424 research outputs found
Stabilization of collapse and revival dynamics by a non-Markovian phonon bath
Semiconductor quantum dots (QDs) have been demonstrated to be versatile
candidates to study the fundamentals of light-matter interaction [1-3]. In
contrast with atom optics, dissipative processes are induced by the inherent
coupling to the environment and are typically perceived as a major obstacle
towards stable performances in experiments and applications [4].
In this paper we show that this is not necessarily the case. In fact, the
memory of the environment can enhance coherent quantum optical effects. In
particular, we demonstrate that the non-Markovian coupling to an incoherent
phonon bath has a stabilizing effect on the coherent QD cavity-quantum
electrodynamics (cQED) by inhibiting irregular oscillations and boosting
regular collapse and revival patterns. For low photon numbers we predict QD
dynamics that deviate dramatically from the well-known atomic Jaynes-Cummings
model. Our proposal opens the way to a systematic and deliberate design of
photon quantum effects via specifically engineered solid-state environments.Comment: 5 pages, 4 figure
The three-dimensional Anderson model of localization with binary random potential
We study the three-dimensional two-band Anderson model of localization and
compare our results to experimental results for amorphous metallic alloys
(AMA). Using the transfer-matrix method, we identify and characterize the
metal-insulator transitions as functions of Fermi level position, band
broadening due to disorder and concentration of alloy composition. The
appropriate phase diagrams of regions of extended and localized electronic
states are studied and qualitative agreement with AMA such as Ti-Ni and Ti-Cu
metallic glasses is found. We estimate the critical exponents nu_W, nu_E and
nu_x when either disorder W, energy E or concentration x is varied,
respectively. All our results are compatible with the universal value nu ~ 1.6
obtained in the single-band Anderson model.Comment: 9 RevTeX4 pages with 11 .eps figures included, submitted to PR
Metal-insulator transitions in anisotropic 2d systems
Several phenomena related to the critical behaviour of non-interacting
electrons in a disordered 2d tight-binding system with a magnetic field are
studied. Localization lengths, critical exponents and density of states are
computed using transfer matrix techniques. Scaling functions of isotropic
systems are recovered once the dimension of the system in each direction is
chosen proportional to the localization length. It is also found that the
critical point is independent of the propagation direction, and that the
critical exponents for the localization length for both propagating directions
are equal to that of the isotropic system (approximately 7/3). We also
calculate the critical value of the scaling function for both the isotropic and
the anisotropic system. It is found that the isotropic value equals the
geometric mean of the two anisotropic values. Detailed numerical studies of the
density of states for the isotropic system reveals that for an appreciable
amount of disorder the critical energy is off the band center.Comment: 6 pages RevTeX, 6 figures included, submitted to Physical Review
Partitioning Schemes and Non-Integer Box Sizes for the Box-Counting Algorithm in Multifractal Analysis
We compare different partitioning schemes for the box-counting algorithm in
the multifractal analysis by computing the singularity spectrum and the
distribution of the box probabilities. As model system we use the Anderson
model of localization in two and three dimensions. We show that a partitioning
scheme which includes unrestricted values of the box size and an average over
all box origins leads to smaller error bounds than the standard method using
only integer ratios of the linear system size and the box size which was found
by Rodriguez et al. (Eur. Phys. J. B 67, 77-82 (2009)) to yield the most
reliable results.Comment: 10 pages, 13 figure
Energy-level statistics at the metal-insulator transition in anisotropic systems
We study the three-dimensional Anderson model of localization with
anisotropic hopping, i.e. weakly coupled chains and weakly coupled planes. In
our extensive numerical study we identify and characterize the metal-insulator
transition using energy-level statistics. The values of the critical disorder
are consistent with results of previous studies, including the
transfer-matrix method and multifractal analysis of the wave functions.
decreases from its isotropic value with a power law as a function of
anisotropy. Using high accuracy data for large system sizes we estimate the
critical exponent . This is in agreement with its value in the
isotropic case and in other models of the orthogonal universality class. The
critical level statistics which is independent of the system size at the
transition changes from its isotropic form towards the Poisson statistics with
increasing anisotropy.Comment: 22 pages, including 8 figures, revtex few typos corrected, added
journal referenc
Multifractal analysis of the metal-insulator transition in anisotropic systems
We study the Anderson model of localization with anisotropic hopping in three
dimensions for weakly coupled chains and weakly coupled planes. The eigenstates
of the Hamiltonian, as computed by Lanczos diagonalization for systems of sizes
up to , show multifractal behavior at the metal-insulator transition even
for strong anisotropy. The critical disorder strength determined from the
system size dependence of the singularity spectra is in a reasonable agreement
with a recent study using transfer matrix methods. But the respective spectrum
at deviates from the ``characteristic spectrum'' determined for the
isotropic system. This indicates a quantitative difference of the multifractal
properties of states of the anisotropic as compared to the isotropic system.
Further, we calculate the Kubo conductivity for given anisotropies by exact
diagonalization. Already for small system sizes of only sites we observe
a rapidly decreasing conductivity in the directions with reduced hopping if the
coupling becomes weaker.Comment: 25 RevTeX pages with 10 PS-figures include
Effects of Scale-Free Disorder on the Anderson Metal-Insulator Transition
We investigate the three-dimensional Anderson model of localization via a
modified transfer-matrix method in the presence of scale-free diagonal disorder
characterized by a disorder correlation function decaying asymptotically
as . We study the dependence of the localization-length exponent
on the correlation-strength exponent . % For fixed disorder ,
there is a critical , such that for ,
and for , remains that of the
uncorrelated system in accordance with the extended Harris criterion. At the
band center, is independent of but equal to that of the
uncorrelated system. The physical mechanisms leading to this different behavior
are discussed.Comment: submitted to Phys. Rev. Let
Finite-size scaling from self-consistent theory of localization
Accepting validity of self-consistent theory of localization by Vollhardt and
Woelfle, we derive the finite-size scaling procedure used for studies of the
critical behavior in d-dimensional case and based on the use of auxiliary
quasi-1D systems. The obtained scaling functions for d=2 and d=3 are in good
agreement with numerical results: it signifies the absence of essential
contradictions with the Vollhardt and Woelfle theory on the level of raw data.
The results \nu=1.3-1.6, usually obtained at d=3 for the critical exponent of
the correlation length, are explained by the fact that dependence L+L_0 with
L_0>0 (L is the transversal size of the system) is interpreted as L^{1/\nu}
with \nu>1. For dimensions d\ge 4, the modified scaling relations are derived;
it demonstrates incorrectness of the conventional treatment of data for d=4 and
d=5, but establishes the constructive procedure for such a treatment.
Consequences for other variants of finite-size scaling are discussed.Comment: Latex, 23 pages, figures included; additional Fig.8 is added with
high precision data by Kramer et a
Expression and prognostic relevance of activated extracellular-regulated kinases (ERK1/2) in breast cancer
Extracellular-regulated kinases (ERK1, ERK2) play important roles in the malignant behaviour of breast cancer cells in vitro. In our present study, 148 clinical breast cancer samples (120 cases with follow-up data) were studied for the expression of ERK1, ERK2 and their phosphorylated forms p-ERK1 and p-ERK2 by immunoblotting, and p-ERK1/2 expression in corresponding paraffin sections was analysed by immunohistochemistry. The results were correlated with established clinical and histological prognostic parameters, follow-up data and expression of seven cell-cycle regulatory proteins as well as MMP1, MMP9, PAI-1 and AP-1 transcription factors, which had been analysed before. High p-ERK1 expression as determined by immunoblots correlated significantly with a low frequency of recurrences and infrequent fatal outcome (P=0.007 and 0.008) and was an independent indicator of long relapse-free and overall survival in multivariate analysis. By immunohistochemistry, strong p-ERK staining in tumour cells was associated with early stages (P=0.020), negative nodal status (P=0.003) and long recurrence-free survival (P=0.017). In contrast, expression of the unphosphorylated kinases ERK1 and ERK2 was not associated with clinical and histological prognostic parameters, except a positive correlation with oestrogen receptor status. Comparison with the expression of formerly analysed cell-cycle- and invasion-associated proteins corroborates our conclusion that activation of ERK1 and ERK2 is not associated with enhanced proliferation and invasion of mammary carcinomas
A Multiplatform Parallel Approach for Lattice Sieving Algorithms
Lattice sieving is currently the leading class of algorithms for solving the shortest vector problem over lattices. The computational difficulty of this problem is the basis for constructing secure post-quantum public-key cryptosystems based on lattices. In this paper, we present a novel massively parallel approach for solving the shortest vector problem using lattice sieving and hardware acceleration.
We combine previously reported algorithms with a proper caching strategy and develop hardware architecture. The main advantage of the proposed approach is eliminating the overhead of the data transfer between a CPU and a hardware accelerator.
The authors believe that this is the first such architecture reported in the literature to date and predict to achieve up to 8 times higher throughput when compared to a multi-core high-performance CPU.
Presented methods can be adapted for other sieving algorithms hard to implement in FPGAs due to the communication and memory bottlenec
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