4,679 research outputs found
Multi-triplet bound states and finite-temperature dynamics in highly frustrated quantum spin ladders
Low-dimensional quantum magnets at finite temperatures present a complex
interplay of quantum and thermal fluctuation effects in a restricted phase
space. While some information about dynamical response functions is available
from theoretical studies of the one-triplet dispersion in unfrustrated chains
and ladders, little is known about the finite-temperature dynamics of
frustrated systems. Experimentally, inelastic neutron scattering studies of the
highly frustrated two-dimensional material SrCu(BO) show an almost
complete destruction of the one-triplet excitation band at a temperature only
1/3 of its gap energy, accompanied by strong scattering intensities for
apparent multi-triplet excitations. We investigate these questions in the
frustrated spin ladder and present numerical results from exact diagonalization
for the dynamical structure factor as a function of temperature. We find
anomalously rapid transfer of spectral weight out of the one-triplet band and
into both broad and sharp spectral features at a wide range of energies,
including below the zero-temperature gap of this excitation. These features are
multi-triplet bound states, which develop particularly strongly near the
quantum phase transition, fall to particularly low energies there, and persist
to all the way to infinite temperature. Our results offer valuable insight into
the physics of finite-temperature spectral functions in SrCu(BO)
and many other highly frustrated spin systems.Comment: 22 pages, 19 figures; published version: many small modification
Efficient Quantum Monte Carlo simulations of highly frustrated magnets: the frustrated spin-1/2 ladder
Quantum Monte Carlo simulations provide one of the more powerful and
versatile numerical approaches to condensed matter systems. However, their
application to frustrated quantum spin models, in all relevant temperature
regimes, is hamstrung by the infamous "sign problem." Here we exploit the fact
that the sign problem is basis-dependent. Recent studies have shown that
passing to a dimer (two-site) basis eliminates the sign problem completely for
a fully frustrated spin model on the two-leg ladder. We generalize this result
to all partially frustrated two-leg spin-1/2 ladders, meaning those where the
diagonal and leg couplings take any antiferromagnetic values. We find that,
although the sign problem does reappear, it remains remarkably mild throughout
the entire phase diagram. We explain this result and apply it to perform
efficient quantum Monte Carlo simulations of frustrated ladders, obtaining
accurate results for thermodynamic quantities such as the magnetic specific
heat and susceptibility of ladders up to L=200 rungs (400 spins 1/2) and down
to very low temperatures.Comment: 26 pages including 12 figures; this version: minor modifications in
sections 3.3 and 4.
Discrete modelling of capillary mechanisms in multi-phase granular media
A numerical study of multi-phase granular materials based upon
micro-mechanical modelling is proposed. Discrete element simulations are used
to investigate capillary induced effects on the friction properties of a
granular assembly in the pendular regime. Capillary forces are described at the
local scale through the Young-Laplace equation and are superimposed to the
standard dry particle interaction usually well simulated through an
elastic-plastic relationship. Both effects of the pressure difference between
liquid and gas phases and of the surface tension at the interface are
integrated into the interaction model. Hydraulic hysteresis is accounted for
based on the possible mechanism of formation and breakage of capillary menisci
at contacts. In order to upscale the interparticular model, triaxial loading
paths are simulated on a granular assembly and the results interpreted through
the Mohr-Coulomb criterion. The micro-mechanical approach is validated with a
capillary cohesion induced at the macroscopic scale. It is shown that
interparticular menisci contribute to the soil resistance by increasing normal
forces at contacts. In addition, more than the capillary pressure level or the
degree of saturation, our findings highlight the importance of the density
number of liquid bonds on the overall behaviour of the material
Hybrid Entrepreneurship
In contrast to previous efforts to model the individual’s movement from wage work into entrepreneurship, we consider that individuals might transition incrementally by retaining their wage job while entering into self-employment. We show that these hybrid entrepreneurs represent a significant share of all entrepreneurial activity. Theoretical arguments are proposed to suggest why hybrid entrants are distinct from self-employment entrants, and why hybrid entry may facilitate subsequent entry into full self-employment. We demonstrate that there are significant theoretical and empirical consequences for this group and our understanding of self-employment entry and labor market dynamics. Using matched employee-employer data over eight years, we test the model on a population of Swedish wage earners in the knowledge-intensive sector.Hybrid entrepreneurship; Self-employment; Labour market dynamics; Transition determinants; Employee-employer data
Optimal Hardy-Littlewood type inequalities for polynomials and multilinear operators
In this paper we obtain quite general and definitive forms for
Hardy-Littlewood type inequalities. Moreover, when restricted to the original
particular cases, our approach provides much simpler and straightforward proofs
and we are able to show that in most cases the exponents involved are optimal.
The technique we used is a combination of probabilistic tools and of an
interpolative approach; this former technique is also employed in this paper to
improve the constants for vector-valued Bohnenblust--Hille type inequalities.Comment: 16 page
Stochastic perturbation of sweeping process and a convergence result for an associated numerical scheme
Here we present well-posedness results for first order stochastic
differential inclusions, more precisely for sweeping process with a stochastic
perturbation. These results are provided in combining both deterministic
sweeping process theory and methods concerning the reflection of a Brownian
motion. In addition, we prove convergence results for a Euler scheme,
discretizing theses stochastic differential inclusions.Comment: 30 page
Lifshitz tails for alloy type models in a constant magnetic field
In this note, we study Lifshitz tails for a 2D Landau Hamiltonian perturbed
by a random alloy-type potential constructed with single site potentials
decaying at least at a Gaussian speed. We prove that, if the Landau level stays
preserved as a band edge for the perturbed Hamiltonian, at the Landau levels,
the integrated density of states has a Lifshitz behavior of the type
Zero-Temperature Properties of the Quantum Dimer Model on the Triangular Lattice
Using exact diagonalizations and Green's function Monte Carlo simulations, we
have studied the zero-temperature properties of the quantum dimer model on the
triangular lattice on clusters with up to 588 sites. A detailed comparison of
the properties in different topological sectors as a function of the cluster
size and for different cluster shapes has allowed us to identify different
phases, to show explicitly the presence of topological degeneracy in a phase
close to the Rokhsar-Kivelson point, and to understand finite-size effects
inside this phase. The nature of the various phases has been further
investigated by calculating dimer-dimer correlation functions. The present
results confirm and complement the phase diagram proposed by Moessner and
Sondhi on the basis of finite-temperature simulations [Phys. Rev. Lett. {\bf
86}, 1881 (2001)].Comment: 10 pages, 16 figure
Characterization of the Sequential Product on Quantum Effects
We present a characterization of the standard sequential product of quantum
effects. The characterization is in term of algebraic, continuity and duality
conditions that can be physically motivated.Comment: 11 pages. Accepted for publication in the Journal of Mathematical
Physic
Spectral analysis of the background in ground-based, long-slit spectroscopy
This paper examines the variations, because of atmospheric extinction, of
broad-band visible spectra, obtained from long-slit spectroscopy, in the
vicinity of some stars, nebulae, and one faint galaxy.Comment: 12 figure
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