3,034 research outputs found
Entropy Change through Rayleigh-B\'enard Convective Transition with Rigid Boundaries
The previous investigation on Rayleigh-B\'enard convection of a dilute
classical gas [T. Kita: J. Phys. Soc. Jpn. {\bf 75} (2006) 124005] is extended
to calculate entropy change of the convective transition with the rigid
boundaries. We obtain results qualitatively similar to those of the stress-free
boundaries. Above the critical Rayleigh number, the roll convection is realized
among possible steady states with periodic structures, carrying the highest
entropy as a function of macroscopic mechanical variables.Comment: 5 pages, 4 figure
A theory of new type of heavy-electron superconductivity in PrOs_4Sb_12: quadrupolar-fluctuation mediated odd-parity pairings
It is shown that unconventional nature of superconducting state of
PrOs_4Sb_12, a Pr-based heavy electron compound with the filled-Skutterudite
structure, can be explained in a unified way by taking into account the
structure of the crystalline-electric-field (CEF) level, the shape of the Fermi
surface determined by the band structure calculation, and a picture of the
quasiparticles in f-configuration with magnetically singlet CEF ground
state. Possible types of pairing are narrowed down by consulting recent
experimental results. In particular, the chiral "p"-wave states such as
p_x+ip_y is favoured under the magnetic field due to the orbital Zeeman effect,
while the "p"-wave states with two-fold symmetery such as p_x can be stabilized
by a feedback effect without the magnetic field. It is also discussed that the
double superconducting transition without the magnetic field is possible due to
the spin-orbit coupling of the "triplet" Cooper pairs in the chiral state.Comment: 12 pages, 2 figures, submitted to J. Phys.: Condens. Matter Lette
Entropy and Spin Susceptibility of s-wave Type-II Superconductors near
A theoretical study is performed on the entropy and the spin
susceptibility near the upper critical field of s-wave
type-II superconductors with arbitrary impurity concentrations. The changes of
these quantities through may be expressed as , for example, where is the average flux density
and denotes entropy in the normal state. It is found that the
slopes and at T=0 are identical, connected
directly with the zero-energy density of states, and vary from 1.72 in the
dirty limit to in the clean limit. This mean-free-path dependence
of and at T=0 is quantitatively the same as that
of the slope for the flux-flow resistivity studied
previously. The result suggests that and near
T=0 are convex downward (upward) in the dirty (clean) limit, deviating
substantially from the linear behavior . The specific-heat
jump at also shows fairly large mean-free-path dependence.Comment: 8 pages, 5 figure
Current-driven Magnetization Reversal in a Ferromagnetic Semiconductor (Ga,Mn)As/GaAs/(Ga,Mn)As Tunnel Junction
Current-driven magnetization reversal in a ferromagnetic semiconductor based
(Ga,Mn)As/GaAs/(Ga,Mn)As magnetic tunnel junction is demonstrated at 30 K.
Magnetoresistance measurements combined with current pulse application on a
rectangular 1.5 x 0.3 um^2 device revealed that magnetization switching occurs
at low critical current densities of 1.1 - 2.2 x 10^5 A/cm^2 despite the
presence of spin-orbit interaction in the p-type semiconductor system. Possible
mechanisms responsible for the effect are discussed.Comment: 16 pages, 4 figure
Self-Consistent Approximations for Superconductivity beyond the Bardeen-Cooper-Schrieffer Theory
We develop a concise self-consistent perturbation expansion for
superconductivity where all the pair processes are naturally incorporated
without drawing "anomalous" Feynman diagrams. This simplification results from
introducing an interaction vertex that is symmetric in the particle-hole
indices besides the ordinary space-spin coordinates. The formalism
automatically satisfies conservation laws, includes the Luttinger-Ward theory
as the normal-state limit, and reproduces the Bardeen-Cooper-Schrieffer theory
as the lowest-order approximation. It enables us to study the thermodynamic,
single-particle, two-particle, and dynamical properties of superconductors with
competing fluctuations based on a single functional of Green's
function in the Nambu space. Specifically, we derive closed equations
in the FLEX-S approximation, i.e., the fluctuation exchange approximation for
superconductivity with all the pair processes, which contains extra terms
besides those in the standard FLEX approximation.Comment: 14 pages, 6 figure
Quasiparticles of d-wave superconductors in finite magnetic fields
We study quasiparticles of d-wave superconductors in the vortex lattice by
self-consistently solving the Bogoliubov-de Gennes equations. It is found for a
pure state that: (i) low-energy quasiparticle bands in the
magnetic Brillouin zone have rather large dispersion even in low magnetic
fields, indicating absense of bound states for an isolated vortex; (ii) in
finite fields with small, the calculated tunneling conductance at
the vortex core shows a double-peak structure near zero bias, as qualitatively
consistent with the STM experiment by Maggio-Aprile et al. [Phys. Rev. Lett.
{\bf 75} (1995) 2754]. We also find that mixing of a - or an s-wave
component, if any, develops gradually without transitions as the field is
increased, having little effect on the tunneling spectra.Comment: 4 pages, 4 figures, LaTe
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Measurements of reactive nitrogen produced by tropical thunderstorms during BIBLE-C
The Biomass Burning and Lightning Experiment phase C (BIBLE-C) aircraft mission was carried out near Darwin, Australia (12°S, 131°E) in December 2000. This was the first aircraft experiment designed to estimate lightning NO production rates in the tropics, where production is considered to be most intense. During the two flights (flights 10 and 13 made on December 9 and 11-12, respectively) enhancements of NOx (NO + NO2) up to 1000 and 1600 parts per trillion by volume (pptv, 10-s data) were observed at altitudes between 11.5 and 14 km. The Geostationary Meteorological Satellite (GMS) cloud (brightness temperature) data and ground-based lightning measurements by the Global Positioning and Tracking System (GPATS) indicate that there were intensive lightning events over the coast of the Gulf of Carpentaria, which took place upstream from our measurement area 10 to 14 h prior to the measurements. For these two flights, air in which NOx exceeded 100 pptv extended over 620 × 140 and 400 × 170 km2 (wind direction x perpendicular direction), respectively, suggesting a significant impact of lightning NO production on NOx levels in the tropics. We estimate the amount of NOx observed between 11.5 and 14 km produced by the thunderstorms to be 3.3 and 1.8 × 1025 NO molecules for flights 10 and 13, respectively. By using the GPATS lightning flash count data, column NO production rates are estimated to be 1.9-4.4 and 21-49 × 1025 NO molecules per single flash for these two flight data sets. In these estimations, it is assumed that the column NO production between 0 and 16 km is greater than the observed values between 11.5 and 14 km by a factor of 3.2, which is derived using results reported by Pickering et al. (1998). There are however large uncertainties in the GPATS lightning data in this study and care must be made when the production rates are referred. Uncertainties in these estimates are discussed. The impact on the ozone production rate is also described. Copyright 2007 by the American Geophysical Union
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