431,218 research outputs found
On the theory of composition in physics
We develop a theory for describing composite objects in physics. These can be
static objects, such as tables, or things that happen in spacetime (such as a
region of spacetime with fields on it regarded as being composed of smaller
such regions joined together). We propose certain fundamental axioms which, it
seems, should be satisfied in any theory of composition. A key axiom is the
order independence axiom which says we can describe the composition of a
composite object in any order. Then we provide a notation for describing
composite objects that naturally leads to these axioms being satisfied. In any
given physical context we are interested in the value of certain properties for
the objects (such as whether the object is possible, what probability it has,
how wide it is, and so on). We associate a generalized state with an object.
This can be used to calculate the value of those properties we are interested
in for for this object. We then propose a certain principle, the composition
principle, which says that we can determine the generalized state of a
composite object from the generalized states for the components by means of a
calculation having the same structure as the description of the generalized
state. The composition principle provides a link between description and
prediction.Comment: 23 pages. To appear in a festschrift for Samson Abramsky edited by
Bob Coecke, Luke Ong, and Prakash Panangade
Intensive variables in the framework of the non-extensive thermostatistics
By assuming an appropriate energy composition law between two systems
governed by the same non-extensive entropy, we revisit the definitions of
temperature and pressure, arising from the zeroth principle of thermodynamics,
in a manner consistent with the thermostatistics structure of the theory. We
show that the definitions of these quantities are sensitive to the composition
law of entropy and internal energy governing the system. In this way, we can
clarify some questions raised about the possible introduction of intensive
variables in the context of non-extensive statistical mechanics.Comment: 14 pages, elsart style, version accepted on Physics Letters
Paramagnetism in the kagome compounds (Zn,Mg,Cd)Cu(OH)Cl
Frustrated magnetism on the kagome lattice has been a fertile ground for rich
and fascinating physics, ranging from experimental evidence of a spin liquid to
theoretical predictions of exotic superconductivity. Among experimentally
realized spin- kagome magnets, herbertsmithite, kapellasite, and
haydeeite [(Zn,Mg)Cu(OH)Cl] are all well described by a
three-parameter Heisenberg model, but they exhibit distinctly different
physics. We address the problem using a pseudofermion functional
renormalization-group approach and analyze the low-energy physics in the
experimentally accessible parameter range. Our analysis places kapellasite and
haydeeite near the boundaries between magnetically ordered and disordered
phases, implying that slight modifications could dramatically affect their
magnetic properties. Inspired by this, we perform \textit{ab initio} density
functional theory calculations of (Zn,Mg,Cd)Cu (OH)Cl at
various pressures. Our results suggest that by varying pressure and composition
one can traverse a paramagnetic regime between different magnetically ordered
phases.Comment: Published version. Main paper (7 pages, 5 figures) and Supplemental
material (7 pages, 4 figures, 3 tables). Change in titl
Composition, volume, and aspect ratio dependence of the strain distribution, band lineups and electron effective masses in self-assembled pyramidal In1-xGaxAs/GaAs and SixGe1-x/Si quantum dots
We present a systematic investigation of the strain distribution of self-assembled pyramidal In1-xGaxAs/GaAs and SixGe1-x/Si quantum dots for the case of growth on a (001) substrate. The dependence of the biaxial and hydrostatic components of the strain on the quantum dot volume, aspect ratio, composition, and percentage of alloying x is studied using a method based on a Green's function technique. The dependence of the carriers' confining potentials and the electronic effective mass on the same parameters is then calculated in the framework of eight-band k .p theory. The results for which comparable published data are available are in good agreement with the theoretical values for strain profiles, confining potentials, and electronic effective mass. © 2002 American Institute of Physics
The Inner Rim of YSO Disks: Effects of dust grain evolution
Dust-grain growth and settling are the first steps towards planet formation.
An understanding of dust physics is therefore integral to a complete theory of
the planet formation process. In this paper, we explore the possibility of
using the dust evaporation front in YSO disks (`the inner rim') as a probe of
the dust physics operating in circumstellar disks. The geometry of the rim
depends sensitively on the composition and spatial distribution of dust. Using
radiative transfer and hydrostatic equilibrium calculations we demonstrate that
dust growth and settling can curve the evaporation front dramatically (from a
cylindrical radius of about 0.5 AU in the disk mid-plane to 1.2 AU in the disk
upper layers for an A0 star). We compute synthetic images and interferometric
visibilities for our representative rim models and show that the current
generation of near-IR long-baseline interferometers (VLTI, CHARA) can strongly
constrain the dust properties of circumstellar disks, shedding light on the
relatively poorly understood processes of grain growth, settling and turbulent
mixing.Comment: 26 pages, 9 figures. Accepted for publication in Ap
Role of Dirac cones in magnetotransport properties of REFeAsO (RE=rare earth) oxypnictides
In this work we study the effect of the rare earth element in iron
oxypnictides of composition REFeAsO (RE=rare earth). On one hand we carry out
Density Functional Theory calculations of the band structure, which evidence
the multiband character of these compounds and the presence of Dirac cones
along the Y-{\Gamma} and Z-R directions of the reciprocal space. On the other
hand, we explore transport behavior by means of resistivity, Hall resistance
and magnetoresistance measurements, which confirm the dominant role of Dirac
cones. By combining our theoretical and experimental approaches, we extract
information on effective masses, scattering rates and Fermi velocities for
different rare earth elements.Comment: 13 pages, 5 figures accepted for publication on European Journal of
Physics
Synthesis of the elements in stars: forty years of progress
Forty years ago Burbidge, Burbidge, Fowler, and Hoyle combined what we would now call fragmentary evidence from nuclear physics, stellar evolution and the abundances of elements and isotopes in the solar system as well as a few stars into a synthesis of remarkable ingenuity. Their review provided a foundation for forty years of research in all of the aspects of low energy nuclear experiments and theory, stellar modeling over a wide range of mass and composition, and abundance studies of many hundreds of stars, many of which have shown distinct evidence of the processes suggested by B2FH. In this review we summarize progress in each of these fields with emphasis on the most recent developments
Hamilton's Turns for the Lorentz Group
Hamilton in the course of his studies on quaternions came up with an elegant
geometric picture for the group SU(2). In this picture the group elements are
represented by ``turns'', which are equivalence classes of directed great
circle arcs on the unit sphere , in such a manner that the rule for
composition of group elements takes the form of the familiar parallelogram law
for the Euclidean translation group. It is only recently that this construction
has been generalized to the simplest noncompact group , the double cover of SO(2,1). The present work develops a theory of
turns for , the double and universal cover of SO(3,1) and ,
rendering a geometric representation in the spirit of Hamilton available for
all low dimensional semisimple Lie groups of interest in physics. The geometric
construction is illustrated through application to polar decomposition, and to
the composition of Lorentz boosts and the resulting Wigner or Thomas rotation.Comment: 13 pages, Late
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