17,777 research outputs found
Exobiology experiment concepts for space station
The exobiology discipline uses ground based and space flight resources to conduct a multidiscipline research effort dedicated to understanding fundamental questions about the origin, evolution, and distribution of life and life related molecules throughout the universe. Achievement of this understanding requires a methodical research strategy which traces the history of the biogenic elements from their origins in stellar formation processes through the chemical evolution of molecules essential for life to the origin and evolution of primitive and, ultimately, complex living species. Implementation of this strategy requires the collection and integration of data from solar system exploration spacecraft and ground based and orbiting observatories and laboratories. The Science Lab Module (SLM) of the Space Station orbiting complex may provide an ideal setting in which to perform certain classes of experiments which form the cornerstone of exobiology research. These experiments could demonstrate the pathways and processes by which biomolecules are synthesized under conditions that simulate the primitive Earth, planetary atmospheres, cometary ices, and interstellar dust grains. Exobiology experiments proposed for the Space Station generally fall into four classes: interactions among gases and grains (nucleation, accretion, gas-grain reactions), high energy chemistry for the production of biomolecules, physical and chemical processes occurring on an artificial comet, and tests of the theory of panspermia
Phase diagram for the quantum Hall state in monolayer graphene
The quantum Hall state in a defect-free graphene sample is studied
within the framework of quantum Hall ferromagnetism. We perform a systematic
analysis of the pseudospin anisotropies, which arise from the valley and
sublattice asymmetric short-range electron-electron (e-e) and electron-phonon
(e-ph) interactions. The phase diagram, obtained in the presence of generic
pseudospin anisotropy and the Zeeman effect, consists of four phases
characterized by the following orders: spin-polarized ferromagnetic, canted
antiferromagnetic, charge density wave, and Kekul\'{e} distortion. We take into
account the Landau level mixing effects and show that they result in the key
renormalizations of parameters. First, the absolute values of the anisotropy
energies become greatly enhanced and can significantly exceed the Zeeman
energy. Second, the signs of the anisotropy energies due to e-e interactions
can change upon renormalization. A crucial consequence of the latter is that
the short-range e-e interactions alone could favor any state on the phase
diagram, depending on the details of interactions at the lattice scale. On the
other hand, the leading e-ph interactions always favor the Kekul\'{e}
distortion order. The possibility of inducing phase transitions by tilting the
magnetic field is discussed.Comment: 25 pages, 19 figs; v2: nearly identical to the published version,
some stylistic improvements, Tables I-IV added, anisotropy energies redefined
as u -> u/2 for aesthetic reaso
Reflection above the barrier as tunneling in momentum space
Quantum mechanics predicts an exponentially small probability that a particle
with energy greater than the height of a potential barrier will nevertheless
reflect from the barrier in violation of classical expectations. This process
can be regarded as tunneling in momentum space, leading to a simple derivation
of the reflection probability.Comment: 7 pages, 3 figures, submitted to American Journal of Physics. Version
2: MIT preprint number added, typographical error in caption to Figure 2
correcte
Cosmic microwave background constraints on the epoch of reionization
We use a compilation of cosmic microwave anisotropy data to constrain the
epoch of reionization in the Universe, as a function of cosmological
parameters. We consider spatially-flat cosmologies, varying the matter density
(the flatness being restored by a cosmological constant), the Hubble
parameter and the spectral index of the primordial power spectrum. Our
results are quoted both in terms of the maximum permitted optical depth to the
last-scattering surface, and in terms of the highest allowed reionization
redshift assuming instantaneous reionization. For critical-density models,
significantly-tilted power spectra are excluded as they cannot fit the current
data for any amount of reionization, and even scale-invariant models must have
an optical depth to last scattering of below 0.3. For the currently-favoured
low-density model with and a cosmological constant, the
earliest reionization permitted to occur is at around redshift 35, which
roughly coincides with the highest estimate in the literature. We provide
general fitting functions for the maximum permitted optical depth, as a
function of cosmological parameters. We do not consider the inclusion of tensor
perturbations, but if present they would strengthen the upper limits we quote.Comment: 9 pages LaTeX file with ten figures incorporated (uses mn.sty and
epsf). Corrects some equation typos, superseding published versio
Phase Structure of 2-Flavor Quark Matter: Heterogeneous Superconductors
We analyze the free energy of charge and color neutral 2-flavor quark matter
within the BCS approximation. We consider both the homogeneous gapless
superconducting phase and the heterogeneous mixed phase where normal and BCS
superconducting phases coexist. We calculate the surface tension between normal
and superconducting phases and use it to compare the free energies of the
gapless and mixed phases. Our calculation, which retains only the leading order
gradient contribution to the free energy, indicates that the mixed phase is
energetically favored over an interesting range of densities of relevance to 2
flavor quark matter in neutron stars.Comment: 11 pages, 4 figures. Major Revisions. Includes a detailed discussion
of the kinetic terms of the effective theory, instabilities of the gapless
phase and the charge neutral phase diagra
Mathematical modelling and experimental validation of electrostatic sensors for rotational speed measurement
Recent research has demonstrated that electrostatic sensors can be applied to the measurement of rotational speed with excellent repeatability and accuracy under a range of conditions. However, the sensing mechanism and fundamental characteristics of the electrostatic sensors are still largely unknown and hence the design of the sensors is not optimised for rotational speed measurement. This paper presents the mathematical modelling of strip electrostatic sensors for rotational speed measurement and associated experimental studies for the validation of the modelling results. In the modelling, an ideal point charge on the surface of the rotating object is regarded as an impulse input to the sensing system. The fundamental characteristics of the sensor, including spatial sensitivity, spatial filtering length and signal bandwidth, are quantified from the developed model. The effects of the geometric dimensions of the electrode, the distance between the electrode and the rotor surface and the rotational speed being measured on the performance of the sensor are analyzed. A close agreement between the modelling results and experimental measurements has been observed under a range of conditions. Optimal design of the electrostatic sensor for a given rotor size is suggested and discussed in accordance with the modelling and experimental results
The plasmonic eigenvalue problem
A plasmon of a bounded domain is a non-trivial
bounded harmonic function on which is
continuous at and whose exterior and interior normal
derivatives at have a constant ratio. We call this ratio a
plasmonic eigenvalue of . Plasmons arise in the description of
electromagnetic waves hitting a metallic particle . We investigate
these eigenvalues and prove that they form a sequence of numbers converging to
one. Also, we prove regularity of plasmons, derive a variational
characterization, and prove a second order perturbation formula. The problem
can be reformulated in terms of Dirichlet-Neumann operators, and as a side
result we derive a formula for the shape derivative of these operators.Comment: 22 pages; replacement 8-March-14: minor corrections; to appear in
Review in Mathematical Physic
Spin-orbit mediated anisotropic spin interaction in interacting electron systems
We investigate interactions between spins of strongly correlated electrons
subject to the spin-orbit interaction. Our main finding is that of a novel,
spin-orbit mediated anisotropic spin-spin coupling of the van der Waals type.
Unlike the standard exchange, this interaction does not require the wave
functions to overlap. We argue that this ferromagnetic interaction is important
in the Wigner crystal state where the exchange processes are severely
suppressed. We also comment on the anisotropy of the exchange between spins
mediated by the spin-orbital coupling.Comment: 4.1 pages, 1 figure; (v2) minor changes, published versio
Consistent Quantum Counterfactuals
An analysis using classical stochastic processes is used to construct a
consistent system of quantum counterfactual reasoning. When applied to a
counterfactual version of Hardy's paradox, it shows that the probabilistic
character of quantum reasoning together with the ``one framework'' rule
prevents a logical contradiction, and there is no evidence for any mysterious
nonlocal influences. Counterfactual reasoning can support a realistic
interpretation of standard quantum theory (measurements reveal what is actually
there) under appropriate circumstances.Comment: Minor modifications to make it agree with published version. Latex 8
pages, 2 figure
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