10,202 research outputs found
A Lee-Yang--inspired functional with a density--dependent neutron-neutron scattering length
Inspired by the low--density Lee-Yang expansion for the energy of a dilute
Fermi gas of density and momentum , we introduce here a
Skyrme--type functional that contains only -wave terms and provides, at the
mean--field level, (i) a satisfactory equation of state for neutron matter from
extremely low densities up to densities close to the equilibrium point, and
(ii) a good--quality equation of state for symmetric matter at density scales
around the saturation point. This is achieved by using a density--dependent
neutron-neutron scattering length ) which satisfies the low--density
limit (for Fermi momenta going to zero) and has a density dependence tuned in
such a way that the low--density constraint is satisfied
at all density scales.Comment: 5 figure
From dilute matter to the equilibrium point in the energy--density--functional theory
Due to the large value of the scattering length in nuclear systems, standard
density--functional theories based on effective interactions usually fail to
reproduce the nuclear Fermi liquid behavior both at very low densities and
close to equilibrium. Guided on one side by the success of the Skyrme density
functional and, on the other side, by resummation techniques used in Effective
Field Theories for systems with large scattering lengths, a new energy--density
functional is proposed. This functional, adjusted on microscopic calculations,
reproduces the nuclear equations of state of neutron and symmetric matter at
various densities. Furthermore, it provides reasonable saturation properties as
well as an appropriate density dependence for the symmetry energy.Comment: 4 figures, 2 table
From bare interactions, low--energy constants and unitary gas to nuclear density functionals without free parameters: application to neutron matter
We further progress along the line of Ref. [Phys. Rev. {\bf A 94}, 043614
(2016)] where a functional for Fermi systems with anomalously large -wave
scattering length was proposed that has no free parameters. The
functional is designed to correctly reproduce the unitary limit in Fermi gases
together with the leading-order contributions in the s- and p-wave channels at
low density. The functional is shown to be predictive up to densities
fm that is much higher densities compared to the Lee-Yang
functional, valid for fm. The form of the functional
retained in this work is further motivated. It is shown that the new functional
corresponds to an expansion of the energy in and to all
orders, where is the effective range and is the Fermi momentum. One
conclusion from the present work is that, except in the extremely low--density
regime, nuclear systems can be treated perturbatively in with
respect to the unitary limit. Starting from the functional, we introduce
density--dependent scales and show that scales associated to the bare
interaction are strongly renormalized by medium effects. As a consequence, some
of the scales at play around saturation are dominated by the unitary gas
properties and not directly to low-energy constants. For instance, we show that
the scale in the s-wave channel around saturation is proportional to the
so-called Bertsch parameter and becomes independent of . We also
point out that these scales are of the same order of magnitude than those
empirically obtained in the Skyrme energy density functional. We finally
propose a slight modification of the functional such that it becomes accurate
up to the saturation density fm
Magnetic blackbody shift of hyperfine transitions for atomic clocks
We derive an expression for the magnetic blackbody shift of hyperfine
transitions such as the cesium primary reference transition which defines the
second. The shift is found to be a complicated function of temperature, and has
a T^2 dependence only in the high-temperature limit. We also calculate the
shift of ground-state p_1/2 hyperfine transitions which have been proposed as
new atomic clock transitions. In this case interaction with the p_3/2
fine-structure multiplet may be the dominant effect
A frequency-adjustable electromagnet for hyperthermia measurements on magnetic nanoparticles
We describe a low-cost and simple setup for hyperthermia measurements on
colloidal solutions of magnetic nanoparticles (ferrofluids) with a
frequency-adjustable magnetic field in the range 5-500 kHz produced by an
electromagnet. By optimizing the general conception and each component (nature
of the wires, design of the electromagnet), a highly efficient setup is
obtained. For instance, in a useful gap of 1.1 cm, a magnetic field of 4.8 mT
is generated at 100 kHz and 500 kHz with an output power of 3.4 W and 75 W,
respectively. A maximum magnetic field of 30 mT is obtained at 100 kHz. The
temperature of the colloidal solution is measured using optical fiber sensors.
To remove contributions due to heating of the electromagnet, a differential
measurement is used. In this configuration the sensitivity is better than 1.5
mW at 100 kHz and 19.3 mT. This setup allows one to measure weak heating powers
on highly diluted colloidal solutions. The hyperthermia characteristics of a
solution of Fe nanoparticles are described, where both the magnetic field and
the frequency dependence of heating power have been measured
High Dimensional Classification with combined Adaptive Sparse PLS and Logistic Regression
Motivation: The high dimensionality of genomic data calls for the development
of specific classification methodologies, especially to prevent over-optimistic
predictions. This challenge can be tackled by compression and variable
selection, which combined constitute a powerful framework for classification,
as well as data visualization and interpretation. However, current proposed
combinations lead to instable and non convergent methods due to inappropriate
computational frameworks. We hereby propose a stable and convergent approach
for classification in high dimensional based on sparse Partial Least Squares
(sparse PLS). Results: We start by proposing a new solution for the sparse PLS
problem that is based on proximal operators for the case of univariate
responses. Then we develop an adaptive version of the sparse PLS for
classification, which combines iterative optimization of logistic regression
and sparse PLS to ensure convergence and stability. Our results are confirmed
on synthetic and experimental data. In particular we show how crucial
convergence and stability can be when cross-validation is involved for
calibration purposes. Using gene expression data we explore the prediction of
breast cancer relapse. We also propose a multicategorial version of our method
on the prediction of cell-types based on single-cell expression data.
Availability: Our approach is implemented in the plsgenomics R-package.Comment: 9 pages, 3 figures, 4 tables + Supplementary Materials 8 pages, 3
figures, 10 table
Urban Agriculture and Other Green Uses: Remaking the Shrinking City
For many decades, the primary challenge of land use law has been how to promote and channel growth and development. Nobody wants stagnation; the cure is growth, and lately the cure has been âsmart growth.â In the last several years, however, some cities have begun openly to address a previously unacknowledged truth: some cities will and do shrink. They lose population and have no foreseeable prospect of ever regaining it. The land use planning community has begun to grapple with the issue of the shrinking city, asking how we can achieve managed, âsmartâ shrinkage To some extent, the answer is to shift density and promote green uses of various kinds This brings us to the legal question Does an organized effort to help a city shrink pose any distinctive legal issues? What constraints might the law impose on a cityâs goals of diminishing its infrastructure responsibilities, downzoning its land to less intensive uses, or taking other steps consistent with a goal of managed shrinkage This paper explores a few of those issues, using Cleveland, Ohio as an example. It considers legal challenges the shrinking city might face, particularly when downzoning urban property to promote urban agriculture and other green uses, focusing on the application of takings law It also briefly considers the fairness issues associated with downzoning and the limitations of the current legal structure for revitalizing Brownfields in a setting where traditional redevelopment is unlikely
Urban Green Uses: The New Renewal
As they confront dramatically reduced population and little prospect of significant near-term growth, several cities in the rust belt have turned to innovative tactics to put excess land to beneficial use. These measures include the creation of active land banks, downzoning for green uses such as urban agriculture, possible consolidation of population and abandonment of utility and public services, and installation of green infrastructure, such as stormwater retention and renewable power generation facilities, on publicly owned land. In the process, these cities face intriguing legal questions: What steps are needed to form an effective land bank? What is the liability of land banks for cleanup of contaminated properties? Are cities required to provide municipal services to unpopulated areas within their boundaries? In the unlikely event that a city uses eminent domain to relocate owners of sparsely-populated areas, what is âjust compensationâ for this action? What issues might arise with zoning land for less intensive uses such as urban farms? Some of the answers are emerging. For example, state authorizing legislation has been enacted to establish the type of active land bank successfully implemented in St. Louis, Cleveland, and other cities, and it appears that cities need not provide infrastructure and services throughout their land area, though they are best advised retain any rights of way or easements that may be needed in the event of future development. Other questions â both legal and practical - have yet to be fully answered, as rust belt cities lead the way in what might tentatively be called The New Renewal â a form of sustainable development that dovetails well with the policies of cities that seek to combat and adapt to climate change
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