2,740 research outputs found
Lorentz Invariant Superluminal Tunneling
It is shown that superluminal optical signalling is possible without
violating Lorentz invariance and causality via tunneling through photonic band
gaps in inhomogeneous dielectrics of a special kind.Comment: 10 pages revtex, no figure, more discussions added, submitted to
Phys. Rev.
Political opportunism and countercyclical fiscal policy in election-year recessions
Political budget cycles (PBCs) have been well documented in the literature, albeit not for all circumstances. Similarly, there is clear evidence on the positive effect of economic growth on electoral success. However, no work has been done on the impact of economic growth on the magnitude of PBCs. The theoretical model argues that a government has an incentive to increase fiscal manipulations when a recession is expected to hit and curtail reelection chances; this amounts to countercyclical policy for opportunistic rather than Keynesian motives. Very robust evidence for this behavior is found in Portuguese municipalities; in election years, budget deficits go up even more and significantly so, when a recession is expected.Fundação para a Ciência e Tecnologia (FCT
Tunneling dynamics in relativistic and nonrelativistic wave equations
We obtain the solution of a relativistic wave equation and compare it with
the solution of the Schroedinger equation for a source with a sharp onset and
excitation frequencies below cut-off. A scaling of position and time reduces to
a single case all the (below cut-off) nonrelativistic solutions, but no such
simplification holds for the relativistic equation, so that qualitatively
different ``shallow'' and ``deep'' tunneling regimes may be identified
relativistically. The nonrelativistic forerunner at a position beyond the
penetration length of the asymptotic stationary wave does not tunnel;
nevertheless, it arrives at the traversal (semiclassical or
B\"uttiker-Landauer) time "tau". The corresponding relativistic forerunner is
more complex: it oscillates due to the interference between two saddle point
contributions, and may be characterized by two times for the arrival of the
maxima of lower and upper envelops. There is in addition an earlier
relativistic forerunner, right after the causal front, which does tunnel.
Within the penetration length, tunneling is more robust for the precursors of
the relativistic equation
One-neutron knockout from Ni
The single-particle structure of Ni and level structure of Ni
were investigated with the \mbox{Be (Ni,Ni+)} reaction at 73 MeV/nucleon. An inclusive cross
section of 41.4(12) mb was obtained for the reaction, compared to a theoretical
prediction of 85.4 mb, hence only 48(2)% of the theoretical cross section is
exhausted. This reduction in the observed spectroscopic strength is consistent
with that found for lighter well-bound nuclei. One-neutron removal
spectroscopic factors of 0.58(11) to the ground state and 3.7(2) to all excited
states of Ni were deduced.Comment: Phys. Rev. C, accepte
Negative phase time for Scattering at Quantum Wells: A Microwave Analogy Experiment
If a quantum mechanical particle is scattered by a potential well, the wave
function of the particle can propagate with negative phase time. Due to the
analogy of the Schr\"odinger and the Helmholtz equation this phenomenon is
expected to be observable for electromagnetic wave propagation. Experimental
data of electromagnetic wells realized by wave guides filled with different
dielectrics confirm this conjecture now.Comment: 10 pages, 6 figure
Transforming teacher education, an activity theory analysis
This paper explores the work of teacher education in England and Scotland. It seeks to locate this work within conflicting socio-cultural views of professional practice and academic work. Drawing on an activity theory framework that integrates the analysis of these contradictory discourses with a study of teacher educators’ practical activities, including the material artefacts that mediate the work, the paper offers a critical perspective on the social organisation of university-based teacher education. Informed by Engeström’s activity theory concept of transformation, the paper extends the discussion of contradictions in teacher education to consider the wider socio-cultural relations of the work. The findings raise important questions about the way in which teacher education work within universities is organised and the division of labour between schools and universities
Rotational states in deformed nuclei: An analytic approach
The consequences of the spontaneous breaking of rotational symmetry are
investigated in a field theory model for deformed nuclei, based on simple
separable interactions. The crucial role of the Ward-Takahashi identities to
describe the rotational states is emphasized. We show explicitly how the rotor
picture emerges from the isoscalar Goldstone modes, and how the two-rotor model
emerges from the isovector scissors modes. As an application of the formalism,
we discuss the M1 sum rules in deformed nuclei, and make connection to
empirical information.Comment: 19 pages, 9 figure
Negative group delay for Dirac particles traveling through a potential well
The properties of group delay for Dirac particles traveling through a
potential well are investigated. A necessary condition is put forward for the
group delay to be negative. It is shown that this negative group delay is
closely related to its anomalous dependence on the width of the potential well.
In order to demonstrate the validity of stationary-phase approach, numerical
simulations are made for Gaussian-shaped temporal wave packets. A restriction
to the potential-well's width is obtained that is necessary for the wave packet
to remain distortionless in the travelling. Numerical comparison shows that the
relativistic group delay is larger than its corresponding non-relativistic one.Comment: 10 pages, 5 figure
Formal matched asymptotics for degenerate Ricci flow neckpinches
Gu and Zhu have shown that Type-II Ricci flow singularities develop from
nongeneric rotationally symmetric Riemannian metrics on , for all . In this paper, we describe and provide plausibility arguments for a
detailed asymptotic profile and rate of curvature blow-up that we predict such
solutions exhibit
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