318 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
Sum rules for charmed baryon masses
The measured masses of the three charge states of the charmed
baryon are found to be in disagreement with a sum rule based on the quark
model, but relying on no detailed assumptions about the form of the
interaction. This poses a significant problem for the charmed baryon sector of
the quark model. Other relations among charmed baryon masses are also
discussed.Comment: 5 pages, latex, no figure
Analytic Relations between Localizable Entanglement and String Correlations in Spin Systems
We study the relation between the recently defined localizable entanglement
and generalized correlations in quantum spin systems. Differently from the
current belief, the localizable entanglement is always given by the average of
a generalized string. Using symmetry arguments we show that in most spin 1/2
and spin 1 systems the localizable entanglement reduces to the spin-spin or
string correlations, respectively. We prove that a general class of spin 1
systems, which includes the Heisenberg model, can be used as perfect quantum
channel. These conclusions are obtained in analytic form and confirm some
results found previously on numerical grounds.Comment: 5 pages, RevTeX
On critical phases in anisotropic spin-1 chains
Quantum spin-1 chains may develop massless phases in presence of Ising-like
and single-ion anisotropies. We have studied c=1 critical phases by means of
both analytical techniques, including a mapping of the lattice Hamiltonian onto
an O(2) nonlinear sigma model, and a multi-target DMRG algorithm which allows
for accurate calculation of excited states. We find excellent quantitative
agreement with the theoretical predictions and conclude that a pure Gaussian
model, without any orbifold construction, describes correctly the low-energy
physics of these critical phases. This combined analysis indicates that the
multicritical point at large single-ion anisotropy does not belong to the same
universality class as the Takhtajan-Babujian Hamiltonian as claimed in the
past. A link between string-order correlation functions and twisting vertex
operators, along the c=1 line that ends at this point, is also suggested.Comment: 9 pages, 3 figures, svjour format, submitted to Eur. Phys. J.
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
OER nelle UniversitĂ italiane: primi risultati di un'indagine conoscitiva del Gruppo CRUI OA-OER
Le OER si inseriscono nell'ambito del movimento Open Access e dell'accesso aperto alla conoscenza, ritenuta
come bene comune e prevedono licenze aperte costruite nel rispetto dei diritti di proprietĂ intellettuale
dell’autore come stabilito dalle convenzioni internazionali e dalle normative europee e nazionali. All’inizio del
2006, nell’ambito della Commissione biblioteche della CRUI, è stato costituito il Gruppo di lavoro per l’Open
Access, coordinato dal prof. Roberto Delle Donne, con il compito di dare attuazione ai principi della
Dichiarazione di Berlino. Il Gruppo CRUI Open Access ha creato un sottogruppo OER. Durante l'avvio della
discussione del Sottogruppo OA-OER, ci si è accordati sull'idea di realizzare un'indagine e di creare una mappa
dello stato dell'arte di OER in Italia. Il contributo presenta i primi risultati dell'indagine sulle OER nelle
UniversitĂ italiane
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