65 research outputs found
An optimal series expansion of the multiparameter fractional Brownian motion
We derive a series expansion for the multiparameter fractional Brownian
motion. The derived expansion is proven to be rate optimal.Comment: 21 pages, no figures, final version, to appear in Journal of
Theoretical Probabilit
Magnetization Plateaus in a Solvable 3-Leg Spin Ladder
We present a solvable ladder model which displays magnetization plateaus at
fractional values of the total magnetization. Plateau signatures are also shown
to exist along special lines. The model has isotropic Heisenberg interactions
with additional many-body terms. The phase diagram can be calculated exactly
for all values of the rung coupling and the magnetic field. We also derive the
anomalous behaviour of the susceptibility near the plateau boundaries. There is
good agreement with the phase diagram obtained recently for the pure Heisenberg
ladders by numerical and perturbative techniques.Comment: 4 pages, revtex, 3 postscript figures, small changes to the text and
references update
A Strong-Coupling Approach to the Magnetization Process of Polymerized Quantum Spin Chains
Polymerized quantum spin chains (i.e. spin chains with a periodic modulation
of the coupling constants) exhibit plateaux in their magnetization curves when
subjected to homogeneous external magnetic fields. We argue that the
strong-coupling limit yields a simple but general explanation for the
appearance of plateaux as well as of the associated quantization condition on
the magnetization. We then proceed to explicitly compute series for the plateau
boundaries of trimerized and quadrumerized spin-1/2 chains. The picture is
completed by a discussion how the universality classes associated to the
transitions at the boundaries of magnetization plateaux arise in many cases
from a first order strong-coupling effective Hamiltonian.Comment: 5 pages REVTeX, three PostScript figures included using psfig.st
Charged Particle Multiplicity in Diffractive Deep Inelastic Scattering
The recent data from H1 Collaboration on hadron multiplicity in diffractive
DIS has been studied in the framework of perturbative QCD as a function of
invariant diffractive mass. The formulas obtained explain the observed excess
of particle production in diffractive DIS relative to that in DIS and
annihilation. It is shown that the results are sensitive to the quark--gluon
structure of the Pomeron. Namely, the data say in favour of a super-hard gluon
distribution at the initial scale.Comment: 12 pages, 3 figures; to be published in Phys. Rev.
Haldane-gap chains in a magnetic field
We consider quasi one dimensional spin-1 Heisenberg chains with crystal field
anisotropy in a uniform magnetic field. We determine the dynamical structure
factor in various limits and obtain a fairly complete qualitative picture of
how it changes with the applied field. In particular, we discuss how the width
of the higher energy single magnon modes depends on the field. We consider the
effects of a weak interchain coupling. We discuss the relevance of our results
for recent neutron scattering experiments on the quasi-1D Haldane-gap compound
NDMAP.Comment: 34 pages, 7 figure
Random bond XXZ chains with modulated couplings
The magnetization behavior of q-periodic antiferromagnetic spin 1/2
Heisenberg chains under uniform magnetic fields is investigated in a background
of disorder exchange distributions. By means of both real space decimation
procedures and numerical diagonalizations in XX chains, it is found that for
binary disorder the magnetization exhibits wide plateaux at values of
1+2(p-1)/q, where p is the disorder strength. In contrast, no spin gaps are
observed in the presence of continuous exchange distributions. We also study
the magnetic susceptibility at low magnetic fields. For odd q-modulations the
susceptibility exhibits a universal singularity, whereas for q even it displays
a non-universal power law behavior depending on the parameters of the
distribution.Comment: 4 pages, 3 figures. Final version to appear in PR
A Brownian particle in a microscopic periodic potential
We study a model for a massive test particle in a microscopic periodic
potential and interacting with a reservoir of light particles. In the regime
considered, the fluctuations in the test particle's momentum resulting from
collisions typically outweigh the shifts in momentum generated by the periodic
force, and so the force is effectively a perturbative contribution. The
mathematical starting point is an idealized reduced dynamics for the test
particle given by a linear Boltzmann equation. In the limit that the mass ratio
of a single reservoir particle to the test particle tends to zero, we show that
there is convergence to the Ornstein-Uhlenbeck process under the standard
normalizations for the test particle variables. Our analysis is primarily
directed towards bounding the perturbative effect of the periodic potential on
the particle's momentum.Comment: 60 pages. We reorganized the article and made a few simplifications
of the conten
Dynamical spin correlations in Heisenberg ladder under magnetic field and correlation functions in SO(5) ladder
The zero-temperature dynamical spin-spin correlation functions are calculated
for the spin-1/2 two-leg Heisenberg ladder in a magnetic field above the lower
critical field Hc1. The dynamical structure factors are calculated which
exhibit both massless and massive excitations. These modes appear in different
sectors characterized by the parity in the rung direction and by the momentum
in the direction of the chains. The structure factors have power-law
singularities at the lower edges of their support. The results are also
applicable to spin-1 Heisenberg chain. The implications are briefly discussed
for various correlation functions and the pi-resonance in the SO(5) symmetric
ladder model.Comment: 15 pages, 6 figures, added references; final version to appear in
Phys. Rev.
Metal-Kondo insulating transitions and transport in one dimension
We study two different metal-insulating transitions possibly occurring in
one-dimensional Kondo lattices. First, we show how doping the pure Kondo
lattice model in the strong-coupling limit, results in a Pokrovsky-Talapov
transition. This produces a conducting state with a charge susceptibility
diverging as the inverse of the doping, that seems in agreement with numerical
datas. Second, in the weak-coupling region, Kondo insulating transitions arise
due to the consequent renormalization of the backward Kondo scattering. Here,
the interplay between Kondo effect and electron-electron interactions gives
rise to significant phenomena in transport, in the high-temperature delocalized
(ballistic) regime. For repulsive interactions, as a perfect signature of Kondo
localization, the conductivity is found to decrease monotonically with
temperature. When interactions become attractive, spin fluctuations in the
electron (Luttinger-type) liquid are suddenly lowered. The latter is less
localized by magnetic impurities than for the repulsive counterpart, and as a
result a large jump in the Drude weight and a maximum in the conductivity arise
in the entrance of the Kondo insulating phase. These can be viewed as remnants
of s-wave superconductivity arising for attractive enough interactions.
Comparisons with transport in the single impurity model are also performed. We
finally discuss the case of randomly distributed magnetic defects, and the
applications on persistent currents of mesoscopic rings.Comment: 21 pages, two columns, 5 figures and 1 table; Final version: To
appear in Physical Review
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