5,351 research outputs found
Surface correlations for two-dimensional Coulomb fluids in a disc
After a brief review of previous work, two exactly solvable two-dimensional
models of a finite Coulomb fluid in a disc are studied. The charge correlation
function near the boundary circle is computed. When the disc radius is large
compared to the bulk correlation length, a correlation function of the surface
charge density can be defined. It is checked, on the solvable models, that this
correlation function does have the generic long-range behaviour, decaying as
the inverse square distance, predicted by macroscopic electrostatics. In the
case of a two-component plasma (Coulomb fluid made of two species of particles
of opposite charges), the density correlation function on the boundary circle
itself is conjectured to have a temperature-independent behaviour, decaying as
the -4 power of the distance.Comment: 15 pages, Latex, submitted to J.Phys.:Condens.Matte
The Casimir force at high temperature
The standard expression of the high-temperature Casimir force between perfect
conductors is obtained by imposing macroscopic boundary conditions on the
electromagnetic field at metallic interfaces. This force is twice larger than
that computed in microscopic classical models allowing for charge fluctuations
inside the conductors. We present a direct computation of the force between two
quantum plasma slabs in the framework of non relativistic quantum
electrodynamics including quantum and thermal fluctuations of both matter and
field. In the semi-classical regime, the asymptotic force at large slab
separation is identical to that found in the above purely classical models,
which is therefore the right result. We conclude that when calculating the
Casimir force at non-zero temperature, fluctuations inside the conductors can
not be ignored.Comment: 7 pages, 0 figure
Scaling and Universality in City Space Syntax: between Zipf and Matthew
We report about universality of rank-integration distributions of open spaces
in city space syntax similar to the famous rank-size distributions of cities
(Zipf's law). We also demonstrate that the degree of choice an open space
represents for other spaces directly linked to it in a city follows a power law
statistic. Universal statistical behavior of space syntax measures uncovers the
universality of the city creation mechanism. We suggest that the observed
universality may help to establish the international definition of a city as a
specific land use pattern.Comment: 24 pages, 5 *.eps figure
The epsilon expansion at next-to-next-to-leading order with small imaginary chemical potential
We discuss chiral perturbation theory for two and three quark flavors in the
epsilon expansion at next-to-next-to-leading order (NNLO) including a small
imaginary chemical potential. We calculate finite-volume corrections to the
low-energy constants and and determine the non-universal
modifications of the theory, i.e., modifications that cannot be mapped to
random matrix theory (RMT). In the special case of two quark flavors in an
asymmetric box we discuss how to minimize the finite-volume corrections and
non-universal modifications by an optimal choice of the lattice geometry.
Furthermore we provide a detailed calculation of a special version of the
massless sunset diagram at finite volume.Comment: 21 pages, 5 figure
The UK market for energy service contracts in 2014â2015
This paper provides an overview of the UK market for energy service contracts in 2014 and highlights the growing role of intermediaries. Using information from secondary literature and interviews, it identifies the businesses offering energy service contracts, the sectors and organisations that are purchasing those contracts, the types of contract that are available, the areas of market growth and the reasons for that growth. The paper finds that the UK market is relatively large, highly diverse, concentrated in particular sectors and types of site and overwhelmingly focused upon established technologies with high rates of return. A major driver is the emergence of procurement frameworks for energy service contracts in the public sector. These act as intermediaries between clients and contractors, thereby lowering transaction costs and facilitating learning. The market is struggling to become established in commercial offices, largely as a result of split incentives, and is unlikely to develop further in this sector without different business models, tenancy arrangements and policy initiatives. Overall, the paper concludes that energy service contracts can play an important role in the transition to a low-carbon economy, especially when supported by intermediaries, but their potential is still limited by high transaction costs
The Effects of Binary Evolution on the Dynamics of Core Collapse and Neutron-Star Kicks
We systematically examine how the presence in a binary affects the final core
structure of a massive star and its consequences for the subsequent supernova
explosion. Interactions with a companion star may change the final rate of
rotation, the size of the helium core, the strength of carbon burning and the
final iron core mass. Stars with initial masses larger than \sim 11\Ms that
experiece core collapse will generally have smaller iron cores at the time of
the explosion if they lost their envelopes due to a previous binary
interaction. Stars below \sim 11\Ms, on the other hand, can end up with larger
helium and metal cores if they have a close companion, since the second
dredge-up phase which reduces the helium core mass dramatically in single stars
does not occur once the hydrogen envelope is lost. We find that the initially
more massive stars in binary systems with masses in the range 8 - 11\Ms are
likely to undergo an electron-capture supernova, while single stars in the same
mass range would end as ONeMg white dwarfs. We suggest that the core collapse
in an electron-capture supernova (and possibly in the case of relatively small
iron cores) leads to a prompt explosion rather than a delayed neutrino-driven
explosion and that this naturally produces neutron stars with low-velocity
kicks. This leads to a dichotomous distribution of neutron star kicks, as
inferred previously, where neutron stars in relatively close binaries attain
low kick velocities. We illustrate the consequences of such a dichotomous kick
scenario using binary population synthesis simulations and discuss its
implications. This scenario has also important consequences for the minimum
initial mass of a massive star that becomes a neutron star. (Abbreviated.)Comment: 8 pages, 3 figures, submitted to ApJ, updated versio
How a Diverse Research Ecosystem Has Generated New Rehabilitation Technologies: Review of NIDILRRâs Rehabilitation Engineering Research Centers
Over 50 million United States citizens (1 in 6 people in the US) have a developmental, acquired, or degenerative disability. The average US citizen can expect to live 20% of his or her life with a disability. Rehabilitation technologies play a major role in improving the quality of life for people with a disability, yet widespread and highly challenging needs remain. Within the US, a major effort aimed at the creation and evaluation of rehabilitation technology has been the Rehabilitation Engineering Research Centers (RERCs) sponsored by the National Institute on Disability, Independent Living, and Rehabilitation Research. As envisioned at their conception by a panel of the National Academy of Science in 1970, these centers were intended to take a âtotal approach to rehabilitationâ, combining medicine, engineering, and related science, to improve the quality of life of individuals with a disability. Here, we review the scope, achievements, and ongoing projects of an unbiased sample of 19 currently active or recently terminated RERCs. Specifically, for each center, we briefly explain the needs it targets, summarize key historical advances, identify emerging innovations, and consider future directions. Our assessment from this review is that the RERC program indeed involves a multidisciplinary approach, with 36 professional fields involved, although 70% of research and development staff are in engineering fields, 23% in clinical fields, and only 7% in basic science fields; significantly, 11% of the professional staff have a disability related to their research. We observe that the RERC program has substantially diversified the scope of its work since the 1970âs, addressing more types of disabilities using more technologies, and, in particular, often now focusing on information technologies. RERC work also now often views users as integrated into an interdependent society through technologies that both people with and without disabilities co-use (such as the internet, wireless communication, and architecture). In addition, RERC research has evolved to view users as able at improving outcomes through learning, exercise, and plasticity (rather than being static), which can be optimally timed. We provide examples of rehabilitation technology innovation produced by the RERCs that illustrate this increasingly diversifying scope and evolving perspective. We conclude by discussing growth opportunities and possible future directions of the RERC program
Individual Eigenvalue Distributions for the Wilson Dirac Operator
We derive the distributions of individual eigenvalues for the Hermitian
Wilson Dirac Operator D5 as well as for real eigenvalues of the Wilson Dirac
Operator DW. The framework we provide is valid in the epsilon regime of chiral
perturbation theory for any number of flavours Nf and for non-zero low energy
constants W6, W7, W8. It is given as a perturbative expansion in terms of the
k-point spectral density correlation functions and integrals thereof, which in
some cases reduces to a Fredholm Pfaffian. For the real eigenvalues of DW at
fixed chirality nu this expansion truncates after at most nu terms for small
lattice spacing "a". Explicit examples for the distribution of the first and
second eigenvalue are given in the microscopic domain as a truncated expansion
of the Fredholm Pfaffian for quenched D5, where all k-point densities are
explicitly known from random matrix theory. For the real eigenvalues of
quenched DW at small "a" we illustrate our method by the finite expansion of
the corresponding Fredholm determinant of size nu.Comment: 20 pages, 5 figures; v2: typos corrected, refs added and discussion
of W6 and W7 extende
On type Ia supernovae and the formation of single low-mass white dwarfs
There is still considerable debate over the progenitors of type Ia supernovae
(SNe Ia). Likewise, it is not agreed how single white dwarfs with masses less
than ~0.5 Msun can be formed in the field, even though they are known to exist.
We consider whether single low-mass white dwarfs (LMWDs) could have been formed
in binary systems where their companions have exploded as a SN Ia. In this
model, the observed single LMWDs are the remnants of giant-branch donor stars
whose envelopes have been stripped off by the supernova explosion. We
investigate the likely remnants of SNe Ia, including the effects of the
explosion on the envelope of the donor star. We also use evolutionary arguments
to examine alternative formation channels for single LMWDs. In addition, we
calculate the expected kinematics of the potential remnants of SNe Ia. SN Ia in
systems with giant-branch donor stars can naturally explain the production of
single LMWDs. It seems difficult for any other formation mechanism to account
for the observations, especially for those single LMWDs with masses less than
~0.4 Msun. Independent of those results, we find that the kinematics of one
potentially useful population containing single LMWDs is consistent with our
model. Studying remnant white-dwarf kinematics seems to be a promising way to
investigate SN Ia progenitors. The existence of single LMWDs appears to
constitute evidence for the production of SNe Ia in binary systems with a
red-giant donor star. Other single white dwarfs with higher space velocities
support a second, probably dominant, population of SN Ia progenitors which
contained main-sequence or subgiant donor stars at the time of explosion. The
runaway stars LP400-22 and US 708 suggest the possibility of a third formation
channnel for some SNe Ia in systems where the donor stars are hot subdwarfs.Comment: Accepted for publication in Astronomy & Astrophysic
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