525 research outputs found
First principles quasiparticle damping rates in bulk lead
First principles calculations of the damping rates (inverse inelastic
lifetimes) of low energy quasiparticles in bulk Pb are presented. Damping rates
are obtained both for excited electrons and holes with energies up to 8 eV on a
set of k vectors throughout the Brillouin zone (BZ). Strong localization
effects in the calculated lifetimes are found. Averaged over the BZ inelastic
lifetimes versus quasiparticle energy are reported as well. In addition, the
effect of the spin-orbit induced splitting in the band structure on the
calculated lifetimes in Pb is investigated.Comment: 10 pages, 8 figures, 5 table
Pressure on charged domain walls and additional imprint mechanism in ferroelectrics
The impact of free charges on the local pressure on a charged ferroelectric
domain wall produced by an electric field has been analyzed. A general formula
for the local pressure on a charged domain wall is derived considering full or
partial compensation of bound polarization charges by free charges. It is shown
that the compensation can lead to a very strong reduction of the pressure
imposed on the wall from the electric field. In some cases this pressure can be
governed by small nonlinear effects. It is concluded that the free charge
compensation of bound polarization charges can lead to substantial reduction of
the domain wall mobility even in the case when the mobility of free charge
carriers is high. This mobility reduction gives rise to an additional imprint
mechanism which may play essential role in switching properties of
ferroelectric materials. The effect of the pressure reduction on the
compensated charged domain walls is illustrated for the case of 180-degree
ferroelectric domain walls and of 90-degree ferroelectric domain walls with the
head-to-head configuration of the spontaneous polarization vectors.Comment: subm. to PRB. This verion is extended by appendi
A dynamically consistent analysis of circulation and transports in the southwestern Weddell Sea
International audienceAn inverse model is applied for the analysis of hydrographic and current meter data collected on the repeat WOCE section SR4 in the Weddell Sea in 1989?1992. The section crosses the Weddell Sea cyclonic gyre from Kapp Norvegia to the northern end of the Antarctic Peninsula. The concepts of geostrophy, conservation of planetary vorticity and hydrostatics are combined with advective balances of active and passive properties to provide a dynamically consistent circulation pattern. Our variational assimilation scheme allows the calculation of three-dimensional velocities in the section plane. Current speeds are small except along the coasts where they reach up to 12 cm/s. We diagnose a gyre transport of 34 Sverdrup which is associated with a poleward heat transport of 28×1012 W corresponding to an average heat flux of 15 Wm?2 in the Weddell Sea south of the transect. This exceeds the estimated local flux on the transect of 2 Wm?2. As the transect is located mostly in the open ocean, we conclude that the shelf areas contribute significantly to the ocean-atmosphere exchange and are consequently key areas for the contribution of the Weddell Sea to global ocean ventilation. Conversion of water masses occuring south of the section transform 6.6±1.1 Sv of the inflowing warm deep water into approximately equal amounts of Weddell Sea deep water and Weddell Sea bottom water. The volume transport of surface water equals in the in- and outflow. This means that almost all newly formed surface water is involved in the deep and bottom water formation. Comparison with the results obtained by pure velocity interpolation combined with a hydrographic data subset indicates major differences in the derived salt transports and the water mass conversion of the surface water. The differences can be explained by deviations in the structure of the upper ocean currents to which shelf areas contribute significantly. Additionally a rigorous variance analysis is performed. When only hydrographic data are used for the inversion both the gyre transport and the poleward heat transport are substantially lower. They amount to less than 40% of our best estimate while the standard deviations of both quantities are 6.5 Sv and 37×1012 W, respectively. With the help of long-term current meter measurements these errors can be reduced to 2 Sv and 8×1012 W. Our result underlines the importance of velocity data or equivalent information that helps to estimate the absolute velocities
Quasiparticle dynamics in ferromagnetic compounds of the Co-Fe and Ni-Fe systems
We report a theoretical study of the quasiparticle lifetime and the
quasiparticle mean free path caused by inelastic electron-electron scattering
in ferromagnetic compounds of the Co-Fe and Ni-Fe systems. The study is based
on spin-polarized calculations, which are performed within the
approximation for equiatomic and Co- and Ni-rich compounds, as well as for
their constituents. We mainly focus on the spin asymmetry of the quasiparticle
properties, which leads to the spin-filtering effect experimentally observed in
spin-dependent transport of hot electrons and holes in the systems under study.
By comparing with available experimental data on the attenuation length, we
estimate the contribution of the inelastic mean free path to the latter.Comment: 10 pages, 10 figure
Exact Asymptotic Results for a Model of Sequence Alignment
Finding analytically the statistics of the longest common subsequence (LCS)
of a pair of random sequences drawn from c alphabets is a challenging problem
in computational evolutionary biology. We present exact asymptotic results for
the distribution of the LCS in a simpler, yet nontrivial, variant of the
original model called the Bernoulli matching (BM) model which reduces to the
original model in the large c limit. We show that in the BM model, for all c,
the distribution of the asymptotic length of the LCS, suitably scaled, is
identical to the Tracy-Widom distribution of the largest eigenvalue of a random
matrix whose entries are drawn from a Gaussian unitary ensemble. In particular,
in the large c limit, this provides an exact expression for the asymptotic
length distribution in the original LCS problem.Comment: 4 pages Revtex, 2 .eps figures include
Random Operator Approach for Word Enumeration in Braid Groups
We investigate analytically the problem of enumeration of nonequivalent
primitive words in the braid group B_n for n >> 1 by analysing the random word
statistics and the target space on the basis of the locally free group
approximation. We develop a "symbolic dynamics" method for exact word
enumeration in locally free groups and bring arguments in support of the
conjecture that the number of very long primitive words in the braid group is
not sensitive to the precise local commutation relations. We consider the
connection of these problems with the conventional random operator theory,
localization phenomena and statistics of systems with quenched disorder. Also
we discuss the relation of the particular problems of random operator theory to
the theory of modular functionsComment: 36 pages, LaTeX, 4 separated Postscript figures, submitted to Nucl.
Phys. B [PM
Longest increasing subsequence as expectation of a simple nonlinear stochastic PDE with a low noise intensity
We report some new observation concerning the statistics of Longest
Increasing Subsequences (LIS). We show that the expectation of LIS, its
variance, and apparently the full distribution function appears in statistical
analysis of some simple nonlinear stochastic partial differential equation
(SPDE) in the limit of very low noise intensity.Comment: 6 pages, 4 figures, reference adde
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