628 research outputs found
Delocalization and conductance quantization in one-dimensional systems
We investigate the delocalization and conductance quantization in finite
one-dimensional chains with only off-diagonal disorder coupled to leads. It is
shown that the appearence of delocalized states at the middle of the band under
correlated disorder is strongly dependent upon the even-odd parity of the
number of sites in the system. In samples with inversion symmetry the
conductance equals for odd samples, and is smaller for even parity.
This result suggests that this even-odd behaviour found previously in the
presence of electron correlations may be unrelated to charging effects in the
sample.Comment: submitted to PR
Electronic structure of intentionally disordered AlAs/GaAs superlattices
We use realistic pseudopotentials and a plane-wave basis to study the
electronic structure of non-periodic, three-dimensional, 2000-atom
(AlAs)_n/(GaAs)_m (001) superlattices, where the individual layer thicknesses
n,m = {1,2,3} are randomly selected. We find that while the band gap of the
equivalent (n = m = 2) ordered superlattice is indirect, random fluctuations in
layer thicknesses lead to a direct gap in the planar Brillouin zone, strong
wavefunction localization along the growth direction, short radiative
lifetimes, and a significant band-gap reduction, in agreement with experiments
on such intentionally grown disordered superlattices.Comment: 10 pages, REVTeX and EPSF macros, 4 figures in postscript. e-mail to
[email protected]
High-Brightness Beams from a Light Source Injector: The Advanced Photon Source Low-Energy Undulator Test Line Linac
The use of existing linacs, and in particular light source injectors, for free-electron laser (FEL) experiments is becoming more common due to the desire to test FELs at ever shorter wavelengths. The high-brightness, high-current beams required by high-gain FELs impose technical specifications that most existing linacs were not designed to meet. Moreover, the need for specialized diagnostics, especially shot-to-shot data acquisition, demands substantial modification and upgrade of conventional linacs. Improvements have been made to the Advanced Photon Source (APS) injector linac in order to produce and characterize high-brightness beams. Specifically, effort has been directed at generating beams suitable for use in the low-energy undulator test line (LEUTL) FEL in support of fourth-generation light source research. The enhancements to the linac technical and diagnostic capabilities that allowed for self-amplified spontaneous emission (SASE) operation of the FEL at 530 nm are described. Recent results, including details on technical systems improvements and electron beam measurement techniques, will be discussed. The linac is capable of accelerating beams to over 650 MeV. The nominal FEL beam parameters used are as follows: 217 MeV energy; 0.1-0.2% rms energy spread; 4-8 um normalized rms emittance; 80-120 A peak current from a 0.2-0.7 nC charge at a 2-7 ps FWHM bunch
Optical excitations of Peierls-Mott insulators with bond disorder
The density-matrix renormalization group (DMRG) is employed to calculate
optical properties of the half-filled Hubbard model with nearest-neighbor
interactions. In order to model the optical excitations of oligoenes, a Peierls
dimerization is included whose strength for the single bonds may fluctuate.
Systems with up to 100 electrons are investigated, their wave functions are
analyzed, and relevant length-scales for the low-lying optical excitations are
identified. The presented approach provides a concise picture for the size
dependence of the optical absorption in oligoenes.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
Some Open Points in Nonextensive Statistical Mechanics
We present and discuss a list of some interesting points that are currently
open in nonextensive statistical mechanics. Their analytical, numerical,
experimental or observational advancement would naturally be very welcome.Comment: 30 pages including 6 figures. Invited paper to appear in the
International Journal of Bifurcation and Chao
Equilibrium Distribution of Heavy Quarks in Fokker-Planck Dynamics
We obtain within Fokker-Planck dynamics an explicit generalization of
Einstein's relation between drag, diffusion and equilibrium distribution for a
spatially homogeneous system, considering both the transverse and longitudinal
diffusion for dimension n>1. We then provide a complete characterization of
when the equilibrium distribution becomes a Boltzmann/J"uttner distribution,
and when it satisfies the more general Tsallis distribution. We apply this
analysis to recent calculations of drag and diffusion of a charm quark in a
thermal plasma, and show that only a Tsallis distribution describes the
equilibrium distribution well. We also provide a practical recipe applicable to
highly relativistic plasmas, for determining both diffusion coefficients so
that a specific equilibrium distribution will arise for a given drag
coefficient.Comment: 4 pages including 2 figure
On a generalised model for time-dependent variance with long-term memory
The ARCH process (R. F. Engle, 1982) constitutes a paradigmatic generator of
stochastic time series with time-dependent variance like it appears on a wide
broad of systems besides economics in which ARCH was born. Although the ARCH
process captures the so-called "volatility clustering" and the asymptotic
power-law probability density distribution of the random variable, it is not
capable to reproduce further statistical properties of many of these time
series such as: the strong persistence of the instantaneous variance
characterised by large values of the Hurst exponent (H > 0.8), and asymptotic
power-law decay of the absolute values self-correlation function. By means of
considering an effective return obtained from a correlation of past returns
that has a q-exponential form we are able to fix the limitations of the
original model. Moreover, this improvement can be obtained through the correct
choice of a sole additional parameter, . The assessment of its validity
and usefulness is made by mimicking daily fluctuations of SP500 financial
index.Comment: 6 pages, 4 figure
Quantum marginal problem and N-representability
A variant of the quantum marginal problem was known from early sixties as
N-representability problem. In 1995 it was designated by National Research
Council of USA as one of ten most prominent research challenges in quantum
chemistry. In spite of this recognition the progress was very slow, until a
couple of years ago the problem came into focus again, now in framework of
quantum information theory. In the paper I give an account of the recent
development.Comment: A talk at 12 Central European workshop on Quantum Optics, July 2005,
Bilkent University, Turke
Three-dimensional effects on extended states in disordered models of polymers
We study electronic transport properties of disordered polymers in the
presence of both uncorrelated and short-range correlated impurities. In our
procedure, the actual physical potential acting upon the electrons is replaced
by a set of nonlocal separable potentials, leading to a Schr\"odinger equation
that is exactly solvable in the momentum representation. We then show that the
reflection coefficient of a pair of impurities placed at neighboring sites
(dimer defect) vanishes for a particular resonant energy. When there is a
finite number of such defects randomly distributed over the whole lattice, we
find that the transmission coefficient is almost unity for states close to the
resonant energy, and that those states present a very large localization
length. Multifractal analysis techniques applied to very long systems
demonstrate that these states are truly extended in the thermodynamic limit.
These results reinforce the possibility to verify experimentally theoretical
predictions about absence of localization in quasi-one-dimensional disordered
systems.Comment: 16 pages, REVTeX 3.0, 5 figures on request from FDA
([email protected]). Submitted to Phys. Rev. B. MA/UC3M/09/9
Seeing versus Doing: How Businesses Manage Tensions in Pursuit of Sustainability
Management of organizational tensions can facilitate the simultaneous advancement of economic, social, and environmental priorities. The approach is based on managers identifying and managing tensions between the three priorities, by employing one of the three strategic responses. Although recent work has provided a theoretical basis for such tension acknowledgment and management, there is a dearth of empirical studies. We interviewed 32 corporate sustainability managers across 25 forestry and wood-products organizations in Australia. Study participants were divided into two groups: (1) those considered effective at corporate sustainability and (2) a status-quo group. Contrary to current theory, our findings showed that acknowledgment of organizational tensions was widespread in the Australian forestry and wood-products industry and not limited to those managers who are effective at managing corporate sustainability. What differed was the degree to which managers did something about the perceived tensions—with the effective group more consistently acting to manage and resolve paradoxical scenarios. Our findings suggest that existing theoretical constructs of tension management may not adequately capture the individual-level complexity involved with managing tensions in practice
- …