4,185 research outputs found
Kohn-Sham equations for nanowires with direct current
The paper describes the derivation of the Kohn-Sham equations for a nanowire
with direct current. A value of the electron current enters the problem as an
input via a subsidiary condition imposed by pointwise Lagrange multiplier.
Using the constrained minimization of the Hohenberg-Kohn energy functional, we
derive a set of self-consistent equations for current carrying orbitals of the
molecular wire
Logarithmic temperature profiles in the ultimate regime of thermal convection
We report on the theory of logarithmic temperature profiles in very strongly
developed thermal convection in the geometry of a Rayleigh-Benard cell with
aspect ratio one and discuss the degree of agreement with the recently measured
profiles in the ultimate state of very large Rayleigh number flow. The
parameters of the log-profile are calculated and compared with the measure
ones. Their physical interpretation as well as their dependence on the radial
position are discussed.Comment: 14 pages, no figur
Response maxima in modulated turbulence
Isotropic and homogeneous turbulence driven by an energy input modulated in
time is studied within a variable range mean-field theory. The response of the
system, observed in the second order moment of the large-scale velocity
difference D(L,t)=>~Re(t)^2$, is calculated for varying
modulation frequencies w and weak modulation amplitudes. For low frequencies
the system follows the modulation of the driving with almost constant
amplitude, whereas for higher driving frequencies the amplitude of the response
decreases on average 1/w. In addition, at certain frequencies the amplitude of
the response either almost vanishes or is strongly enhanced. These frequencies
are connected with the frequency scale of the energy cascade and multiples
thereof.Comment: 11 pages, 6 figure
Velocity profiles in strongly turbulent Taylor-Couette flow
We derive the velocity profiles in strongly turbulent Taylor-Couette flow for
the general case of independently rotating cylinders. The theory is based on
the Navier-Stokes equations in the appropriate (cylinder) geometry. In
particular, we derive the axial and the angular velocity profiles as functions
of distance from the cylinder walls and find that both follow a logarithmic
profile, with downwards-bending curvature corrections, which are more
pronounced for the angular velocity profile as compared to the axial velocity
profile, and which strongly increase with decreasing ratio between inner
and outer cylinder radius. In contrast, the azimuthal velocity does not follow
a log-law. We then compare the angular and azimuthal velocity profiles with the
recently measured profiles in the ultimate state of (very) large Taylor
numbers. Though the {\em qualitative} trends are the same -- down-bending for
large wall distances and (properly shifted and non-dimensionalized) angular
velocity profile being closer to a log-law than (properly shifted
and non-dimensionalized) azimuthal velocity profile -- {\em
quantitative} deviations are found for large wall distances. We attribute these
differences to the Taylor rolls and the height dependence of the profiles,
neither of which are considered in the theoretical approach
Non-Oberbeck-Boussinesq effects in two-dimensional Rayleigh-Benard convection in glycerol
We numerically analyze Non-Oberbeck-Boussinesq (NOB) effects in
two-dimensional Rayleigh-Benard flow in glycerol, which shows a dramatic change
in the viscosity with temperature. The results are presented both as functions
of the Rayleigh number (Ra) up to (for fixed temperature difference
between the top and bottom plates) and as functions of
"non-Oberbeck-Boussinesqness'' or "NOBness'' () up to 50 K (for fixed
Ra). For this large NOBness the center temperature is more than 5 K
larger than the arithmetic mean temperature between top and bottom plate
and only weakly depends on Ra. To physically account for the NOB deviations of
the Nusselt numbers from its Oberbeck-Boussinesq values, we apply the
decomposition of into the product of two effects, namely
first the change in the sum of the top and bottom thermal BL thicknesses, and
second the shift of the center temperature as compared to . While
for water the origin of the deviation is totally dominated by the second
effect (cf. Ahlers et al., J. Fluid Mech. 569, pp. 409 (2006)) for glycerol the
first effect is dominating, in spite of the large increase of as compared
to .Comment: 6 pages, 7 figure
Molecular hydrogen as baryonic dark matter
High-angular resolution CO observations of small-area molecular structures
(SAMS) are presented. The feature-less structures seen in the single-dish
measurements break up into several smaller clumps in the interferometer map. At
an adopted distance of 100pc their sizes are of order a few hundred AU, some of
which are still unresolved at an angular resolution of about 3". The clumps
have a fractal structure with a fractal index between 1.7 and 2.0. Their
kinetic temperature is between 7K and 18K. Adopting standard conversion factors
masses are about 1/10 Jupiter-masses for individual clumps and densities are
higher than 20000cm^{-3}. The clumps are highly overpressured and it is unknown
what creates or maintains such structures.Comment: 8 pages, 1 figure, accepted by Astrophysical Journal Letter
Classification of phase transitions of finite Bose-Einstein condensates in power law traps by Fisher zeros
We present a detailed description of a classification scheme for phase
transitions in finite systems based on the distribution of Fisher zeros of the
canonical partition function in the complex temperature plane. We apply this
scheme to finite Bose-systems in power law traps within a semi-analytic
approach with a continuous one-particle density of states for different values of and to a three dimensional harmonically
confined ideal Bose-gas with discrete energy levels. Our results indicate that
the order of the Bose-Einstein condensation phase transition sensitively
depends on the confining potential.Comment: 7 pages, 9 eps-figures, For recent information on physics of small
systems see "http://www.smallsystems.de
Herman-Kluk propagator is free from zero-point energy leakage
Semiclassical techniques constitute a promising route to approximate quantum
dynamics based on classical trajectories starting from a quantum-mechanically
correct distribution. One of their main drawbacks is the so-called zero-point
energy (ZPE) leakage, that is artificial redistribution of energy from the
modes with high frequency and thus high ZPE to that with low frequency and ZPE
due to classical equipartition. Here, we show that an elaborate semiclassical
formalism based on the Herman-Kluk propagator is free from the ZPE leakage
despite utilizing purely classical propagation. This finding opens the road to
correct dynamical simulations of systems with a multitude of degrees of freedom
that cannot be treated fully quantum-mechanically due to the exponential
increase of the numerical effort.Comment: 6 pages 2 figure
Tunneling in a cavity
The mechanism of coherent destruction of tunneling found by Grossmann et al.
[Phys. Rev. Lett. 67, 516 (1991)] is studied from the viewpoint of quantum
optics by considering the photon statistics of a single mode cavity field which
is strongly coupled to a two-level tunneling system (TS). As a function of the
interaction time between TS and cavity the photon statistics displays the
tunneling dynamics. In the semi-classical limit of high photon occupation
number , coherent destruction of tunneling is exhibited in a slowing down of
an amplitude modulation for certain parameter ratios of the field. The
phenomenon is explained as arising from interference between displaced number
states in phase space which survives the large limit due to identical
scaling between orbit width and displacement.Comment: 4 pages Revtex, 2 PS-figures, appears in The Physical Review
A New Computing Laboratory to Evaluate and Test Hardware, Software and Systems: Intellectual Property Based Organizational Setting No. 1
The proliferation of minicomputers and the seductive nature of belief in hardware solutions to a complex problem, has led the authors to suggest the creating of a new computing laboratory (NCL). The laboratory would permit clients to seek help in the system design aspects of their problem and to test the proposed hardware/software/systems solution on a NCL workbench. The importance of a entrepreneurial management style for the enterprise is stressed
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