3,440 research outputs found
How Participation Creates Citizens: Participatory Governance as Performative Practice
Participation is a prominent feature of many decision-making and planning processes. Among its proclaimed benefits is its potential to strengthen public support and involvement. However, participation is also known for having unintended consequences which lead to failures in meeting its objectives. This article takes a critical perspective on participation by discussing how participation may influence the ways in which citizens can become involved. Participation unavoidably involves (1) restrictions about who should be involved and about the space for negotiation, (2) assumptions about what the issue at stake is, and (3) expectations about what the outcome of participation should be and how the participants are expected to behave. This is illustrated by a case study about the Dutch nature area, the Drentsche Aa. The case study demonstrates how the participatory process that took place and the restrictions, assumptions, and expectations that were involved resulted in six forms of citizen involvement, both intended and unintended, which ranged between creativity, passivity, and entrenchment. Based on these findings, the article argues that participation does not merely serve as a neutral place in which citizens are represented, but instead creates different categories of citizens. Recognizing this means reconceiving participation as performative practice. Such a perspective goes beyond overly optimistic views of participation as a technique whose application can be perfected, as well as pessimistic views of participation as repression or domination. Instead, it appreciates both intended and unintended forms of citizen involvement as meaningful and legitimate, and recognizes citizenship as being constituted in interaction in the context of participatio
Nonequilibrium time evolution of the spectral function in quantum field theory
Transport or kinetic equations are often derived assuming a quasi-particle
(on-shell) representation of the spectral function. We investigate this
assumption using a three-loop approximation of the 2PI effective action in real
time, without a gradient expansion or on-shell approximation. For a scalar
field in 1+1 dimensions the nonlinear evolution, including the integration over
memory kernels, can be solved numerically. We find that a spectral function
approximately described by a nonzero width emerges dynamically. During the
nonequilibrium time evolution the Wigner transformed spectral function is
slowly varying, even in presence of strong qualitative changes in the effective
particle distribution. These results may be used to make further analytical
progress towards a quantum Boltzmann equation including off-shell effects and a
nonzero width.Comment: 20 pages with 6 eps figures, explanation and references added; to
appear in Phys.Rev.
Colossal proximity effect in a superconducting triplet spin valve based on halfmetallic ferromagnetic CrO2
Ferromagnets can sustain supercurrents through the formation of equal spin
triplet Cooper pairs and the mechanism of odd-frequency pairing. Since such
pairs are not broken by the exchange energy of the ferromagnet, superconducting
triplet correlations are long-ranged and spin-polarized, with promises for
superconducting spintronics devices. The main challenge is to understand how
triplets are generated at the superconductor (S)/ ferromagnet (F) interface.
Here we use the concept of a so-called triplet spin valve (TSV) to investigate
the conversion of singlets in a conventional superconductor to triplets in the
halfmetallic ferromagnet CrO_2. TSV's are composed of two ferromagnetic layers
(separated by a thin normal metal (N) layer) and a superconductor
(F_1/N/F_2/S). The package F_1/N/F_2 generates triplets in F_1 when the
magnetization directions of the F_{1,2}-layers are not collinear. This drains
singlet pairs from the S-layer, and triplet generation is therefore signalled
by a decrease of the critical temperature . Recently, experiments with
TSV's were reported with Co draining layers, using in-plane fields, and finding
T_c-shifts up to 100~mK. Using CrO_2 instead of Co and rotating a magnetic
field from in-plane to out-of-plane, we find strong T_c variations of almost a
Kelvin up to fields of the order of a Tesla. Such strong drainage is consistent
with the large lengths over which supercurrents can flow in CrO_2, which are
significantly larger than in conventional ferromagnets. Our results point to
the special interest of halfmetals for superconducting spintronics.Comment: 6 pages, 5 figures; supplementary information separat
Spin dynamics in a superconductor / ferromagnet proximity system
The ferromagnetic resonance of thin sputtered Ni80Fe20 films grown on Nb is
measured. By varying the temperature and thickness of the Nb the role of the
superconductivity on the whole ferromagnetic layer in these heterostructures is
explored. The change in the spin transport properties below the superconducting
transition of the Nb is found to manifest itself in the Ni80Fe20 layer by a
sharpening in the resonance of the ferromagnet, or a decrease in the effective
Gilbert damping co-efficient. This dynamic proximity effect is in contrast to
low frequency studies in these systems, where the effect of the superconductor
is confined to a small region in the ferromagnet. We interpret this in terms of
the spin pumping model.Comment: 4 pages, 4 figures, to be submitted for publicatio
Ward Identities for the 2PI effective action in QED
We study the issue of symmetries and associated Ward-like identities in the
context of two-particle-irreducible (2PI) functional techniques for abelian
gauge theories. In the 2PI framework, the -point proper vertices of the
theory can be obtained in various different ways which, although equivalent in
the exact theory, differ in general at finite approximation order. We derive
generalized (2PI) Ward identities for these various -point functions and
show that such identities are exactly satisfied at any approximation order in
2PI QED. In particular, we show that 2PI-resummed vertex functions, i.e.
field-derivatives of the so-called 2PI-resummed effective action, exactly
satisfy standard Ward identities. We identify another set of -point
functions in the 2PI framework which exactly satisfy the standard Ward
identities at any approximation order. These are obtained as field-derivatives
of the two-point function \bcG^{-1}[\phi], which defines the extremum of the
2PI effective action. We point out that the latter is not constrained by the
underlying symmetry. As a consequence, the well-known fact that the
corresponding gauge-field polarization tensor is not transverse in momentum
space for generic approximations does not constitute a violation of (2PI) Ward
identities. More generally, our analysis demonstrates that approximation
schemes based on 2PI functional techniques respect all the Ward identities
associated with the underlying abelian gauge symmetry. Our results apply to
arbitrary linearly realized global symmetries as well.Comment: 33 pages, 2 figure
Alcohol representations are socially situated: an investigation of beverage representations by using a property generation task
Previous research suggests that people's representations of alcoholic beverages play an important role in drinking behavior. However, relatively little is known about the contents of these representations. Here, we introduce the property generation task as a tool to explore these representations in detail. In a laboratory study (N = 110), and a bar field-study (N = 56), participants listed typical properties of alcoholic beverages, sugary beverages, and water. Each of these properties was then categorized using a previously developed, hierarchical coding scheme. For example, the property “sweet” was categorized as referring to “taste”, which falls under “sensory experience”, which falls under “consumption situation”. Afterwards, participants completed measures of drinking behavior and alcohol craving. Results showed that alcoholic beverages were strongly represented in terms of consumption situations, with 57% and 69% of properties relating to consumption in the laboratory and the bar study, respectively. Specifically, alcoholic beverages were more strongly represented in terms of the social context of consumption (e.g., “with friends”) than the other beverages. In addition, alcoholic beverages were strongly represented in terms of sensory experiences (e.g. “sweet”) and positive outcomes (e.g. “creates fun”), as were the sugary beverages and water. In Study 1, the extent to which alcoholic beverages were represented in terms of social context was positively associated with craving and regularly consuming alcohol. The property generation task provides a useful tool to access people's idiosyncratic representations of alcoholic beverages. This may further our understanding of drinking behavior, and help to tailor research and interventions to reduce drinking of alcoholic and other high-calorie beverages
Transport coefficients from the 2PI effective action
We show that the lowest nontrivial truncation of the two-particle irreducible
(2PI) effective action correctly determines transport coefficients in a weak
coupling or 1/N expansion at leading (logarithmic) order in several
relativistic field theories. In particular, we consider a single real scalar
field with cubic and quartic interactions in the loop expansion, the O(N) model
in the 2PI-1/N expansion, and QED with a single and many fermion fields.
Therefore, these truncations will provide a correct description, to leading
(logarithmic) order, of the long time behavior of these systems, i.e. the
approach to equilibrium. This supports the promising results obtained for the
dynamics of quantum fields out of equilibrium using 2PI effective action
techniques.Comment: 5 pages, explanation in introduction expanded, summary added; to
appear in PR
Shear Viscosity in the O(N) Model
We compute the shear viscosity in the O(N) model at first nontrivial order in
the large N expansion. The calculation is organized using the 1/N expansion of
the 2PI effective action (2PI-1/N expansion) to next-to-leading order, which
leads to an integral equation summing ladder and bubble diagrams. We also
consider the weakly coupled theory for arbitrary N, using the three-loop
expansion of the 2PI effective action. In the limit of weak coupling and
vanishing mass, we find an approximate analytical solution of the integral
equation. For general coupling and mass, the integral equation is solved
numerically using a variational approach. The shear viscosity turns out to be
close to the result obtained in the weak-coupling analysis.Comment: 37 pages, few typos corrected; to appear in JHE
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