7,061 research outputs found
Lifshitz Tails in Constant Magnetic Fields
We consider the 2D Landau Hamiltonian perturbed by a random alloy-type
potential, and investigate the Lifshitz tails, i.e. the asymptotic behavior of
the corresponding integrated density of states (IDS) near the edges in the
spectrum of . If a given edge coincides with a Landau level, we obtain
different asymptotic formulae for power-like, exponential sub-Gaussian, and
super-Gaussian decay of the one-site potential. If the edge is away from the
Landau levels, we impose a rational-flux assumption on the magnetic field,
consider compactly supported one-site potentials, and formulate a theorem which
is analogous to a result obtained in the case of a vanishing magnetic field
Global Bounds for the Lyapunov Exponent and the Integrated Density of States of Random Schr\"odinger Operators in One Dimension
In this article we prove an upper bound for the Lyapunov exponent
and a two-sided bound for the integrated density of states at an
arbitrary energy of random Schr\"odinger operators in one dimension.
These Schr\"odinger operators are given by potentials of identical shape
centered at every lattice site but with non-overlapping supports and with
randomly varying coupling constants. Both types of bounds only involve
scattering data for the single-site potential. They show in particular that
both and decay at infinity at least like
. As an example we consider the random Kronig-Penney model.Comment: 9 page
The Role of Knowledge in Information Technology Project Governance
IT governance, the distribution of decision-making concerning IT, has primarily been studied at the organizational or, more recently, the business unit level. However, many IT decisions take place in the context of IT projects, making it important to understand governance issues at the project level. This research develops a project-level conceptualization of IT governance that draws from both the governance and project management literatures. A model of how IT project governance arrangements are influenced by the distribution of business and IT knowledge and the impact on project performance is also proposed. This model will be tested using matched surveys from business and IT managers involved in IT projects
Modulatory effects of magnetic vestibular stimulation on resting-state networks can be explained by subject-specific orientation of inner-ear anatomy in the MR static magnetic field
Strong static magnetic fields, as used in magnetic resonance imaging (MRI), stimulate the vestibular inner ear leading to a state of imbalance within the vestibular system that causes nystagmus. This magnetic vestibular stimulation (MVS) also modulates fluctuations of resting-state functional MRI (RS-fMRI) networks. MVS can be explained by a Lorentz force model, indicating that MVS is the result of the interaction of the static magnetic field strength and direction (called \textquotedblB0 magnetic field\textquotedbl in MRI) with the inner ear's continuous endolymphatic ionic current. However, the high variability between subjects receiving MVS (measured as nystagmus slow-phase velocity and RS-fMRI amplitude modulations) despite matching head position, remains to be explained. Furthermore, within the imaging community, an \textquotedbleasy-to-acquire-and-use\textquotedbl proxy accounting for modulatory MVS effects in RS-fMRI fluctuations is needed. The present study uses MRI data of 60 healthy volunteers to examine the relationship between RS-fMRI fluctuations and the individual orientation of inner-ear anatomy within the static magnetic field of the MRI. The individual inner-ear anatomy and orientation were assessed via high-resolution anatomical CISS images and related to fluctuations of RS-fMRI networks previously associated with MVS. More specifically, we used a subject-specific proxy for MVS (pMVS) that corresponds to the orientation of the individual inner-ear anatomy within the static magnetic field direction (also called \textquotedblz-direction\textquotedbl in MR imaging). We found that pMVS explained a considerable fraction of the total variance in RS-fMRI fluctuations (for instance, from 11% in the right cerebellum up to 36% in the cerebellar vermis). In addition to pMVS, we examined the angle of Reid's plane, as determined from anatomical imaging as an alternative and found that this angle (with the same sinus transformation as for pMVS) explained considerably less variance, e.g., from 2 to 16%. In our opinion, an excess variability due to MVS should generally be addressed in fMRI research analogous to nuisance regression for movement, pulsation, and respiration effects. We suggest using the pMVS parameter to deal with modulations of RS-fMRI fluctuations due to MVS. MVS-induced variance can easily be accounted by using high-resolution anatomical imaging of the inner ear and including the proposed pMVS parameter in fMRI group-level analysis
Electron localization by self-assembled GaSb/GaAs quantum dots.
We have studied the photoluminescence from type-II GaSb/GaAs self-assembled quantum dots in magnetic fields up to 50 T. Our results show that at low laser power, electrons are more weakly bound to the dots than to the wetting layer, but that at high laser power, the situation is reversed. We attribute this effect to an enhanced Coulomb interaction between a single electron and dots that are multiply charged with holes
A Higgs Mechanism for Gravity
In this paper we elaborate on the idea of an emergent spacetime which arises
due to the dynamical breaking of diffeomorphism invariance in the early
universe. In preparation for an explicit symmetry breaking scenario, we
consider nonlinear realizations of the group of analytical diffeomorphisms
which provide a unified description of spacetime structures. We find that
gravitational fields, such as the affine connection, metric and coordinates,
can all be interpreted as Goldstone fields of the diffeomorphism group. We then
construct a Higgs mechanism for gravity in which an affine spacetime evolves
into a Riemannian one by the condensation of a metric. The symmetry breaking
potential is identical to that of hybrid inflation but with the non-inflaton
scalar extended to a symmetric second rank tensor. This tensor is required for
the realization of the metric as a Higgs field. We finally comment on the role
of Goldstone coordinates as a dynamical fluid of reference.Comment: 15 pages, 2 figures, 3 tables, appendix C on on-shell d.o.f. added,
references adde
A Higgs Mechanism for Gravity. Part II: Higher Spin Connections
We continue the work of hep-th/0503024 in which gravity is considered as the
Goldstone realization of a spontaneously broken diffeomorphism group. We
complete the discussion of the coset space Diff(d,R)/SO(1,d-1) formed by the
d-dimensional group of analytic diffeomorphisms and the Lorentz group. We find
that this coset space is parameterized by coordinates, a metric and an infinite
tower of higher-spin-like or generalized connections. We then study effective
actions for the corresponding symmetry breaking which gives mass to the higher
spin connections. Our model predicts that gravity is modified at high energies
by the exchange of massive higher spin particles.Comment: 17 pages; discussion on local Poincare invariance and matter currents
added; references adde
- …