420 research outputs found
A discrete time-dependent method for metastable atoms in intense fields
The full-dimensional time-dependent Schrodinger equation for the electronic
dynamics of single-electron systems in intense external fields is solved
directly using a discrete method.
Our approach combines the finite-difference and Lagrange mesh methods. The
method is applied to calculate the quasienergies and ionization probabilities
of atomic and molecular systems in intense static and dynamic electric fields.
The gauge invariance and accuracy of the method is established. Applications to
multiphoton ionization of positronium and hydrogen atoms and molecules are
presented. At very high intensity above saturation threshold, we extend the
method using a scaling technique to estimate the quasienergies of metastable
states of the hydrogen molecular ion. The results are in good agreement with
recent experiments.Comment: 10 pages, 9 figure, 4 table
Stability and Complexity of Minimising Probabilistic Automata
We consider the state-minimisation problem for weighted and probabilistic
automata. We provide a numerically stable polynomial-time minimisation
algorithm for weighted automata, with guaranteed bounds on the numerical error
when run with floating-point arithmetic. Our algorithm can also be used for
"lossy" minimisation with bounded error. We show an application in image
compression. In the second part of the paper we study the complexity of the
minimisation problem for probabilistic automata. We prove that the problem is
NP-hard and in PSPACE, improving a recent EXPTIME-result.Comment: This is the full version of an ICALP'14 pape
Evaluation of body composition as a potential biomarker in spinal muscular atrophy
INTRODUCTION:
We aimed to investigate the correlation between body composition (BC) and spinal muscular atrophy (SMA)-specific motor function assessments.
METHODS:
Patients with SMA types I or II, aged 1 to 10\u2009years, were recruited in this cross-sectional study. The protocol included anthropometric measurements, and dual-energy X-ray absoprtiometry to assess fat mass (FM), lean mass (LM), fat-free mass (FFM), FM and FFM indexes (FMI, FFMI), and motor function assessments (Children's Hospital of Philadelphia Infant Test of Neuromuscular Disorders scale for SMAI, and Hammersmith Functional Motor Scale-Expanded for SMAII).
RESULTS:
Eighty-eight children were included. All had a higher FM percentage than reference values. Motor function was moderately correlated with body mass index (BMI), FFMI, and LMI in SMAI, and weakly correlated with FFMI, LMI, and LM:FM ratio in SMAII.
DISCUSSION:
BC shows promise as a potential biomarker for SMA, but further studies are needed
Markov chain analysis of random walks on disordered medium
We study the dynamical exponents and for a particle diffusing
in a disordered medium (modeled by a percolation cluster), from the regime of
extreme disorder (i.e., when the percolation cluster is a fractal at )
to the Lorentz gas regime when the cluster has weak disorder at and
the leading behavior is standard diffusion. A new technique of relating the
velocity autocorrelation function and the return to the starting point
probability to the asymptotic spectral properties of the hopping transition
probability matrix of the diffusing particle is used, and the latter is
numerically analyzed using the Arnoldi-Saad algorithm. We also present evidence
for a new scaling relation for the second largest eigenvalue in terms of the
size of the cluster, , which provides a
very efficient and accurate method of extracting the spectral dimension
where .Comment: 34 pages, REVTEX 3.
Time delay between photoemission from the 2p and 2s subshells of Neon
The R-Matrix incorporating Time (RMT) method is a new method for solving the
time-dependent Schroedinger equation for multi-electron atomic systems exposed
to intense short-pulse laser light. We have employed the RMT method to
investigate the time delay in the photoemission of an electron liberated from a
2p orbital in a neon atom with respect to one released from a 2s orbital
following absorption of an attosecond XUV pulse. Time delays due to XUV pulses
in the range 76-105 eV are presented. For an XUV pulse at the experimentally
relevant 105.2 eV, we calculate the time delay to be 10.2 +/- 1.3 attoseconds,
somewhat larger than estimated by other theoretical calculations, but still a
factor two smaller than experiment. We repeated the calculation for a photon
energy of 89.8 eV with a larger basis set capable of modelling
correlated-electron dynamics within the neon atom and the residual Ne(+) ion. A
time delay of 14.5 +/- 1.5 attoseconds was observed, compared to a 16.7 +/- 1.5
attosecond result using a single-configuration representation of the residual
Ne(+) ion.Comment: 4 pages, 3 figures, 1 tabl
Stability and Decay Rates of Non-Isotropic Attractive Bose-Einstein Condensates
Non-Isotropic Attractive Bose-Einstein condensates are investigated with
Newton and inverse Arnoldi methods. The stationary solutions of the
Gross-Pitaevskii equation and their linear stability are computed. Bifurcation
diagrams are calculated and used to find the condensate decay rates
corresponding to macroscopic quantum tunneling, two-three body inelastic
collisions and thermally induced collapse.
Isotropic and non-isotropic condensates are compared. The effect of
anisotropy on the bifurcation diagram and the decay rates is discussed.
Spontaneous isotropization of the condensates is found to occur. The influence
of isotropization on the decay rates is characterized near the critical point.Comment: revtex4, 11 figures, 2 tables. Submitted to Phys. Rev.
Spectrum of the non-abelian phase in Kitaev's honeycomb lattice model
The spectral properties of Kitaev's honeycomb lattice model are investigated
both analytically and numerically with the focus on the non-abelian phase of
the model. After summarizing the fermionization technique which maps spins into
free Majorana fermions, we evaluate the spectrum of sparse vortex
configurations and derive the interaction between two vortices as a function of
their separation. We consider the effect vortices can have on the fermionic
spectrum as well as on the phase transition between the abelian and non-abelian
phases. We explicitly demonstrate the -fold ground state degeneracy in the
presence of well separated vortices and the lifting of the degeneracy due
to their short-range interactions. The calculations are performed on an
infinite lattice. In addition to the analytic treatment, a numerical study of
finite size systems is performed which is in exact agreement with the
theoretical considerations. The general spectral properties of the non-abelian
phase are considered for various finite toroidal systems.Comment: 32 pages, 13 figures; corrected typos and changed SU(2)_2 to Isin
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