5,131 research outputs found
Astrophysical significance of the anisotropic kinetic alpha effect
The generation of large scale flows by the anisotropic kinetic alpha (AKA)
effect is investigated in simulations with a suitable time-dependent space- and
time-periodic anisotropic forcing lacking parity invariance. The forcing
pattern moves relative to the fluid, which leads to a breaking of the Galilean
invariance as required for the AKA effect to exist. The AKA effect is found to
produce a clear large scale flow pattern when the Reynolds number, R, is small
as only a few modes are excited in linear theory. In this case the
non-vanishing components of the AKA tensor are dynamically independent of the
Reynolds number. For larger values of R, many more modes are excited and the
components of the AKA tensor are found to decrease rapidly with increasing
value of R. However, once there is a magnetic field (imposed and of sufficient
strength, or dynamo-generated and saturated) the field begins to suppress the
AKA effect, regardless of the value of R. It is argued that the AKA effect is
unlikely to be astrophysically significant unless the magnetic field is weak
and R is small.Comment: 8 pages, 10 figures, submitted to A&
Evidence for an axion-like particle from PKS 1222+216?
The surprising discovery by MAGIC of an intense, rapidly varying emission in
the energy range 70 - 400 GeV from the flat spectrum radio quasar PKS 1222+216
represents a challenge for all interpretative scenarios. Indeed, in order to
avoid absorption of \gamma rays in the dense ultraviolet radiation field of the
broad line region (BLR), one is forced to invoke some unconventional
astrophysical picture, like for instance the existence of a very compact (r\sim
10^{14} cm) emitting blob at a large distance (R \sim10^{18} cm) from the jet
base. We offer the investigation of a scenario based on the standard blazar
model for PKS 1222+216 where \gamma rays are produced close to the central
engine, but we add the new assumption that inside the source photons can
oscillate into axion-like particles (ALPs), which are a generic prediction of
several extensions of the Standard Model of elementary particle interactions.
As a result, a considerable fraction of very-high-energy photons can escape
absorption from the BLR through the mechanism of photon-ALP oscillations much
in the same way as they largely avoid absorption from extragalactic background
light when propagating over cosmic distances in the presence of large-scale
magnetic fields in the nG range. In addition we show that the above MAGIC
observations and the simultaneous Fermi/LAT observations in the energy range
0.3 - 3 GeV can both be explained by a standard spectral energy distribution
for experimentally allowed values of the model parameters. In particular, we
need a very light ALP just like in the case of photon-ALP oscillations in
cosmic space. Moreover, we find it quite tantalizing that the most favorable
value of the photon-ALP coupling happens to be the same in both situations.
Although our ALPs cannot contribute to the cold dark matter, they are a viable
candidate for the quintessential dark energy. [abridged]Comment: 32 pages, 10 figures, accepted for publication in Physical Review
Flame front propagation IV: Random Noise and Pole-Dynamics in Unstable Front Propagation II
The current paper is a corrected version of our previous paper
arXiv:adap-org/9608001. Similarly to previous version we investigate the
problem of flame propagation. This problem is studied as an example of unstable
fronts that wrinkle on many scales. The analytic tool of pole expansion in the
complex plane is employed to address the interaction of the unstable growth
process with random initial conditions and perturbations. We argue that the
effect of random noise is immense and that it can never be neglected in
sufficiently large systems. We present simulations that lead to scaling laws
for the velocity and acceleration of the front as a function of the system size
and the level of noise, and analytic arguments that explain these results in
terms of the noisy pole dynamics.This version corrects some very critical
errors made in arXiv:adap-org/9608001 and makes more detailed description of
excess number of poles in system, number of poles that appear in the system in
unit of time, life time of pole. It allows us to understand more correctly
dependence of the system parameters on noise than in arXiv:adap-org/9608001Comment: 23 pages, 4 figures,revised, version accepted for publication in
journal "Combustion, Explosion and Shock Waves". arXiv admin note:
substantial text overlap with arXiv:nlin/0302021, arXiv:adap-org/9608001,
arXiv:nlin/030201
Flow and magnetic structures in a kinematic ABC-dynamo
Dynamo theory describes the magnetic field induced by the rotating, convecting and electrically conducting fluid in a celestial body. The classical ABC-flow model represents fast dynamo action, required to sustain such a magnetic field. In this letter, Lagrangian coherent structures (LCSs) in the ABC-flow are detected through Finite-time Lyapunov exponents (FTLE). The flow skeleton is identified by extracting intersections between repelling and attracting LCSs. For the case A = B = C = 1, the skeleton structures are made up from lines connecting two different types of stagnation points in the ABC-flow. The corresponding kinematic ABC-dynamo problem is solved using a spectral method, and the distribution of cigar-like magnetic structures visualized. Inherent links are found to exist between LCSs in the ABC-flow and induced magnetic structures, which provides insight into the mechanism behind the ABC-dynamo
Control of bulk superconductivity via surface-bound electric fields in ion-gated niobium nitride thin films
Ionic gating is a very popular tool to investigate and control the electric transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, often reaching values comparable to those occurring in metallic systems. Despite finding large success in tuning the phase diagram of low-carrier density systems, including cuprates and iron-based superconductors, its applicability to conventional metallic superconductors has received significantly less attention. In my talk, I will present the work which has been carried out in my research group over several years to investigate how ionic gating can tune the properties of metallic superconductor, using niobium nitride (NbN) as an emblematic case.
By fabricating EDL-FETs on NbN thin films with thickness ranging between 10 and 40 nm, we observed that small positive and negative shifts in the critical temperature Tc could be induced by changing the gate-voltage polarity, and that the magnitude of these shifts increased upon decreasing the film thickness. These findings indicated that, despite the gate-induced electric field being confined in a thin layer at the surface by electrostatic screening, the perturbation to the superconducting state extends in a region much larger than a single unit cell. Indeed, the dependence of Tc on the gate voltage and thickness could be reconciled with the Eliashberg theory of superconductivity only if this thin surface layer is coupled to the underlying, unperturbed bulk via proximity effect. We also determined that the thickness of this surface layer (i.e. the screening length of the electric field) strongly increases for large gate electric fields, reaching values of the order of 3 nm at the highest doping. Ab-initio DFT calculations reproduced these results and linked this anomalous increase of the screening length to a distortion of the pristine charge density in the material upon the application of sufficiently large electric fields.
This proximity-effect-induced transformation of the quasi-2D perturbation to the electron density into a 3D bulk modification of the superconducting properties seems to be a general behavior in gated superconductors that could hinder the possibility to obtain large Tc shifts in films thicker than the screening length. Consequently, we are currently focusing on exploring the tunability of ultrathin (< 5nm-thick) NbN films in order to maximize the gate-induced Tc shift, where we developed a novel technique of self-encapsulation in ultrathin niobium oxide to ensure the full reversibility of the gate modulation in these extremely sensitive devices
Photoisomerisation and light-induced morphological switching of a polyoxometalateâazobenzene hybrid
The functionalization of a spherical Keplerate-type polyoxometalate {Mo72V30} with a cationic azobenzene surfactant has been achieved through ionic self-assembly. The photoisomerisation reaction of this complex, which emerges in a light-triggered aggregationâdisaggregation process, has been followed by 1H NMR spectroscopy, dynamic light scattering, absorption spectroscopy and scanning electron microscopy analyses
Measurement of the t t-bar production cross section in the dilepton channel in pp collisions at sqrt(s) = 7 TeV
The t t-bar production cross section (sigma[t t-bar]) is measured in
proton-proton collisions at sqrt(s) = 7 TeV in data collected by the CMS
experiment, corresponding to an integrated luminosity of 2.3 inverse
femtobarns. The measurement is performed in events with two leptons (electrons
or muons) in the final state, at least two jets identified as jets originating
from b quarks, and the presence of an imbalance in transverse momentum. The
measured value of sigma[t t-bar] for a top-quark mass of 172.5 GeV is 161.9 +/-
2.5 (stat.) +5.1/-5.0 (syst.) +/- 3.6(lumi.) pb, consistent with the prediction
of the standard model.Comment: Replaced with published version. Included journal reference and DO
Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV
A search for the standard model Higgs boson in the H to ZZ to 2l 2nu decay
channel, where l = e or mu, in pp collisions at a center-of-mass energy of 7
TeV is presented. The data were collected at the LHC, with the CMS detector,
and correspond to an integrated luminosity of 4.6 inverse femtobarns. No
significant excess is observed above the background expectation, and upper
limits are set on the Higgs boson production cross section. The presence of the
standard model Higgs boson with a mass in the 270-440 GeV range is excluded at
95% confidence level.Comment: Submitted to JHE
Combined search for the quarks of a sequential fourth generation
Results are presented from a search for a fourth generation of quarks
produced singly or in pairs in a data set corresponding to an integrated
luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in
2011. A novel strategy has been developed for a combined search for quarks of
the up and down type in decay channels with at least one isolated muon or
electron. Limits on the mass of the fourth-generation quarks and the relevant
Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a
simple extension of the standard model with a sequential fourth generation of
fermions. The existence of mass-degenerate fourth-generation quarks with masses
below 685 GeV is excluded at 95% confidence level for minimal off-diagonal
mixing between the third- and the fourth-generation quarks. With a mass
difference of 25 GeV between the quark masses, the obtained limit on the masses
of the fourth-generation quarks shifts by about +/- 20 GeV. These results
significantly reduce the allowed parameter space for a fourth generation of
fermions.Comment: Replaced with published version. Added journal reference and DO
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