2,008 research outputs found
On the two-magnon bound states for the quantum Heisenberg chain with variable range exchange
The spectrum of finite-difference two-magnon operator is investigated for
quantum S=1/2 chain with variable range exchange of the form . It is found that usual bound state appears for some values
of the total pseudomomentum of two magnons as for the Heisenberg chain with
nearest-neighbor spin interaction. Besides this state, a new type of bound
state with oscillating wave function appears at larger values of the total
pseudomomentum.Comment: 8 pages, latex, no figure
Spectral correlations in systems undergoing a transition from periodicity to disorder
We study the spectral statistics for extended yet finite quasi 1-d systems
which undergo a transition from periodicity to disorder. In particular we
compute the spectral two-point form factor, and the resulting expression
depends on the degree of disorder. It interpolates smoothly between the two
extreme limits -- the approach to Poissonian statistics in the (weakly)
disordered case, and the universal expressions derived for the periodic case.
The theoretical results agree very well with the spectral statistics obtained
numerically for chains of chaotic billiards and graphs.Comment: 16 pages, Late
Monitoring and Pay: An Experiment on Employee Performance under Endogenous Supervision
We present an experimental test of a shirking model where monitoring intensity is endogenous and effort a continuous variable. Wage level, monitoring intensity and consequently the desired enforceable effort level are jointly determined by the maximization problem of the firm. As a result, monitoring and pay should be complements. In our experiment, between and within treatment variation is qualitatively in line with the normative predictions of the model under standard assumptions. Yet, we also find evidence for reciprocal behavior. Our data analysis shows, however, that it does not pay for the employer to solely rely on the reciprocity of employees.incentive contracts; supervision; efficiency wages;experiment; incomplete contracts; reciprocity
Improved and Perfect Actions in Discrete Gravity
We consider the notion of improved and perfect actions within Regge calculus.
These actions are constructed in such a way that they - although being defined
on a triangulation - reproduce the continuum dynamics exactly, and therefore
capture the gauge symmetries of General Relativity. We construct the perfect
action in three dimensions with cosmological constant, and in four dimensions
for one simplex. We conclude with a discussion about Regge Calculus with curved
simplices, which arises naturally in this context.Comment: 28 pages, 2 figure
New relations between spinor and scalar one-loop effective Lagrangians in constant background fields
Simple new relations are presented between the one-loop effective Lagrangians
of spinor and scalar particles in constant curvature background fields, both
electromagentic and gravitational. These relations go beyond the well-known
cases for self-dual background fields
Semiclassical propagator of the Wigner function
Propagation of the Wigner function is studied on two levels of semiclassical
propagation, one based on the van-Vleck propagator, the other on phase-space
path integration. Leading quantum corrections to the classical Liouville
propagator take the form of a time-dependent quantum spot. Its oscillatory
structure depends on whether the underlying classical flow is elliptic or
hyperbolic. It can be interpreted as the result of interference of a
\emph{pair} of classical trajectories, indicating how quantum coherences are to
be propagated semiclassically in phase space. The phase-space path-integral
approach allows for a finer resolution of the quantum spot in terms of Airy
functions.Comment: 4 pages, 3 figure
Short wavelength quantum electrodynamical correction to cold plasma-wave propagation
The effect of short wavelength quantum electrodynamic (QED) correction on
plasma-wave propagation is investigated. The effect on plasma oscillations and
on electromagnetic waves in an unmagnetized as well as a magnetized plasma is
investigated. The effects of the short wavelength QED corrections are most
significant for plasma oscillations and for extraordinary modes. In particular,
the QED correction allow plasma oscillations to propagate, and the
extra-ordinary mode looses its stop band. The significance of our results is
discussed.Comment: 12 pages, 5 figure
Gauge invariant perturbations around symmetry reduced sectors of general relativity: applications to cosmology
We develop a gauge invariant canonical perturbation scheme for perturbations
around symmetry reduced sectors in generally covariant theories, such as
general relativity. The central objects of investigation are gauge invariant
observables which encode the dynamics of the system. We apply this scheme to
perturbations around a homogeneous and isotropic sector (cosmology) of general
relativity. The background variables of this homogeneous and isotropic sector
are treated fully dynamically which allows us to approximate the observables to
arbitrary high order in a self--consistent and fully gauge invariant manner.
Methods to compute these observables are given. The question of backreaction
effects of inhomogeneities onto a homogeneous and isotropic background can be
addressed in this framework. We illustrate the latter by considering
homogeneous but anisotropic Bianchi--I cosmologies as perturbations around a
homogeneous and isotropic sector.Comment: 39 pages, 1 figur
- ā¦