2,496 research outputs found
Representations of the discrete inhomogeneous Lorentz group and Dirac wave equation on the lattice
We propose the fundamental and two dimensional representation of the Lorentz
groups on a (3+1)-dimensional hypercubic lattice, from which representations of
higher dimensions can be constructed. For the unitary representation of the
discrete translation group we use the kernel of the Fourier transform. From the
Dirac representation of the Lorentz group (including reflections) we derive in
a natural way the wave equation on the lattice for spin 1/2 particles. Finally
the induced representation of the discrete inhomogeneous Lorentz group is
constructed by standard methods and its connection with the continuous case is
discussed.Comment: LaTeX, 20 pages, 1 eps figure, uses iopconf.sty (late submission
A cotunneling mechanism for all-electrical Electron Spin Resonance of single adsorbed atoms
The recent development of all-electrical electron spin resonance (ESR) in a
scanning tunneling microscope (STM) setup has opened the door to vast
applications. Despite the fast growing number of experimental works on STM-ESR,
the fundamental principles remains unclear. By using a cotunneling picture, we
show that the spin resonance signal can be explained as a time-dependent
variation of the tunnel barrier induced by the alternating electric driving
field. We demonstrate how this variation translates into the resonant frequency
response of the direct current. Our cotunneling theory explains the main
experimental findings. Namely, the linear dependence of the Rabi flop rate with
the alternating bias amplitude, the absence of resonant response for
spin-unpolarized currents, and the weak dependence on the actual atomic
species.Comment: 11 pages, 3 figure
Many-body effects in magnetic inelastic electron tunneling spectroscopy
Magnetic inelastic electron tunneling spectroscopy (IETS) shows sharp
increases in conductance when a new conductance channel associated to a change
in magnetic structure is open. Typically, the magnetic moment carried by an
adsorbate can be changed by collision with a tunneling electron; in this
process the spin of the electron can flip or not. A previous one-electron
theory [Phys. Rev. Lett. {\bf 103}, 176601 (2009)] successfully explained both
the conductance thresholds and the magnitude of the conductance variation. The
elastic spin flip of conduction electrons by a magnetic impurity leads to the
well known Kondo effect. In the present work, we compare the theoretical
predictions for inelastic magnetic tunneling obtained with a one-electron
approach and with a many-body theory including Kondo-like phenomena. We apply
our theories to a singlet-triplet transition model system that contains most of
the characteristics revealed in magnetic IETS. We use two self-consistent
treatments (non-crossing approximation and self-consistent ladder
approximation). We show that, although the one-electron limit is properly
recovered, new intrinsic many-body features appear. In particular, sharp peaks
appear close to the inelastic thresholds; these are not localized exactly at
thresholds and could influence the determination of magnetic structures from
IETS experiments.Analysis of the evolution with temperature reveals that these
many-body features involve an energy scale different from that of the usual
Kondo peaks. Indeed, the many-body features perdure at temperatures much larger
than the one given by the Kondo energy scale of the system.Comment: 10 pages and 6 figure
Specsim: The MIRI Medium Resolution Spectrometer Simulator
MIRI, the Mid-InfraRed Instrument, is one of four instruments being built for
the James Webb Space Telescope, and is developed jointly between an
EuropeanConsortium and the US. In this paper we present a software data
simulator for one of MIRI's four instruments: the Integral Field Unit (IFU)
Medium Resolution Spectrometer (MIRI-MRS), the first mid-infrared IFU
spectrograph, and one of the first IFUs to be used in a space mission. To give
the MIRI community a preview of the properties of the MIRI-MRS data products
before the telescope is operational, the Specsim tool has been developed to
model, in software, the operation of the spectrometer. Specsim generates
synthetic data frames approximating those which will be taken by the instrument
in orbit. The program models astronomical sources and generates detector frames
using the predicted and measured optical properties of the telescope and MIRI.
These frames can then be used to illustrate and inform a range of operational
activities, including data calibration strategies and the development and
testing of the data reduction software for the MIRI-MRS. Specsim will serve as
a means of communication between the many consortium members by providing a way
to easily illustrate the performance of the spectrometer under different
circumstances, tolerances of components and design scenarios.Comment: 8 pages, 5 figures; A high resolution version is available at
http://www.roe.ac.uk/~npfl/Publications/lgw+06.ps.gz (Changed URL of high-res
version
Surface embedding, topology and dualization for spin networks
Spin networks are graphs derived from 3nj symbols of angular momentum. The
surface embedding, the topology and dualization of these networks are
considered. Embeddings into compact surfaces include the orientable sphere S^2
and the torus T, and the not orientable projective space P^2 and Klein's bottle
K. Two families of 3nj graphs admit embeddings of minimal genus into S^2 and
P^2. Their dual 2-skeletons are shown to be triangulations of these surfaces.Comment: LaTeX 17 pages, 6 eps figures (late submission to arxiv.org
Analysing inter-relationships among water, governance, human development variables in developing countries
The "Integrated Water Resources Management" principle was formally laid down at the International Conference on Water and Sustainable development in Dublin 1992. One of the main results of this conference is that improving Water and Sanitation Services (WSS), being a complex and interdisciplinary issue, passes through collaboration and coordination of different sectors (environment, health, economic activities, governance, and international cooperation). These sectors influence or are influenced by the access to WSS. The understanding of these interrelations appears as crucial for decision makers in the water sector. In this framework, the Joint Research Centre (JRC) of the European Commission (EC) has developed a new database (WatSan4Dev database) containing 42 indicators (called variables in this paper) from environmental, socio-economic, governance and financial aid flows data in developing countries. This paper describes the development of the WatSan4Dev dataset, the statistical processes needed to improve the data quality, and finally, the analysis to verify the database coherence is presented. Based on 25 relevant variables, the relationships between variables are described and organised into five factors (HDP – Human Development against Poverty, AP – Human Activity Pressure on water resources, WR – Water Resources, ODA – Official Development Aid, CEC – Country Environmental Concern). Linear regression methods are used to identify key variables having influence on water supply and sanitation. First analysis indicates that the informal urbanisation development is an important factor negatively influencing the percentage of the population having access to WSS. Health, and in particular children's health, benefits from the improvement of WSS. Irrigation is also enhancing Water Supply service thanks to multi-purpose infrastructure. Five country profiles are also created to deeper understand and synthetize the amount of information gathered. This new classification of countries is useful in identifying countries with a less advanced position and weaknesses to be tackled. The relevance of indicators gathered to represent environmental and water resources state is questioned in the discussion section. The paper concludes with the necessity to increase the reliability of current indicators and calls for further research on specific indicators, in particular on water quality at national scale, in order to better include environmental state in analysis to WSS
Valoración de la importancia de los datos obtenidos durante la exploración de un paciente con tumores mamarios
Finite-Dimensional Calculus
We discuss topics related to finite-dimensional calculus in the context of
finite-dimensional quantum mechanics. The truncated Heisenberg-Weyl algebra is
called a TAA algebra after Tekin, Aydin, and Arik who formulated it in terms of
orthofermions. It is shown how to use a matrix approach to implement analytic
representations of the Heisenberg-Weyl algebra in univariate and multivariate
settings. We provide examples for the univariate case. Krawtchouk polynomials
are presented in detail, including a review of Krawtchouk polynomials that
illustrates some curious properties of the Heisenberg-Weyl algebra, as well as
presenting an approach to computing Krawtchouk expansions. From a mathematical
perspective, we are providing indications as to how to implement in finite
terms Rota's "finite operator calculus".Comment: 26 pages. Added material on Krawtchouk polynomials. Additional
references include
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