2,327 research outputs found
Neutron activation analysis traces copper artifacts to geographical point of origin
Impurities remaining in the metallic copper are identified and quantified by spectrographic and neutron activation analysis. Determination of the type of ore used for the copper artifact places the geographic point of origin of the artifact
Microcanonical entropy for small magnetisations
Physical quantities obtained from the microcanonical entropy surfaces of
classical spin systems show typical features of phase transitions already in
finite systems. It is demonstrated that the singular behaviour of the
microcanonically defined order parameter and susceptibility can be understood
from a Taylor expansion of the entropy surface. The general form of the
expansion is determined from the symmetry properties of the microcanonical
entropy function with respect to the order parameter. The general findings are
investigated for the four-state vector Potts model as an example of a classical
spin system.Comment: 15 pages, 7 figure
Phase transitions and configuration space topology
Equilibrium phase transitions may be defined as nonanalytic points of
thermodynamic functions, e.g., of the canonical free energy. Given a certain
physical system, it is of interest to understand which properties of the system
account for the presence of a phase transition, and an understanding of these
properties may lead to a deeper understanding of the physical phenomenon. One
possible approach of this issue, reviewed and discussed in the present paper,
is the study of topology changes in configuration space which, remarkably, are
found to be related to equilibrium phase transitions in classical statistical
mechanical systems. For the study of configuration space topology, one
considers the subsets M_v, consisting of all points from configuration space
with a potential energy per particle equal to or less than a given v. For
finite systems, topology changes of M_v are intimately related to nonanalytic
points of the microcanonical entropy (which, as a surprise to many, do exist).
In the thermodynamic limit, a more complex relation between nonanalytic points
of thermodynamic functions (i.e., phase transitions) and topology changes is
observed. For some class of short-range systems, a topology change of the M_v
at v=v_t was proved to be necessary for a phase transition to take place at a
potential energy v_t. In contrast, phase transitions in systems with long-range
interactions or in systems with non-confining potentials need not be
accompanied by such a topology change. Instead, for such systems the
nonanalytic point in a thermodynamic function is found to have some
maximization procedure at its origin. These results may foster insight into the
mechanisms which lead to the occurrence of a phase transition, and thus may
help to explore the origin of this physical phenomenon.Comment: 22 pages, 6 figure
Symmetries of microcanonical entropy surfaces
Symmetry properties of the microcanonical entropy surface as a function of
the energy and the order parameter are deduced from the invariance group of the
Hamiltonian of the physical system. The consequences of these symmetries for
the microcanonical order parameter in the high energy and in the low energy
phases are investigated. In particular the breaking of the symmetry of the
microcanonical entropy in the low energy regime is considered. The general
statements are corroborated by investigations of various examples of classical
spin systems.Comment: 15 pages, 5 figures include
Excitation of the 3.071mm Hyperfine Line in Li-Like 57-Fe in Astrophysical Plasmas
As noted first by Sunyaev & Churazov (1984), the 3.071 mm hyperfine line from
might be observable in astrophysical plasmas. We assess the
atomic processes which might contribute to the excitation of this line. We
determine the intensity of the hyperfine line from an isothermal, coronal
plasma in collisional ionization equilibrium and for a coronal plasma cooling
isobarically due to its own radiation. Comparisons of the hyperfine line to
other lines emitted by the same ion, Fe, are shown to be useful for
deriving the isotopic fraction of Fe. We calculate the ratios of the
hyperfine line to the 2s--2p EUV lines at 192 \AA and 255 \AA, and the 2s--3p
X-ray doublet at 10.6 \AA.Comment: 28 pages text+figures, Accepted to ApJ in Jan 98, also at
http://www.astro.virginia.edu/~nld2n/research.htm
Effect of Quantum Confinement on Electron Tunneling through a Quantum Dot
Employing the Anderson impurity model, we study tunneling properties through
an ideal quantum dot near the conductance minima. Considering the Coulomb
blockade and the quantum confinement on an equal footing, we have obtained
current contributions from various types of tunneling processes; inelastic
cotunneling, elastic cotunneling, and resonant tunneling of thermally activated
electrons. We have found that the inelastic cotunneling is suppressed in the
quantum confinement limit, and thus the conductance near its minima is
determined by the elastic cotunneling at low temperature (,
: dot-reservoir coupling constant), or by the resonant tunneling of
single electrons at high temperature ().Comment: 11 pages Revtex, 2 Postscript figures, To appear in Phys.Rev.
The Possibilist Transactional Interpretation and Relativity
A recent ontological variant of Cramer's Transactional Interpretation, called
"Possibilist Transactional Interpretation" or PTI, is extended to the
relativistic domain. The present interpretation clarifies the concept of
'absorption,' which plays a crucial role in TI (and in PTI). In particular, in
the relativistic domain, coupling amplitudes between fields are interpreted as
amplitudes for the generation of confirmation waves (CW) by a potential
absorber in response to offer waves (OW), whereas in the nonrelativistic
context CW are taken as generated with certainty. It is pointed out that
solving the measurement problem requires venturing into the relativistic domain
in which emissions and absorptions take place; nonrelativistic quantum
mechanics only applies to quanta considered as 'already in existence' (i.e.,
'free quanta'), and therefore cannot fully account for the phenomenon of
measurement, in which quanta are tied to sources and sinks.Comment: Final version with some minor corrections as published in Foundations
of Physics. This paper has significant overlap with Chapter 6 of my book on
the Transactional Interpretation, forthcoming from Cambridge University
Press:
http://www.cambridge.org/us/knowledge/isbn/item6860644/?site_locale=en_US
(Additional preview material is available at rekastner.wordpress.com)
Comments welcom
Finite-size behaviour of the microcanonical specific heat
For models which exhibit a continuous phase transition in the thermodynamic
limit a numerical study of small systems reveals a non-monotonic behaviour of
the microcanonical specific heat as a function of the system size. This is in
contrast to a treatment in the canonical ensemble where the maximum of the
specific heat increases monotonically with the size of the system. A
phenomenological theory is developed which permits to describe this peculiar
behaviour of the microcanonical specific heat and allows in principle the
determination of microcanonical critical exponents.Comment: 15 pages, 7 figures, submitted to J. Phys.
Quantum Nonlocality: Not Eliminated by the Heisenberg Picture
It is argued that the Heisenberg picture of standard quantum mechanics does
not save Einstein locality as claimed in Deutsch and Hayden (2000). In
particular, the EPR-type correlations that DH obtain by comparing two qubits in
a local manner are shown to exist before that comparison. In view of this
result, the local comparison argument would appear to ineffective in supporting
their locality claim.Comment: Final version; to appear in Foundations of Physic
Discovery of Raman-scattered lines in the massive luminous emission-line star LHA 115-S 18
LHA 115-S 18 is a very peculiar emission-line star exhibiting the B[e]
phenomenon. Located in the Small Magellanic Cloud, its spectrum shows features
of an extremely wide range of excitation and ionization stages, extending from
highly ionized atomic lines (Si IV, C IV, He II) in the UV and optical regions
to molecular emission bands of CO and TiO in the optical and IR regions. The
most distinguishing spectral characteristic of LHA 115-S 18 is the high
variability detected in the He II {\lambda}4686 emission line, which can be a
very conspicuous or completely invisible feature.
In this work, we report on another peculiarity of LHA 115-S 18. From
high-resolution optical spectra taken between 2000 and 2008, we discovered the
appearance and strengthening of two emission features at {\lambda}6825 \AA, and
{\lambda}7082 \AA,, which we identified as Raman-scattered lines. This is the
first time these lines have been detected in the spectrum of a massive luminous
B[e] star. As the classification of LHA 115-S 18 is highly controversial, we
discuss how the discovery of the appearance of Raman-scattered lines in this
peculiar star might help us to solve this puzzle.Comment: Letter accepted for publication in MNRAS. 5 pages, 3 figure
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