992 research outputs found
Indistinguishable Particles in Quantum Mechanics: An Introduction
In this article, we discuss the identity and indistinguishability of quantum
systems and the consequent need to introduce an extra postulate in Quantum
Mechanics to correctly describe situations involving indistinguishable
particles. This is, for electrons, the Pauli Exclusion Principle, or in
general, the Symmetrization Postulate. Then, we introduce fermions and bosons
and the distributions respectively describing their statistical behaviour in
indistinguishable situations. Following that, we discuss the spin-statistics
connection, as well as alternative statistics and experimental evidence for all
these results, including the use of bunching and antibunching of particles
emerging from a beam splitter as a signature for some bosonic or fermionic
states.Comment: To appear in Contemp. Phy
Why do some intermediate polars show soft X-ray emission? A survey of XMM-Newton spectra
We make a systematic analysis of the XMM-Newton X-ray spectra of intermediate
polars (IPs) and find that, contrary to the traditional picture, most show a
soft blackbody component. We compare the results with those from AM Her stars
and deduce that the blackbody emission arises from reprocessing of hard X-rays,
rather than from the blobby accretion sometimes seen in AM Hers. Whether an IP
shows a blackbody component appears to depend primarily on geometric factors: a
blackbody is not seen in those that have accretion footprints that are always
obscured by accretion curtains or are only visible when foreshortened on the
white-dwarf limb. Thus we argue against previous suggestions that the blackbody
emission characterises a separate sub-group of IPs which are more akin to AM
Hers, and develop a unified picture of the blackbody emission in these stars.Comment: 9 pages, 6 figures. Accepted for publication in Ap
Confluence Modulo Equivalence in Constraint Handling Rules
Previous results on proving confluence for Constraint Handling Rules are
extended in two ways in order to allow a larger and more realistic class of CHR
programs to be considered confluent. Firstly, we introduce the relaxed notion
of confluence modulo equivalence into the context of CHR: while confluence for
a terminating program means that all alternative derivations for a query lead
to the exact same final state, confluence modulo equivalence only requires the
final states to be equivalent with respect to an equivalence relation tailored
for the given program. Secondly, we allow non-logical built-in predicates such
as var/1 and incomplete ones such as is/2, that are ignored in previous work on
confluence.
To this end, a new operational semantics for CHR is developed which includes
such predicates. In addition, this semantics differs from earlier approaches by
its simplicity without loss of generality, and it may also be recommended for
future studies of CHR.
For the purely logical subset of CHR, proofs can be expressed in first-order
logic, that we show is not sufficient in the present case. We have introduced a
formal meta-language that allows reasoning about abstract states and
derivations with meta-level restrictions that reflect the non-logical and
incomplete predicates. This language represents subproofs as diagrams, which
facilitates a systematic enumeration of proof cases, pointing forward to a
mechanical support for such proofs
Similarity in cognitive complexity and attraction to friends and lovers: Experimental and correlational studies
Abstract OnlyTwo studies are reported examining whether similarities in cognitive complexity foster different forms of interpersonal attraction. Study 1 provided an experimental test of the hypothesis that perceivers would be more attracted to targets with similar levels of complexity than to targets with dissimilar levels of complexity. Participants read interpersonal impressions reflecting low and high levels of cognitive complexity and completed 3 assessments of attraction (social, task, and intellectual) to the source of the impressions. As predicted, there were significant interactions between perceiver complexity and target complexity such that high-complexity perceivers were more attracted to high-complexity targets than were low-complexity perceivers, whereas low-complexity perceivers were more attracted to low-complexity targets than were high-complexity perceivers. Unexpectedly, however, low-complexity perceivers were more attracted to a high-complexity target than a low-complexity target. Study 2 examined the effects of similarities in cognitive complexity on attraction among 126 pairs of dating partners. Partners having similar levels of cognitive complexity expressed significantly greater intellectual attraction to one another than partners having dissimilar levels of cognitive complexity
Inelastic collapse of a randomly forced particle
We consider a randomly forced particle moving in a finite region, which
rebounds inelastically with coefficient of restitution r on collision with the
boundaries. We show that there is a transition at a critical value of r,
r_c\equiv e^{-\pi/\sqrt{3}}, above which the dynamics is ergodic but beneath
which the particle undergoes inelastic collapse, coming to rest after an
infinite number of collisions in a finite time. The value of r_c is argued to
be independent of the size of the region or the presence of a viscous damping
term in the equation of motion.Comment: 4 pages, REVTEX, 2 EPS figures, uses multicol.sty and epsf.st
Thermoelectric enhancement in PbTe with K, Na co-doping from tuning the interaction of the light and heavy hole valence bands
The effect of K and K-Na substitution for Pb atoms in the rock salt lattice
of PbTe was investigated to test a hypothesis for development of resonant
states in the valence band that may enhance the thermoelectric power. We
combined high temperature Hall-effect, electrical conductivity and thermal
conductivity measurements to show that K-Na co-doping do not form resonance
states but2 can control the energy difference of the maxima of the two primary
valence sub-bands in PbTe. This leads to an enhanced interband interaction with
rising temperature and a significant rise in the thermoelectric figure of merit
of p-type PbTe. The experimental data can be explained by a combination of a
single and two-band model for the valence band of PbTe depending on hole
density that varies in the range of 1-15 x 10^19 cm^-3.Comment: 8 figure
Constructing Hybrid Baryons with Flux Tubes
Hybrid baryon states are described in quark potential models as having
explicit excitation of the gluon degrees of freedom. Such states are described
in a model motivated by the strong coupling limit of Hamiltonian lattice gauge
theory, where three flux tubes meeting at a junction play the role of the glue.
The adiabatic approximation for the quark motion is used, and the flux tubes
and junction are modeled by beads which are attracted to each other and the
quarks by a linear potential, and vibrate in various string modes. Quantum
numbers and estimates of the energies of the lightest hybrid baryons are
provided.Comment: 4 pages, RevTeX. Submitted to Physical Review Letter
On the geometry of quantum indistinguishability
An algebraic approach to the study of quantum mechanics on configuration
spaces with a finite fundamental group is presented. It uses, in an essential
way, the Gelfand-Naimark and Serre-Swan equivalences and thus allows one to
represent geometric properties of such systems in algebraic terms. As an
application, the problem of quantum indistinguishability is reformulated in the
light of the proposed approach. Previous attempts aiming at a proof of the
spin-statistics theorem in non-relativistic quantum mechanics are explicitly
recast in the global language inherent to the presented techniques. This leads
to a critical discussion of single-valuedness of wave functions for systems of
indistinguishable particles. Potential applications of the methods presented in
this paper to problems related to quantization, geometric phases and phase
transitions in spin systems are proposed.Comment: 24 page
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