, indicating two independent viscometric functions in PEF. With an appropriate definition, it is observed that the two elongational viscosities converge to each other at very low elongation rates, i.e., in the Newtonian regime. For all three alkanes, tension-thinning behavior is observed. At high elongation rates, chains appear to be fully stretched. This is supported by the result of the mean-square end-to-end distance of chains ??? Rete2 ??? and the mean-square radius of gyration of chains ??? Rg2 ???, and further supported by the result of the intramolecular Lennard-Jones (LJ) potential energy. It is also observed that ??? Rete2 ??? and ??? Rg2 ??? show a different trend as a function of strain rate from those in shear flow: after reaching a plateau value, ??? Rete2 ??? and ??? Rg2 ??? are found to increase further as elongation rate increases. A minimum in the hydrostatic pressure is observed for hexadecane and tetracosane at about ??̇ (m ??2 ??)12 =0.02. This phenomenon is shown to be associated with the intermolecular LJ potential energy. The bond-bending and torsional energies display similar trends, but a different behavior is observed for the bond-stretching energy. An important observation common in these three bonded-intramolecular interactions is that all three modes are suppressed to a small value at high elongation rates. We conjecture that a liquid-crystal-like, nematic structure is present in these systems at high elongation rates, which is characterized by a strong chain alignment with a fully stretched conformation.open323

A Spin Model for Investigating Chirality

Spin chirality has generated great interest recently both from possible applications to flux phases and intrinsically, as an example of a several-site magnetic order parameter that can be long-ranged even where simpler order parameters are not. Previous work (motivated by the flux phases) has focused on antiferromagnetic chiral order; we construct a model in which the chirality orders ferromagnetically and investigate the model's behavior as a function of spin. Enlisting the aid of exact diagonalization, spin-waves, perturbation theory, and mean fields, we conclude that the model likely has long-ranged chiral order for spins 1 and greater and no non-trivial chiral order for spin 1/2.Comment: uuencoded gzipped tarred plain tex fil

Spin-wave Scattering in the Effective Lagrangian Perspective

Nonrelativistic systems exhibiting collective magnetic behavior are analyzed in the framework of effective Lagrangians. The method, formulating the dynamics in terms of Goldstone bosons, allows to investigate the consequences of spontaneous symmetry breaking from a unified point of view. Low energy theorems concerning spin-wave scattering in ferro- and antiferromagnets are established, emphasizing the simplicity of actual calculations. The present work includes approximate symmetries and discusses the modification of the low energy structure imposed by an external magnetic and an anisotropy field, respectively. Throughout the paper, analogies between condensed matter physics and Lorentz-invariant theories are pointed out, demonstrating the universal feature of the effective Lagrangian technique.Comment: Published versio

Spin-wave interaction in two-dimensional ferromagnets with dipolar forces

We discuss the spin-wave interaction in two-dimensional (2D) Heisenberg ferromagnet (FM) with dipolar forces at $T_C\gg T\ge0$ using 1/S expansion. A comprehensive analysis is carried out of the first 1/S corrections to the spin-wave spectrum. In particular, similar to 3D FM discussed in our previous paper A.V. Syromyatnikov, PRB {\bf 74}, 014435 (2006), we obtain that the spin-wave interaction leads to the {\it gap} in the spectrum $\epsilon_{\bf k}$ renormalizing greatly the bare gapless spectrum at small momenta $k$. Expressions for the spin-wave damping $\Gamma_{\bf k}$ are derived self-consistently and it is concluded that magnons are well-defined quasi-particles in both quantum and classical 2D FMs at small $T$. We observe thermal enhancement of both $\Gamma_{\bf k}$ and $\Gamma_{\bf k}/\epsilon_{\bf k}$ at small momenta. In particular, a peak appears in $\Gamma_{\bf k}$ and $\Gamma_{\bf k}/\epsilon_{\bf k}$ at small $k$ and at any given direction of $\bf k$. If $S\sim1$ the height of the peak in $\Gamma_{\bf k}/\epsilon_{\bf k}$ is not larger than a value proportional to $T/D\ll1$, where $D$ is the spin-wave stiffness. In the case of large spins $S\gg1$ the peak in $\Gamma_{\bf k}/\epsilon_{\bf k}$ cannot be greater than that of the classical 2D FM found at $k=0$ which height is small only {\it numerically}: $\Gamma_{\bf 0}/\epsilon_{\bf 0}\approx0.16$ for the simple square lattice. Frustrating next-nearest-neighbor exchange coupling increases $\Gamma_{\bf 0}/\epsilon_{\bf 0}$ in classical 2D FM only slightly. We find expressions for spin Green's functions and the magnetization. The latter differs from the well-known result by S.V. Maleev, Sov. Phys. JETP {\bf 43}, 1240 (1976). The effect of the exchange anisotropy is also discussed briefly

Multiple Field-Induced Phase Transitions in a Geometrically-Frustrated Dipolar Magnet - Gd2Ti2O7

Field-driven phase transitions generally arise from competition between Zeeman energy and exchange or crystal-field anisotropy. Here we present the phase diagram of a frustrated pyrochlore magnet Gd2Ti2O7, where crystal field splitting is small compared to the dipolar energy. We find good agreement between zero-temperature critical fields and those obtained from a mean-field model. Here, dipolar interactions couple real-space and spin-space, so the transitions in Gd2Ti2O7 arise from field-induced "cooperative anisotropy" reflecting the broken spatial symmetries of the pyrochlore lattice.Comment: 10pages,5figures: pdf file attached PACS 75.30.Kz, 75.50.Ee, 75.10.-

Rheological and structural studies of linear polyethylene melts under planar elongational flow using nonequilibrium molecular dynamics simulations

We present various rheological and structural properties of three polyethylene liquids, C50 H102, C78 H158, and C128 H258, using nonequilibrium molecular dynamics simulations of planar elongational flow. All three melts display tension-thinning behavior of both elongational viscosities, ??1 and ??2. This tension thinning appears to follow the power law with respect to the elongation rate, i.e., ????? ??̇ b, where the exponent b is shown to be approximately -0.4 for ??1 and ??2. More specifically, b of ??1 is shown to be slightly larger than that of ??2 and to increase in magnitude with the chain length, while b of ??2 appeared to be independent of the chain length. We also investigated separately the contribution of each mode to the two elongational viscosities. For all three liquids, the intermolecular Lennard-Jones (LJ), intramolecular LJ, and bond-stretching modes make positive contributions to both ??1 and ??2, while the bond-torsional and bond-bending modes make negative contributions to both ??1 and ??2. The contribution of each of the five modes decreases in magnitude with increasing elongation rate. The hydrostatic pressure shows a clear minimum at a certain elongation rate for each liquid, and the elongation rate at which the minimum occurs appears to increase with the chain length. The behavior of the hydrostatic pressure with respect to the elongation rate is shown to correlate with the intermolecular LJ energy from a microscopic viewpoint. On the other hand, ??? Rete2 ??? and ??? Rg2 ??? appear to be correlated with the intramolecular LJ energy. The study of the effect of the elongational field on the conformation tensor c̃ shows that the degree of increase of tr (c̃) -3 with the elongation rate becomes stronger as the chain length increases. Also, the well-known linear reaction between ?? and c̃ does not seem to be satisfactory. It seems that a simple relation between ?? and c̃ would not be valid, in general, for arbitrary flows.open29

Surface spin-flop and discommensuration transitions in antiferromagnets

Phase diagrams as a function of anisotropy $D$ and magnetic field $H$ are obtained for discommensurations and surface states for an antiferromagnet in which $H$ is parallel to the easy axis, by modeling it using the ground states of a one-dimensional chain of classical XY spins. A surface spin-flop phase exists for all $D$, but the interval in $H$ over which it is stable becomes extremely small as $D$ goes to zero. First-order transitions, separating different surface states and ending in critical points, exist inside the surface spin-flop region. They accumulate at a field $H'$ (depending on $D$) significantly less than the value $H_{SF}$ for a bulk spin-flop transition. For $H' < H < H_{SF}$ there is no surface spin-flop phase in the strict sense; instead, the surface restructures by, in effect, producing a discommensuration infinitely far away in the bulk. The results are used to explain in detail the phase transitions occurring in systems consisting of a finite, even number of layers.Comment: Revtex 17 pages, 15 figure

Magnetic susceptibility of EuTe/PbTe Heisenberg superlattices: experimental and theoretical studies

We report results on the temperature dependence of the susceptibilities of a set of MBE-grown short-period EuTe/PbTe antiferromagnetic superlattices having different EuTe layer thicknesses. In-plane and orthogonal susceptibilities have been measured and display a strong anisotropy at low temperature, confirming the occurrence of a magnetic phase transition in the thicker samples, as seen also in neutron diffraction studies. We suggest that dipolar interactions stabilize antiferromagnetic long-range order in an otherwise isotropic system and we present numerical and analytical results for the low-temperature orthogonal susceptibility.Comment: 30 pages, 8 ps figures, RevTe