5,176 research outputs found
Cluster-induced crater formation
Using molecular-dynamics simulation, we study the crater volumes induced by
energetic impacts ( km/s) of projectiles containing up to N=1000
atoms. We find that for Lennard-Jones bonded material the crater volume depends
solely on the total impact energy . Above a threshold \Eth, the volume
rises linearly with . Similar results are obtained for metallic materials.
By scaling the impact energy to the target cohesive energy , the crater
volumes become independent of the target material. To a first approximation,
the crater volume increases in proportion with the available scaled energy,
. The proportionality factor is termed the cratering efficiency and
assumes values of around 0.5.Comment: 9 page
Wilson chains are not thermal reservoirs
Wilson chains, based on a logarithmic discretization of a continuous
spectrum, are widely used to model an electronic (or bosonic) bath for Kondo
spins and other quantum impurities within the numerical renormalization group
method and other numerical approaches. In this short note we point out that
Wilson chains can not serve as thermal reservoirs as their temperature changes
by a number of order Delta E when a finite amount of energy Delta E is added.
This proves that for a large class of non-equilibrium problems they cannot be
used to predict the long-time behavior.Comment: 2 page
Charge gaps and quasiparticle bands of the ionic Hubbard model
The ionic Hubbard model on a cubic lattice is investigated using analytical
approximations and Wilson's renormalization group for the charge excitation
spectrum. Near the Mott insulating regime, where the Hubbard repulsion starts
to dominate all energies, the formation of correlated bands is described. The
corresponding partial spectral weights and local densities of states show
characteristic features, which compare well with a hybridized-band picture
appropriate for the regime at small , which at half-filling is known as a
band insulator. In particular, a narrow charge gap is obtained at half-filling,
and the distribution of spectral quasi-particle weight reflects the fundamental
hybridization mechanism of the model
Gravity vs radiation model: on the importance of scale and heterogeneity in commuting flows
We test the recently introduced radiation model against the gravity model for
the system composed of England and Wales, both for commuting patterns and for
public transportation flows. The analysis is performed both at macroscopic
scales, i.e. at the national scale, and at microscopic scales, i.e. at the city
level. It is shown that the thermodynamic limit assumption for the original
radiation model significantly underestimates the commuting flows for large
cities. We then generalize the radiation model, introducing the correct
normalisation factor for finite systems. We show that even if the gravity model
has a better overall performance the parameter-free radiation model gives
competitive results, especially for large scales.Comment: in press Phys. Rev. E, 201
E-ARK: Harmonising pan-European archival processes to ensure continuous access to e-government records and information
There has been a widespread shift to electronic ways of conducting business that has transformed existing relationships between governments, governments and citizens, and governments and business. This move to electronic interactions is supported by new busi- ness systems that streamline and automate transactions, enable integration of information and service delivery and enhance collaboration between participants. Such changes in the way government business is carried out have significant implications for how public ad- ministrations document their activities and make that information available to both gov- ernment and citizens to aid future decision making and accountability. Because digital rec- ords are particularly vulnerable to technological obsolescence and media decay, ensuring future access to the information created by government is a challenging issue for all juris- dictions. This paper focus on the E-ARK project, a European endeavour to standardise and create tools for consistently transferring digital records between business systems and digi- tal archives. The E-ARK approach has the potential to simplify and make consistent diverse approaches to solving the issue of how to transfer information between the ICT systems in use in government, and the archives charged with the responsibility for ongoing and man- agement of the information considered to be of long-term significance.This work was co-funded by KEEP SOLUTIONS, LDA
Evolution with hole doping of the electronic excitation spectrum in the cuprate superconductors
The recent scanning tunnelling results of Alldredge et al on Bi-2212 and of
Hanaguri et al on Na-CCOC are examined from the perspective of the BCS/BEC
boson-fermion resonant crossover model for the mixed-valent HTSC cuprates. The
model specifies the two energy scales controlling the development of HTSC
behaviour and the dichotomy often now alluded to between nodal and antinodal
phenomena in the HTSC cuprates. Indication is extracted from the data as to how
the choice of the particular HTSC system sees these two basic energy scales
(cursive-U, the local pair binding energy and, Delta-sc, the nodal BCS-like gap
parameter) evolve with doping and change in degree of metallization of the
structurally and electronically perturbed mixed-valent environment.Comment: 19 pages, 5 figure
Kondo effect in a carbon nanotube with spin-orbit interaction and valley mixing: A DM-NRG study
We investigate the effects of spin-orbit interaction (SOI) and valley mixing
on the transport and dynamical properties of a carbon nanotube (CNT) quantum
dot in the Kondo regime. As these perturbations break the pseudo-spin symmetry
in the CNT spectrum but preserve time-reversal symmetry, they induce a finite
splitting between formerly degenerate Kramers pairs. Correspondingly,
a crossover from the SU(4) to the SU(2)-Kondo effect occurs as the strength of
these symmetry breaking parameters is varied. Clear signatures of the crossover
are discussed both at the level of the spectral function as well as of the
conductance. In particular, we demonstrate numerically and support with scaling
arguments, that the Kondo temperature scales inversely with the splitting
in the crossover regime. In presence of a finite magnetic field, time
reversal symmetry is also broken. We investigate the effects of both parallel
and perpendicular fields (with respect to the tube's axis), and discuss the
conditions under which Kondo revivals may be achieved.Comment: 13 pages, 17 figure
Star cluster formation and star formation: the role of environment and star-formation efficiencies
“The original publication is available at www.springerlink.com”. Copyright Springer. DOI: 10.1007/s10509-009-0088-5By analyzing global starburst properties in various kinds of starburst and post-starburst galaxies and relating them to the properties of the star cluster populations they form, I explore the conditions for the formation of massive, compact, long-lived star clusters. The aim is to determine whether the relative amount of star formation that goes into star cluster formation as opposed to field star formation, and into the formation of massive long-lived clusters in particular, is universal or scales with star-formation rate, burst strength, star-formation efficiency, galaxy or gas mass, and whether or not there are special conditions or some threshold for the formation of star clusters that merit to be called globular clusters a few billion years later.Peer reviewe
The interstellar C18O/C17O ratio in the solar neighbourhood: The rho Oph cloud
Observations of up to ten carbon monoxide (CO and isotopomers) transitions
are presented to study the interstellar C18O/C17O ratio towards 21 positions in
the nearby (d~140pc) low-mass star forming cloud rho Oph. A map of the C18O
J=1-0 distribution of parts of the cloud is also shown. An average
12C18O/12C17O isotopomeric ratio of 4.11 +/- 0.14, reflecting the 18O/17O
isotope ratio, is derived from Large Velocity Gradient (LVG) calculations. From
LTE column densities we derive a ratio of 4.17 +/-0.26. These calculations also
show that the kinetic temperature decreases from about 30 K in the cloud
envelope to about 10 K in the cloud cores. This decrease is accompanied by an
increase of the average molecular hydrogen density from 10^4 cm-3 to >10^5
cm-3. Towards some lines of sight C18O optical depths reach values of order
unity.Comment: 13 pages, 9 figures; accepted for publication in A&
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