1,089 research outputs found
Giant magnetic anisotropy at nanoscale: overcoming the superparamagnetic limit
It has been recently observed for palladium and gold nanoparticles, that the
magnetic moment at constant applied field does not change with temperature over
the range comprised between 5 and 300 K. These samples with size smaller than
2.5 nm exhibit remanence up to room temperature. The permanent magnetism for so
small samples up to so high temperatures has been explained as due to blocking
of local magnetic moment by giant magnetic anisotropies. In this report we
show, by analysing the anisotropy of thiol capped gold films, that the orbital
momentum induced at the surface conduction electrons is crucial to understand
the observed giant anisotropy. The orbital motion is driven by localised charge
and/or spin through spin orbit interaction, that reaches extremely high values
at the surfaces. The induced orbital moment gives rise to an effective field of
the order of 103 T that is responsible of the giant anisotropy.Comment: 15 pages, 2 figures, submitted to PR
Maximally symmetric stabilizer MUBs in even prime-power dimensions
One way to construct a maximal set of mutually unbiased bases (MUBs) in a
prime-power dimensional Hilbert space is by means of finite phase-space
methods. MUBs obtained in this way are covariant with respect to some subgroup
of the group of all affine symplectic phase-space transformations. However,
this construction is not canonical: as a consequence, many different choices of
covariance sugroups are possible. In particular, when the Hilbert space is
dimensional, it is known that covariance with respect to the full group
of affine symplectic phase-space transformations can never be achieved. Here we
show that in this case there exist two essentially different choices of maximal
subgroups admitting covariant MUBs. For both of them, we explicitly construct a
family of covariant MUBs. We thus prove that, contrary to the odd
dimensional case, maximally covariant MUBs are very far from being unique.Comment: 22 page
5D gravitational waves from complexified black rings
In this paper we construct and briefly study the 5D time-dependent solutions
of general relativity obtained via double analytic continuation of the black
hole (Myers-Perry) and of the black ring solutions with a double
(Pomeransky-Senkov) and a single rotation (Emparan-Reall). The new solutions
take the form of a generalized Einstein-Rosen cosmology representing
gravitational waves propagating in a closed universe. In this context the
rotation parameters of the rings can be interpreted as the extra wave
polarizations, while it is interesting to state that the waves obtained from
Myers-Perry Black holes exhibit an extra boost-rotational symmetry in higher
dimensions which signals their better behavior at null infinity. The analogue
to the C-energy is analyzed.Comment: 18 pages, 4 figures. References added, introduction and conclusions
are amended, some issues related to singularity structure and symmetries are
discussed. Matches the print version to appear in JHE
Tilted String Cosmologies
Global symmetries of the string effective action are employed to generate
tilted, homogeneous Bianchi type VI_h string cosmologies from a previously
known stiff perfect fluid solution to Einstein gravity. The dilaton field is
not constant on the surfaces of homogeneity. The future asymptotic state of the
models is interpreted as a plane wave and is itself an exact solution to the
string equations of motion to all orders in the inverse string tension. An
inhomogeneous generalization of the Bianchi type III model is also found.Comment: 9 pages, Standard Latex Source. To appear in Physics Letters B Minor
change: Authors now alphabetically liste
On quasi-local charges and Newman--Penrose type quantities in Yang--Mills theories
We generalize the notion of quasi-local charges, introduced by P. Tod for
Yang--Mills fields with unitary groups, to non-Abelian gauge theories with
arbitrary gauge group, and calculate its small sphere and large sphere limits
both at spatial and null infinity. We show that for semisimple gauge groups no
reasonable definition yield conserved total charges and Newman--Penrose (NP)
type quantities at null infinity in generic, radiative configurations. The
conditions of their conservation, both in terms of the field configurations and
the structure of the gauge group, are clarified. We also calculate the NP
quantities for stationary, asymptotic solutions of the field equations with
vanishing magnetic charges, and illustrate these by explicit solutions with
various gauge groups.Comment: 22 pages, typos corrected, appearing in Classical and Quantum Gravit
Tetrads in SU(3) X SU(2) X U(1) Yang-Mills geometrodynamics
The relationship between gauge and gravity amounts to understanding
underlying new geometrical local structures. These structures are new tetrads
specially devised for Yang-Mills theories, Abelian and Non-Abelian in
four-dimensional Lorentzian spacetimes. In the present manuscript a new tetrad
is introduced for the Yang-Mills SU(3) X SU(2) X U(1) formulation. These new
tetrads establish a link between local groups of gauge transformations and
local groups of spacetime transformations. New theorems are proved regarding
isomorphisms between local internal SU(3) X SU(2) X U(1) groups and local
tensor products of spacetime LB1 and LB2 groups of transformations. The new
tetrads and the stress-energy tensor allow for the introduction of three new
local gauge invariant objects. Using these new gauge invariant objects and in
addition a new general local duality transformation, a new algorithm for the
gauge invariant diagonalization of the Yang-Mills stress-energy tensor is
developed.Comment: There is a new appendix. The unitary transformations by local SU(2)
subgroup elements of a local group coset representative is proved to be a new
local group coset representative. This proof is relevant to the study of the
memory of the local tetrad SU(3) generated gauge transformations. Therefore,
it is also relevant to the group theorems proved in the paper. arXiv admin
note: substantial text overlap with arXiv:gr-qc/060204
Structure, classifcation, and conformal symmetry, of elementary particles over non-archimedean space-time
It is known that no length or time measurements are possible in sub-Planckian
regions of spacetime. The Volovich hypothesis postulates that the
micro-geometry of spacetime may therefore be assumed to be non-archimedean. In
this letter, the consequences of this hypothesis for the structure,
classification, and conformal symmetry of elementary particles, when spacetime
is a flat space over a non-archimedean field such as the -adic numbers, is
explored. Both the Poincar\'e and Galilean groups are treated. The results are
based on a new variant of the Mackey machine for projective unitary
representations of semidirect product groups which are locally compact and
second countable. Conformal spacetime is constructed over -adic fields and
the impossibility of conformal symmetry of massive and eventually massive
particles is proved
On the coexistence of position and momentum observables
We investigate the problem of coexistence of position and momentum
observables. We characterize those pairs of position and momentum observables
which have a joint observable
Asymptotic Behaviour of Inhomogeneous String Cosmologies
The asymptotic behaviour at late times of inhomogeneous axion-dilaton
cosmologies is investigated. The space-times considered here admit two abelian
space-like Killing vectors. These space-times evolve towards an anisotropic
universe containing gravitational radiation. Furthermore, a peeling-off
behaviour of the Weyl tensor and the antisymmetric tensor field strength is
found. The relation to the pre-big-bang scenario is briefly discussed.Comment: 15 pages, Late
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