1,948 research outputs found
Universal behaviour of ideal and interacting quantum gases in two dimensions
I discuss ideal and interacting quantum gases obeying general fractional
exclusion statistics. For systems with constant density of single-particle
states, described in the mean field approximation, the entropy depends neither
on the microscopic exclusion statistics, nor on the interaction. Such systems
are called {\em thermodynamically equivalent} and I show that the microscopic
reason for this equivalence is a one-to-one correspondence between the excited
states of these systems. This provides a method, different from the
bosonisation technique, to transform between systems of different exclusion
statistics. In the last section the macroscopic aspects of this method are
discussed.
In Appendix A I calculate the fluctuation of the ground state population of a
condensed Bose gas in grandcanonical ensemble and mean field approximation,
while in Appendix B I show a situation where although the system exhibits
fractional exclusion properties on microscopic energy intervals, a rigorous
calculation of the population of single particle states reveals a condensation
phenomenon. This also implies a malfunction of the usual and simplified
calculation technique of the most probable statistical distributions.Comment: About 14 journal pages, with 1 figure. Changes: Body of paper: same
content, with slight rephrasing. Apendices are new. In the original
submission I just mentioned the condensation, which is now detailed in
Appendix B. They were intended for a separate paper. Reason for changes:
rejection from Phys. Rev. Lett., resubmission to J. Phys. A: Math. Ge
Number Fluctuation in an interacting trapped gas in one and two dimensions
It is well-known that the number fluctuation in the grand canonical ensemble,
which is directly proportional to the compressibility, diverges for an ideal
bose gas as T -> 0. We show that this divergence is removed when the atoms
interact in one dimension through an inverse square two-body interaction. In
two dimensions, similar results are obtained using a self-consistent
Thomas-Fermi (TF) model for a repulsive zero-range interaction. Both models may
be mapped on to a system of non-interacting particles obeying the Haldane-Wu
exclusion statistics. We also calculate the number fluctuation from the ground
state of the gas in these interacting models, and compare the grand canonical
results with those obtained from the canonical ensemble.Comment: 11 pages, 1 appendix, 3 figures. Submitted to J. Phys. B: Atomic,
Molecular & Optica
Electromagnetic nucleon form factors in instant and point form
We present a study of the electromagnetic structure of the nucleons with
constituent quark models in the framework of relativistic quantum mechanics. In
particular, we address the construction of spectator-model currents in the
instant and point forms. Corresponding results for the elastic nucleon
electromagnetic form factors as well as charge radii and magnetic moments are
presented. We also compare results obtained by different realistic nucleon wave
functions stemming from alternative constituent quark models. Finally, we
discuss the theoretical uncertainties that reside in the construction of
spectator-model transition operators.Comment: 13 pages, 9 figures, updated and extended version for publicatio
Simple Analytical Particle and Kinetic Energy Densities for a Dilute Fermionic Gas in a d-Dimensional Harmonic Trap
We derive simple analytical expressions for the particle density
and the kinetic energy density for a system of noninteracting
fermions in a dimensional isotropic harmonic oscillator potential. We test
the Thomas-Fermi (TF, or local-density) approximation for the functional
relation using the exact and show that it locally
reproduces the exact kinetic energy density , {\it including the shell
oscillations,} surprisingly well everywhere except near the classical turning
point. For the special case of two dimensions (2D), we obtain the unexpected
analytical result that the integral of yields the {\it
exact} total kinetic energy.Comment: 4 pages, 4 figures; corrected versio
Production and detection of doubly charmed tetraquarks
The feasibility of tetraquark detection is studied. For the cc\bar{u}\bar{d}
tetraquark we show that in present (SELEX, Tevatron, RHIC) and future
facilities (LHCb, ALICE) the production rate is promising and we propose some
detectable decay channels.Comment: 6 pages, 5 figure
Microstructural evolution in the intercritical heat affected zone of a boron containing modified 9Cr-1Mo steel
Type IV cracking observed in high Cr ferritic steels is attributed to poor creep properties of the intercritical heat affected zone (ICHAZ) of the weld joint which in turn associated with the partial transformation that take place in this zone during the weld thermal cycle and resulting refinement of the microstructure. Recent studies on steels with controlled addition of boron has shown that creep strength of ICHAZ in these steels are comparable to that of the base metal and the microstructure of this zone is significantly different from that of the steels with out boron. Hence in this paper microstructural evolution in the simulated ICHAZ of two different P91 steels, one without boron and another with boron and reduced nitrogen content has been studied. Results indicate that during heating part of the weld thermal cycle, austenite nucleate and grow into two different morphologies, globular and acicular which transform to martensite during cooling. The former is more prevalent along the prior austenite grain boundaries (PAGB) of the base metal while the latter along the lath boundaries of the tempered martensite within the grains. Results also show the transformation to austenite is delayed in the boron containing steels and austenite with both the morphologies are formed. However, growth of the globular austenite, formed along the PAGB is sluggish and hence PAGBs are discernable even after the transformation. This explains the difference in the microstructure of the of ICHAZ of steels with boron and without boron and this could also be the reason for the improved creep resistance of the weld joint of high Cr ferritic steels with controlled addition of boron
Electron-deuteron scattering in the equal-time formalism: beyond the impulse approximation
Using a three-dimensional formalism that includes relativistic kinematics,
the effects of negative-energy states, approximate boosts of the two-body
system, and current conservation, we calculate the electromagnetic form factors
of the deuteron up to Q^2 of 4 GeV^2. This is done using a dynamical boost for
two-body systems with spin. We first compute form factors in impulse
approxmation, but then also add an isoscalar meson-exchange current of pion
range that involves the gamma-pi contact operator associated with pseudovector
pi-N coupling. We also consider effects of the rho-pi-gamma meson-exchange
current. The experimentally measured quantities A, B, and t20 are calculated
over the kinematic range probed in recent Jefferson Laboratory experiments. The
rho-pi-gamma meson-exchange current provides significant strength in A at large
Q^2 and the gamma-pi contact-term exchange current shifts t20, providing good
agreement with the JLab data. Relativistic effects and the gamma-pi
meson-exchange current do not provide an explanation of the B observable, but
the rho-pi-gamma current could help to provide agreement if a nonstandard value
is used for the tensor rho-N coupling that enters this contribution.Comment: 15 pages, 10 figures. (v2) Added references on rho-pi-gamma current
as well as comparison to recent Novosibirsk data on T20. Implemented
\includegraphics in place of \BoxedEPSF. (v3) Modified in order to clarify
the nature of the boost we implemented for particles with spin. Other minor
changes. Version to be published in Physical Review
Exclusion Statistics in a trapped two-dimensional Bose gas
We study the statistical mechanics of a two-dimensional gas with a repulsive
delta function interaction, using a mean field approximation. By a direct
counting of states we establish that this model obeys exclusion statistics and
is equivalent to an ideal exclusion statistics gas.Comment: 3 pages; minor changes in notation; typos correcte
Exclusion Statistics in a two-dimensional trapped Bose gas
We briefly explain the notion of exclusion statistics and in particular
discuss the concept of an ideal exclusion statistics gas. We then review a
recent work where it is demonstrated that a {\em two-dimensional} Bose gas with
repulsive delta function interactions obeys ideal exclusion statistics, with a
fractional parameter related to the interaction strength.Comment: 10 pages, RevTeX. Proceedings of the Salerno workshop "Theory of
Quantum Gases and Quantum Coherence", to appear in a special issue of J.Phys.
B, Dec. 200
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