5,882 research outputs found
Electron localisation in static and time-dependent one-dimensional model systems
Electron localization is the tendency of an electron in a many-body system to
exclude other electrons from its vicinity. Using a new natural measure of
localization based on the exact manyelectron wavefunction, we find that
localization can vary considerably between different ground-state systems, and
can also be strongly disrupted, as a function of time, when a system is driven
by an applied electric field. We use our new measure to assess the well-known
electron localization function (ELF), both in its approximate single-particle
form (often applied within density-functional theory) and its full
many-particle form. The full ELF always gives an excellent description of
localization, but the approximate ELF fails in time-dependent situations, even
when the exact Kohn-Sham orbitals are employed.Comment: 7 pages, 4 figure
GW self-screening error and its correction using a local density functional
The self-screening error in electronic structure theory is the part of the self-interaction error that would remain within the GW approximation if the exact dynamically screened Coulomb interaction W were used, causing each electron to artificially screen its own presence. This introduces error into the electron density and ionization potential. We propose a simple, computationally efficient correction to GW calculations in the form of a local density functional, obtained using a series of finite training systems; in tests, this eliminates the self-screening errors in the electron density and ionization potential
Reconstructing the global topology of the universe from the cosmic microwave background
If the universe is multiply-connected and sufficiently small, then the last
scattering surface wraps around the universe and intersects itself. Each circle
of intersection appears as two distinct circles on the microwave sky. The
present article shows how to use the matched circles to explicitly reconstruct
the global topology of space.Comment: 6 pages, 2 figures, IOP format. To be published in the proceedings of
the Cleveland Cosmology and Topology Workshop 17-19 Oct 1997. Submitted to
Class. Quant. Gra
Learning From Early Attempts to Generalize Darwinian Principles to Social Evolution
Copyright University of Hertfordshire & author.Evolutionary psychology places the human psyche in the context of evolution, and addresses the Darwinian processes involved, particularly at the level of genetic evolution. A logically separate and potentially complementary argument is to consider the application of Darwinian principles not only to genes but also to social entities and processes. This idea of extending Darwinian principles was suggested by Darwin himself. Attempts to do this appeared as early as the 1870s and proliferated until the early twentieth century. But such ideas remained dormant in the social sciences from the 1920s until after the Second World War. Some lessons can be learned from this earlier period, particularly concerning the problem of specifying the social units of selection or replication
Isospin splitting of the nucleon mean field
The isospin splitting of the nucleon mean field is derived from the Brueckner
theory extended to asymmetric nuclear matter. The Argonne V18 has been adopted
as bare interaction in combination with a microscopic three body force. The
isospin splitting of the effective mass is determined from the
Brueckner-Hartree-Fock self-energy: It is linear acording to the Lane ansatz
and such that for neutron-rich matter. The symmetry potential
is also determined and a comparison is made with the predictions of the
Dirac-Brueckner approach and the phenomenological interactions. The theoretical
predictions are also compared with the empirical parametrizations of neutron
and proton optical-model potentials based on the experimental nucleon-nucleus
scattering and the phenomenological ones adopted in transport-model simulations
of heavy-ion collisions. The direct contribution of the rearrangement term due
to three-body forces to the single particle potential and symmetry potential is
discussed.Comment: 8 pages, 10 figure
Nucleon-nucleon cross sections in neutron-rich matter and isospin transport in heavy-ion reactions at intermediate energies
Nucleon-nucleon (NN) cross sections are evaluated in neutron-rich matter
using a scaling model according to nucleon effective masses. It is found that
the in-medium NN cross sections are not only reduced but also have a different
isospin dependence compared with the free-space ones. Because of the
neutron-proton effective mass splitting the difference between nn and pp
scattering cross sections increases with the increasing isospin asymmetry of
the medium. Within the transport model IBUU04, the in-medium NN cross sections
are found to influence significantly the isospin transport in heavy-ion
reactions. With the in-medium NN cross sections, a symmetry energy of
was found most acceptable
compared with both the MSU isospin diffusion data and the presently acceptable
neutron-skin thickness in Pb. The isospin dependent part of isobaric nuclear incompressibility was further narrowed down to
MeV. The possibility of determining simultaneously the in-medium
NN cross sections and the symmetry energy was also studied. The proton
transverse flow, or even better the combined transverse flow of neutrons and
protons, can be used as a probe of the in-medium NN cross sections without much
hindrance from the uncertainties of the symmetry energy.Comment: 32 pages including 14 figures. Submitted to Phys. Rev.
The application of ultrasonic NDT techniques in tribology
The use of ultrasonic reflection is emerging as a technique for studying tribological contacts. Ultrasonic waves can be transmitted non-destructively through machine components and their behaviour at an interface describes the characteristics of that contact. This paper is a review of the current state of understanding of the mechanisms of ultrasonic reflection at interfaces, and how this has been used to investigate the processes of dry rough surface contact and lubricated contact. The review extends to cover how ultrasound has been used to study the tribological function of certain engineering machine elements
Effects of isospin and momentum dependent interactions on thermal properties of asymmetric nuclear matter
Thermal properties of asymmetric nuclear matter are studied within a
self-consistent thermal model using an isospin and momentum dependent
interaction (MDI) constrained by the isospin diffusion data in heavy-ion
collisions, a momentum-independent interaction (MID), and an isoscalar
momentum-dependent interaction (eMDYI). In particular, we study the temperature
dependence of the isospin-dependent bulk and single-particle properties, the
mechanical and chemical instabilities, and liquid-gas phase transition in hot
asymmetric nuclear matter. Our results indicate that the temperature dependence
of the equation of state and the symmetry energy are not so sensitive to the
momentum dependence of the interaction. The symmetry energy at fixed density is
found to generally decrease with temperature and for the MDI interaction the
decrement is essentially due to the potential part. It is further shown that
only the low momentum part of the single-particle potential and the nucleon
effective mass increases significantly with temperature for the
momentum-dependent interactions. For the MDI interaction, the low momentum part
of the symmetry potential is significantly reduced with increasing temperature.
For the mechanical and chemical instabilities as well as the liquid-gas phase
transition in hot asymmetric nuclear matter, our results indicate that the
boundary of these instabilities and the phase-coexistence region generally
shrink with increasing temperature and is sensitive to the density dependence
of the symmetry energy and the isospin and momentum dependence of the nuclear
interaction, especially at higher temperatures.Comment: 21 pages, 29 figure
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