9,441 research outputs found
Why Is Law Enforcement Decentralized?
Law enforcement is decentralized. It is so despite documented interjurisdictional externalities which would justify its centralization. To explain this fact, we construct a political economy model of law enforcement. Under decentralization, law enforcement in each region is in accord with the preferences of regional citizens, but interjurisdictional externalities are neglected. Under centralization, law enforcement for all regions is chosen by a legislature of regional representatives which may take externalities into account. However, the majority rule applies for decisions made by the central legislature and this implies that the allocation of enforcement resources may be skewed in favour of those who belong to the required majority. We show that the choice between centralization and decentralization depends on the technology of law enforcement and the nature of the interjurisdictional externalities.Crime, Law enforcement, Decentralization, Externalities
Magnetic responses of randomly depleted spin ladders
The magnetic responses of a spin-1/2 ladder doped with non-magnetic
impurities are studied using various methods and including the regime where
frustration induces incommensurability. Several improvements are made on the
results of the seminal work of Sigrist and Furusaki [J. Phys. Soc. Jpn. 65,
2385 (1996)]. Deviations from the Brillouin magnetic curve due to interactions
are also analyzed. First, the magnetic profile around a single impurity and
effective interactions between impurities are analyzed within the bond-operator
mean-field theory and compared to density-matrix renormalization group
calculations. Then, the temperature behavior of the Curie constant is studied
in details. At zero-temperature, we give doping-dependent corrections to the
results of Sigrist and Furusaki on general bipartite lattice and compute
exactly the distribution of ladder cluster due to chain breaking effects. Using
exact diagonalization and quantum Monte-Carlo methods on the effective model,
the temperature dependence of the Curie constant is compared to a random dimer
model and a real-space renormalization group scenario. Next, the low-part of
the magnetic curve corresponding to the contribution of impurities is computed
using exact diagonalization. The random dimer model is shown to capture the
bulk of the curve, accounting for the deviation from the Brillouin response. At
zero-temperature, the effective model prediction agrees relatively well with
density-matrix renormalization group calculations. Finite-temperature effects
are displayed within the effective model and for large depleted ladder models
using quantum Monte-Carlo simulations. In all, the effect of incommensurability
does not display a strong qualitative effect on both the magnetic
susceptibility and the magnetic curve. Consequences for experiments on the
BiCu2PO6 compound and other spin-gapped materials are briefly discussed.Comment: 24 pages, 20 figure
Mixed lipid bilayers with locally varying spontaneous curvature and bending
A model of lipid bilayers made of a mixture of two lipids with different
average compositions on both leaflets, is developed. A Landau hamiltonian
describing the lipid-lipid interactions on each leaflet, with two lipidic
fields and , is coupled to a Helfrich one, accounting for the
membrane elasticity, via both a local spontaneous curvature, which varies as
, and a bending modulus equal to
. This model allows us to define curved
patches as membrane domains where the asymmetry in composition,
, is large, and thick and stiff patches where is
large. These thick patches are good candidates for being lipidic rafts, as
observed in cell membranes, which are composed primarily of saturated lipids
forming a liquid-ordered domain and are known to be thick and flat
nano-domains. The lipid-lipid structure factors and correlation functions are
computed for globally spherical membranes and planar ones. Phase diagrams are
established, within a Gaussian approximation, showing the occurrence of two
types of Structure Disordered phases, with correlations between either curved
or thick patches, and an Ordered phase, corresponding to the divergence of the
structure factor at a finite wave vector. The varying bending modulus plays a
central role for curved membranes, where the driving force is
balanced by the line tension, to form raft domains of size ranging from 10 to
100~nm. For planar membranes, raft domains emerge via the cross-correlation
with curved domains. A global picture emerges from curvature-induced
mechanisms, described in the literature for planar membranes, to coupled
curvature- and bending-induced mechanisms in curved membranes forming a closed
vesicle
Melting of a frustration-induced dimer crystal and incommensurability in the J_1-J_2 two-leg ladder
The phase diagram of an antiferromagnetic ladder with frustrating
next-nearest neighbor couplings along the legs is determined using numerical
methods (exact diagonalization and density-matrix renormalization group)
supplemented by strong-coupling and mean-field analysis. Interestingly, this
model displays remarkable features, bridging the physics of the J_1-J_2 chain
and of the unfrustated ladder. The phase diagram as a function of the
transverse coupling J_{\perp} and the frustration J_2 exhibits an Ising
transition between a columnar phase of dimers and the usual rung-singlet phase
of two-leg ladders. The transition is driven by resonating valence bond
fluctuations in the singlet sector while the triplet spin gap remains finite
across the transition. In addition, frustration brings incommensurability in
the real-space spin correlation functions, the onset of which evolves smoothly
from the J_1-J_2 chain value to zero in the large-J_{\perp} limit. The onset of
incommensurability in the spin structure-factor and in the dispersion relation
is also analyzed. The physics of the frustrated rung-singlet phase is well
understood using perturbative expansions and mean-field theories in the
large-J_{\perp} limit. Lastly, we discuss the effect of the non-trivial magnon
dispersion relation on the thermodynamical properties of the system. The
relation of this model and its physics to experimental observations on
compounds which are currently investigated, such as BiCu_2PO_6, is eventually
addressed.Comment: 13 pages, 13 figure
Identifying Fiscal Policy (In)effectiveness from the Differential Counter-Cyclicality of Government Spending in the Interwar Period
Differences across decades in the counter-cyclical stance of fiscal policy can identify whether the growth in government spending affects output growth and so speeds recovery from a recession. We study government-spending reaction functions from the 1920s and 1930s for twenty countries. There are two main findings. First, surprisingly, government spending was less counter-cyclical in the 1930s than in the 1920s. Second, the growth of government spending did not have a significant effect on output growth, so that there is little evidence that this feature of fiscal policy played a stabilizing role in the interwar period.fiscal policy, business-cycle history, Great Depression, interwar period
Phase diagram of hard-core bosons on clean and disordered 2-leg ladders: Mott insulator - Luttinger liquid - Bose glass
One dimensional free-fermions and hard-core bosons are often considered to be
equivalent. Indeed, when restricted to nearest-neighbor hopping on a chain the
particles cannot exchange themselves, and therefore hardly experience their own
statistics. Apart from the off-diagonal correlations which depends on the
so-called Jordan-Wigner string, real-space observables are similar for
free-fermions and hard-core bosons on a chain. Interestingly, by coupling only
two chains, thus forming a two-leg ladder, particle exchange becomes allowed,
and leads to a totally different physics between free-fermions and hard-core
bosons. Using a combination of analytical (strong coupling, field theory,
renormalization group) and numerical (quantum Monte Carlo, density-matrix
renormalization group) approaches, we study the apparently simple but
non-trivial model of hard-core bosons hopping in a two-leg ladder geometry. At
half-filling, while a band insulator appears for fermions at large interchain
hopping tperp >2t only, a Mott gap opens up for bosons as soon as tperp\neq0
through a Kosterlitz-Thouless transition. Away from half-filling, the situation
is even more interesting since a gapless Luttinger liquid mode emerges in the
symmetric sector with a non-trivial filling-dependent Luttinger parameter
1/2\leq Ks \leq 1. Consequences for experiments in cold atoms, spin ladders in
a magnetic field, as well as disorder effects are discussed. In particular, a
quantum phase transition is expected at finite disorder strength between a 1D
superfluid and an insulating Bose glass phase.Comment: 24 pages, 23 figure
Bulking II: Classifications of Cellular Automata
This paper is the second part of a series of two papers dealing with bulking:
a way to define quasi-order on cellular automata by comparing space-time
diagrams up to rescaling. In the present paper, we introduce three notions of
simulation between cellular automata and study the quasi-order structures
induced by these simulation relations on the whole set of cellular automata.
Various aspects of these quasi-orders are considered (induced equivalence
relations, maximum elements, induced orders, etc) providing several formal
tools allowing to classify cellular automata
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