401 research outputs found
Critical Behavior of the Random Potts Chain
We study the critical behavior of the random q-state Potts quantum chain by
density matrix renormalization techniques. Critical exponents are calculated by
scaling analysis of finite lattice data of short chains () averaging
over all possible realizations of disorder configurations chosen according to a
binary distribution. Our numerical results show that the critical properties of
the model are independent of q in agreement with a renormalization group
analysis of Senthil and Majumdar (Phys. Rev. Lett.{\bf 76}, 3001 (1996)). We
show how an accurate analysis of moments of the distribution of magnetizations
allows a precise determination of critical exponents, circumventing some
problems related to binary disorder. Multiscaling properties of the model and
dynamical correlation functions are also investigated.Comment: LaTeX2e file with Revtex, 9 pages, 8 eps figures, 4 tables; typos
correcte
Relaxational dynamics in 3D randomly diluted Ising models
We study the purely relaxational dynamics (model A) at criticality in
three-dimensional disordered Ising systems whose static critical behaviour
belongs to the randomly diluted Ising universality class. We consider the
site-diluted and bond-diluted Ising models, and the +- J Ising model along the
paramagnetic-ferromagnetic transition line. We perform Monte Carlo simulations
at the critical point using the Metropolis algorithm and study the dynamic
behaviour in equilibrium at various values of the disorder parameter. The
results provide a robust evidence of the existence of a unique model-A dynamic
universality class which describes the relaxational critical dynamics in all
considered models. In particular, the analysis of the size-dependence of
suitably defined autocorrelation times at the critical point provides the
estimate z=2.35(2) for the universal dynamic critical exponent. We also study
the off-equilibrium relaxational dynamics following a quench from T=\infty to
T=T_c. In agreement with the field-theory scenario, the analysis of the
off-equilibrium dynamic critical behavior gives an estimate of z that is
perfectly consistent with the equilibrium estimate z=2.35(2).Comment: 38 page
On the universality of the fluctuation-dissipation ratio in non-equilibrium critical dynamics
The two-time nonequilibrium correlation and response functions in 1D kinetic
classical spin systems with non-conserved dynamics and quenched to their
zero-temperature critical point are studied. The exact solution of the kinetic
Ising model with Glauber dynamics for a wide class of initial states allows for
an explicit test of the universality of the non-equilibrium limit
fluctuation-dissipation ratio X_{\infty}. It is shown that the value of
X_{\infty} depends on whether the initial state has finitely many domain walls
or not and thus two distinct dynamic universality classes can be identified in
this model. Generic 1D kinetic spin systems with non-conserved dynamics fall
into the same universality classes as the kinetic Glauber-Ising model provided
the dynamics is invariant under the C-symmetry of simultaneous spin and
magnetic-field reversal. While C-symmetry is satisfied for magnetic systems, it
need not be for lattice gases which may therefore display hitherto unexplored
types of non-universal kinetics
On the definition of a unique effective temperature for non-equilibrium critical systems
We consider the problem of the definition of an effective temperature via the
long-time limit of the fluctuation-dissipation ratio (FDR) after a quench from
the disordered state to the critical point of an O(N) model with dissipative
dynamics. The scaling forms of the response and correlation functions of a
generic observable are derived from the solutions of the corresponding
Renormalization Group equations. We show that within the Gaussian approximation
all the local observables have the same FDR, allowing for a definition of a
unique effective temperature. This is no longer the case when fluctuations are
taken into account beyond that approximation, as shown by a computation up to
the first order in the epsilon-expansion for two quadratic observables. This
implies that, contrarily to what often conjectured, a unique effective
temperature can not be defined for this class of models.Comment: 32 pages, 5 figures. Minor changes, published versio
Universality class of 3D site-diluted and bond-diluted Ising systems
We present a finite-size scaling analysis of high-statistics Monte Carlo
simulations of the three-dimensional randomly site-diluted and bond-diluted
Ising model. The critical behavior of these systems is affected by
slowly-decaying scaling corrections which make the accurate determination of
their universal asymptotic behavior quite hard, requiring an effective control
of the scaling corrections. For this purpose we exploit improved Hamiltonians,
for which the leading scaling corrections are suppressed for any thermodynamic
quantity, and improved observables, for which the leading scaling corrections
are suppressed for any model belonging to the same universality class.
The results of the finite-size scaling analysis provide strong numerical
evidence that phase transitions in three-dimensional randomly site-diluted and
bond-diluted Ising models belong to the same randomly dilute Ising universality
class. We obtain accurate estimates of the critical exponents, ,
, , , ,
, and of the leading and next-to-leading correction-to-scaling
exponents, and .Comment: 45 pages, 22 figs, revised estimate of n
Aperiodicity-Induced Second-Order Phase Transition in the 8-State Potts Model
We investigate the critical behavior of the two-dimensional 8-state Potts
model with an aperiodic distribution of the exchange interactions between
nearest-neighbor rows. The model is studied numerically through intensive Monte
Carlo simulations using the Swendsen-Wang cluster algorithm. The transition
point is located through duality relations, and the critical behavior is
investigated using FSS techniques at criticality. For strong enough
fluctuations of the aperiodic sequence under consideration, a second order
phase transition is found. The exponents and are
obtained at the new fixed point.Comment: LaTeX file with Revtex, 4 pages, 5 eps figures, to appear in Phys.
Rev. Let
7-Substituted 2-Nitro-5,6-dihydroimidazo[2,1-b][1,3]oxazines: Novel Antitubercular Agents Lead to a New Preclinical Candidate for Visceral Leishmaniasis.
Within a backup program for the clinical investigational agent pretomanid (PA-824), scaffold hopping from delamanid inspired the discovery of a novel class of potent antitubercular agents that unexpectedly possessed notable utility against the kinetoplastid disease visceral leishmaniasis (VL). Following the identification of delamanid analogue DNDI-VL-2098 as a VL preclinical candidate, this structurally related 7-substituted 2-nitro-5,6-dihydroimidazo[2,1-b][1,3]oxazine class was further explored, seeking efficacious backup compounds with improved solubility and safety. Commencing with a biphenyl lead, bioisosteres formed by replacing one phenyl by pyridine or pyrimidine showed improved solubility and potency, whereas more hydrophilic side chains reduced VL activity. In a Leishmania donovani mouse model, two racemic phenylpyridines (71 and 93) were superior, with the former providing >99% inhibition at 12.5 mg/kg (b.i.d., orally) in the Leishmania infantum hamster model. Overall, the 7R enantiomer of 71 (79) displayed more optimal efficacy, pharmacokinetics, and safety, leading to its selection as the preferred development candidate
Magnetic critical behavior of two-dimensional random-bond Potts ferromagnets in confined geometries
We present a numerical study of 2D random-bond Potts ferromagnets. The model
is studied both below and above the critical value which discriminates
between second and first-order transitions in the pure system. Two geometries
are considered, namely cylinders and square-shaped systems, and the critical
behavior is investigated through conformal invariance techniques which were
recently shown to be valid, even in the randomness-induced second-order phase
transition regime Q>4. In the cylinder geometry, connectivity transfer matrix
calculations provide a simple test to find the range of disorder amplitudes
which is characteristic of the disordered fixed point. The scaling dimensions
then follow from the exponential decay of correlations along the strip. Monte
Carlo simulations of spin systems on the other hand are generally performed on
systems of rectangular shape on the square lattice, but the data are then
perturbed by strong surface effects. The conformal mapping of a semi-infinite
system inside a square enables us to take into account boundary effects
explicitly and leads to an accurate determination of the scaling dimensions.
The techniques are applied to different values of Q in the range 3-64.Comment: LaTeX2e file with Revtex, revised versio
Fluctuation relations in non-equilibrium stationary states of Ising models
Fluctuation relations for the entropy production in non equilibrium
stationary states of Ising models are investigated by Monte Carlo simulations.
Systems in contact with heat baths at two different temperatures or subject to
external driving will be studied. In the first case, by considering different
kinetic rules and couplings with the baths, the behavior of the probability
distributions of the heat exchanged in a time with the thermostats, both
in the disordered and in the low temperature phase, are discussed. The
fluctuation relation is always verified in the large limit and
deviations from linear response theory are observed. Finite- corrections
are shown to obey a scaling behavior. In the other case the system is in
contact with a single heat bath but work is done by shearing it. Also for this
system the statistics collected for the mechanical work shows the validity of
the fluctuation relation and preasymptotic corrections behave analogously to
the case with two baths.Comment: 9 figure
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