4,099 research outputs found
Long range order for lattice dipoles
We consider a system of classical Heisenberg spins on a cubic lattice in
dimensions three or more, interacting via the dipole-dipole interaction. We
prove that at low enough temperature the system displays orientational long
range order, as expected by spin wave theory. The proof is based on reflection
positivity methods. In particular, we demonstrate a previously unproven
conjecture on the dispersion relation of the spin waves, first proposed by
Froehlich and Spencer, which allows one to apply infrared bounds for estimating
the long distance behavior of the spin-spin correlation functions.Comment: 9 page
Semidirect product of CCR and CAR algebras and asymptotic states in quantum electrodynamics
A C*-algebra containing the CCR and CAR algebras as its subalgebras and
naturally described as the semidirect product of these algebras is discussed. A
particular example of this structure is considered as a model for the algebra
of asymptotic fields in quantum electrodynamics, in which Gauss' law is
respected. The appearence in this algebra of a phase variable related to
electromagnetic potential leads to the universal charge quantization.
Translationally covariant representations of this algebra with energy-momentum
spectrum in the future lightcone are investigated. It is shown that vacuum
representations are necessarily nonregular with respect to total
electromagnetic field. However, a class of translationally covariant,
irreducible representations is constructed excplicitly, which remain as close
as possible to the vacuum, but are regular at the same time. The spectrum of
energy-momentum fills the whole future lightcone, but there are no vectors with
energy-momentum lying on a mass hyperboloid or in the origin.Comment: 42 pages, LaTeX; minor corrections, a reference adde
Stellar evolution of massive stars at very low metallicities
Recently, measurements of abundances in extremely metal poor (EMP) stars have
brought new constraints on stellar evolution models. In an attempt to explain
the origin of the abundances observed, we computed pre--supernova evolution
models, explosion models and the related nucleosynthesis. In this paper, we
start by presenting the pre-SN models of rotating single stars with
metallicities ranging from solar metallicity down to almost metal free. We then
review key processes in core-collapse and bounce, before we integrate them in a
simplistic parameterization for 3D MHD models, which are well underway and
allow one to follow the evolution of the magnetic fields during collapse and
bounce. Finally, we present explosive nucleosynthesis results including
neutrino interactions with matter, which are calculated using the outputs of
the explosion models.
The main results of the pre-SN models are the following. First, primary
nitrogen is produced in large amount in models with an initial metallicity
. Second, at the same metallicity of and for models with
an initial mass larger than about 60 Mo, rotating models may experience heavy
mass loss (up to more than half of the initial mass of the star). The chemical
composition of these winds can qualitatively reproduce the abundance patterns
observed at the surface of carbon-rich EMP stars. Explosive nucleosynthesis
including neutrino-matter interactions produce improved abundances for iron
group elements, in particular for scandium and zinc. It also opens the way to a
new neutrino and proton rich process (p-process) able to contribute to the
nucleosynthesis of elements with A > 64. (Abridged)Comment: 29 pages, 10 figures, Reviews of Modern Astronomy 19, proceedings for
79th Annual Scientific Meeting of the Deutsche Astronomische Gesellschaft
200
Probing Non-Abelian Statistics in nu=12/5 Quantum Hall State
The tunneling current and shot noise of the current between two Fractional
Quantum Hall (FQH) edges in the FQH state in electronic
Mach-Zehnder interferometer are studied. It is shown that the tunneling current
and shot noise can be used to probe the existence of parafermion
statistics in the FQH state. More specifically, the dependence of
the current on the Aharonov-Bohm flux in the Read-Rezayi state is asymmetric
under the change of the sign of the applied voltage. This property is absent in
the Abelian Laughlin states. Moreover the Fano factor can exceed 12.7 electron
charges in the FQH state . This number well exceeds the maximum
possible Fano factor in all Laughlin states and the Moore-Read
state which was shown previously to be and respectively.Comment: 10 pages, 6 figure
Adaiabtic theorems and reversible isothermal processes
Isothermal processes of a finitely extended, driven quantum system in contact
with an infinite heat bath are studied from the point of view of quantum
statistical mechanics. Notions like heat flux, work and entropy are defined for
trajectories of states close to, but distinct from states of joint thermal
equilibrium. A theorem characterizing reversible isothermal processes as
quasi-static processes (''isothermal theorem'') is described. Corollaries
concerning the changes of entropy and free energy in reversible isothermal
processes and on the 0th law of thermodynamics are outlined
The Spin-Statistics Theorem for Anyons and Plektons in d=2+1
We prove the spin-statistics theorem for massive particles obeying braid
group statistics in three-dimensional Minkowski space. We start from first
principles of local relativistic quantum theory. The only assumption is a gap
in the mass spectrum of the corresponding charged sector, and a restriction on
the degeneracy of the corresponding mass.Comment: 21 pages, 2 figures. Citation added; Minor modifications of Appendix
Expression and alternative splicing of the neural cell adhesion molecule NCAM in human granulosa cells during luteinization
Freshly aspirated human granulosa cells from pre-ovulatory follicles and granulosa cells luteinized in culture possess the neural cell adhesion molecule (NCAM) of approximate molecular mass of 140,000 and NCAM mRNA as confirmed by S1-nuclease protection assays and RT-PCR. Moreover, in the process of luteinization the NCAM isoform pattern is modified. Isoforms containing an insert of 10 amino acids (termed VASE) in the extracellular domain of NCAM were supplemented by alternatively spliced isoforms without this insert. NCAM immunoreactivity, at light and electron microscope levels, was associated with the cell membrane of most granulosa cells which formed clusters. During time in culture an increasing subpopulation of granulosa cells, devoid of NCAM immunoreactivity, spread out and formed monolayers. This differential expression and the alternative splicing of NCAM during luteinization of granulosa cells raise the possibility that NCAM could be involved in folliculogenesis and the formation of the corpus luteum in the human
Spin - or, actually: Spin and Quantum Statistics
The history of the discovery of electron spin and the Pauli principle and the
mathematics of spin and quantum statistics are reviewed. Pauli's theory of the
spinning electron and some of its many applications in mathematics and physics
are considered in more detail. The role of the fact that the tree-level
gyromagnetic factor of the electron has the value g = 2 in an analysis of
stability (and instability) of matter in arbitrary external magnetic fields is
highlighted. Radiative corrections and precision measurements of g are
reviewed. The general connection between spin and statistics, the CPT theorem
and the theory of braid statistics are described.Comment: 50 pages, no figures, seminar on "spin
Pushing 1D CCSNe to explosions: model and SN 1987A
We report on a method, PUSH, for triggering core-collapse supernova
explosions of massive stars in spherical symmetry. We explore basic explosion
properties and calibrate PUSH such that the observables of SN1987A are
reproduced. Our simulations are based on the general relativistic hydrodynamics
code AGILE combined with the detailed neutrino transport scheme IDSA for
electron neutrinos and ALS for the muon and tau neutrinos. To trigger
explosions in the otherwise non-exploding simulations, we rely on the
neutrino-driven mechanism. The PUSH method locally increases the energy
deposition in the gain region through energy deposition by the heavy neutrino
flavors. Our setup allows us to model the explosion for several seconds after
core bounce. We explore the progenitor range 18-21M. Our studies
reveal a distinction between high compactness (HC) and low compactness (LC)
progenitor models, where LC models tend to explore earlier, with a lower
explosion energy, and with a lower remnant mass. HC models are needed to obtain
explosion energies around 1 Bethe, as observed for SN1987A. However, all the
models with sufficiently high explosion energy overproduce Ni. We
conclude that fallback is needed to reproduce the observed nucleosynthesis
yields. The nucleosynthesis yields of Ni depend sensitively on the
electron fraction and on the location of the mass cut with respect to the
initial shell structure of the progenitor star. We identify a progenitor and a
suitable set of PUSH parameters that fit the explosion properties of SN1987A
when assuming 0.1M of fallback. We predict a neutron star with a
gravitational mass of 1.50M. We find correlations between explosion
properties and the compactness of the progenitor model in the explored
progenitors. However, a more complete analysis will require the exploration of
a larger set of progenitors with PUSH.Comment: revised version as accepted by ApJ (results unchanged, text modified
for clarification, a few references added); 26 pages, 20 figure
Degradation of LaMnO{3-y} surface layer in LaMnO{3-y}/ metal interface
We report electrical measurements showing the degradation processes of
LaMnO (LaMnO) in LaMnO/normal metal interface in both point contact and
planar-type junctions. Immediately after the preparation of the interface, the
degradation process was followed by measuring the evolution of the junction
resistance versus time. This process is characterized by the appearance of a
second maximum in the resistance vs. temperature (R-T) dependence at
temperatures lower than the Curie temperature T, at which the
metal-insulator transition occurs in the bulk. These effects are explained in
terms of the formation of a depleted interface layer in LaMnO caused by an
out-diffusion of oxygen from the manganite surface to the normal metal. This
assumption is confirmed by XPS measurement. Similar results on LaSrMnO
interfaces are also obtained.Comment: 4 pages, 3 figures, accepted for publication in Appl. Phys.
Lett.(2002
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