71 research outputs found
Dynamics of vortex tangle without mutual friction in superfluid He
A recent experiment has shown that a tangle of quantized vortices in
superfluid He decayed even at mK temperatures where the normal fluid was
negligible and no mutual friction worked. Motivated by this experiment, this
work studies numerically the dynamics of the vortex tangle without the mutual
friction, thus showing that a self-similar cascade process, whereby large
vortex loops break up to smaller ones, proceeds in the vortex tangle and is
closely related with its free decay. This cascade process which may be covered
with the mutual friction at higher temperatures is just the one at zero
temperature Feynman proposed long ago. The full Biot-Savart calculation is made
for dilute vortices, while the localized induction approximation is used for a
dense tangle. The former finds the elementary scenario: the reconnection of the
vortices excites vortex waves along them and makes them kinked, which could be
suppressed if the mutual friction worked. The kinked parts reconnect with the
vortex they belong to, dividing into small loops. The latter simulation under
the localized induction approximation shows that such cascade process actually
proceeds self-similarly in a dense tangle and continues to make small vortices.
Considering that the vortices of the interatomic size no longer keep the
picture of vortex, the cascade process leads to the decay of the vortex line
density. The presence of the cascade process is supported also by investigating
the classification of the reconnection type and the size distribution of
vortices. The decay of the vortex line density is consistent with the solution
of the Vinen's equation which was originally derived on the basis of the idea
of homogeneous turbulence with the cascade process. The obtained result is
compared with the recent Vinen's theory.Comment: 16 pages, 16 figures, submitted to PR
Nanofriction mechanisms derived from the dependence of friction on load and sliding velocity from air to UHV on hydrophilic silicon
This paper examines friction as a function of the sliding velocity and
applied normal load from air to UHV in a scanning force microscope (SFM)
experiment in which a sharp silicon tip slides against a flat Si(100) sample.
Under ambient conditions, both surfaces are covered by a native oxide, which is
hydrophilic. During pump-down in the vacuum chamber housing the SFM, the
behavior of friction as a function of the applied normal load and the sliding
velocity undergoes a change. By analyzing these changes it is possible to
identify three distinct friction regimes with corresponding contact properties:
(a) friction dominated by the additional normal forces induced by capillarity
due to the presence of thick water films, (b) higher drag force from ordering
effects present in thin water layers and (c) low friction due to direct
solid-solid contact for the sample with the counterbody. Depending on
environmental conditions and the applied normal load, all three mechanisms may
be present at one time. Their individual contributions can be identified by
investigating the dependence of friction on the applied normal load as well as
on the sliding velocity in different pressure regimes, thus providing
information about nanoscale friction mechanisms
The anomaly line bundle of the self-dual field theory
In this work, we determine explicitly the anomaly line bundle of the abelian
self-dual field theory over the space of metrics modulo diffeomorphisms,
including its torsion part. Inspired by the work of Belov and Moore, we propose
a non-covariant action principle for a pair of Euclidean self-dual fields on a
generic oriented Riemannian manifold. The corresponding path integral allows to
study the global properties of the partition function over the space of metrics
modulo diffeomorphisms. We show that the anomaly bundle for a pair of self-dual
fields differs from the determinant bundle of the Dirac operator coupled to
chiral spinors by a flat bundle that is not trivial if the underlying manifold
has middle-degree cohomology, and whose holonomies are determined explicitly.
We briefly sketch the relevance of this result for the computation of the
global gravitational anomaly of the self-dual field theory, that will appear in
another paper.Comment: 41 pages. v2: A few typos corrected. Version accepted for publication
in CM
Breakup Density in Spectator Fragmentation
Proton-proton correlations and correlations of protons, deuterons and tritons
with alpha particles from spectator decays following 197Au + 197Au collisions
at 1000 MeV per nucleon have been measured with two highly efficient detector
hodoscopes. The constructed correlation functions, interpreted within the
approximation of a simultaneous volume decay, indicate a moderate expansion and
low breakup densities, similar to assumptions made in statistical
multifragmentation models.
PACS numbers: 25.70.Pq, 21.65.+f, 25.70.Mn, 25.75.GzComment: 11 pages, LaTeX with 3 included figures; Also available from
http://www-kp3.gsi.de/www/kp3/aladin_publications.htm
Vertex Operators for Closed Superstrings
We construct an iterative procedure to compute the vertex operators of the
closed superstring in the covariant formalism given a solution of IIA/IIB
supergravity. The manifest supersymmetry allows us to construct vertex
operators for any generic background in presence of Ramond-Ramond (RR) fields.
We extend the procedure to all massive states of open and closed superstrings
and we identify two new nilpotent charges which are used to impose the gauge
fixing on the physical states. We solve iteratively the equations of the vertex
for linear x-dependent RR field strengths. This vertex plays a role in studying
non-constant C-deformations of superspace. Finally, we construct an action for
the free massless sector of closed strings, and we propose a form for the
kinetic term for closed string field theory in the pure spinor formalism.Comment: TeX, harvmac, amssym.tex, 41 pp; references adde
Particles-vortex interactions and flow visualization in He4
Recent experiments have demonstrated a remarkable progress in implementing
and use of the Particle Image Velocimetry (PIV) and particle tracking
techniques for the study of turbulence in He4. However, an interpretation of
the experimental data in the superfluid phase requires understanding how the
motion of tracer particles is affected by the two components, the viscous
normal fluid and the inviscid superfluid. Of a particular importance is the
problem of particle interactions with quantized vortex lines which may not only
strongly affect the particle motion, but, under certain conditions, may even
trap particles on quantized vortex cores. The article reviews recent
theoretical, numerical, and experimental results in this rapidly developing
area of research, putting critically together recent results, and solving
apparent inconsistencies. Also discussed is a closely related technique of
detection of quantized vortices negative ion bubbles in He4.Comment: To appear in the J Low Temperature Physic
Conformational Changes in Single-Strand DNA as a Function of Temperature by SANS
AbstractSmall-angle neutron scattering (SANS) measurements were performed on a solution of single-strand DNA, 5′-ATGCTGATGC-3′, in sodium phosphate buffer solution at 10°C temperature increments from 25°C to 80°C. Cylindrical, helical, and random coil shape models were fitted to the SANS measurements at each temperature. All the shapes exhibited an expansion in the diameter direction causing a slightly shortened pitch from 25°C to 43°C, an expansion in the pitch direction with a slight decrease in the diameter from 43°C to 53°C, and finally a dramatic increase in the pitch and diameter from 53°C to 80°C. Differential scanning calorimeter scans of the sequence in solution exhibited a reversible two-state transition profile with a transition temperature of 47.5±0.5°C, the midpoint of the conformational changes observed in the SANS measurements, and a calorimetric transition enthalpy of 60±3kJ mol−1 that indicates a broad transition as is observed in the SANS measurements. A transition temperature of 47±1°C was also obtained from ultraviolet optical density measurements of strand melting scans of the single-strand DNA. This transition corresponds to unstacking of the bases of the sequence and is responsible for the thermodynamic discrepancy between its binding stability to its complementary sequence determined directly at ambient temperatures and determined from extrapolated values of the melting of the duplex at high temperature
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