4,280 research outputs found
Interrelations between Stochastic Equations for Systems with Pair Interactions
Several types of stochastic equations are important in thermodynamics,
chemistry, evolutionary biology, population dynamics and quantitative social
science. For systems with pair interactions four different types of equations
are derived, starting from a master equation for the state space: First,
general mean value and (co)variance equations. Second, Boltzmann-like
equations. Third, a master equation for the configuration space allowing
transition rates which depend on the occupation numbers of the states. Fourth,
a Fokker-Planck equation and a ``Boltzmann-Fokker-Planck equation''. The
interrelations of these equations and the conditions for their validity are
worked out clearly. A procedure for a selfconsistent solution of the nonlinear
equations is proposed. Generalizations to interactions between an arbitrary
number of systems are discussed.Comment: For related work see
http://www.theo2.physik.uni-stuttgart.de/helbing.htm
The Origins of Phase Transitions in Small Systems
The identification and classification of phases in small systems, e.g.
nuclei, social and financial networks, clusters, and biological systems, where
the traditional definitions of phase transitions are not applicable, is
important to obtain a deeper understanding of the phenomena observed in such
systems. Within a simple statistical model we investigate the validity and
applicability of different classification schemes for phase transtions in small
systems. We show that the whole complex temperature plane contains necessary
information in order to give a distinct classification.Comment: 3 pages, 4 figures, revtex 4 beta 5, for further information see
http://www.smallsystems.d
Quantum critical points with the Coulomb interaction and the dynamical exponent: when and why z=1
A general scenario that leads to Coulomb quantum criticality with the
dynamical critical exponent z=1 is proposed. I point out that the long-range
Coulomb interaction and quenched disorder have competing effects on z, and that
the balance between the two may lead to charged quantum critical points at
which z=1 exactly. This is illustrated with the calculation for the Josephson
junction array Hamiltonian in dimensions D=3-\epsilon. Precisely in D=3,
however, the above simple result breaks down, and z>1. Relation to other
theoretical studies is discussed.Comment: RevTex, 4 pages, 1 ps figur
Utilisation of Postnatal Care among Rural Women in Nepal
Background: Postnatal care is uncommon in Nepal, and where it is available the quality is often poor. Adequate utilisation of postnatal care can help reduce mortality and morbidity among mothers and their babies. Therefore, our study assessed the utilisation of postnatal care at a rural community level.
Methods: A descriptive, cross-sectional study was carried out in two neighbouring villages in early 2006. A total of 150 women who had delivered in the previous 24 months were asked to participate in the study using a semi-structured questionnaire.
Results: The proportion of women who had received postnatal care after delivery was low (34%). Less than one in five women (19%) received care within 48 hours of giving birth. Women in one village had less access to postnatal care than women in the neighbouring one. Lack of awareness was the main barrier to the utilisation of postnatal care.
The woman's own occupation and ethnicity, the number of pregnancies and children and the husband's socio-economic status, occupation and education were significantly associated with the utilisation of postnatal care.
Multivariate analysis showed that wealth as reflected in occupation and having attended antenatal are important factors associated with the uptake of postnatal care. In addition, women experiencing health problems appear strongly motivated to seek postnatal care.
Conclusion: The postnatal care has a low uptake and is often regarded as inadequate in Nepal. This is an important message to both service providers and health-policy makers. Therefore, there is an urgent need to assess the actual quality of postnatal care provided. Also there appears to be a need for awareness-raising programmes highlighting the availability of current postnatal care where this is of sufficient quality
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Complete Experimental Structure Determination of the p(3x2)pg Phase of Glycine on Cu{110}
We present a quantitative low energy electron diffraction (LEED) surface-crystallograpic
study of the complete adsorption geometry of glycine adsorbed on Cu{110} in the ordered
p(3×2) phase. The glycine molecules form bonds to the surface through the N atoms of the
amino group and the two O atoms of the de-protonated carboxylate group, each with separate
Cu atoms such that every Cu atom in the first layer is involved in a bond. Laterally, N atoms are
nearest to the atop site (displacement 0.41 Å). The O atoms are asymmetrically displaced from
the atop site by 0.54 Å and 1.18 Å with two very different O-Cu bond lengths of 1.93 Å and
2.18 Å. The atom positions of the upper-most Cu layers show small relaxations within 0.07 Å
of the bulk-truncated surface geometry. The unit cell of the adsorbate layer consists of two
glycine molecules, which are related by a glide-line symmetry operation. This study clearly
shows that a significant coverage of adsorbate structures without this glide-line symmetry must
be rejected, both on the grounds of the energy dependence of the spot intensities (LEED-IV
curves) and of systematic absences in the LEED pattern
How classical is the quantum universe?
We discuss two topics that are usually considered to be exclusively
"quantum": the Schroedinger equation, and the uncertainty principle. We show
(or rather recall) that the Schroedinger equation can be derived from
Hamilton's equations using the metaplectic representation. We also show that
the uncertainty principle, stated in the form of the
Robertson-Schroedinger-Heisenberg inequalities can be formulated in perfectly
classical terms using the topological notion of symplectic capacity
Neutron Scattering and Its Application to Strongly Correlated Systems
Neutron scattering is a powerful probe of strongly correlated systems. It can
directly detect common phenomena such as magnetic order, and can be used to
determine the coupling between magnetic moments through measurements of the
spin-wave dispersions. In the absence of magnetic order, one can detect diffuse
scattering and dynamic correlations. Neutrons are also sensitive to the
arrangement of atoms in a solid (crystal structure) and lattice dynamics
(phonons). In this chapter, we provide an introduction to neutrons and neutron
sources. The neutron scattering cross section is described and formulas are
given for nuclear diffraction, phonon scattering, magnetic diffraction, and
magnon scattering. As an experimental example, we describe measurements of
antiferromagnetic order, spin dynamics, and their evolution in the
La(2-x)Ba(x)CuO(4) family of high-temperature superconductors.Comment: 31 pages, chapter for "Strongly Correlated Systems: Experimental
Techniques", edited by A. Avella and F. Mancin
A plea for the development of an universally accepted modular tooth wear evaluation system
Wild-Type Phosphoribosylpyrophosphate Synthase (PRS) from Mycobacterium tuberculosis: A Bacterial Class II PRS?
The 5-phospho-α-D-ribose 1-diphosphate (PRPP) metabolite plays essential roles in several biosynthetic pathways, including histidine, tryptophan, nucleotides, and, in mycobacteria, cell wall precursors. PRPP is synthesized from α-D-ribose 5-phosphate (R5P) and ATP by the Mycobacterium tuberculosis prsA gene product, phosphoribosylpyrophosphate synthase (MtPRS). Here, we report amplification, cloning, expression and purification of wild-type MtPRS. Glutaraldehyde cross-linking results suggest that MtPRS predominates as a hexamer, presenting varied oligomeric states due to distinct ligand binding. MtPRS activity measurements were carried out by a novel coupled continuous spectrophotometric assay. MtPRS enzyme activity could be detected in the absence of Pi. ADP, GDP and UMP inhibit MtPRS activity. Steady-state kinetics results indicate that MtPRS has broad substrate specificity, being able to accept ATP, GTP, CTP, and UTP as diphosphoryl group donors. Fluorescence spectroscopy data suggest that the enzyme mechanism for purine diphosphoryl donors follows a random order of substrate addition, and for pyrimidine diphosphoryl donors follows an ordered mechanism of substrate addition in which R5P binds first to free enzyme. An ordered mechanism for product dissociation is followed by MtPRS, in which PRPP is the first product to be released followed by the nucleoside monophosphate products to yield free enzyme for the next round of catalysis. The broad specificity for diphosphoryl group donors and detection of enzyme activity in the absence of Pi would suggest that MtPRS belongs to Class II PRS proteins. On the other hand, the hexameric quaternary structure and allosteric ADP inhibition would place MtPRS in Class I PRSs. Further data are needed to classify MtPRS as belonging to a particular family of PRS proteins. The data here presented should help augment our understanding of MtPRS mode of action. Current efforts are toward experimental structure determination of MtPRS to provide a solid foundation for the rational design of specific inhibitors of this enzyme
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