80 research outputs found
Degradation mechanism analysis in temperature stress tests on III-V ultra-high concentrator solar cells using a 3D distributed model
A temperature stress test was carried out on GaAs single-junction solar cells to analyze the degradation suffered when working at ultra-high concentrations. The acceleration of the degradation was realized at two different temperatures: 130 °C and 150 °C. In both cases, the degradation trend was the same, and only gradual failures were observed. A fit of the dark IâV curve at 25 °C with a 3D distributed model before and after the test was done. The fit with the 3D distributed model revealed degradation at the perimeter because the recombination current in the depletion region of the perimeter increased by about fourfold after the temperature stress test. Therefore, this test did not cause any morphological change in the devices, and although the devices were isolated with silicone, the perimeter region was revealed as the most fragile component of the solar cell. Consequently, the current flowing beneath the busbar favors the progression of defects in the device in the perimeter region
RG flows from Spin(7), CY 4-fold and HK manifolds to AdS, Penrose limits and pp waves
We obtain explicit realizations of holographic renormalization group (RG)
flows from M-theory, from E^{2,1} \times Spin(7) at UV to AdS_4 \times
\tilde{S^7} (squashed S^7) at IR, from E^{2,1} \times CY4 at UV to AdS_4 \times
Q^{1,1,1} at IR, and from E^{2,1} \times HK (hyperKahler) at UV to AdS_4 \times
N^{0,1,0} at IR. The dual type IIA string theory configurations correspond to
D2-D6 brane systems where D6 branes wrap supersymmetric four-cycles. We also
study the Penrose limits and obtain the pp-wave backgrounds for the above
configurations. Besides, we study some examples of non-supersymmetric and
supersymmetric flows in five-dimensional gauge theories.Comment: 42 pages, 6 eps figures, typos and misprints correcte
The Cauchy problem of f(R) gravity
The initial value problem of metric and Palatini f(R)gravity is studied by
using the dynamical equivalence between these theories and Brans-Dicke gravity.
The Cauchy problem is well-formulated for metric f(R)gravity in the presence of
matter and well-posed in vacuo. For Palatini f(R)gravity, instead, the Cauchy
problem is not well-formulated.Comment: 16 latex pages, to appear in Class. Quantum Grav; typographical
errors corrected, new references adde
General Brane Geometries from Scalar Potentials: Gauged Supergravities and Accelerating Universes
We find broad classes of solutions to the field equations for d-dimensional
gravity coupled to an antisymmetric tensor of arbitrary rank and a scalar field
with non-vanishing potential. Our construction generates these configurations
from the solution of a single nonlinear ordinary differential equation, whose
form depends on the scalar potential. For an exponential potential we find
solutions corresponding to brane geometries, generalizing the black p-branes
and S-branes known for the case of vanishing potential. These geometries are
singular at the origin with up to two (regular) horizons. Their asymptotic
behaviour depends on the parameters of the model. When the singularity has
negative tension or the cosmological constant is positive we find
time-dependent configurations describing accelerating universes. Special cases
give explicit brane geometries for (compact and non-compact) gauged
supergravities in various dimensions, as well as for massive 10D supergravity,
and we discuss their interrelation. Some examples lift to give new solutions to
10D supergravity. Limiting cases with a domain wall structure preserve part of
the supersymmetries of the vacuum. We also consider more general potentials,
including sums of exponentials. Exact solutions are found for these with up to
three horizons, having potentially interesting cosmological interpretation. We
give several additional examples which illustrate the power of our techniques.Comment: 54 pages, 6 figures. Uses JHEP3. Published versio
Search for subsolar-mass black hole binaries in the second part of Advanced LIGOâs and Advanced Virgoâs third observing run
We describe a search for gravitational waves from compact binaries with at least one component with mass 0.2âMââ1.0âMâ and mass ratio q â„ 0.1 in Advanced LIGO and Advanced Virgo data collected between 1 November 2019, 15:00 UTC and 27 March 2020, 17:00 UTC. No signals were detected. The most significant candidate has a false alarm rate of 0.2yrâ1
â . We estimate the sensitivity of our search over the entirety of Advanced LIGOâs and Advanced Virgoâs third observing run, and present the most stringent limits to date on the merger rate of binary black holes with at least one subsolar-mass component. We use the upper limits to constrain two fiducial scenarios that could produce subsolar-mass black holes: primordial black holes (PBH) and a model of dissipative dark matter. The PBH model uses recent prescriptions for the merger rate of PBH binaries that include a rate suppression factor to effectively account for PBH early binary disruptions. If the PBHs are monochromatically distributed, we can exclude a dark matter fraction in PBHs fPBH âłâ0.6 (at 90% confidence) in the probed subsolar-mass range. However, if we allow for broad PBH mass distributions we are unable to rule out fPBH = 1. For the dissipative model, where the dark matter has chemistry that allows a small fraction to cool and collapse into black holes, we find an upper bound fDBH < 10â5 on the fraction of atomic dark matter collapsed into black holes
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