33,206 research outputs found

    Cluster expansion for dimerized spin systems

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    We have studied dimerized spin systems by realizing the cluster expansion to high order. We have extended our previous dimer expansion for one-dimensional systems to cover weakly interacting chains for a quantitative description of three dimensional materials like PHCC and KCuCl_3. By comparison with recent inelastic neutron scattering data we are able to determine the exchange energies between individual spins. We have further investigated the incommensurate region of zigzag chains with isotropic exchange coupling constants near the disorder-line where the dispersion curve exhibits a minimum at a finite wavevector. Our approach clearly shows the gradual transition between the minimum of the dispersion at wavevector 0 and wavevector Pi within this region. The extent of the incommensurate regime is given analytically in an expansion in the coupling constants.Comment: 3 pages, 3 figures; contribution to ICNS2001; uses svjour.clo, svglobal.clo (included

    Nonvacuum pseudoparticles, quantum tunneling and metastability

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    It is shown that nonvacuum pseudoparticles can account for quantum tunneling and metastability. In particular the saddle-point nature of the pseudoparticles is demonstrated, and the evaluation of path-integrals in their neighbourhood. Finally the relation between instantons and bounces is used to derive a result conjectured by Bogomolny and Fateyev.Comment: Latex, 16 pages, no figure

    Charge radius and dipole response of 11^{11}Li

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    We investigate the consistency of the measured charge radius and dipole response of 11^{11}Li within a three-body model. We show how these observables are related to the mean square distance between the 9^9Li core and the center of mass of the two valence neutrons. In this representation we find by considering the effect of smaller corrections that the discrepancy between the results of the two measurements is of the order of 1.5σ\sigma. We also investigate the sensitivity to the three-body structure of 11^{11}Li and find that the charge radius measurement favors a model with a 50% s-wave component in the ground state of the two-neutron halo, whereas the dipole response is consistent with a smaller s-wave component of about 25% value.Comment: 6 pages, 3 figure

    Gluon Distribution Functions for Very Large Nuclei at Small Transverse Momentum

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    We show that the gluon distribution function for very large nuclei may be computed for small transverse momentum as correlation functions of an ultraviolet finite two dimensional Euclidean field theory. This computation is valid to all orders in the density of partons per unit area, but to lowest order in αs\alpha_s. The gluon distribution function is proportional to 1/x1/x, and the effect of the finite density of partons is to modify the dependence on transverse momentum for small transverse momentum.Comment: TPI--MINN--93--52/T, NUC--MINN--93--28/T, UMN--TH--1224/93, LaTex, 11 page

    Application of Instantons: Quenching of Macroscopic Quantum Coherence and Macroscopic Fermi-Particle Configurations

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    Starting from the coherent state representation of the evolution operator with the help of the path-integral, we derive a formula for the low-lying levels E=Ï”0−2â–łÏ”cos(s+Ο)πE = \epsilon_0 - 2\triangle\epsilon cos (s+\xi)\pi of a quantum spin system. The quenching of macroscopic quantum coherence is understood as the vanishing of cos(s+Ο)πcos (s+\xi)\pi in disagreement with the suppression of tunneling (i.e. â–łÏ”=0\triangle\epsilon = 0) as claimed in the literature. A new configuration called the macroscopic Fermi-particle is suggested by the character of its wave function. The tunneling rate ((2â–łÏ”)/(π)(2\triangle\epsilon)/(\pi)) does not vanish, not for integer spin s nor for a half-integer value of s, and is calculated explicitly (for the position dependent mass) up to the one-loop approximation.Comment: 13 pages, LaTex, no figure

    Instanton Induced Tunneling Amplitude at Excited States with the LSZ Method

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    Quantum tunneling between degenerate ground states through the central barrier of a potential is extended to excited states with the instanton method. This extension is achieved with the help of an LSZ reduction technique as in field theory and may be of importance in the study of macroscopic quantum phenomena in magnetic systems.Comment: 8 pages, LaTex, no figure
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