166,571 research outputs found
Time-dependent density functional theory for strong electromagnetic fields in crystalline solids
We apply the coupled dynamics of time-dependent density functional theory and
Maxwell equations to the interaction of intense laser pulses with crystalline
silicon. As a function of electromagnetic field intensity, we see several
regions in the response. At the lowest intensities, the pulse is reflected and
transmitted in accord with the dielectric response, and the characteristics of
the energy deposition is consistent with two-photon absorption. The absorption
process begins to deviate from that at laser intensities ~ 10^13 W/cm^2, where
the energy deposited is of the order of 1 eV per atom. Changes in the
reflectivity are seen as a function of intensity. When it passes a threshold of
about 3 \times 1012 W/cm2, there is a small decrease. At higher intensities,
above 2 \times 10^13 W/cm^2, the reflectivity increases strongly. This behavior
can be understood qualitatively in a model treating the excited electron-hole
pairs as a plasma.Comment: 27 pages; 11 figure
Chemical Constraints on the Water and Total Oxygen Abundances in the Deep Atmosphere of Saturn
Thermochemical equilibrium and kinetic calculations for the trace gases CO,
PH3, and SiH4 give three independent constraints on the water and total oxygen
abundances of Saturn's deep atmosphere. A lower limit to the water abundance of
H2O/H2 > 1.7 x 10^-3 is given by CO chemistry while an upper limit of H2O/H2 <
5.5 x 10^-3 is given by PH3 chemistry. A combination of the CO and PH3
constraints indicates a water enrichment on Saturn of 1.9 to 6.1 times the
solar system abundance (H2O/H2 = 8.96 x 10^-4). The total oxygen abundance must
be at least 1.7 times the solar system abundance (O/H2 = 1.16 x 10^-3) in order
for the SiH4 to remain below a detection limit of SiH4/H2 < 2 x 10^-10. A
combination of the CO, PH3, and SiH4 constraints suggests that the total oxygen
abundance on Saturn is 3.2 to 6.4 times the solar system abundance. Our results
indicate that oxygen on Saturn is less enriched than other heavy elements (such
as C and P) relative to a solar system composition. This work was supported by
NASA NAG5-11958.Comment: 19 pages, 2 figures, accepted for publication in the Astrophysical
Journa
Antiferromagnetic Spin Fluctuation above the Superconducting Dome and the Full-Gaps Superconducting State in LaFeAsO1-xFx Revealed by 75As-Nuclear Quadrupole Resonance
We report a systematic study by 75As nuclear-quadrupole resonance in
LaFeAsO1-xFx. The antiferromagnetic spin fluctuation (AFSF) found above the
magnetic ordering temperature TN = 58 K for x = 0.03 persists in the regime
0.04 < x < 0.08 where superconductivity sets in. A dome-shaped x-dependence of
the superconducting transition temperature Tc is found, with the highest Tc =
27 K at x = 0.06 which is realized under significant AFSF. With increasing x
further, the AFSF decreases, and so does Tc. These features resemble closely
the cuprates La2-xSrxCuO4. In x = 0.06, the spin-lattice relaxation rate (1/T1)
below Tc decreases exponentially down to 0.13 Tc, which unambiguously indicates
that the energy gaps are fully-opened. The temperature variation of 1/T1 below
Tc is rendered nonexponential for other x by impurity scattering.Comment: 5 pages, 5 figures, more references adde
A proof-theoretic trust and reputation model for VANET
Vehicular Ad Hoc Networks (VANETs) are an important component of intelligent transportation systems, which are set to become part of global transportation infrastructure in the near future. In the context of such networks, security requirements need to rely on a combination of reputation of communicating agents and trust relations over the messaging framework. This is crucial in order to maintain dynamic and safe behaviour under all circumstances. Formal correctness, resolution of contradictions and proven safety of transitive operations in the presence of reputation and trust within the infrastructure remain mostly unexplored issues. This could lead to potentially disastrous situations, putting lives at risk. In this paper we provide a proof-theoretic interpretation of a reputation and trust model for VANET. This allows for formal verification through translation into the Coq proof assistant, and can guarantee consistency of messaging protocols and security of transitive transmissions
A computationally grounded, weighted doxastic logic
Modelling, reasoning and verifying complex situations involving a system of agents is crucial in all phases of the development of a number of safety-critical systems. In particular, it is of fundamental importance to have tools and techniques to reason about the doxastic and epistemic states of agents, to make sure that the agents behave as intended. In this paper we introduce a computationally grounded logic called COGWED and we present two types of semantics that support a range of practical situations. We provide model checking algorithms, complexity characterisations and a prototype implementation. We validate our proposal against a case study from the avionic domain: we assess and verify the situational awareness of pilots flying an aircraft with several automated components in off-nominal conditions
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