Half-ordered state in the anisotropic Haldane-gap antiferromagnet NDMAP

Neutron diffraction experiments performed on the Haldane gap material NDMAP in high magnetic fields applied at an angle to the principal anisotropy axes reveal two consecutive field-induced phase transitions. The low-field phase is the gapped Haldane state, while at high fields the system exhibits 3-dimensional long-range Neel order. In a peculiar phase found at intermediate fields only half of all the spin chains participate in the long-range ordering, while the other half remains disordered and gapped.Comment: 4 pages, 2 figures, submitted to Phys. Rev.

Algebras for parameterised monads

Parameterised monads have the same relationship to adjunctions with parameters as monads do to adjunctions. In this paper, we investigate algebras for parameterised monads. We identify the Eilenberg-Moore category of algebras for parameterised monads and prove a generalisation of Beck’s theorem characterising this category. We demonstrate an application of this theory to the semantics of type and effect systems

Molecular Design of Polymer Coatings Capable of Photo-Triggered Stress Relaxation via Dynamic Covalent Bond Exchange

Polymer coatings are frequently used to modify surface properties of inorganic substrates. However, the disparity in physical properties between polymer film and substrate often leads to residual stress development, which can be deleterious to the overall performance of coated materials. This work reports the molecular design of polymer films that dissipate stress upon irradiation with ultraviolet (UV) light. These polymers are synthesized by post-polymerization modification of the reactive polymer, poly(2-vinyl-4,4-dimethyl azlactone), to introduce dynamic crosslinks capable of light-initiated addition transfer fragmentation chemistry. Using a custom-built optical cantilever, contrasting film stress responses are observed between films containing dynamic bonds and analogous control films after UV light irradiation, which indicate successful stress relaxation. Further experiments demonstrate the complete relaxation of residual stress in dynamic films after an extended exposure, thereby generating a “stress-free” film. Films fabricated using this approach can be easily tailored to incorporate additional moieties to introduce desired surface properties for future application in a wide array of coatings

Universal Behavior of One-Dimensional Gapped Antiferromagnets in Staggered Magnetic Field

We study the properties of one-dimensional gapped Heisenberg antiferromagnets in the presence of an arbitrary strong staggered magnetic field. For these systems we predict a universal form for the staggered magnetization curve. This function, as well as the effect the staggered field has on the energy gaps in longitudinal and transversal excitation spectra, are determined from the universal form of the effective potential in O(3)-symmetric 1+1--dimensional field theory. Our theoretical findings are in excellent agreement with recent neutron scattering data on R_2 Ba Ni O_5 (R = magnetic rare earth) linear-chain mixed spin antiferromagnets.Comment: 4 pages, 2 figure

Absence of gap for infinite half--integer spin ladders with an odd number of legs

A proof is presented for the absence of gap for spin $1/2$ ladders with an odd number of legs, in the infinite leg length limit. This result is relevant to the current discussion of coupled one--dimensional spin systems, a physical realization of which are vanadyl pyrophosphate, (VO)$_2$P$_2$O$_7$, and stoichiometric Sr$_{n-1}$ Cu$_{n+1}$ O$_{2n}$ (with $n=3,5,7,9,\dots$).Comment: REVTeX, 8 page

Observation of a Transient Magnetization Plateau in a Quantum Antiferromagnet on the Kagome Lattice

The magnetization process of an S=1/2 antiferromagnet on the kagome lattice, [Cu_3(titmb)_2(OCOCH_3)_6]H_2O {titmb= 1,3,5-tris(imidazol-1-ylmethyl)-2,4,6 trimethylbenzene} has been measured at very low temperatures in both pulsed and steady fields. We have found a new dynamical behavior in the magnetization process: a plateau at one third of the saturation magnetization appears in the pulsed field experiments for intermediate sweep rates of the magnetic field and disappears in the steady field experiments. A theoretical analysis using exact diagonalization yields J_1=-19K and J_2=6K, for the nearest neighbor and second nearest neighbor interactions, respectively. This set of exchange parameters explains the very low saturation field and the absence of the plateau in the thermodynamic equilibrium as well as the two-peak feature in the magnetic heat capacity. Supported by numerical results we argue that a dynamical order by disorder phenomenon could explain the transient appearance of the 1/3 plateau in pulsed field experiments.Comment: 7 pages, 5 figure

Quasi-elastic neutron scattering in the high-field phase of a Haldane antiferromagnet

Inelastic neutron scattering experiments on the Haldane-gap quantum antiferromagnet NDMAP are performed in magnetic fields below and above the critical field Hc at which the gap closes. Quasi-elastic neutron scattering is found for H>Hc indicating topological excitations in the high field phase.Comment: Added to discussion section. v2: Updated figure

Direct Observation of the Quantum Energy Gap in S = 1/2 Tetragonal Cuprate Antiferromagnets

Using an electron spin resonance spectrometer covering a wide range of frequency and magnetic field, we have measured the low energy excitations of the S=1/2 tetragonal antiferromagnets, Sr_{2}CuO_{2}Cl_{2} and Sr_{2}Cu_{3}O_{4}Cl_{2}. Our observation of in-plane energy gaps of order 0.1 meV at zero external magnetic field are consistent with a spin wave calculation, which includes several kinds of quantum fluctuations that remove frustration. Results agree with other experiments and with exchange anisotropy parameters determined from a five band Hubbard model.Comment: 4 pages, 3 figure