334 research outputs found
Approaching the Problem of Time with a Combined Semiclassical-Records-Histories Scheme
I approach the Problem of Time and other foundations of Quantum Cosmology
using a combined histories, timeless and semiclassical approach. This approach
is along the lines pursued by Halliwell. It involves the timeless probabilities
for dynamical trajectories entering regions of configuration space, which are
computed within the semiclassical regime. Moreover, the objects that Halliwell
uses in this approach commute with the Hamiltonian constraint, H. This approach
has not hitherto been considered for models that also possess nontrivial linear
constraints, Lin. This paper carries this out for some concrete relational
particle models (RPM's). If there is also commutation with Lin - the Kuchar
observables condition - the constructed objects are Dirac observables.
Moreover, this paper shows that the problem of Kuchar observables is explicitly
resolved for 1- and 2-d RPM's. Then as a first route to Halliwell's approach
for nontrivial linear constraints that is also a construction of Dirac
observables, I consider theories for which Kuchar observables are formally
known, giving the relational triangle as an example. As a second route, I apply
an indirect method that generalizes both group-averaging and Barbour's best
matching. For conceptual clarity, my study involves the simpler case of
Halliwell 2003 sharp-edged window function. I leave the elsewise-improved
softened case of Halliwell 2009 for a subsequent Paper II. Finally, I provide
comments on Halliwell's approach and how well it fares as regards the various
facets of the Problem of Time and as an implementation of QM propositions.Comment: An improved version of the text, and with various further references.
25 pages, 4 figure
Triangleland. II. Quantum Mechanics of Pure Shape
Relational particle models are of value in the absolute versus relative
motion debate. They are also analogous to the dynamical formulation of general
relativity, and as such are useful for investigating conceptual strategies
proposed for resolving the problem of time in quantum general relativity.
Moreover, to date there are few explicit examples of these at the quantum
level. In this paper I exploit recent geometrical and classical dynamics work
to provide such a study based on reduced quantization in the case of pure shape
(no scale) in 2-d for 3 particles (triangleland) with multiple harmonic
oscillator type potentials. I explore solutions for these making use of exact,
asymptotic, perturbative and numerical methods. An analogy to the mathematics
of the linear rigid rotor in a background electric field is useful throughout.
I argue that further relational models are accessible by the methods used in
this paper, and for specific uses of the models covered by this paper in the
investigation of the problem of time (and other conceptual and technical
issues) in quantum general relativity.Comment: Journal Reference added, minor updates to References and Figure
Ascertaining the notion of board accountability in Chinese listed companies
Accountability is a concept that has been frequently referred to in Anglo-American systems and in the OECD’s corporate governance documents, as well as in the English translations of corporate governance documents from non-English speaking jurisdictions. It is in the Anglo-American literature, in particular, where the word finds prominence. It has been suggested in China that accountability is one of the basic principles of corporate governance that needs to be consistently enforced. But does this mean that board accountability, as it has been provided for in the Anglo-American system, is actually an element of Chinese corporate governance? If not, should it be adopted? Or should China develop a concept that is more appropriately included as a critical part of its own particular corporate governance needs? The paper aims to address these matters in order to ascertain where Chinese corporate governance stands on accountability as far as the boards of large listed companies are concerned, and what it should do. We opine that while there are elements of accountability in Chinese corporate governance, it does not have the form of accountability embraced in Anglo-American systems. But, it is argued, as China moves from having a system totally based on administrative governance to one that is based more on economic governance the kind of approach that applies in Anglo-American jurisdictions is likely to become more relevant. Within a hybrid corporate governance system combining elements of both administrative and economic governance, we develop a unique “wenze system” with forms and characters of accountability that is likely to develop to address the needs of corporate governance in China and the fostering of its listed companies
Extremely long quasiparticle spin lifetimes in superconducting aluminium using MgO tunnel spin injectors
There has been an intense search in recent years for long-lived
spin-polarized carriers for spintronic and quantum-computing devices. Here we
report that spin polarized quasi-particles in superconducting aluminum layers
have surprisingly long spin-lifetimes, nearly a million times longer than in
their normal state. The lifetime is determined from the suppression of the
aluminum's superconductivity resulting from the accumulation of spin polarized
carriers in the aluminum layer using tunnel spin injectors. A Hanle effect,
observed in the presence of small in-plane orthogonal fields, is shown to be
quantitatively consistent with the presence of long-lived spin polarized
quasi-particles. Our experiments show that the superconducting state can be
significantly modified by small electric currents, much smaller than the
critical current, which is potentially useful for devices involving
superconducting qubits
"Feed from the Service": Corruption and Coercion in the State-University Relations in Central Eurasia
Education in Central Eurasia has become one of the industries, most affected by corruption. Corruption in academia, including bribery, extortions, embezzlement, nepotism, fraud, cheating, and plagiarism, is reflected in the region’s media and addressed in few scholarly works. This paper considers corruption in higher education as a product of interrelations between the government and academia. A substantial block of literature considers excessive corruption as an indicator of a weak state. In contrast to standard interpretations, this paper argues that in non-democratic societies corruption is used on a systematic basis as a mechanism of direct and indirect administrative control over higher education institutions. Informal approval of corrupt activities in exchange for loyalty and compliance with the regime may be used in the countries of Central Eurasia for the purposes of political indoctrination. This paper presents the concept of corruption and coercion in the state-university relations in Central Eurasia and outlines the model which incorporates this concept and the “feed from the service” approach. It presents implications of this model for the state-university relations and the national educational systems in Central Eurasia in general and offers some suggestions on curbing corruption
Critical Behavior of the Supersolid transition in Bose-Hubbard Models
We study the phase transitions of interacting bosons at zero temperature
between superfluid (SF) and supersolid (SS) states. The latter are
characterized by simultaneous off-diagonal long-range order and broken
translational symmetry. The critical phenomena is described by a
long-wavelength effective action, derived on symmetry grounds and verified by
explicit calculation. We consider two types of supersolid ordering:
checkerboard (X) and collinear (C), which are the simplest cases arising in two
dimensions on a square lattice. We find that the SF--CSS transition is in the
three-dimensional XY universality class. The SF--XSS transition exhibits
non-trivial new critical behavior, and appears, within a
expansion to be driven generically first order by fluctuations. However, within
a one--loop calculation directly in a strong coupling fixed point with
striking ``non-Bose liquid'' behavior is found. At special isolated
multi-critical points of particle-hole symmetry, the system falls into the 3d
Ising universality class.Comment: RevTeX, 24 pages, 16 figures. Also available at
http://www.cip.physik.tu-muenchen.de/tumphy/d/T34/Mitarbeiter/frey.htm
A Natural Supersymmetric Model with MeV Dark Matter
It has previously been proposed that annihilating dark matter particles with
MeV-scale masses could be responsible for the flux of 511 keV photons observed
from the region of the Galactic Bulge. The conventional wisdom, however, is
that it is very challenging to construct a viable particle physics model
containing MeV dark matter. In this letter, we challenge this conclusion by
describing a simple and natural supersymmetric model in which the lightest
supersymmetric particle naturally has a MeV-scale mass and the other
phenomenological properties required to generate the 511 keV emission. In
particular, the small ( ) effective couplings between dark
matter and the Standard Model fermions required in this scenario naturally lead
to radiative corrections that generate MeV-scale masses for both the dark
matter candidate and the mediator particle.Comment: 4 pages, 1 figure. v2: Small modification to discussion of spectru
Yukawa Unified Supersymmetric SO(10) Model: Cosmology, Rare Decays and Collider Searches
It has recently been pointed out that viable sparticle mass spectra can be
generated in Yukawa unified SO(10) supersymmetric grand unified models
consistent with radiative breaking of electroweak symmetry. Model solutions are
obtained only if , and positive -term
contributions to scalar masses from SO(10) gauge symmetry breaking are used. In
this paper, we attempt to systematize the parameter space regions where
solutions are obtained. We go on to calculate the relic density of neutralinos
as a function of parameter space. No regions of the parameter space explored
were actually cosmologically excluded, and very reasonable relic densities were
found in much of parameter space. Direct neutralino detection rates could
exceed 1 event/kg/day for a Ge detector, for low values of GUT scale
gaugino mass . We also calculate the branching fraction for decays, and find that it is beyond the 95% CL experimental limits in
much, but not all, of the parameter space regions explored. However, recent
claims have been made that NLO effects can reverse the signs of certain
amplitudes in the calculation, leading to agreement between
theory and experiment in Yukawa unified SUSY models. For the Fermilab Tevatron
collider, significant regions of parameter space can be explored via
and searches. There also exist some limited regions of
parameter space where a trilepton signal can be seen at TeV33. Finally, there
exist significant regions of parameter space where direct detection of bottom
squark pair production can be made, especially for large negative values of the
GUT parameter .Comment: Added comparison to Blazek/Raby results and added Comments on de Boer
et al. b->s gamma result
Graphene and non-Abelian quantization
In this article we employ a simple nonrelativistic model to describe the low
energy excitation of graphene. The model is based on a deformation of the
Heisenberg algebra which makes the commutator of momenta proportional to the
pseudo-spin. We solve the Landau problem for the resulting Hamiltonian which
reduces, in the large mass limit while keeping fixed the Fermi velocity, to the
usual linear one employed to describe these excitations as massless Dirac
fermions. This model, extended to negative mass, allows to reproduce the
leading terms in the low energy expansion of the dispersion relation for both
nearest and next-to-nearest neighbor interactions. Taking into account the
contributions of both Dirac points, the resulting Hall conductivity, evaluated
with a -function approach, is consistent with the anomalous integer
quantum Hall effect found in graphene. Moreover, when considered in first order
perturbation theory, it is shown that the next-to-leading term in the
interaction between nearest neighbor produces no modifications in the spectrum
of the model while an electric field perpendicular to the magnetic field
produces just a rigid shift of this spectrum.
PACS: 03.65.-w, 81.05.ue, 73.43.-fComment: 23 pages, 4 figures. Version to appear in the Journal of Physics A.
The title has been changed into "Graphene and non-Abelian quantization". The
motivation and presentation of the paper has been changed. An appendix and
Section 6 on the evaluation of the Hall conductivity have been added.
References adde
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