1,951 research outputs found
An economic geography of the United States: from commutes to megaregions
The emergence in the United States of large-scale “megaregions” centered on major
metropolitan areas is a phenomenon often taken for granted in both scholarly studies
and popular accounts of contemporary economic geography. This paper uses a data set
of more than 4,000,000 commuter flows as the basis for an empirical approach to the
identification of such megaregions. We compare a method which uses a visual heuristic
for understanding areal aggregation to a method which uses a computational
partitioning algorithm, and we reflect upon the strengths and limitations of both. We
discuss how choices about input parameters and scale of analysis can lead to different
results, and stress the importance of comparing computational results with “common
sense” interpretations of geographic coherence. The results provide a new perspective
on the functional economic geography of the United States from a megaregion
perspective, and shed light on the old geographic problem of the division of space into
areal units
Data showing regional differences in rat brain monoaminergic function
Chemical neurotransmitters (such as dopamine) modulate cognitive function via ascending projections to various cortical and sub-cortical brain regions. This report describes and links to a relatively large dataset (up to N=112) compiled from control (untreated) brain samples taken during a series of experimental in vivo studies. The dataset is freely available, to explore the normal interrelationships between levels of neurotransmitter (e.g., dopamine, serotonin), across brain regions implicated in both normal reward and drug addiction, as well as in disorders such as schizophrenia (e.g., nucleus accumbens, frontal cortex). Most experimental studies run with a relatively small control group, so there is a lack of baseline data on the expected levels of neurotransmitters and their metabolites in different brain regions. Accordingly, the available dataset has been compiled from a number of studies run in the same laboratory, and using closely similar behavioural procedures, sampling selected brain regions of a priori interest. These collated data can be used to explore differences in the distribution of the monoamines and their metabolites, patterns of neurotransmitter intercorrelations, both between and within different brain structures and including some consideration of laterality effects
Resonances and the thermonuclear reaction rate
We present an approximate analytic expression for thermonuclear reaction rate
of charged particles when the cross section contains a single narrow or wide
resonance described by a Breit-Wigner shape. The resulting expression is
uniformly valid as the effective energy and resonance energy coalesce. We use
our expressions to calculate the reaction rate for
C(p,)N.Comment: 4 pages, 1 figure, presented at the VIII International Conference on
Nucleus-Nucleus in Moscow (Russia) on June 17-21, 200
The mass insertion approximation without squark degeneracy
We study the applicability of the mass insertion approximation (MIA) for
calculations of neutral meson mixing when squark masses are not degenerate and,
in particular, in models of alignment. We show that the MIA can give results
that are much better than an order of magnitude estimate as long as the masses
are not strongly hierarchical. We argue that, in an effective two-squark
framework, m_q=(m_1+m_2)/2 is the best choice for the MIA expansion point,
rather than, for example, m_q^2=(m_1^2+m_2^2)/2.Comment: 7 pages, revtex
Hexatic-Herringbone Coupling at the Hexatic Transition in Smectic Liquid Crystals: 4- Renormalization Group Calculations Revisited
Simple symmetry considerations would suggest that the transition from the
smectic-A phase to the long-range bond orientationally ordered hexatic
smectic-B phase should belong to the XY universality class. However, a number
of experimental studies have constantly reported over the past twenty years
"novel" critical behavior with non-XY critical exponents for this transition.
Bruinsma and Aeppli argued in Physical Review Letters {\bf 48}, 1625 (1982),
using a renormalization-group calculation, that short-range
molecular herringbone correlations coupled to the hexatic ordering drive this
transition first order via thermal fluctuations, and that the critical behavior
observed in real systems is controlled by a `nearby' tricritical point. We have
revisited the model of Bruinsma and Aeppli and present here the results of our
study. We have found two nontrivial strongly-coupled herringbone-hexatic fixed
points apparently missed by those authors. Yet, those two new nontrivial
fixed-points are unstable, and we obtain the same final conclusion as the one
reached by Bruinsma and Aeppli, namely that of a fluctuation-driven first order
transition. We also discuss the effect of local two-fold distortion of the bond
order as a possible missing order parameter in the Hamiltonian.Comment: 1 B/W eps figure included. Submitted to Physical Review E. Contact:
[email protected]
Unbounded representations of -deformation of Cuntz algebra
We study a deformation of the Cuntz-Toeplitz -algebra determined by the
relations . We define well-behaved unbounded
*-representations of the *-algebra defined by relations above and classify all
such irreducible representations up to unitary equivalence.Comment: 13 pages, Submitted to Lett. Math. Phy
Hydrodynamics of Spatially Ordered Superfluids
We derive the hydrodynamic equations for the supersolid and superhexatic
phases of a neutral two-dimensional Bose fluid. We find, assuming that the
normal part of the fluid is clamped to an underlying substrate, that both
phases can sustain third-sound modes and that in the supersolid phase there are
additional modes due to the superfluid motion of point defects (vacancies and
interstitials).Comment: 24 pages of ReVTeX and 7 uuencoded figures. Submitted for publication
in Phys. Rev.
Sparticle masses in deflected mirage mediation
We discuss the sparticle mass patterns that can be realized in deflected
mirage mediation scenario of supersymmetry breaking, in which the moduli,
anomaly, and gauge mediations all contribute to the MSSM soft parameters.
Analytic expression of low energy soft parameters and also the sfermion mass
sum rules are derived, which can be used to interpret the experimentally
measured sparticle masses within the framework of the most general mixed
moduli-gauge-anomaly mediation. Phenomenological aspects of some specific
examples are also discussed.Comment: 43 pages, 17 figures, references adde
Simultaneous Diagonal and Off Diagonal Order in the Bose--Hubbard Hamiltonian
The Bose-Hubbard model exhibits a rich phase diagram consisting both of
insulating regimes where diagonal long range (solid) order dominates as well as
conducting regimes where off diagonal long range order (superfluidity) is
present. In this paper we describe the results of Quantum Monte Carlo
calculations of the phase diagram, both for the hard and soft core cases, with
a particular focus on the possibility of simultaneous superfluid and solid
order. We also discuss the appearance of phase separation in the model. The
simulations are compared with analytic calculations of the phase diagram and
spin wave dispersion.Comment: 28 pages plus 24 figures, uuencoded Revtex+postscript file
Classical Vs Quantum Probability in Sequential Measurements
We demonstrate in this paper that the probabilities for sequential
measurements have features very different from those of single-time
measurements. First, they cannot be modelled by a classical stochastic process.
Second, they are contextual, namely they depend strongly on the specific
measurement scheme through which they are determined. We construct
Positive-Operator-Valued measures (POVM) that provide such probabilities. For
observables with continuous spectrum, the constructed POVMs depend strongly on
the resolution of the measurement device, a conclusion that persists even if we
consider a quantum mechanical measurement device or the presence of an
environment. We then examine the same issues in alternative interpretations of
quantum theory. We first show that multi-time probabilities cannot be naturally
defined in terms of a frequency operator. We next prove that local hidden
variable theories cannot reproduce the predictions of quantum theory for
sequential measurements, even when the degrees of freedom of the measuring
apparatus are taken into account. Bohmian mechanics, however, does not fall in
this category. We finally examine an alternative proposal that sequential
measurements can be modelled by a process that does not satisfy the Kolmogorov
axioms of probability. This removes contextuality without introducing
non-locality, but implies that the empirical probabilities cannot be always
defined (the event frequencies do not converge). We argue that the predictions
of this hypothesis are not ruled out by existing experimental results
(examining in particular the "which way" experiments); they are, however,
distinguishable in principle.Comment: 56 pages, latex; revised and restructured. Version to appear in
Found. Phy
- …