768 research outputs found
Three-magnetic fields
A completely new mechanism to generate the observed amount of large-scale
cosmological magnetic fields is introduced in the context of three-form
inflation. The amplification of the fields occurs via fourth order dynamics of
the vector perturbations and avoids the backreaction problem that plagues most
previously introduced mechanisms.Comment: 4 pages, 2 figures -- v2 as published (title changed in the published
version to "Cosmic magnetization in three-form inflation"
Doubly-boosted vector cosmologies from disformal metrics
A systematic dynamical system approach is applied to study the cosmology of
anisotropic Bianchi I universes in which a vector field is assumed to operate
on a disformal frame. This study yields a number of new fixed points, among
which anisotropic scaling solutions. Within the simplifying assumption of
(nearly) constant-slope potentials these are either not stable attractors, do
not describe accelerating expansion or else they feature too large anisotropies
to be compatible with observations. Nonetheless, some solutions do have an
appeal for cosmological applications in that isotropy is retained due to rapid
oscillations of the vector field.Comment: 11 pages, 4 figures, prepared during the NORDITA Extended Theories of
Gravity program. arXiv admin note: substantial text overlap with
arXiv:1407.344
Instabilities in tensorial nonlocal gravity
We discuss the cosmological implications of nonlocal modifications of general
relativity containing tensorial structures. Assuming the presence of standard
radiation- and matter-dominated eras, we show that, except in very particular
cases, the nonlocal terms contribute a rapidly growing energy density. These
models therefore generically do not have a stable cosmological evolution.Comment: 10 pages, 2 figures. v2: version published in PR
Unifying Einstein and Palatini gravities
We consider a novel class of gravity theories where the connection is
related to the conformally scaled metric with
a scaling that depends on the scalar curvature only. We call them
C-theories and show that the Einstein and Palatini gravities can be obtained as
special limits. In addition, C-theories include completely new physically
distinct gravity theories even when . With nonlinear ,
C-theories interpolate and extrapolate the Einstein and Palatini cases and may
avoid some of their conceptual and observational problems. We further show that
C-theories have a scalar-tensor formulation, which in some special cases
reduces to simple Brans-Dicke-type gravity. If matter fields couple to the
connection, the conservation laws in C-theories are modified. The stability of
perturbations about flat space is determined by a simple condition on the
lagrangian.Comment: 17 pages, no figure
Spin polarization in a T-shape conductor induced by strong Rashba spin-orbit coupling
We investigate numerically the spin polarization of the current in the
presence of Rashba spin-orbit interaction in a T-shaped conductor proposed by
A.A. Kiselev and K.W. Kim (Appl. Phys. Lett. {\bf 78} 775 (2001)). The
recursive Green function method is used to calculate the three terminal spin
dependent transmission probabilities. We focus on single-channel transport and
show that the spin polarization becomes nearly 100 % with a conductance close
to for sufficiently strong spin-orbit coupling. This is interpreted
by the fact that electrons with opposite spin states are deflected into an
opposite terminal by the spin dependent Lorentz force. The influence of the
disorder on the predicted effect is also discussed. Cases for multi-channel
transport are studied in connection with experiments
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