392 research outputs found
Diagnosis and management of transient ischaemic attack and ischaemic stroke in the acute phase
Effect of multilayer barriers on the optical properties of GaInNAs single quantum-well structures grown by metalorganic vapor phase epitaxy
We report on the effects of combined strain-compensating and strain-mediating layers of various widths on the optical properties of 1.3 ÎŒm GaInNAsâGaAs single quantum well structures grown by metalorganic vapor phase epitaxy (MOVPE). While the emission wavelength of GaInNAsâGaAs quantum wells can be redshifted by the adoption of strain-compensated GaNAs layers, the material quality is degraded by the increased stress at the wellâbarrier interface. This detrimental effect can be cured by inserting a strain-mediating InGaAs layer between them. Contrary to what is expected, however, the emission wavelength is blueshifted by the insertion of the InGaAs layer, which is attributed to the reduced N incorporation due to the improved interface quality. Our results indicate that the optical properties of MOVPE-grown GaInNAsâGaAs quantum wells can be optimized in quantum efficiency and emission wavelength by combination of strain-compensating and strain-mediating layers with suitable characteristics
Spectroscopic characterization of 1.3”m GaInNAs quantum-well structures grown by metal-organic vapor phase epitaxy
We report optical studies of high-quality 1.3âÎŒm strain-compensated GaInNAs/GaAs single-quantum-well structures grown by metalorganic vapor phase epitaxy. Photoluminescence excitation (PLE) spectroscopy shows clearly the electronic structure of the two-dimensional quantum well. The transition energies between quantized states of the electrons and holes are in agreement with theoretical calculations based on the band anti-crossing model in which the localized N states interact with the extended states in the conduction band. We also investigated the polarization properties of the luminescence by polarized edge-emission measurements. Luminescence bands with different polarization characters arising from the electron to heavy-hole and light-hole transitions, respectively, have been identified and verify the transition assignment observed in the PLE spectrum
Optical spectroscopy of gan microcavities with thicknesses controlled using a plasma etch-back
The effect of an etch-back step to control the cavity length within GaN-based microcavities formed between two dielectric Bragg mirrors was investigated using photoluminescence and reflectivity. The structures are fabricated using a combination of a laser lift-off technique to separate epitaxial III-N layers from their sapphire substrates and electron-beam evaporation to deposit silica/zirconia multilayer mirrors. The photoluminescence measurements reveal cavity modes from both etched and nonetched microcavities. Similar cavity finesses are measured for 2.0 and 0.8 mm GaN cavities fabricated from the same wafer, indicating that the etchback has had little effect on the microcavity quality. For InGaN quantum well samples the etchback is shown to allow controllable reduction of the cavity length. Two etch steps of 100 nm are demonstrated with an accuracy of approximately 5%. The etchback, achieved using inductively coupled plasma and wet chemical etching, allows removal of the low-quality GaN nucleation layer, control of the cavity length, and modification of the surface resulting from lift-off
Minimal Supersymmetric Pati-Salam Theory: Determination of Physical Scales
We systematically study the minimal supersymmetric Pati-Salam theory, paying
special attention to the unification constraints. We find that the SU(4)_c
scale M_c and the Left-Right scale M_R lie in the range 10^{10} GeV < M_c <
10^{14} GeV, 10^{3} GeV < M_R <10^{10} GeV (with single-step breaking at
10^{10} GeV), giving a potentially accessible scale of parity breaking. The
theory includes the possibility of having doubly-charged supermultiplets at the
supersymmetry breaking scale; color octet states with mass of order M_R^2/M_c;
magnetic monopoles of intermediate mass that do not conflict with cosmology,
and a 'clean' (type I) form for the see-saw mechanism of neutrino mass.Comment: 5 page
Magnetic fields and Sunyaev-Zel'dovich effect in galaxy clusters
In this work we study the contribution of magnetic fields to the Sunyaev
Zeldovich (SZ) effect in the intracluster medium. In particular we calculate
the SZ angular power spectrum and the central temperature decrement. The effect
of magnetic fields is included in the hydrostatic equilibrium equation by
splitting the Lorentz force into two terms one being the force due to magnetic
pressure which acts outwards and the other being magnetic tension which acts
inwards. A perturbative approach is adopted to solve for the gas density
profile for weak magnetic fields (< 4 micro G}). This leads to an enhancement
of the gas density in the central regions for nearly radial magnetic field
configurations. Previous works had considered the force due to magnetic
pressure alone which is the case only for a special set of field
configurations. However, we see that there exists possible sets of
configurations of ICM magnetic fields where the force due to magnetic tension
will dominate. Subsequently, this effect is extrapolated for typical field
strengths (~ 10 micro G) and scaling arguments are used to estimate the angular
power due to secondary anisotropies at cluster scales. In particular we find
that it is possible to explain the excess power reported by CMB experiments
like CBI, BIMA, ACBAR at l > 2000 with sigma_8 ~ 0.8 (WMAP 5 year data) for
typical cluster magnetic fields. In addition we also see that the magnetic
field effect on the SZ temperature decrement is more pronounced for low mass
clusters ( ~ 2 keV). Future SZ detections of low mass clusters at few arc
second resolution will be able to probe this effect more precisely. Thus, it
will be instructive to explore the implications of this model in greater detail
in future works.Comment: 20 pages, 8 figure
Implications of the HERA Events for the R-Parity Breaking SUSY Signals at Tevatron
The favoured R-parity violating SUSY scenarios for the anomalous HERA events
correspond to top and charm squark production via the and
couplings. In both cases the corresponding electronic
branching fractions of the squarks are expected to be . Consequently the
canonical leptoquark signature is incapable of probing these scenarios at the
Tevatron collider over most of the MSSM parameter space. We suggest alternative
signatures for probing them at Tevatron, which seem to be viable over the
entire range of MSSM parameters.Comment: 20 pages Latex file with 4 ps files containing 4 figure
Neutrino masses in R-parity violating supersymmetric models
We study neutrino masses and mixing in R-parity violating supersymmetric
models with generic soft supersymmetry breaking terms. Neutrinos acquire masses
from various sources: Tree level neutrino--neutralino mixing and loop effects
proportional to bilinear and/or trilinear R-parity violating parameters. Each
of these contributions is controlled by different parameters and have different
suppression or enhancement factors which we identified. Within an Abelian
horizontal symmetry framework these factors are related and specific
predictions can be made. We found that the main contributions to the neutrino
masses are from the tree level and the bilinear loops and that the observed
neutrino data can be accommodated once mild fine-tuning is allowed.Comment: 18 pages; minor typos corrected. To be published in Physical Review
Lepton Flavour Violating Leptonic/Semileptonic Decays of Charged Leptons in the Minimal Supersymmetric Standard Model
We consider the leptonic and semileptonic (SL) lepton flavour violating (LFV)
decays of the charged leptons in the minimal supersymmetric standard model
(MSSM). The formalism for evaluation of branching fractions for the SL LFV
charged-lepton decays with one or two pseudoscalar mesons, or one vector meson
in the final state, is given. Previous amplitudes for the SL LFV charged-lepton
decays in MSSM are improved, for instance the -penguin amplitude is
corrected to assure the gauge invariance. The decays are studied not only in
the model-independent formulation of the theory in the frame of MSSM, but also
within the frame of the minimal supersymmetric SO(10) model within which the
parameters of the MSSM are determined. The latter model gives predictions for
the neutrino-Dirac Yukawa coupling matrix, once free parameters in the model
are appropriately fixed to accommodate the recent neutrino oscillation data.
Using this unambiguous neutrino-Dirac Yukawa couplings, we calculate the LFV
leptonic and SL decay processes assuming the minimal supergravity scenario. A
very detailed numerical analysis is done to constrain the MSSM parameters.
Numerical results for SL LFV processes are given, for instance for tau -> e
(mu) pi0, tau -> e (mu) eta, tau -> e (mu) eta', tau -> e (mu) rho0, tau -> e
(mu) phi, tau -> e (mu) omega, etc.Comment: 36 pages, 3 tables, 5 .eps figure
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