33,645 research outputs found
Cluster expansion for dimerized spin systems
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
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 Li
We investigate the consistency of the measured charge radius and dipole
response of Li within a three-body model. We show how these observables
are related to the mean square distance between the Li 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 1.5. We also
investigate the sensitivity to the three-body structure of 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
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 . The gluon distribution function is proportional to ,
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
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 of a quantum spin
system. The quenching of macroscopic quantum coherence is understood as the
vanishing of in disagreement with the suppression of tunneling
(i.e. ) 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
() 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
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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