6,219 research outputs found
Critical Casimir Forces in Colloidal Suspensions
Some time ago, Fisher and de Gennes pointed out that long-ranged correlations
in a fluid close to its critical point Tc cause distinct forces between
immersed colloidal particles which can even lead to flocculation [C. R. Acad.
Sc. Paris B 287, 207 (1978)]. Here we calculate such forces between pairs of
spherical particles as function of both relevant thermodynamic variables, i.e.,
the reduced temperature t = (T-Tc)/Tc and the field h conjugate to the order
parameter. This provides the basis for specific predictions concerning the
phase behavior of a suspension of colloidal particles in a near-critical
solvent.Comment: 29 pages, 14 figure
Polymer depletion effects near mesoscopic particles
The behavior of mesoscopic particles dissolved in a dilute solution of long,
flexible, and nonadsorbing polymer chains is studied by field-theoretic
methods. For spherical and cylindrical particles the solvation free energy for
immersing a single particle in the solution is calculated explicitly. Important
features are qualitatively different for self-avoiding polymer chains as
compared with ideal chains. The results corroborate the validity of the
Helfrich-type curvature expansion for general particle shapes and allow for
quantitative experimental tests. For the effective interactions between a small
sphere and a wall, between a thin rod and a wall, and between two small spheres
quantitative results are presented. A systematic approach for studying
effective many-body interactions is provided. The common Asakura-Oosawa
approximation modelling the polymer coils as hard spheres turns out to fail
completely for small particles and still fails by about 10% for large
particles.Comment: 68 pages, 6 figure
The column density towards LMC X-1
We measure the neutral absorption towards the black hole X-ray binary system
LMC X-1 from six archival soft X-ray spectra obtained with the gratings and/or
CCD detectors on Chandra, XMM-Newton, and Swift. Four spectral models for the
soft continuum have been investigated. While the powerlaw model may
overestimate NH considerably, the others give consistent results. Taking the
lower metalicity of the Large Magellanic Cloud into account, we find equivalent
hydrogen column densities of N_H = (1.0-1.3)*10^22 cm^-2, with a systematic
dependence on the orbital phase. This variation of the neutral absorption can
nearly explain the orbital modulation of the soft X-ray flux recently detected
with the All Sky Monitor (ASM) on the Rossi X-ray Timing Explorer (RXTE).Comment: 4 pages, accepted for publication as a Letter in Astronomy and
Astrophysic
THE TIGHT-BINDING APPROACH TO THE DIELECTRIC RESPONSE IN THE MULTIBAND SYSTEMS
Starting from the random phase approximation for the weakly coupled multiband
tightly-bounded electron systems, we calculate the dielectric matrix in terms
of intraband and interband transitions. The advantages of this representation
with respect to the usual plane-wave decomposition are pointed out. The
analysis becomes particularly transparent in the long wavelength limit, after
performing the multipole expansion of bare Coulomb matrix elements. For
illustration, the collective modes and the macroscopic dielectric function for
a general cubic lattice are derived. It is shown that the dielectric
instability in conducting narrow band systems proceeds by a common softening of
one transverse and one longitudinal mode. Furthermore, the self-polarization
corrections which appear in the macroscopic dielectric function for finite band
systems, are identified as a combined effect of intra-atomic exchange
interactions between electrons sitting in different orbitals and a finite
inter-atomic tunneling.Comment: 20 pages, LaTeX, no figure
Beam Dynamics Study of a Cooling Experiment based on the 88 MHz CERN Cooling Channel
This note presents a beam dynamics analysis of a possible muon cooling experiment based on 88 MHz cavities. The proposed set-up is a subsection of the cooling channel in the CERN reference scheme for a neutrino factory. We present two different set-ups using 8 and 4 cavities. For each of these set-ups we have carried out a beam dynamics study based on engineering designs for the cavities and solenoids. The study includes a parameter scan which allows to evaluate the performance of the systems for various input beam parameters and settings
Spectroscopy of the stellar wind in the Cygnus X-1 system
The X-ray luminosity of black holes is produced through the accretion of
material from their companion stars. Depending on the mass of the donor star,
accretion of the material falling onto the black hole through the inner
Lagrange point of the system or accretion by the strong stellar wind can occur.
Cygnus X-1 is a high mass X-ray binary system, where the black hole is powered
by accretion of the stellar wind of its supergiant companion star HDE226868. As
the companion is close to filling its Roche lobe, the wind is not symmetric,
but strongly focused towards the black hole. Chandra-HETGS observations allow
for an investigation of this focused stellar wind, which is essential to
understand the physics of the accretion flow. We compare observations at the
distinct orbital phases of 0.0, 0.2, 0.5 and 0.75. These correspond to
different lines of sights towards the source, allowing us to probe the
structure and the dynamics of the wind.Comment: conference proceeding from Integral/Bart Workshop Karlsbad, CZ,
14.4-18.4 201
Modified critical correlations close to modulated and rough surfaces
Correlation functions are sensitive to the presence of a boundary. Surface
modulations give rise to modified near surface correlations, which can be
measured by scattering probes. To determine these correlations, we develop a
perturbative calculation in deformations in height from a flat surface. The
results, combined with a renormalization group around four dimensions, are also
used to predict critical behavior near a self-affinely rough surface. We find
that a large enough roughness exponent can modify surface critical behavior.Comment: 4 pages, 1 figure. Revised version as published in Phys. Rev. Lett.
86, 4596 (2001
- …