6,430 research outputs found
Gas of self-avoiding loops on the brickwork lattice
An exact calculation of the phase diagram for a loop gas model on the
brickwork lattice is presented. The model includes a bending energy. In the
dense limit, where all the lattice sites are occupied, a phase transition
occuring at an asymmetric Lifshitz tricritical point is observed as the
temperature associated with the bending energy is varied. Various critical
exponents are calculated. At lower densities, two lines of transitions (in the
Ising universality class) are observed, terminated by a tricritical point,
where there is a change in the modulation of the correlation function. To each
tricritical point an associated disorder line is found.Comment: 19 pages, 6 figures. to appear in J. Phys. A : Math. & Ge
Multichannel oscillations and relations between LSND, KARMEN and MiniBooNE, with and without CP violation
We show by examples that multichannel mixing can affect both the parameters
extracted from neutrino oscillation experiments, and that more general
conclusions derived by fitting the experimental data under the assumption that
only two channels are involved in the mixing. Implications for MiniBooNE are
noted and an example based on maximal CP violation displays profound
implications for the two data sets (muon-neutrino and muon-antineutrino) of
that experiment.Comment: 5 pages 4 figure
Elementary excitations in the gapped phase of a frustrated S=1/2 spin ladder: from spinons to the Haldane triplet
We use the variational matrix-product ansatz to study elementary excitations
in the S=1/2 ladder with additional diagonal coupling, equivalent to a single
S=1/2 chain with alternating exchange and next-nearest neighbor interaction. In
absence of alternation the elementary excitation consists of two free S=1/2
particles ("spinons") which are solitons in the dimer order. When the
nearest-neighbor exchange alternates, the "spinons" are confined into one S=1
excitation being a soliton in the generalized string order. Variational results
are found to be in a qualitative agreement with the exact diagonalization data
for 24 spins. We argue that such an approach gives a reasonably good
description in a wide range of the model parameters.Comment: RevTeX, 13 pages, 11 embedded figures, uses psfig and multico
Density Matrix Renormalization Group Study of the Disorder Line in the Quantum ANNNI Model
We apply Density Matrix Renormalization Group methods to study the phase
diagram of the quantum ANNNI model in the region of low frustration where the
ferromagnetic coupling is larger than the next-nearest-neighbor
antiferromagnetic one. By Finite Size Scaling on lattices with up to 80 sites
we locate precisely the transition line from the ferromagnetic phase to a
paramagnetic phase without spatial modulation. We then measure and analyze the
spin-spin correlation function in order to determine the disorder transition
line where a modulation appears. We give strong numerical support to the
conjecture that the Peschel-Emery one-dimensional line actually coincides with
the disorder line. We also show that the critical exponent governing the
vanishing of the modulation parameter at the disorder transition is .Comment: 4 pages, 5 eps figure
Frustrated quantum Heisenberg ferrimagnetic chains
We study the ground-state properties of weakly frustrated Heisenberg
ferrimagnetic chains with nearest and next-nearest neighbor antiferromagnetic
exchange interactions and two types of alternating sublattice spins S_1 > S_2,
using 1/S spin-wave expansions, density-matrix renormalization group, and
exact- diagonalization techniques. It is argued that the zero-point spin
fluctuations completely destroy the classical commensurate- incommensurate
continuous transition. Instead, the long-range ferrimagnetic state disappears
through a discontinuous transition to a singlet state at a larger value of the
frustration parameter. In the ferrimagnetic phase we find a disorder point
marking the onset of incommensurate real-space short-range spin-spin
correlations.Comment: 16 pages (LaTex 2.09), 6 eps figure
Collision-Induced Non-Adiabatic Transitions Between The Ion-Pair States Of Molecular Iodine: A Challenge For Experiment And Theory
The ion-pair states of molecular iodine provide a unique system for studying the efficiency, selectivity, and mechanisms of collision-induced non-adiabatic transitions. Non-adiabatic transitions between the first-tier ion-pair states in collisions with molecular partners and rare gases are analyzed and discussed. The qualitative features of the rate constants and product state distributions under single collision conditions are summarized and interpreted in terms of appropriate theoretical approaches. Two mechanisms for the non-adiabatic transitions are clearly identified. The first, operative for collisions involving molecular partners possessing permanent or transition electrostatic moments, is highly selective. It connects the initially prepared level in the E 0(g)(+) electronic state with the near-resonant vibronic level of the D 0(u)(+) state with a minimum change of the total angular momentum. In an extreme quasi-resonant case when the gap between initial and final rovibronic level is less than 1 cm(-1), this mechanism has a giant cross section, 40 times that of a gas kinetic collision. An electrostatic model, which includes the coupling of the giant E-D transition dipole moment with a moment of the colliding partner and the semiclassical Born approximation, provides a plausible interpretation of this mechanism. A second mechanism is shown to govern collisions with rare gas atoms. It results in population of several ion-pair states and broad distributions over rovibronic levels. This mechanism is successfully interpreted by quantum scattering calculations based on the diatomics-in-molecule diabatic potential energy surfaces and coupling matrix elements. The calculations provide good agreement with experimental measurements and reveal different mechanisms for the population of different electronic states. Unexplained features of the non-adiabatic dynamics and directions of future work are outlined
Frustrated antiferromagnetic quantum spin chains for spin length S > 1
We investigate frustrated antiferromagnetic Heisenberg quantum spin chains at
T=0 for S=3/2 and S=2 using the DMRG method. We localize disorder and Lifshitz
points, confirming that quantum disorder points can be seen as quantum remnants
of classical phase transitions. Both in the S=3/2 and the S=2 chain, we observe
the disappearance of effectively free S=1/2 and S=1 end spins respectively. The
frustrated spin chain is therefore a suitable system for clearly showing the
existence of free end spins S'=[S/2] also in half-integer antiferromagnetic
spin chains with S>1/2. We suggest that the first order transition found for
S=1 in our previous work is present in all frustrated spin chains with S>1/2,
characterized by the disappearance of effectively free end spins with S'=[S/2].Comment: 6 pages, 8 ps figures, uses RevTeX, submitted to PR
MSW-like Enhancements without Matter
We study the effects of a scalar field, coupled only to neutrinos, on
oscillations among weak interaction current eigenstates. The effect of a real
scalar field appears as effective masses for the neutrino mass eigenstates, the
same for \nbar as for \n. Under some conditions, this can lead to a
vanishing of , giving rise to MSW-like effects. We discuss some
examples and show that it is possible to resolve the apparent discrepancy in
spectra required by r-process nucleosynthesis in the mantles of supernovae and
by Solar neutrino solutions.Comment: 9 pages, latex, 1 figur
Cutting tool temperatures in contour turning : transient analysis and experimental verification
This paper describes a model for predicting cutting tool temperatures under transient conditions. It is applicable to processes such as contour turning, in which the cutting speed, feed rate, and depth of cut may vary continuously with time. The model is intended for use in process development and trouble shooting. Therefore, emphasis is given in the model development to enable rapid computation and to avoid the need to specify parameters such as thermal contact resistances and convection coefficients which are not known in practice. Experiments were conducted to validate the predictive model. The model predictions with two different boundary conditions bound the experimental results. An example is presented which shows the utility of the model for process planning
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