6,048 research outputs found
Methanol masers reveal the magnetic field of the high-mass protostar IRAS 18089-1732
Context. The importance of the magnetic field in high-mass-star formation is
not yet fully clear and there are still many open questions concerning its role
in the accretion processes and generation of jets and outflows. In the past few
years, masers have been successfully used to probe the magnetic field
morphology and strength at scales of a few au around massive protostars, by
measuring linear polarisation angles and Zeeman splitting. The massive
protostar IRAS 18089-1732 is a well studied high-mass-star forming region,
showing a hot core chemistry and a disc-outflow system. Previous SMA
observations of polarised dust revealed an ordered magnetic field oriented
around the disc of IRAS 18089-1732. Aims. We want to determine the magnetic
field in the dense region probed by 6.7 GHz methanol maser observations and
compare it with observations in dust continuum polarisation, to investigate how
the magnetic field in the compact maser region relates to the large-scale field
around massive protostars. Methods. We reduced MERLIN observations at 6.7 GHz
of IRAS 18089-1732 and we analysed the polarised emission by methanol masers.
Results. Our MERLIN observations show that the magnetic field in the 6.7 GHz
methanol maser region is consistent with the magnetic field constrained by the
SMA dust polarisation observations. A tentative detection of circularly
polarised line emission is also presented. Conclusions. We found that the
magnetic field in the maser region has the same orientation as in the disk.
Thus the large-scale field component, even at the au scale of the masers,
dominates over any small-scale field fluctuations. We obtained, from the
circular polarisation tentative detection, a field strength along the line of
sight of 5.5 mG which appeared to be consistent with the previous estimates.Comment: 12 pages, 7 figures, accepted for publication in A&
Dynamics of quantum systems
A relation between the eigenvalues of an effective Hamilton operator and the
poles of the matrix is derived which holds for isolated as well as for
overlapping resonance states. The system may be a many-particle quantum system
with two-body forces between the constituents or it may be a quantum billiard
without any two-body forces. Avoided crossings of discrete states as well as of
resonance states are traced back to the existence of branch points in the
complex plane. Under certain conditions, these branch points appear as double
poles of the matrix. They influence the dynamics of open as well as of
closed quantum systems. The dynamics of the two-level system is studied in
detail analytically as well as numerically.Comment: 21 pages 7 figure
Phase transitions in open quantum systems
We consider the behaviour of open quantum systems in dependence on the
coupling to one decay channel by introducing the coupling parameter
being proportional to the average degree of overlapping. Under critical
conditions, a reorganization of the spectrum takes place which creates a
bifurcation of the time scales with respect to the lifetimes of the resonance
states. We derive analytically the conditions under which the reorganization
process can be understood as a second-order phase transition and illustrate our
results by numerical investigations. The conditions are fulfilled e.g. for a
picket fence with equal coupling of the states to the continuum. Energy
dependencies within the system are included. We consider also the generic case
of an unfolded Gaussian Orthogonal Ensemble. In all these cases, the
reorganization of the spectrum occurs at the critical value of
the control parameter globally over the whole energy range of the spectrum. All
states act cooperatively.Comment: 28 pages, 22 Postscript figure
Elastic contact between self-affine surfaces: Comparison of numerical stress and contact correlation functions with analytic predictions
Contact between an elastic manifold and a rigid substrate with a self-affine
fractal surface is reinvestigated with Green's function molecular dynamics.
Stress and contact autocorrelation functions (ACFs) are found to decrease
algebraically. A rationale is provided for the observed similarity in the
exponents for stress and contact ACFs. Both exponents differ substantially from
analytic predictions over the range of Hurst roughness exponents studied. The
effect of increasing the range of interactions from a hard sphere repulsion to
exponential decay is analyzed. Results for exponential interactions are
accurately described by recent systematic corrections to Persson's contact
mechanics theory. The relation between the area of simply connected contact
patches and the normal force is also studied. Below a threshold size the
contact area and force are consistent with Hertzian contact mechanics, while
area and force are linearly related in larger contact patches.Comment: 12 pages, 9 figure
Path integral approach to random motion with nonlinear friction
Using a path integral approach, we derive an analytical solution of a
nonlinear and singular Langevin equation, which has been introduced previously
by P.-G. de Gennes as a simple phenomenological model for the stick-slip motion
of a solid object on a vibrating horizontal surface. We show that the optimal
(or most probable) paths of this model can be divided into two classes of
paths, which correspond physically to a sliding or slip motion, where the
object moves with a non-zero velocity over the underlying surface, and a
stick-slip motion, where the object is stuck to the surface for a finite time.
These two kinds of basic motions underlie the behavior of many more complicated
systems with solid/solid friction and appear naturally in de Gennes' model in
the path integral framework.Comment: 18 pages, 3 figure
Lineshape of the thermopower of quantum dots
Quantum dots are an important model system for thermoelectric phenomena, and
may be used to enhance the thermal-to-electric energy conversion efficiency in
functional materials. It is therefore important to obtain a detailed
understanding of a quantum-dot's thermopower as a function of the Fermi energy.
However, so far it has proven difficult to take effects of co-tunnelling into
account in the interpretation of experimental data. Here we show that a
single-electron tunnelling model, using knowledge of the dot's electrical
conductance which in fact includes all-order co-tunneling effects, predicts the
thermopower of quantum dots as a function of the relevant energy scales, in
very good agreement with experiment.Comment: 10 pages, 5 figure
Stick-slip motion of solids with dry friction subject to random vibrations and an external field
We investigate a model for the dynamics of a solid object, which moves over a
randomly vibrating solid surface and is subject to a constant external force.
The dry friction between the two solids is modeled phenomenologically as being
proportional to the sign of the object's velocity relative to the surface, and
therefore shows a discontinuity at zero velocity. Using a path integral
approach, we derive analytical expressions for the transition probability of
the object's velocity and the stationary distribution of the work done on the
object due to the external force. From the latter distribution, we also derive
a fluctuation relation for the mechanical work fluctuations, which incorporates
the effect of the dry friction.Comment: v1: 23 pages, 9 figures; v2: Reference list corrected; v3: Published
version, typos corrected, references adde
On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion
Surface roughness has a huge impact on many important phenomena. The most
important property of rough surfaces is the surface roughness power spectrum
C(q). We present surface roughness power spectra of many surfaces of practical
importance, obtained from the surface height profile measured using optical
methods and the Atomic Force Microscope. We show how the power spectrum
determines the contact area between two solids. We also present applications to
sealing, rubber friction and adhesion for rough surfaces, where the power
spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some
figures are in Postscript Level
Heat to Electricity Conversion by a Graphene Stripe with Heavy Chiral Fermions
A conversion of thermal energy into electricity is considered in the
electrically polarized graphene stripes with zigzag edges where the heavy
chiral fermion (HCF) states are formed. The stripes are characterized by a high
electric conductance Ge and by a significant Seebeck coefficient S. The
electric current in the stripes is induced due to a non-equilibrium thermal
injection of "hot" electrons. This thermoelectric generation process might be
utilized for building of thermoelectric generators with an exceptionally high
figure of merit Z{\delta}T \simeq 100 >> 1 and with an appreciable electric
power densities \sim 1 MW/cm2.Comment: 8 pages, 3 figure
Master equation approach to friction at the mesoscale
At the mesoscale friction occurs through the breaking and formation of local
contacts. This is often described by the earthquake-like model which requires
numerical studies. We show that this phenomenon can also be described by a
master equation, which can be solved analytically in some cases and provides an
efficient numerical solution for more general cases. We examine the effect of
temperature and aging of the contacts and discuss the statistical properties of
the contacts for different situations of friction and their implications,
particularly regarding the existence of stick-slip.Comment: To be published in Physical Review
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