3,322 research outputs found
Short communications: Bat-hunting behaviour of the Dark Chanting Goshawk Melierax metabates
No abstrac
The conservation of energy-momentum and the mass for the graviton
In this work we give special attention to the bimetric theory of gravitation
with massive gravitons proposed by Visser in 1998. In his theory, a prior
background metric is necessary to take in account the massive term. Although in
the great part of the astrophysical studies the Minkowski metric is the best
choice to the background metric, it is not possible to consider this metric in
cosmology. In order to keep the Minkowski metric as background in this case, we
suggest an interpretation of the energy-momentum conservation in Visser's
theory, which is in accordance with the equivalence principle and recovers
naturally the special relativity in the absence of gravitational sources.
Although we do not present a general proof of our hypothesis we show its
validity in the simple case of a plane and dust-dominated universe, in which
the `massive term' appears like an extra contribution for the energy density.Comment: 9 pages, accepted for publishing in GR
Wormhole geometries supported by a nonminimal curvature-matter coupling
Wormhole geometries in curvature-matter coupled modified gravity are
explored, by considering an explicit nonminimal coupling between an arbitrary
function of the scalar curvature, R, and the Lagrangian density of matter. It
is the effective stress-energy tensor containing the coupling between matter
and the higher order curvature derivatives that is responsible for the null
energy condition violation, and consequently for supporting the respective
wormhole geometries. The general restrictions imposed by the null energy
condition violation are presented in the presence of a nonminimal R-matter
coupling. Furthermore, obtaining exact solutions to the gravitational field
equations is extremely difficult due to the nonlinearity of the equations,
although the problem is mathematically well-defined. Thus, we outline several
approaches for finding wormhole solutions, and deduce an exact solution by
considering a linear R nonmiminal curvature-matter coupling and by considering
an explicit monotonically decreasing function for the energy density. Although
it is difficult to find exact solutions of matter threading the wormhole
satisfying the energy conditions at the throat, an exact solution is found
where the nonminimal coupling does indeed minimize the violation of the null
energy condition of normal matter at the throat.Comment: 8 pages, 3 figures. V2: 9 pages, error and typos corrected;
discussion and references added; to appear in PR
Superconductivity in heavy-fermion U(Pt,Pd)3 and its interplay with magnetism
The effect of Pd doping on the superconducting phase diagram of the
unconventional superconductor UPt3 has been measured by (magneto)resistance,
specific heat, thermal expansion and magnetostriction. Experiments on single-
and polycrystalline U(Pt1-xPdx)3 for x<= 0.006 show that the superconducting
transition temperatures of the A phase, Tc+, and of the B phase, Tc-, both
decrease, while the splitting DTc increases at a rate of 0.30(2)K/at.%Pd. We
find that DTc(x) correlates with an increase of the weak magnetic moment m(x)
upon Pd doping. This provides further evidence for Ginzburg-Landau scenarios
with magnetism as the symmetry breaking field, i.e. the 2D E representation and
the 1D odd parity model. Only for small splittings DTc is proportional to
m^2(Tc+) (DTc<= 0.05 K) as predicted. The results at larger splittings call for
Ginzburg-Landau expansions beyond 4th order. The tetracritical point in the B-T
plane persists till at least x= 0.002 for B perpendicular to c, while it is
rapidly suppressed for B||c. Upon alloying the A and B phases gain stability at
the expense of the C phase.Comment: 25 pages text (PS), 8 pages with 14 figures (PS), submitted to
Phys.Rev.
"Cosmological" quasiparticle production in harmonically trapped superfluid gases
We show that a variety of cosmologically motivated effective quasiparticle
space-times can be produced in harmonically trapped superfluid Bose and Fermi
gases. We study the analogue of cosmological particle production in these
effective space-times, induced by trapping potentials and coupling constants
possessing an arbitrary time dependence. The WKB probabilities for phonon
creation from the superfluid vacuum are calculated, and an experimental
procedure to detect quasiparticle production by measuring density-density
correlation functions is proposed.Comment: 8 pages, 1 figure; references updated, as published in Physical
Review
Kondo hybridisation and the origin of metallic states at the (001) surface of SmB6
SmB6, a well-known Kondo insulator, has been proposed to be an ideal
topological insulator with states of topological character located in a clean,
bulk electronic gap, namely the Kondo hybridisation gap. Seeing as the Kondo
gap arises from many body electronic correlations, this would place SmB6 at the
head of a new material class: topological Kondo insulators. Here, for the first
time, we show that the k-space characteristics of the Kondo hybridisation
process is the key to unravelling the origin of the two types of metallic
states observed directly by ARPES in the electronic band structure of
SmB6(001). One group of these states is essentially of bulk origin, and cuts
the Fermi level due to the position of the chemical potential 20 meV above the
lowest lying 5d-4f hybridisation zone. The other metallic state is more
enigmatic, being weak in intensity, but represents a good candidate for a
topological surface state. However, before this claim can be substantiated by
an unequivocal measurement of its massless dispersion relation, our data raises
the bar in terms of the ARPES resolution required, as we show there to be a
strong renormalisation of the hybridisation gaps by a factor 2-3 compared to
theory, following from the knowledge of the true position of the chemical
potential and a careful comparison with the predictions from recent
LDA+Gutzwiler calculations. All in all, these key pieces of evidence act as
triangulation markers, providing a detailed description of the electronic
landscape in SmB6, pointing the way for future, ultrahigh resolution ARPES
experiments to achieve a direct measurement of the Dirac cones in the first
topological Kondo insulator.Comment: 9 pages, 4 Figures and supplementary material (including Movies and
CORPES13 "best prize" poster
Analog gravity from field theory normal modes?
We demonstrate that the emergence of a curved spacetime ``effective
Lorentzian geometry'' is a common and generic result of linearizing a field
theory around some non-trivial background. This investigation is motivated by
considering the large number of ``analog models'' of general relativity that
have recently been developed based on condensed matter physics, and asking
whether there is something more fundamental going on. Indeed, linearization of
a classical field theory (a field theoretic ``normal mode analysis'') results
in fluctuations whose propagation is governed by a Lorentzian-signature curved
spacetime ``effective metric''. For a single scalar field, this procedure
results in a unique effective metric, which is quite sufficient for simulating
kinematic aspects of general relativity (up to and including Hawking
radiation). Quantizing the linearized fluctuations, the one-loop effective
action contains a term proportional to the Einstein--Hilbert action, suggesting
that while classical physics is responsible for generating an ``effective
geometry'', quantum physics can be argued to induce an ``effective dynamics''.
The situation is strongly reminiscent of Sakharov's ``induced gravity''
scenario, and suggests that Einstein gravity is an emergent low-energy
long-distance phenomenon that is insensitive to the details of the high-energy
short-distance physics. (We mean this in the same sense that hydrodynamics is a
long-distance emergent phenomenon, many of whose predictions are insensitive to
the short-distance cutoff implicit in molecular dynamics.)Comment: Revtex 4 (beta 5); 12 pages in single-column forma
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