2,159 research outputs found
Continuum in the Excitation Spectrum of the S=1 Compound CsNiCl_3
Recent neutron scattering experiments on CsNiCl_3 reveal some features which
are not well described by the nonlinear sigma model nor by numerical
simulations on isolated S=1 spin chains. In particular, in real systems the
intensity of the continuum of multiparticle excitations, at T=6K, is about 5
times greater than predicted. Also the gap is slightly higher and the
correlation length is smaller. We propose a theoretical scenario where the
interchain interaction is approximated by a staggered magnetic field, yielding
to a correct prediction of the observed quantities.Comment: 4 pages, 2 figures (.eps), RevTe
Low-Dimensional Spin Systems: Hidden Symmetries, Conformal Field Theories and Numerical Checks
We review here some general properties of antiferromagnetic Heisenberg spin
chains, emphasizing and discussing the role of hidden symmetries in the
classification of the various phases of the models. We present also some recent
results that have been obtained with a combined use of Conformal Field Theory
and of numerical Density Matrix Renormalization Group techniques.Comment: To be published in the proceedings of the XIII Conference on
"Symmetries in Physics", held in Bregenz (Voralberg, Austria), 21-24/7/2003.
Plain LaTeX2e, 4 EPS figure
Alternative Hamiltonian Desciptions and Statistical Mechanics
We argue here that, as it happens in Classical and Quantum Mechanics, where
it has been proven that alternative Hamiltonian descriptions can be compatible
with a given set of equations of motion, the same holds true in the realm of
Statistical Mechanics, i.e. that alternative Hamiltonian descriptions do lead
to the same thermodynamical description of any physical system.Comment: 11 page
Spin Chains in an External Magnetic Field. Closure of the Haldane Gap and Effective Field Theories
We investigate both numerically and analytically the behaviour of a spin-1
antiferromagnetic (AFM) isotropic Heisenberg chain in an external magnetic
field. Extensive DMRG studies of chains up to N=80 sites extend previous
analyses and exhibit the well known phenomenon of the closure of the Haldane
gap at a lower critical field H_c1. We obtain an estimate of the gap below
H_c1. Above the lower critical field, when the correlation functions exhibit
algebraic decay, we obtain the critical exponent as a function of the net
magnetization as well as the magnetization curve up to the saturation (upper
critical) field H_c2. We argue that, despite the fact that the SO(3) symmetry
of the model is explicitly broken by the field, the Haldane phase of the model
is still well described by an SO(3) nonlinear sigma-model. A mean-field theory
is developed for the latter and its predictions are compared with those of the
numerical analysis and with the existing literature.Comment: 11 pages, 4 eps figure
Wigner distributions for finite dimensional quantum systems: An algebraic approach
We discuss questions pertaining to the definition of `momentum', `momentum
space', `phase space', and `Wigner distributions'; for finite dimensional
quantum systems. For such systems, where traditional concepts of `momenta'
established for continuum situations offer little help, we propose a physically
reasonable and mathematically tangible definition and use it for the purpose of
setting up Wigner distributions in a purely algebraic manner. It is found that
the point of view adopted here is limited to odd dimensional systems only. The
mathematical reasons which force this situation are examined in detail.Comment: Latex, 13 page
Reconstructing mass profiles of simulated galaxy clusters by combining Sunyaev-Zeldovich and X-ray images
We present a method to recover mass profiles of galaxy clusters by combining
data on thermal Sunyaev-Zeldovich (tSZ) and X-ray imaging, thereby avoiding to
use any information on X-ray spectroscopy. This method, which represents a
development of the geometrical deprojection technique presented in Ameglio et
al. (2007), implements the solution of the hydrostatic equilibrium equation. In
order to quantify the efficiency of our mass reconstructions, we apply our
technique to a set of hydrodynamical simulations of galaxy clusters. We propose
two versions of our method of mass reconstruction. Method 1 is completely
model-independent, while Method 2 assumes instead the analytic mass profile
proposed by Navarro et al. (1997) (NFW). We find that the main source of bias
in recovering the mass profiles is due to deviations from hydrostatic
equilibrium, which cause an underestimate of the mass of about 10 per cent at
r_500 and up to 20 per cent at the virial radius. Method 1 provides a
reconstructed mass which is biased low by about 10 per cent, with a 20 per cent
scatter, with respect to the true mass profiles. Method 2 proves to be more
stable, reducing the scatter to 10 per cent, but with a larger bias of 20 per
cent, mainly induced by the deviations from equilibrium in the outskirts. To
better understand the results of Method 2, we check how well it allows to
recover the relation between mass and concentration parameter. When analyzing
the 3D mass profiles we find that including in the fit the inner 5 per cent of
the virial radius biases high the halo concentration. Also, at a fixed mass,
hotter clusters tend to have larger concentration. Our procedure recovers the
concentration parameter essentially unbiased but with a scatter of about 50 per
cent.Comment: 13 pages, 11 figures, submitted to MNRA
Phase-space descriptions of operators and the Wigner distribution in quantum mechanics II. The finite dimensional case
A complete solution to the problem of setting up Wigner distribution for
N-level quantum systems is presented. The scheme makes use of some of the ideas
introduced by Dirac in the course of defining functions of noncommuting
observables and works uniformly for all N. Further, the construction developed
here has the virtue of being essentially input-free in that it merely requires
finding a square root of a certain N^2 x N^2 complex symmetric matrix, a task
which, as is shown, can always be accomplished analytically. As an
illustration, the case of a single qubit is considered in some detail and it is
shown that one recovers the result of Feynman and Wootters for this case
without recourse to any auxiliary constructs.Comment: 14 pages, typos corrected, para and references added in introduction,
submitted to Jour. Phys.
Alternative linear structures for classical and quantum systems
The possibility of deforming the (associative or Lie) product to obtain
alternative descriptions for a given classical or quantum system has been
considered in many papers. Here we discuss the possibility of obtaining some
novel alternative descriptions by changing the linear structure instead. In
particular we show how it is possible to construct alternative linear
structures on the tangent bundle TQ of some classical configuration space Q
that can be considered as "adapted" to the given dynamical system. This fact
opens the possibility to use the Weyl scheme to quantize the system in
different non equivalent ways, "evading", so to speak, the von Neumann
uniqueness theorem.Comment: 32 pages, two figures, to be published in IJMP
Integrated interpretation of seismic and resistivity images across the «Val d'Agri» graben (Italy)
Val d'Agri is a «recent SSW - NNE graben» located in the middle of the Southern Apennines thrust belt «chain» and emplaced in Plio-Pleistocene.The recent sedimentation of the valley represents a local critical geophysical problem. Several strong near surface velocity anomalies and scattering degrades seismic data in different ways and compromises the seismic visibility. In 1998, ENI and Enterprise, with the contribution of the European Community (ESIT R & D project - Enhance Seismic In Thrust Belt; EU Thermie fund) acquired two «experimental seismic and Resistivity lines» across the valley. The purpose of the project was to look for methods able to enhance seismic data quality and optimize the data processing flow for «thrust belt» areas. During the work, it was clear that some part of the seismic data processing flow could be used for the detailed geological interpretation of the near subsurface too. In fact, the integrated interpretation of the near surface tomography velocity/depth seismic section, built for enhancing the resolution of static corrections, with the HR resistivity profile, acquired for enhancing the seismic source coupling, allowed a quite detailed lithological interpretation of the main shallow velocity changes and the 2D reconstruction of the structural setting of the valley
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