103 research outputs found
Solvable Kinetic Gaussian Model in External Field
In this paper, the single-spin transition dynamics is used to investigate the
kinetic Gaussian model in a periodic external field. We first derive the
fundamental dynamic equations, and then treat an isotropic d-dimensional
hypercubic lattice Gaussian spin system with Fourier's transformation method.
We obtain exactly the local magnetization and the equal-time pair correlation
function. The critical characteristics of the dynamical, the complex
susceptibility, and the dynamical response are discussed. The results show that
the time evolution of the dynamical quantities and the dynamical responses of
the system strongly depend on the frequency and the wave vector of the external
field.Comment: 11 page
Glauber Critical Dynamics: Exact Solution of the Kinetic Gaussian Model
In this paper, we have exactly solved Glauber critical dynamics of the
Gaussian model on three dimensions. Of course, it is much easy to apply to low
dimensional case. The key steps are that we generalize the spin change
mechanism from Glauber's single-spin flipping to single-spin transition and
give a normalized version of the transition probability . We have also
investigated the dynamical critical exponent and found surprisingly that the
dynamical critical exponent is highly universal which refer to that for one-
two- and three-dimensions they have same value independent of spatial
dimensionality in contrast to static (equilibrium) critical exponents.Comment: 9 page
Interplay between quasi-periodicity and disorder in quantum spin chains in a magnetic field
We study the interplay between disorder and a quasi periodic coupling array
in an external magnetic field in a spin-1/2 XXZ chain. A simple real space
decimation argument is used to estimate the magnetization values where plateaux
show up. The latter are in good agreement with exact diagonalization results on
fairly long XX chains. Spontaneous susceptibility properties are also studied,
finding a logarithmic behaviour similar to the homogeneously disordered case.Comment: 5 RevTeX pages, 5 Postscript figures include
Counterfactual Evaluation of Slate Recommendations with Sequential Reward Interactions
Users of music streaming, video streaming, news recommendation, and
e-commerce services often engage with content in a sequential manner. Providing
and evaluating good sequences of recommendations is therefore a central problem
for these services. Prior reweighting-based counterfactual evaluation methods
either suffer from high variance or make strong independence assumptions about
rewards. We propose a new counterfactual estimator that allows for sequential
interactions in the rewards with lower variance in an asymptotically unbiased
manner. Our method uses graphical assumptions about the causal relationships of
the slate to reweight the rewards in the logging policy in a way that
approximates the expected sum of rewards under the target policy. Extensive
experiments in simulation and on a live recommender system show that our
approach outperforms existing methods in terms of bias and data efficiency for
the sequential track recommendations problem
Kinetics of a non-glauberian Ising model: global observables and exact results
We analyse the spin-flip dynamics in kinetic Ising chains with
Kimball-Deker-Haake (KDH) transition rates, and evaluate exactly the evolution
of global quantities like magnetisation and its fluctuations, and the two-time
susceptibilities and correlations of the global spin and the global three-spin.
Information on the ageing behaviour after a quench to zero temperature is
extracted
Quasi-periodic spin chains in a magnetic field
We study the interplay between a (quasi) periodic coupling array and an
external magnetic field in a spin-1/2 XXZ chain. A new class of magnetization
plateaux are obtained by means of Abelian bosonization methods which give rise
to a sufficient quantization condition. The investigation of magnetic phase
diagrams via exact diagonalization of finite clusters finds a complete
agreement with the continuum treatment in a variety of situations.Comment: 4 pages RevTeX, 5 PostScript figures included. Final version to
appear in PR
Universal Critical Behavior of Aperiodic Ferromagnetic Models
We investigate the effects of geometric fluctuations, associated with
aperiodic exchange interactions, on the critical behavior of -state
ferromagnetic Potts models on generalized diamond hierarchical lattices. For
layered exchange interactions according to some two-letter substitutional
sequences, and irrelevant geometric fluctuations, the exact recursion relations
in parameter space display a non-trivial diagonal fixed point that governs the
universal critical behavior. For relevant fluctuations, this fixed point
becomes fully unstable, and we show the apperance of a two-cycle which is
associated with a novel critical behavior. We use scaling arguments to
calculate the critical exponent of the specific heat, which turns out
to be different from the value for the uniform case. We check the scaling
predictions by a direct numerical analysis of the singularity of the
thermodynamic free-energy. The agreement between scaling and direct
calculations is excellent for stronger singularities (large values of ). The
critical exponents do not depend on the strengths of the exchange interactions.Comment: 4 pages, 1 figure (included), RevTeX, submitted to Phys. Rev. E as a
Rapid Communicatio
Preoperative CT versus diffusion weighted magnetic resonance imaging of the liver in patients with rectal cancer:a prospective randomized trial
Introduction. Colorectal cancer is one of the most frequent cancers in the world and liver metastases are seen in up to 19% of patients with colorectal cancers. Detection of liver metastases is not only vital for sufficient treatment and survival, but also for a better estimation of prognosis. The aim of this study was to evaluate the feasibility of diffusion weighted MRI of the liver as part of a combined MR evaluation of patients with rectal cancers and compare it with the standard preoperative evaluation of the liver with CT.Methods. Consecutive patients diagnosed with rectal cancers were asked to participate in the study. Preoperative CT and diffusion weighted MR (DWMR) were compared to contrast enhanced laparoscopic ultrasound (CELUS).Results. A total of 35 patients were included, 15 patients in Group-1 having the standard CT evaluation of the liver and 20 patients in Group-2 having the standard CT evaluation of the liver and DWMR of the liver. Compared with CELUS, the per-patient sensitivity/specificity was 50/100% for CT, and for DWMR: 100/94% and 100/100% for Reader 1 and 2, respectively. The per-lesion sensitivity of CT and DWMR were 17% and 89%, respectively compared with CELUS. Furthermore, one patient had non-resectable metastases after DWMR despite being diagnosed with resectable metastases after CT. Another patient was diagnosed with multiple liver metastases during CELUS, despite a negative CT-scan.Discussion. DWMR is feasible for preoperative evaluation of liver metastases. The current standard preoperative evaluation with CT-scan results in disadvantages like missed metastases and futile operations. We recommend that patients with rectal cancer, who are scheduled for MR of the rectum, should have a DWMR of the liver performed at the same time
New Dynamic Monte Carlo Renormalization Group Method
The dynamical critical exponent of the two-dimensional spin-flip Ising model
is evaluated by a Monte Carlo renormalization group method involving a
transformation in time. The results agree very well with a finite-size scaling
analysis performed on the same data. The value of is
obtained, which is consistent with most recent estimates
Nonequilibrium relaxation of the two-dimensional Ising model: Series-expansion and Monte Carlo studies
We study the critical relaxation of the two-dimensional Ising model from a
fully ordered configuration by series expansion in time t and by Monte Carlo
simulation. Both the magnetization (m) and energy series are obtained up to
12-th order. An accurate estimate from series analysis for the dynamical
critical exponent z is difficult but compatible with 2.2. We also use Monte
Carlo simulation to determine an effective exponent, z_eff(t) = - {1/8} d ln t
/d ln m, directly from a ratio of three-spin correlation to m. Extrapolation to
t = infinity leads to an estimate z = 2.169 +/- 0.003.Comment: 9 pages including 2 figure
- …