9,601 research outputs found
Poly-MTO, {(CH_3)_{0.92} Re O_3}_\infty, a Conducting Two-Dimensional Organometallic Oxide
Polymeric methyltrioxorhenium, {(CH_{3})_{0.92}ReO_{3}}_{\infty} (poly-MTO),
is the first member of a new class of organometallic hybrids which adopts the
structural pattern and physical properties of classical perovskites in two
dimensions (2D). We demonstrate how the electronic structure of poly-MTO can be
tailored by intercalation of organic donor molecules, such as
tetrathiafulvalene (TTF) or bis-(ethylendithio)-tetrathiafulvalene (BEDT-TTF),
and by the inorganic acceptor SbF. Integration of donor molecules leads to
a more insulating behavior of poly-MTO, whereas SbF insertion does not
cause any significant change in the resistivity. The resistivity data of pure
poly-MTO is remarkably well described by a two-dimensional electron system.
Below 38 K an unusual resistivity behavior, similar to that found in doped
cuprates, is observed: The resistivity initially increases approximately as
ln) before it changes into a dependence below 2 K.
As an explanation we suggest a crossover from purely two-dimensional
charge-carrier diffusion within the \{ReO\} planes at high
temperatures to three-dimensional diffusion at low temperatures in a
disorder-enhanced electron-electron interaction scenario (Altshuler-Aronov
correction). Furthermore, a linear positive magnetoresistance was found in the
insulating regime, which is caused by spatial localization of itinerant
electrons at some of the Re atoms, which formally adopt a electronic
configuration. X-ray diffraction, IR- and ESR-studies, temperature dependent
magnetization and specific heat measurements in various magnetic fields suggest
that the electronic structure of poly-MTO can safely be approximated by a
purely 2D conductor.Comment: 15 pages, 16 figures, 2 table
Subsidizing Religious Participation through Groups: A Model of the âMegachurchâ Strategy for Growth
Either despite or because of their non-traditional approach, megachurches have grown significantly in the United States since 1980. This paper models religious participation as an imperfect public good which, absent intervention, yields suboptimal participation by members from the churchâs perspective. Megachurches address this problem in part by employing secular-based group activities to subsidize religious participation that then translates into an increase in the attendeesâ religious investment. This strategy not only allows megachurches to attract and retain new members when many traditional churches are losing members but also results in higher levels of an individualâs religious capital. As a result, the megachurch may raise expectations of membersâ levels of commitment and faith practices. Data from the FACT2000 survey provide evidence that megachurches employ groups more extensively than other churches, and this approach is consistent with a strategy to use groups to help subsidize individualsâ religious investment. Religious capital rises among members of megachurches relative to members of non-megachurches as a result of this strategy
Range corrections for two-neutron halo nuclei in effective theory
The range corrections to the universal properties and structure of
two-neutron halo nuclei are investigated within an effective quantum mechanics
framework. Treating the nucleus as an effective three-body system, we make a
systematic improvement upon previous calculations by calculating the linear
range corrections at next-to-leading order. Since the effective ranges for the
neutron-core interactions are not known, we estimate the effective range to be
set by the inverse of the pion mass. We investigate the possibility of excited
Efimov states in two-neutron halo nuclei and calculate their mean square radii
to next-to-leading order. We find that the effective range corrections are
generally small and the leading order predictions are very robust.Comment: 19 pages, 4 eps figures, revtex4, final version to appear in Nucl.
Phys.
Slowing and cooling molecules and neutral atoms by time-varying electric field gradients
A method of slowing, accelerating, cooling, and bunching molecules and
neutral atoms using time-varying electric field gradients is demonstrated with
cesium atoms in a fountain. The effects are measured and found to be in
agreement with calculation. Time-varying electric field gradient slowing and
cooling is applicable to atoms that have large dipole polarizabilities,
including atoms that are not amenable to laser slowing and cooling, to Rydberg
atoms, and to molecules, especially polar molecules with large electric dipole
moments. The possible applications of this method include slowing and cooling
thermal beams of atoms and molecules, launching cold atoms from a trap into a
fountain, and measuring atomic dipole polarizabilities.Comment: 13 pages, 10 figures. Scheduled for publication in Nov. 1 Phys. Rev.
Time as an operator/observable in nonrelativistic quantum mechanics
The nonrelativistic Schroedinger equation for motion of a structureless
particle in four-dimensional space-time entails a well-known expression for the
conserved four-vector field of local probability density and current that are
associated with a quantum state solution to the equation. Under the physical
assumption that each spatial, as well as the temporal, component of this
current is observable, the position in time becomes an operator and an
observable in that the weighted average value of the time of the particle's
crossing of a complete hyperplane can be simply defined: ... When the
space-time coordinates are (t,x,y,z), the paper analyzes in detail the case
that the hyperplane is of the type z=constant. Particles can cross such a
hyperplane in either direction, so it proves convenient to introduce an
indefinite metric, and correspondingly a sesquilinear inner product with
non-Hilbert space structure, for the space of quantum states on such a surface.
>... A detailed formalism for computing average crossing times on a z=constant
hyperplane, and average dwell times and delay times for a zone of interaction
between a pair of z=constant hyperplanes, is presented.Comment: 31 pages, no figures. Differs from published version by minor
corrections and additions, and two citation
One at a time, live tracking of NGF axonal transport using quantum dots
Retrograde axonal transport of nerve growth factor (NGF) signals is critical for the survival, differentiation, and maintenance of peripheral sympathetic and sensory neurons and basal forebrain cholinergic neurons. However, the mechanisms by which the NGF signal is propagated from the axon terminal to the cell body are yet to be fully elucidated. To gain insight into the mechanisms, we used quantum dot-labeled NGF (QD-NGF) to track the movement of NGF in real time in compartmentalized culture of rat dorsal root ganglion (DRG) neurons. Our studies showed that active transport of NGF within the axons was characterized by rapid, unidirectional movements interrupted by frequent pauses. Almost all movements were retrograde, but short-distance anterograde movements were occasionally observed. Surprisingly, quantitative analysis at the single molecule level demonstrated that the majority of NGF-containing endosomes contained only a single NGF dimer. Electron microscopic analysis of axonal vesicles carrying QD-NGF confirmed this finding. The majority of QD-NGF was found to localize in vesicles 50â150 nm in diameter with a single lumen and no visible intralumenal membranous components. Our findings point to the possibility that a single NGF dimer is sufficient to sustain signaling during retrograde axonal transport to the cell body
Orbital Magnetism in Ensembles of Parabolic Potentials
We study the magnetic susceptibility of an ensemble of non-interacting
electrons confined by parabolic potentials and subjected to a perpendicular
magnetic field at finite temperatures. We show that the behavior of the average
susceptibility is qualitatively different from that of billiards. When averaged
over the Fermi energy the susceptibility exhibits a large paramagnetic response
only at certain special field values, corresponding to comensurate classical
frequencies, being negligible elsewhere. We derive approximate analytical
formulae for the susceptibility and compare the results with numerical
calculations.Comment: 4 pages, 4 figures, REVTE
Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites
The effect of an exchange field on electrical transport in thin films of
metallic ferromagnetic manganites has been investigated. The exchange field was
induced both by direct exchange coupling in a ferromagnet/antiferromagnet
multilayer and by indirect exchange interaction in a ferromagnet/paramagnet
superlattice. The electrical resistance of the manganite layers was found to be
determined by the absolute value of the vector sum of the effective exchange
field and the external magnetic field.Comment: 5 pages, 4 figure
X-ray Bright Active Galactic Nuclei in Massive Galaxy Clusters I: Number Counts and Spatial Distribution
We present an analysis of the X-ray bright point source population in 43
massive clusters of galaxies observed with the Chandra X-ray Observatory. We
have constructed a catalog of 4210 rigorously selected X-ray point sources in
these fields, which span a survey area of 4.2 square degrees. This catalog
reveals a clear excess of sources when compared to deep blank-field surveys,
which amounts to roughly 1 additional source per cluster, likely Active
Galactic Nuclei (AGN) associated with the clusters. The excess sources are
concentrated within the virial radii of the clusters, with the largest excess
observed near the cluster centers. The average radial profile of the excess
X-ray sources of the cluster are well described by a power law (N(r) ~ r^\beta)
with an index of \beta ~ -0.5. An initial analysis using literature results on
the mean profile of member galaxies in massive X-ray selected clusters
indicates that the fraction of galaxies hosting X-ray AGN rises with increasing
clustercentric radius, being approximately 5 to 10 times higher near the virial
radius than in the central regions. This trend is qualitatively similar to that
observed for star formation in cluster member galaxies.Comment: 18 Pages, 10 Figures, Submitted to MNRAS. Please contact Steven
Ehlert ([email protected]) for higher resolution figures. Updated to
reflect small changes requested by referee. This version has been accepted
into MNRA
- âŠ