42,219 research outputs found
Comment on ``Spin Dependent Hopping and Colossal Negative Magnetoresistance in Epitaxial Films in Fields up to 50 T''
Recently Wagner et al. [Phys. Rev. Lett. Vol. 81, P. 3980 (1998)] proposed
that Mott's original model be modified to incorporate a hopping barrier which
depends on the misorientation between the spins of electrons at the initial and
the final states in an elementary process. They further claimed that using the
model they can explain the observed scaling behavior--
negative-magnetoresistivity scaling proportional to the Brillouin function
in the ferromagnetic state and to in the paramagnetic
state. In this comment we argue that the modification needed for Mott's
original model is different from that proposed by Wagner et al. and further
show that our picture will successfully explain the observed scaling in the two
regimes.Comment: 1 pag
Wedges, Cones, Cosmic Strings, and the Reality of Vacuum Energy
One of J. Stuart Dowker's most significant achievements has been to observe
that the theory of diffraction by wedges developed a century ago by Sommerfeld
and others provided the key to solving two problems of great interest in
general-relativistic quantum field theory during the last quarter of the
twentieth century: the vacuum energy associated with an infinitely thin,
straight cosmic string, and (after an interchange of time with a space
coordinate) the apparent vacuum energy of empty space as viewed by an
accelerating observer. In a sense the string problem is more elementary than
the wedge, since Sommerfeld's technique was to relate the wedge problem to that
of a conical manifold by the method of images. Indeed, Minkowski space, as well
as all cone and wedge problems, are related by images to an infinitely sheeted
master manifold, which we call Dowker space. We review the research in this
area and exhibit in detail the vacuum expectation values of the energy density
and pressure of a scalar field in Dowker space and the cone and wedge spaces
that result from it. We point out that the (vanishing) vacuum energy of
Minkowski space results, from the point of view of Dowker space, from the
quantization of angular modes, in precisely the way that the Casimir energy of
a toroidal closed universe results from the quantization of Fourier modes; we
hope that this understanding dispels any lingering doubts about the reality of
cosmological vacuum energy.Comment: 28 pages, 16 figures. Special volume in honor of J. S. Dowke
Detection of SUSY Signals in Stau Neutralino Co-annihilation Region at the LHC
We study the prospects of detecting the signal in the stau neutralino
co-annihilation region at the LHC using tau leptons. The co-annihilation signal
is characterized by the stau and neutralino mass difference (dM) to be 5-15 GeV
to be consistent with the WMAP measurement of the cold dark matter relic
density as well as all other experimental bounds within the minimal
supergravity model. Focusing on tau's from neutralino_2 --> tau stau --> tau
tau neutralino_1 decays in gluino and squark production, we consider inclusive
MET+jet+3tau production, with two tau's above a high E_T threshold and a third
tau above a lower threshold. Two observables, the number of opposite-signed tau
pairs minus the number of like-signed tau pairs and the peak position of the
di-tau invariant mass distribution, allow for the simultaneous determination of
dM and M_gluino. For dM = 9 GeV and M_gluino = 850 GeV with 30 fb^-1 of data,
we can measure dM to 15% and M_gluino to 6%.Comment: 4 pages LaTex, 3 figures. To appear in Proceedings of SUSY06, the
14th International Conference on Supersymmetry and the Unification of
Fundamental Interactions, UC Irvine, California, 12-17 June 2006. A typo in a
reference is correcte
Symplectic and orthogonal Lie algebra technology for bosonic and fermionic oscillator models of integrable systems
To provide tools, especially L-operators, for use in studies of rational
Yang-Baxter algebras and quantum integrable models when the Lie algebras so(N)
(b_n, d_n) or sp(2n) (c_n) are the invariance algebras of their R matrices,
this paper develops a presentation of these Lie algebras convenient for the
context, and derives many properties of the matrices of their defining
representations and of the ad-invariant tensors that enter their multiplication
laws. Metaplectic-type representations of sp(2n) and so(N) on bosonic and on
fermionic Fock spaces respectively are constructed. Concise general expressions
(see (5.2) and (5.5) below) for their L-operators are obtained, and used to
derive simple formulas for the T operators of the rational RTT algebra of the
associated integral systems, thereby enabling their efficient treatment by
means of the algebraic Bethe ansatz.Comment: 24 pages, LaTe
Effects of drinking-water filtration on Cryptosporidium Seroepidemiology, Scotland
Continuous exposure to low levels of Cryptosporidium oocysts is associated with production of protective antibodies. We investigated prevalence of antibodies against the 27-kDa Cryptosporidium oocyst antigen among blood donors in 2 areas of Scotland supplied by drinking water from different sources with different filtration standards: Glasgow (not filtered) and Dundee (filtered). During 2006–2009, seroprevalence and risk factor data were collected; this period includes 2007, when enhanced filtration was introduced to the Glasgow supply. A serologic response to the 27-kDa antigen was found for ≈75% of donors in the 2 cohorts combined. Mixed regression modeling indicated a 32% step-change reduction in seroprevalence of antibodies against Cryptosporidium among persons in the Glasgow area, which was associated with introduction of enhanced filtration treatment. Removal of Cryptosporidium oocysts from water reduces the risk for waterborne exposure, sporadic infections, and outbreaks. Paradoxically, however, oocyst removal might lower immunity and increase the risk for infection from other sources
Antechamber facilitates loading and unloading of vacuum furnace
Antechamber facilitates the use of a furnace in which materials are heat treated in a high vacuum or a gas atmosphere. It has a high vacuum pumping system, a means for backfilling with a selected gas, an access door, glove ports, and a motor driven platform
Comment on ''the controlled charge ordering and evidence of the metallic state in PrCaMnO films''
In a recent paper (2000 \QTR{it}{J. Phys.: Condens. Matter} \QTR{bf}{12}
L133) Lee \QTR{it}{et al.} have studied the transport properties of
PrCaMnO thin films. They claimed that they are able to
controlled the charge-ordered (CO) state by the lattice strains. We propose
herein another alternative since another indexation of the orientation of the
film can be found leading to almost no distortion of the cell, as compared to
the bulk compound.Comment: 2 page
Identifying "communities" within energy landscapes
Potential energy landscapes can be represented as a network of minima linked
by transition states. The community structure of such networks has been
obtained for a series of small Lennard-Jones clusters. This community structure
is compared to the concept of funnels in the potential energy landscape. Two
existing algorithms have been used to find community structure, one involving
removing edges with high betweenness, the other involving optimization of the
modularity. The definition of the modularity has been refined, making it more
appropriate for networks such as these where multiple edges and
self-connections are not included. The optimization algorithm has also been
improved, using Monte Carlo methods with simulated annealing and basin hopping,
both often used successfully in other optimization problems. In addition to the
small clusters, two examples with known heterogeneous landscapes, LJ_13 with
one labelled atom and LJ_38, were studied with this approach. The network
methods found communities that are comparable to those expected from landscape
analyses. This is particularly interesting since the network model does not
take any barrier heights or energies of minima into account. For comparison,
the network associated with a two-dimensional hexagonal lattice is also studied
and is found to have high modularity, thus raising some questions about the
interpretation of the community structure associated with such partitions.Comment: 13 pages, 11 figure
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