1,815 research outputs found
Propulsion apparatus and method using boil-off gas from a cryogenic liquid
A propulsion system and method are disclosed for controlling the attitude and drag of a space vehicle. A helium dewar contains liquid helium which cools an experiment package. The helium is heated or vented to keep the temperature between 1.5 and 1.7 degrees K to maintain adequate helium boil-off gas as a propellant without adversely affecting the experiment package which is contained in the helium dewar for protection from solar heating. The apparatus includes auxiliary heater and temperature sensor for controlling the temperature of the helium. The boil-off gas propellant is delivered to thruster modules to control vehicle attutude and compensate for drag
Perfect bell nozzle parametric and optimization curves
Nozzle contour data for untruncated Bell nozzles with expansion area ratios to 6100 and a specific heat ratio of 1.2 are provided. Curves for optimization of nozzles for maximum thrust coefficient within a given length, surface area, or area ratio are included. The nozzles are two dimensional axisymmetric and calculations were performed using the method of characteristics. Drag due to wall friction was included in the final thrust coefficient
Anatomy of the quantum melting of the two dimensional Wigner crystal
The Fermi liquid-Wigner crystal transition in a two dimensional electronic
system is revisited with a focus on the nature of the fixed node approximation
done in quantum Monte Carlo calculations. Recently, we proposed (Phys. Rev.
Lett. 94, 046801 (2005)) that for intermediate densities, a hybrid phase (with
the symmetry of the crystal but otherwise liquid like properties) is more
stable than both the liquid and the crystal phase. Here we confirm this result
both in the thermodynamic and continuum limit. The liquid-hybrid transition
takes place at rs=31.5 +/- 0.5. We find that the stability of the hybrid phase
with respect to the crystal one is tightly linked to its delocalized nature. We
discuss the implications of our results for various transition scenarii
(quantum hexatic phase, supersolid, multiple exchange, microemulsions) proposed
in the literature.Comment: 14 pages, 16 figure
P-wave Pairing and Colossal Magnetoresistance in Manganese Oxides
We point out that the existing experimental data of most manganese oxides
show the {\sl frustrated} p-wave superconducting condensation in the
ferromagnetic phase in the sense that the superconducting coherence is not long
enough to cover the whole system. The superconducting state is similar to the
state in superfluid He-3. The sharp drop of resistivity, the steep jump
of specific heat, and the gap opening in tunneling are well understood in terms
of the p-wave pairing. In addition, colossal magnetoresistance (CMR) is
naturally explained by the superconducting fluctuations with increasing
magnetic fields. The finite resistivity may be due to some magnetic
inhomogeneities. This study leads to the possibility of room temperature
superconductivity.Comment: LaTex, 14 pages, For more information, please send me an e-mail.
e-mail adrress : [email protected]
Many-body position operator in lattice fermionic systems with periodic boundary conditions
A total position operator in the position representation is derived for
lattice fermionic systems with periodic boundary conditions. The operator is
shown to be Hermitian, the generator of translations in momentum space, and its
time derivative is shown to correspond to the total current operator in a
periodic system. The operator is such that its moments can be calculated up to
any order. To demonstrate its utility finite size scaling is applied to the
Brinkman-Rice transition as well as metallic and insulating Gutzwiller
wavefunctions.Comment: to appear in Journal of Physics A: Mathematical and General
(reference will be added later
Dietary carotenoid availability, sexual signalling and functional fertility in sticklebacks
In species where males express carotenoid-based sexual signals, more intensely coloured males may be signalling their enhanced ability to combat oxidative stress. This may include mitigating deleterious oxidative damage to their sperm, and so be directly related to their functional fertility. Using a split-clutch in vitro fertilization technique and dietary carotenoid manipulation, we demonstrate that in non-competitive fertilization assays, male three-spined sticklebacks (Gasterosteus aculeatus) that are fed higher (but biologically relevant) levels of carotenoids had a significantly increased fertilization success, irrespective of maternal carotenoid intake. Furthermore, within diet groups, a male's fertilization success was positively related to the expression of his carotenoid-based nuptial coloration, with more intensely coloured males having higher functional fertility. These data provide, to our knowledge, the first demonstration that dietary access to carotenoids influences fertilization success, and suggest that females could use a male's nuptial coloration as an indicator of his functional fertility
Topological Insulators with Inversion Symmetry
Topological insulators are materials with a bulk excitation gap generated by
the spin orbit interaction, and which are different from conventional
insulators. This distinction is characterized by Z_2 topological invariants,
which characterize the groundstate. In two dimensions there is a single Z_2
invariant which distinguishes the ordinary insulator from the quantum spin Hall
phase. In three dimensions there are four Z_2 invariants, which distinguish the
ordinary insulator from "weak" and "strong" topological insulators. These
phases are characterized by the presence of gapless surface (or edge) states.
In the 2D quantum spin Hall phase and the 3D strong topological insulator these
states are robust and are insensitive to weak disorder and interactions. In
this paper we show that the presence of inversion symmetry greatly simplifies
the problem of evaluating the Z_2 invariants. We show that the invariants can
be determined from the knowledge of the parity of the occupied Bloch
wavefunctions at the time reversal invariant points in the Brillouin zone.
Using this approach, we predict a number of specific materials are strong
topological insulators, including the semiconducting alloy Bi_{1-x} Sb_x as
well as \alpha-Sn and HgTe under uniaxial strain. This paper also includes an
expanded discussion of our formulation of the topological insulators in both
two and three dimensions, as well as implications for experiments.Comment: 16 pages, 7 figures; published versio
Tilting instability and other anomalies in the flux-lattice in some magnetic superconductors
The flux-line lattice in the compound , which has a tendency to
ferromagnetic order in the a-b plane is studied with external magnetic field
direction close to the c-axis. We show the existence of an instability where
the direction of flux-lines spontaneously tilts away from that of the applied
field near the onset of ferromagnetic order. The enhanced fluctuations in the
flux lattice and the square flux lattice recently observed are explained and
further experiments suggested.Comment: 12 pages, Latex file, no figur
Metamagnetic Quantum Criticality
A renormalization group treatment of metamagnetic quantum criticality in
metals is presented. In clean systems the universality class is found to be of
the overdamped, conserving (dynamical exponent z=3) Ising type. Detailed
results are obtained for the field and temperature dependence of physical
quantities including the differential susceptibility, resistivity and specific
heat near the transition. An application of the theory is made to Sr3Ru2O7,
which appears to exhibit a metamagnetic critical end-point at a very low
temperature and a field of order 5-7T.Comment: 4 pages latex (Revtex 4) and 3 eps figure
Rival male chemical cues evoke changes in male pre- and post-copulatory investment in a flour beetle
Copyright © The Author 2015. Published by Oxford University Press on behalf of the
International Society for Behavioral Ecology.Supplementary material can be found at http://www.beheco.oxfordjournals.org/The files experiment 1, 2 and 3 are data files related to this paper ‘Rival male chemical cues evoke changes in male pre- and post-copulatory investment in a flour beetle’.Males can gather information on the risk and intensity of sperm competition from their social environment. Recent studies have implicated chemosensory cues, for instance cuticular hydrocarbons (CHCs) in insects, as a key source of this information. Here, using the broad-horned flour beetle (Gnatocerus cornutus), we investigated the importance of contact-derived rival male CHCs in informing male perception of sperm competition risk and intensity. We experimentally perfumed virgin females with male CHCs via direct intersexual contact and measured male pre- and post-copulatory investment in response to this manipulation. Using chemical analysis, we verified that this treatment engendered changes to perfumed female CHC profiles, but did not make perfumed females “smell” mated. Despite this, males responded to these chemical changes. Males increased courtship effort under low levels of perceived competition (from 1–3 rivals), but significantly decreased courtship effort as perceived competition rose (from 3–5 rivals). Furthermore, our measurement of ejaculate investment showed that males allocated significantly more sperm to perfumed females than to control females. Together, these results suggest that changes in female chemical profile elicited by contact with rival males do not provide males with information on female mating status, but rather inform males of the presence of rivals within the population and thus provide a means for males to indirectly assess the risk of sperm competition.Natural Environment Research CouncilRoyal SocietyLeverhulme Early Career Fellowshi
- …