840 research outputs found
Multifractal Properties of Aperiodic Ising Model: role of geometric fluctuations
The role of the geometric fluctuations on the multifractal properties of the
local magnetization of aperiodic ferromagnetic Ising models on hierachical
lattices is investigated. The geometric fluctuations are introduced by
generalized Fibonacci sequences. The local magnetization is evaluated via an
exact recurrent procedure encompassing a real space renormalization group
decimation. The symmetries of the local magnetization patterns induced by the
aperiodic couplings is found to be strongly (weakly) different, with respect to
the ones of the corresponding homogeneous systems, when the geometric
fluctuations are relevant (irrelevant) to change the critical properties of the
system. At the criticality, the measure defined by the local magnetization is
found to exhibit a non-trivial F(alpha) spectra being shifted to higher values
of alpha when relevant geometric fluctuations are considered. The critical
exponents are found to be related with some special points of the F(alpha)
function and agree with previous results obtained by the quite distinct
transfer matrix approach.Comment: 10 pages, 7 figures, 3 Tables, 17 reference
Short Range Ising Spin Glasses: a critical exponent study
The critical properties of short-range Ising spin-glass models, defined on a
diamond hierarchical lattice of graph fractal dimension , 3, and 4,
and scaling factor 2 are studied via a method based on the Migdal-Kadanoff
renormalization-group scheme. The order parameter critical exponent is
directly estimated from the data of the local Edwards- Anderson (EA) order
parameter, obtained through an exact recursion procedure. The scaling of the EA
order parameter, leading to estimates of the exponent of the correlation
length is also performed. Four distinct initial distributions of the quenched
coupling constants (Gaussian, bimodal, uniform and exponential) are considered.
Deviations from a universal behaviour are observed and analysed in the
framework of the renormalized flow in a two dimensional appropriate parameter
space.Comment: 9 pages, 01 figure (ps
lessons from Brazilian air disasters
Purpose: The focus of this study was to analyze crisis management in a context of high-reliability organizations (HRO) evidenced in two cases of Brazilian air disasters. Aspects of human and technological natures were examined, addressing the complex sociotechnical system. Design/methodology/approach: This in-depth case study addressed the two most serious air disasters on Brazilian territory. The first case involved a midair collision between Gol Flight 1907 and the Legacy jet. In the second case, TAM flight 3054 had difficulty braking when landing at the airport and crashed into a building. Data were collected from official disaster documents. Findings: The results revealed that the management and operational activities aimed to maintain the necessary conditions that prioritize a high level of reliability. High reliability mainly involves concern over failure, reluctance to accept simplified interpretations, sensitivity to operations, commitment to resilience and detailed structure specifications. Practical implications: The implications are based on alerting highly reliable organizations, emphasizing the focus on managing more reliably, resiliently and conscientiously. Changes will be required in the operations of organizations seeking to learn to manage unexpected events and respond quickly to continually improve the responsiveness of their services. Originality/value: In the perspective of an intrinsic case study for crisis management in a context of HRO and disaster risk management, the originality of this study lies in its examination of the paradoxical nature of control within the systems of dangerous operations in complex organizations, as well as their contradictions in a high-reliability system.authorsversionpublishe
Tricritical Points in the Sherrington-Kirkpatrick Model in the Presence of Discrete Random Fields
The infinite-range-interaction Ising spin glass is considered in the presence
of an external random magnetic field following a trimodal (three-peak)
distribution. The model is studied through the replica method and phase
diagrams are obtained within the replica-symmetry approximation. It is shown
that the border of the ferromagnetic phase may present first-order phase
transitions, as well as tricritical points at finite temperatures. Analogous to
what happens for the Ising ferromagnet under a trimodal random field, it is
verified that the first-order phase transitions are directly related to the
dilution in the fields (represented by ). The ferromagnetic boundary at
zero temperature also exhibits an interesting behavior: for , a single tricritical point occurs, whereas if
the critical frontier is completely continuous; however, for
, a fourth-order critical point appears. The stability
analysis of the replica-symmetric solution is performed and the regions of
validity of such a solution are identified; in particular, the Almeida-Thouless
line in the plane field versus temperature is shown to depend on the weight
.Comment: 23pages, 7 ps figure
Ising spin glass under continuous-distribution random magnetic fields: Tricritical points and instability lines
The effects of random magnetic fields are considered in an Ising spin-glass
model defined in the limit of infinite-range interactions. The probability
distribution for the random magnetic fields is a double Gaussian, which
consists of two Gaussian distributions centered respectively, at and
, presenting the same width . It is argued that such a
distribution is more appropriate for a theoretical description of real systems
than its simpler particular two well-known limits, namely the single Gaussian
distribution (), and the bimodal one (). The
model is investigated by means of the replica method, and phase diagrams are
obtained within the replica-symmetric solution. Critical frontiers exhibiting
tricritical points occur for different values of , with the possibility
of two tricritical points along the same critical frontier. To our knowledge,
it is the first time that such a behavior is verified for a spin-glass model in
the presence of a continuous-distribution random field, which represents a
typical situation of a real system. The stability of the replica-symmetric
solution is analyzed, and the usual Almeida-Thouless instability is verified
for low temperatures. It is verified that, the higher-temperature tricritical
point always appears in the region of stability of the replica-symmetric
solution; a condition involving the parameters and , for the
occurrence of this tricritical point only, is obtained analytically. Some of
our results are discussed in view of experimental measurements available in the
literature.Comment: 23 pages, 8 figures, accept for publication in Phys. Rev.
Finite top quark mass effects in NNLO Higgs boson production at LHC
We present next-to-next-to-leading order corrections to the inclusive
production of the Higgs bosons at the CERN Large Hadron Collider (LHC)
including finite top quark mass effects. Expanding our analytic results for the
partonic cross section around the soft limit we find agreement with a very
recent publication by Harlander and Ozeren \cite{Harlander:2009mq}.Comment: 15 page
- …