3,023 research outputs found
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
Variational bound on energy dissipation in turbulent shear flow
We present numerical solutions to the extended Doering-Constantin variational
principle for upper bounds on the energy dissipation rate in plane Couette
flow, bridging the entire range from low to asymptotically high Reynolds
numbers. Our variational bound exhibits structure, namely a pronounced minimum
at intermediate Reynolds numbers, and recovers the Busse bound in the
asymptotic regime. The most notable feature is a bifurcation of the minimizing
wavenumbers, giving rise to simple scaling of the optimized variational
parameters, and of the upper bound, with the Reynolds number.Comment: 4 pages, RevTeX, 5 postscript figures are available as one .tar.gz
file from [email protected]
Deceptive signals of phase transitions in small magnetic clusters
We present an analysis of the thermodynamic properties of small transition
metal clusters and show how the commonly used indicators of phase transitions
like peaks in the specific heat or magnetic susceptibility can lead to
deceptive interpretations of the underlying physics. The analysis of the
distribution of zeros of the canonical partition function in the whole complex
temperature plane reveals the nature of the transition. We show that signals in
the magnetic susceptibility at positive temperatures have their origin at zeros
lying at negative temperatures.Comment: 4 pages, 5 figures, revtex4, for further information see
http://www.smallsystems.d
Modified semiclassical approximation for trapped Bose gases
A generalization of the semiclassical approximation is suggested allowing for
an essential extension of its region of applicability. In particular, it
becomes possible to describe Bose-Einstein condensation of a trapped gas in
low-dimensional traps and in traps of low confining dimensions, for which the
standard semiclassical approximation is not applicable. The results of the
modified approach are shown to coincide with purely quantum-mechanical
calculations for harmonic traps, including the one-dimensional harmonic trap.
The advantage of the semiclassical approximation is in its simplicity and
generality. Power-law potentials of arbitrary powers are considered. Effective
thermodynamic limit is defined for any confining dimension. The behaviour of
the specific heat, isothermal compressibility, and density fluctuations is
analyzed, with an emphasis on low confining dimensions, where the usual
semiclassical method fails. The peculiarities of the thermodynamic
characteristics in the effective thermodynamic limit are discussed.Comment: Revtex file, 13 page
Localization and entanglement of two interacting electrons in a quantum-dot molecule
The localization of two interacting electrons in a coupled-quantum-dots
semiconductor structure is demonstrated through numerical calculations of the
time evolution of the two-electron wave function including the Coulomb
interaction between the electrons. The transition from the ground state to a
localized state is induced by an external, time-dependent, uniform electric
field. It is found that while an appropriate constant field can localize both
electrons in one of the wells, oscillatory fields can induce roughly equal
probabilities for both electrons to be localized in either well, generating an
interesting type of localized and entangled state. We also show that shifting
the field suddenly to an appropriate constant value can maintain in time both
types of localization.Comment: 4 pages, 4 figure
On phases in weakly interacting finite Bose systems
We study precursors of thermal phase transitions in finite systems of
interacting Bose gases. For weakly repulsive interactions there is a phase
transition to the one-vortex state. The distribution of zeros of the partition
function indicates that this transition is first order, and the precursors of
the phase transition are already displayed in systems of a few dozen bosons.
Systems of this size do not exhibit new phases as more vortices are added to
the system.Comment: 7 pages, 2 figure
Transitions and Probes in Turbulent Helium
Previous analysis of a Paris turbulence experiment \cite{zoc94,tab95} shows a
transition at the Taylor Reynolds number \rel \approx 700. Here correlation
function data is analyzed which gives further evidence for this transition. It
is seen in both the power spectrum and in structure function measurements. Two
possible explanations may be offered for this observed transition: that it is
intrinsic to the turbulence flow in this closed box experiment or that it is an
effect of a change in the flow around the anemometer. We particularly examine a
pair of ``probe effects''. The first is a thermal boundary layer which does
exist about the probe and does limit the probe response, particularly at high
frequencies. Arguments based on simulations of the response and upon
observations of dissipation suggests that this effect is only crucial beyond
\rel\approx 2000. The second effect is produced by vortex shedding behind the
probe. This has been seen to produce a large modification in some of the power
spectra for large \rel. It might also complicate the interpretation of the
experimental results. However, there seems to be a remaining range of data for
\rel < 1300 uncomplicated by these effects, and which are thus suggestive of
an intrinsic transition.Comment: uuencoded .ps files. submitted to PRE. 12 figures are sent upon
request to jane wang ([email protected]
Universality in fully developed turbulence
We extend the numerical simulations of She et al. [Phys.\ Rev.\ Lett.\ 70,
3251 (1993)] of highly turbulent flow with Taylor-Reynolds number
up to , employing a reduced wave
vector set method (introduced earlier) to approximately solve the Navier-Stokes
equation. First, also for these extremely high Reynolds numbers ,
the energy spectra as well as the higher moments -- when scaled by the spectral
intensity at the wave number of peak dissipation -- can be described by
{\it one universal} function of for all . Second, the ISR
scaling exponents of this universal function are in agreement with
the 1941 Kolmogorov theory (the better, the large is), as is the
dependence of . Only around viscous damping leads to
slight energy pileup in the spectra, as in the experimental data (bottleneck
phenomenon).Comment: 14 pages, Latex, 5 figures (on request), 3 tables, submitted to Phys.
Rev.
Classification of phase transitions in small systems
We present a classification scheme for phase transitions in finite systems
like atomic and molecular clusters based on the Lee-Yang zeros in the complex
temperature plane. In the limit of infinite particle numbers the scheme reduces
to the Ehrenfest definition of phase transitions and gives the right critical
indices. We apply this classification scheme to Bose-Einstein condensates in a
harmonic trap as an example of a higher order phase transitions in a finite
system and to small Ar clusters.Comment: 12 pages, 4 figures, accepted for publication in Phys. Rev. Let
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