9,972 research outputs found
Quantum Gravity Phenomenology, Lorentz Invariance and Discreteness
Contrary to what is often stated, a fundamental spacetime discreteness need
not contradict Lorentz invariance. A causal set's discreteness is in fact
locally Lorentz invariant, and we recall the reasons why. For illustration, we
introduce a phenomenological model of massive particles propagating in a
Minkowski spacetime which arises from an underlying causal set. The particles
undergo a Lorentz invariant diffusion in phase space, and we speculate on
whether this could have any bearing on the origin of high energy cosmic rays.Comment: 13 pages. Replaced version with corrected fundamental solution,
missing m's (mass) and c's (speed of light) added and reference on diffusion
on the three sphere changed. Note with additional references added and
addresses updated, as in published versio
Dynamical study of the hyperextended scalar-tensor theory in the empty Bianchi type I model
The dynamics of the hyperextended scalar-tensor theory in the empty Bianchi
type I model is investigated. We describe a method giving the sign of the first
and second derivatives of the metric functions whatever the coupling function.
Hence, we can predict if a theory gives birth to expanding, contracting,
bouncing or inflationary cosmology. The dynamics of a string inspired theory
without antisymetric field strength is analysed. Some exact solutions are
found.Comment: 18 pages, 3 figure
Topology Change and Causal Continuity
The result that, for a scalar quantum field propagating on a ``trousers''
topology in 1+1 dimensions, the crotch singularity is a source for an infinite
burst of energy has been used to argue against the occurrence of topology
change in quantum gravity. We draw attention to a conjecture due to Sorkin that
it may be the particular type of topology change involved in the trousers
transition that is problematic and that other topology changes may not cause
the same difficulties. The conjecture links the singular behaviour to the
existence of ``causal discontinuities'' in the spacetime and relies on a
classification of topology changes using Morse theory. We investigate various
topology changing transitions, including the pair production of black holes and
of topological geons, in the light of these ideas.Comment: Latex, 28 pages, 10 figures, small changes in text (one figure
removed), conclusions remain unchanged. Accepted for publication in Physical
Review
A Molecular Phylogenetic Study of Generic and Subgeneric Relationships in the Southwest Australian Endemics Conostylis and Blancoa (Haemodoraceae)
We sequenced the plastid gene matK and the nuclear ribosomal spacer ITS for 39 of the 47+ species of Conostylis as well as its monotypic sister genus Blancoa, which some authors have included within Conostylis. Conostylis received 99% bootstrap support as monophyletic, with 100% support that Blancoa is its sister. Within Conostylis, the study provides strong support for two large sister clades, which we refer to as clades A (100%) and B (99%). Clade A consists of C. subgen. Conostylis plus the recently discovered C. glabra of C. subgen. Pendula sect. Divaricata (100%), and C. subgen. Pendula sect. Appendicula (100%). Clade B consists of species mostly placed within the remainder of C. subgen. Pendula but also contains members of the other small subgenera. Subgenus Pendula can be recircumscribed as monophyletic by excluding sect. Appendicula, Conostylis phathyrantha, and C. glabra and including subgen. Androstemma and subgen. Greenia. The status of the other two minor subgenera—C. subgen. Brachycaulon and C. subgen. Bicolorata—requires further investigation. Conostylis sect. Divaricata is polyphyletic. Ancient vicariance events are postulated for Conostylis involving separation of major clades in the northern and southern kwongan regions of southwestern Australia. The phylogenetic pattern in Conostylis is consistent across several lineages with the prolonged persistence of relictual taxa combined with explosive more recent speciation, the latter pronounced in the northern kwongan. There is evidence of significant divergence in major speciation mechanisms and chromosome number change among the three most species-rich subgenera/sections (dysploidy in Pendula and Appendicula vs. diploid speciation in Conostylis). Further investigation is needed to evaluate these ideas and elucidate the patterns of speciation in this most diverse genus of Haemodoraceae
Pressure Evolution of the Ferromagnetic and Field Re-entrant Superconductivity in URhGe
Fine pressure () and magnetic field () tuning on the ferromagnetic
superconductor URhGe are reported in order to clarify the interplay between the
mass enhancement, low field superconductivity (SC) and field reentrant
superconductivity (RSC) by electrical resistivity measurements. With increasing
, the transition temperature and the upper critical field of the low field
SC decrease slightly, while the RSC dome drastically shifts to higher fields
and shrinks. The spin reorientation field also increases. At a
pressure GPa, the RSC has collapsed while the low field SC persists
and may disappear only above 4 GPa. Via careful studies of the
inelastic resistivity term, it is demonstrated that this drastic change
is directly related with the dependence of the effective mass which
determines the critical field of the low field SC and RSC on the basis of
triplet SC without Pauli limiting field.Comment: 5 pages, 6 figures, to appear in Journal of the Physical Society of
Japa
Hidden Quantum Critical Point in a Ferromagnetic Superconductor
We consider a coexistence phase of both Ferromagnetism and superconductivity
and solve the self-consistent mean-field equations at zero temperature. The
superconducting gap is shown to vanish at the Stoner point whereas the
magnetization doesn't. This indicates that the para-Ferro quantum critical
point becomes a hidden critical point. The effective mass in such a phase gets
enhanced whereas the spin wave stiffness is reduced as compared to the pure FM
phase. The spin wave stiffness remains finite even at the para-Ferro quantum
critical point.Comment: 4 pages, Phys. Rev. B (Rapid) accepte
Notes on Euclidean Wilson loops and Riemann Theta functions
The AdS/CFT correspondence relates Wilson loops in N=4 SYM theory to minimal
area surfaces in AdS5 space. In this paper we consider the case of Euclidean
flat Wilson loops which are related to minimal area surfaces in Euclidean AdS3
space. Using known mathematical results for such minimal area surfaces we
describe an infinite parameter family of analytic solutions for closed Wilson
loops. The solutions are given in terms of Riemann theta functions and the
validity of the equations of motion is proven based on the trisecant identity.
The world-sheet has the topology of a disk and the renormalized area is written
as a finite, one-dimensional contour integral over the world-sheet boundary. An
example is discussed in detail with plots of the corresponding surfaces.
Further, for each Wilson loops we explicitly construct a one parameter family
of deformations that preserve the area. The parameter is the so called spectral
parameter. Finally, for genus three we find a map between these Wilson loops
and closed curves inside the Riemann surface.Comment: 35 pages, 7 figures, pdflatex. V2: References added. Typos corrected.
Some points clarifie
High-Field Superconductivity at an Electronic Topological Transition in URhGe
The emergence of superconductivity at high magnetic fields in URhGe is
regarded as a paradigm for new state formation approaching a quantum critical
point. Until now, a divergence of the quasiparticle mass at the metamagnetic
transition was considered essential for superconductivity to survive at
magnetic fields above 30 tesla. Here we report the observation of quantum
oscillations in URhGe revealing a tiny pocket of heavy quasiparticles that
shrinks continuously with increasing magnetic field, and finally disappears at
a topological Fermi surface transition close to or at the metamagnetic field.
The quasiparticle mass decreases and remains finite, implying that the Fermi
velocity vanishes due to the collapse of the Fermi wavevector. This offers a
novel explanation for the re-emergence of superconductivity at extreme magnetic
fields and makes URhGe the first proven example of a material where magnetic
field-tuning of the Fermi surface, rather than quantum criticality alone,
governs quantum phase formation.Comment: A revised version has been accepted for publication in Nature Physic
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