497 research outputs found
Magnetic particles confined in a modulated channel: structural transitions tunable by tilting a magnetic field
The ground state of colloidal magnetic particles in a modulated channel are
investigated as function of the tilt angle of an applied magnetic field. The
particles are confined by a parabolic potential in the transversal direction
while in the axial direction a periodic substrate potential is present. By
using Monte Carlo (MC) simulations, we construct a phase diagram for the
different crystal structures as a function of the magnetic field orientation,
strength of the modulated potential and the commensurability factor of the
system. Interestingly, we found first and second order phase transitions
between different crystal structures, which can be manipulated by the
orientation of the external magnetic field. A re-entrant behavior is found
between two- and four-chain configurations, with continuous second order
transitions. Novel configurations are found consisting of frozen in solitons.
By changing the orientation and/or strength of the magnetic field and/or the
strength and the spatial frequency of the periodic substrate potential, the
system transits through different phases.Comment: Submitted to Phys. Rev. E (10 pages, 12 figures
Ginzburg-Landau theory of the zig-zag transition in quasi-one-dimensional classical Wigner crystals
We present a mean-field description of the zig-zag phase transition of a
quasi-one-dimensional system of strongly interacting particles, with
interaction potential , that are confined by a power-law
potential (). The parameters of the resulting one-dimensional
Ginzburg-Landau theory are determined analytically for different values of
and . Close to the transition point for the zig-zag phase
transition, the scaling behavior of the order parameter is determined. For
the zig-zag transition from a single to a double chain is of second
order, while for the one chain configuration is always unstable and
for the one chain ordered state becomes unstable at a certain
critical density resulting in jumps of single particles out of the chain.Comment: 12 pages, 11 figure
Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem
Accretion flows onto massive stars must transfer mass so quickly that they
are themselves gravitationally unstable, forming dense clumps and filaments.
These density perturbations interact with young massive stars, emitting
ionizing radiation, alternately exposing and confining their HII regions. As a
result, the HII regions are predicted to flicker in flux density over periods
of decades to centuries rather than increasing monotonically in size as
predicted by simple Spitzer solutions. We have recently observed the Sgr B2
region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and
BnA) at a resolution of 0.25''. These observations were made to compare in
detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr
B2 contains 41 UC HII regions, 6 of which are hypercompact. The new
observations of Sgr B2 allow comparison of relative peak flux densites for the
HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the
most source-rich massive star forming regions in the Milky Way. The new 1.3 cm
continuum images indicate that four of the 41 UC HII regions exhibit
significant changes in their peak flux density, with one source (K3) dropping
in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing
in peak flux density. The results are consistent with statistical predictions
from simulations of high mass star formation, suggesting that they offer a
solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
Measurement of the hyperfine splitting of the 6S level in rubidium
We present a measurement of the hyperfine splitting of the 6S excited
level of rubidium using two photon absorption spectroscopy in a glass cell. The
values we obtain for the magnetic dipole constant A are 239.18(03) MHz and
807.66(08) MHz for Rb and Rb, respectively. The combination of
the magnetic moments of the two isotopes and our measurements show a hyperfine
anomaly in this atomic excited state. The observed hyperfine anomaly difference
has a value of due to the finite distribution of
nuclear magnetization, the Bohr-Weisskopf effect.Comment: 12 pages, 14 figure
Backscattered electron images, X-ray maps and Monte Carlo simulations applied to the study of plagioclase composition in volcanic rocks
Zoning patterns in plagioclases are related to abrupt changes in the anorthite content along the crystal growing direction. Accurate characterization of these patterns by electron microprobe is useful to identify magma chamber processes such as recharge, mingling and whole-chamber overturn events.
In this work, a new procedure to obtain high resolution quantitative maps of anorthite concentration in single plagioclase crystals is developed. The methodology consists in performing a calibration of backscattered electron images using quantitative X-ray maps. The ultimate resolution of charac- teristic X-rays and backscattered electron signals is studied by Monte Carlo simulation. The method is applied to characterize the chemical composition of a volcanic plagioclase from the Cerro Vilama, Argentina. The results obtained are more precise than the values given by the methods commonly used in the study of plagioclase composition, i.e. the classical profiling by electron microprobe point analysis or the modern backscattered electron image calibration by means of quantitative energy or wavelength dispersive X-ray analysis at a few selected points.Facultad de Ingeniería (FI)Laboratorio de Investigaciones de Metalurgia Física (LIMF)Centro de Investigación y Desarrollo en Ciencias Aplicadas (CINDECA
High Resolution CO Observations of Massive Star Forming Regions
Context. To further understand the processes involved in the formation of
massive stars, we have undertaken a study of the gas dynamics surrounding three
massive star forming regions. By observing the large scale structures at high
resolution, we are able to determine properties such as driving source, and
spatially resolve the bulk dynamical properties of the gas such as infall and
outflow. Aims. With high resolution observations, we are able to determine
which of the cores in a cluster forming massive stars is responsible for the
large scale structures. Methods. We present CO observations of three massive
star forming regions with known HII regions and show how the CO traces both
infall and outflow. By combining data taken in two SMA configurations with JCMT
observations, we are able to see large scale structures at high resolution.
Results. We find large (0.26-0.40 pc), massive (2-3 M_sun) and energetic (13-17
\times 10^44 erg) outflows emanating from the edges of two HII regions
suggesting they are being powered by the protostar(s) within. We find infall
signatures in two of our sources with mass infall rates of order 10-4 M_sun/yr.
Conclusions. We suggest that star formation is ongoing in these sources despite
the presence of HII regions. We further conclude that the source(s) within a
single HII region are responsible for the observed large scale structures; that
these large structures are not the net effect of multiple outflows from
multiple HII regions and hot cores.Comment: 8 pages,2 figures, accepted for publication in A&
Comparative study of hydroxyapatite coatings obtained by Sol-Gel and electrophoresis on titanium sheets
A comparative study of two coating methods on titanium (Ti) substrates, sol-gel and electrophoresis processes, was performed. Before coating, two different surface treatments were employed. The hydroxyapatite (HA) powder were characterized by infrared spectroscopy (FTIR) and X-ray diffraction (XRD) while coated samples were examined by using scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS). Both techniques seem to be suitable to coat Ti. A previous surface treatment on metallic titanium seems to be fundamental to enhance coat uniformity and adhesion that was estimated by the adhesive tape test. The decrease in calcination temperature did not affect the coating adhesion.The authors thank the financial support of CAPES, CNPq, FAPERJ, and the cooperative project between CNPq/Brasil and MES/Cuba.Peer reviewe
- …