938 research outputs found
Geometry for the accelerating universe
The Lorentzian spacetime metric is replaced by an area metric which naturally
emerges as a generalized geometry in quantum string and gauge theory. Employing
the area metric curvature scalar, the gravitational Einstein-Hilbert action is
re-interpreted as dynamics for an area metric. Without the need for dark energy
or fine-tuning, area metric cosmology explains the observed small acceleration
of the late Universe.Comment: 4 pages, 1 figur
Quantum Phase Transitions in the Itinerant Ferromagnet ZrZn
We report a study of the ferromagnetism of ZrZn, the most promising
material to exhibit ferromagnetic quantum criticality, at low temperatures
as function of pressure . We find that the ordered ferromagnetic moment
disappears discontinuously at =16.5 kbar. Thus a tricritical point
separates a line of first order ferromagnetic transitions from second order
(continuous) transitions at higher temperature. We also identify two lines of
transitions of the magnetisation isotherms up to 12 T in the plane where
the derivative of the magnetization changes rapidly. These quantum phase
transitions (QPT) establish a high sensitivity to local minima in the free
energy in ZrZn, thus strongly suggesting that QPT in itinerant
ferromagnets are always first order
Reflection-Free One-Way Edge Modes in a Gyromagnetic Photonic Crystal
We point out that electromagnetic one-way edge modes analogous to quantum
Hall edge states, originally predicted by Raghu and Haldane in 2D gyroelectric
photonic crystals possessing Dirac point-derived bandgaps, can appear in more
general settings. In particular, we show that the TM modes in a gyromagnetic
photonic crystal can be formally mapped to electronic wavefunctions in a
periodic electromagnetic field, so that the only requirement for the existence
of one-way edge modes is that the Chern number for all bands below a gap is
non-zero. In a square-lattice gyromagnetic Yttrium-Iron-Garnet photonic crystal
operating at microwave frequencies, which lacks Dirac points, time-reversal
breaking is strong enough that the effect should be easily observable. For
realistic material parameters, the edge modes occupy a 10% band gap. Numerical
simulations of a one-way waveguide incorporating this crystal show 100%
transmission across strong defects, such as perfect conductors several lattice
constants wide, larger than the width of the waveguide.Comment: 4 pages, 3 figures (Figs. 1 and 2 revised.
Eddington-Born-Infeld action for dark energy and dark matter
We argue that Einstein gravity coupled to a Born-Infeld theory provides an
attractive candidate to represent dark matter and dark energy. For cosmological
models, the Born-Infeld field has an equation of state which interpolates
between matter, w=0 (small times), and a cosmological constant w=-1 (large
times). On galactic scales, the Born-Infeld field predicts asymptotically flat
rotation curves.Comment: A sign mistake in section on galactic scales is pointed out. This
sign invalidates the content of that section. See comment on manuscrip
The `s-rule' exclusion principle and vacuum interpolation in worldvolume dynamics
We show how the worldvolume realization of the Hanany-Witten effect for a
supersymmetric D5-brane in a D3 background also provides a classical
realization of the `s-rule' exclusion principle. Despite the supersymmetry, the
force on the D5-brane vanishes only in the D5 `ground state', which is shown to
interpolate between 6-dimensional Minkowski space and an -invariant
geometry. The M-theory analogue of these results is briefly
discussed.Comment: 25 pages, 9 figures, LaTeX JHEP styl
Multiple first-order metamagnetic transitions and quantum oscillations in ultrapure
We present measurements on ultra clean single crystals of the bilayered
ruthenate metal Sr3Ru2O7, which has a magnetic-field-tuned quantum critical
point. Quantum oscillations of differing frequencies can be seen in the
resistivity both below and above its metamagnetic transition. This frequency
shift corresponds to a small change in the Fermi surface volume that is
qualitatively consistent with the small moment change in the magnetisation
across the metamagnetic transition. Very near the metamagnetic field, unusual
behaviour is seen. There is a strong enhancement of the resistivity in a narrow
field window, with a minimum in the resistivity as a function of temperature
below 1 K that becomes more pronounced as the disorder level decreases. The
region of anomalous behaviour is bounded at low temperatures by two first-order
phase transitions. The implications of the results are discussed. PACS:
68.35.Rh, 71.27.+a, 72.15.-v, 74.70.PqComment: 12 pages 4 figures, submitte
Cosmology as Geodesic Motion
For gravity coupled to N scalar fields with arbitrary potential V, it is
shown that all flat (homogeneous and isotropic) cosmologies correspond to
geodesics in an (N+1)-dimensional `augmented' target space of Lorentzian
signature (1,N), timelike if V>0, null if V=0 and spacelike if V<0.
Accelerating cosmologies correspond to timelike geodesics that lie within an
`acceleration subcone' of the `lightcone'. Non-flat (k=-1,+1) cosmologies are
shown to evolve as projections of geodesic motion in a space of dimension
(N+2), of signature (1,N+1) for k=-1 and signature (2,N) for k=+1. This
formalism is illustrated by cosmological solutions of models with an
exponential potential, which are comprehensively analysed; the late-time
behviour for other potentials of current interest is deduced by comparison.Comment: 26 pages, 2 figures, journal version with additional reference
Gravity a la Born-Infeld
A simple technique for the construction of gravity theories in Born-Infeld
style is presented, and the properties of some of these novel theories are
investigated. They regularize the positive energy Schwarzschild singularity,
and a large class of models allows for the cancellation of ghosts. The possible
correspondence to low energy string theory is discussed. By including curvature
corrections to all orders in alpha', the new theories nicely illustrate a
mechanism that string theory might use to regularize gravitational
singularities.Comment: 21 pages, 2 figures, new appendix B with corrigendum: Class. Quantum
Grav. 21 (2004) 529
Performance of a diagnostic algorithm for fibrotic hypersensitivity pneumonitis. A case-control study.
The differential diagnosis fibrotic hypersensitivity pneumonitis (HP) versus idiopathic pulmonary fibrosis (IPF) is important but challenging. Recent diagnostic guidelines for HP emphasize including multidisciplinary discussion (MDD) in the diagnostic process, however MDD is not comprehensively available. We aimed to establish the diagnostic accuracy and prognostic validity of a previously proposed HP diagnostic algorithm that foregoes MDD.
We tested the algorithm in patients with an MDD diagnosis of fibrotic HP or IPF (case control study) and determined diagnostic test performances for diagnostic confidences of ≥ 90% and ≥ 70%. Prognostic validity was established using Cox proportional hazards models.
Thirty-one patients with fibrotic HP and 50 IPF patients were included. The algorithm-derived ≥ 90% confidence level for HP had high specificity (0.94, 95% confidence interval [CI] 0.83-0.99), but low sensitivity (0.35 [95%CI 0.19-0.55], J-index 0.29). Test performance was improved for the ≥ 70% confidence level (J-index 0.64) with a specificity of 0.90 (95%CI 0.78-0.97), and a sensitivity of 0.74 (95%CI 0.55-0.88). MDD fibrotic HP diagnosis was strongly associated with lower risk of death (adjusted hazard ratio [HR] 0.10 [0.01-0.92], p = 0.04), whereas the algorithm-derived ≥ 70% and ≥ 90% confidence diagnoses were not significantly associated with survival (adjusted HR 0.37 [0.07-1.80], p = 0.22, and adjusted HR 0.41 [0.05-3.25], p = 0.39, respectively).
The algorithm-derived ≥ 70% diagnostic confidence had satisfactory test performance for MDD-HP diagnosis, with insufficient sensitivity for ≥ 90% confidence. The lowest risk of death in the MDD-derived HP diagnosis validates the reference standard and suggests that a diagnostic algorithm not including MDD, might not replace the latter
Electron Transport through Disordered Domain Walls: Coherent and Incoherent Regimes
We study electron transport through a domain wall in a ferromagnetic nanowire
subject to spin-dependent scattering. A scattering matrix formalism is
developed to address both coherent and incoherent transport properties. The
coherent case corresponds to elastic scattering by static defects, which is
dominant at low temperatures, while the incoherent case provides a
phenomenological description of the inelastic scattering present in real
physical systems at room temperature. It is found that disorder scattering
increases the amount of spin-mixing of transmitted electrons, reducing the
adiabaticity. This leads, in the incoherent case, to a reduction of conductance
through the domain wall as compared to a uniformly magnetized region which is
similar to the giant magnetoresistance effect. In the coherent case, a
reduction of weak localization, together with a suppression of spin-reversing
scattering amplitudes, leads to an enhancement of conductance due to the domain
wall in the regime of strong disorder. The total effect of a domain wall on the
conductance of a nanowire is studied by incorporating the disordered regions on
either side of the wall. It is found that spin-dependent scattering in these
regions increases the domain wall magnetoconductance as compared to the effect
found by considering only the scattering inside the wall. This increase is most
dramatic in the narrow wall limit, but remains significant for wide walls.Comment: 23 pages, 12 figure
- …