381 research outputs found
Modeling Disordered Quantum Systems with Dynamical Networks
It is the purpose of the present article to show that so-called network
models, originally designed to describe static properties of disordered
electronic systems, can be easily generalized to quantum-{\em dynamical}
models, which then allow for an investigation of dynamical and spectral
aspects. This concept is exemplified by the Chalker-Coddington model for the
Quantum Hall effect and a three-dimensional generalization of it. We simulate
phase coherent diffusion of wave packets and consider spatial and spectral
correlations of network eigenstates as well as the distribution of
(quasi-)energy levels. Apart from that it is demonstrated how network models
can be used to determine two-point conductances. Our numerical calculations for
the three-dimensional model at the Metal-Insulator transition point delivers
among others an anomalous diffusion exponent of .
The methods presented here in detail have been used partially in earlier work.Comment: 16 pages, Rev-TeX. to appear in Int. J. Mod. Phys.
Spectral Compressibility at the Metal-Insulator Transition of the Quantum Hall Effect
The spectral properties of a disordered electronic system at the
metal-insulator transition point are investigated numerically. A recently
derived relation between the anomalous diffusion exponent and the
spectral compressibility at the mobility edge, , is
confirmed for the integer quantum Hall delocalization transition. Our
calculations are performed within the framework of an unitary network-model and
represent a new method to investigate spectral properties of disordered
systems.Comment: 5 pages, RevTeX, 3 figures, Postscript, strongly revised version to
be published in PR
Quantum Two-State Dynamics Driven by Stationary Non-Markovian Discrete Noise: Exact Results
We consider the problem of stochastic averaging of a quantum two-state
dynamics driven by non-Markovian, discrete noises of the continuous time random
walk type (multistate renewal processes). The emphasis is put on the proper
averaging over the stationary noise realizations corresponding, e.g., to a
stationary environment. A two state non-Markovian process with an arbitrary
non-exponential distribution of residence times (RTDs) in its states with a
finite mean residence time provides a paradigm. For the case of a two-state
quantum relaxation caused by such a classical stochastic field we obtain the
explicit exact, analytical expression for the averaged Laplace-transformed
relaxation dynamics. In the limit of Markovian noise (implying an exponential
RTD), all previously known results are recovered. We exemplify new more general
results for the case of non-Markovian noise with a biexponential RTD. The
averaged, real-time relaxation dynamics is obtained in this case by numerically
exact solving of a resulting algebraic polynomial problem. Moreover, the case
of manifest non-Markovian noise with an infinite range of temporal
autocorrelation (which in principle is not accessible to any kind of
perturbative treatment) is studied, both analytically (asymptotic long-time
dynamics) and numerically (by a precise numerical inversion of the
Laplace-transformed averaged quantum relaxation).Comment: Chemical Physics, in pres
High frequency intrinsic modes in El Ni\~no Southern Oscillation Index
Recent data of the Southern Oscillation Index have been analyzed. The
power spectrum indicates major intrinsic geophysical short periods. We find
interesting ``high frequency'' oscillations at 24, 27, 37, 76, 100 and 365
days. In particular the 24 days peaks may correspond to the Branstator-Kushnir
wave, the 27 days may be due to the moon effect rotation, the 37 days peaks is
most probably related to the Madden and Julian Oscillation. It is not yet clear
the explanations for the 76 days which may be associated with interseasonal
oscillation in the tropical atmosphere; the 100 days could be resulting from a
mere beat between the 37 and 27 periods, or the 76 and 365 days. Next these
periods are used to reconstruct the signal and to produce a forecast for the
next 9 months, at the time of writing. After cleansing the signal of those
periodicities a detrended fluctuation analysis is performed to reveal the
nature of the stochastic structures in the signal and whether specific
correlation can be found. We study the evolution of the distribution of first
return times, in particular between . A markedly significant
difference from the expected distribution for uncorrelated events is found.Comment: accepted for publication on Physica
Point-Contact Conductances at the Quantum Hall Transition
On the basis of the Chalker-Coddington network model, a numerical and
analytical study is made of the statistics of point-contact conductances for
systems in the integer quantum Hall regime. In the Hall plateau region the
point-contact conductances reflect strong localization of the electrons, while
near the plateau transition they exhibit strong mesoscopic fluctuations. By
mapping the network model on a supersymmetric vertex model with GL(2|2)
symmetry, and postulating a two-point correlator in keeping with the rules of
conformal field theory, we derive an explicit expression for the distribution
of conductances at criticality. There is only one free parameter, the power law
exponent of the typical conductance. Its value is computed numerically to be
X_t = 0.640 +/- 0.009. The predicted conductance distribution agrees well with
the numerical data. For large distances between the two contacts, the
distribution can be described by a multifractal spectrum solely determined by
X_t. Our results demonstrate that multifractality can show up in appropriate
transport experiments.Comment: 18 pages, 15 figures included, revised versio
A clinically relevant gene signature in triple negative and basal-like breast cancer
Introduction: Current prognostic gene expression profiles for breast cancer mainly reflect proliferation status and are most useful in ER-positive cancers. Triple negative breast cancers (TNBC) are clinically heterogeneous and prognostic markers and biology-based therapies are needed to better treat this disease.
Methods: We assembled Affymetrix gene expression data for 579 TNBC and performed unsupervised analysis to define metagenes that distinguish molecular subsets within TNBC. We used n = 394 cases for discovery and n = 185 cases for validation. Sixteen metagenes emerged that identified basal-like, apocrine and claudin-low molecular subtypes, or reflected various non-neoplastic cell populations, including immune cells, blood, adipocytes, stroma, angiogenesis and inflammation within the cancer. The expressions of these metagenes were correlated with survival and multivariate analysis was performed, including routine clinical and pathological variables.
Results: Seventy-three percent of TNBC displayed basal-like molecular subtype that correlated with high histological grade and younger age. Survival of basal-like TNBC was not different from non basal-like TNBC. High expression of immune cell metagenes was associated with good and high expression of inflammation and angiogenesis-related metagenes were associated with poor prognosis. A ratio of high B-cell and low IL-8 metagenes identified 32% of TNBC with good prognosis (hazard ratio (HR) 0.37, 95% CI 0.22 to 0.61; P < 0.001) and was the only significant predictor in multivariate analysis including routine clinicopathological variables.
Conclusions: We describe a ratio of high B-cell presence and low IL-8 activity as a powerful new prognostic marker for TNBC. Inhibition of the IL-8 pathway also represents an attractive novel therapeutic target for this disease
- …