760 research outputs found
Hidden heat transfer in equilibrium states implies directed motion in nonequilibrium states
We study a class of heat engines including Feynman's ratchet, which exhibits
a directed motion of a particle in nonequilibrium steady states maintained by
two heat baths. We measure heat transfer from each heat bath separately, and
average them using a careful procedure that reveals the nature of the heat
transfer associated with directed steps of the particle. Remarkably we find
that steps are associated with nonvanishing heat transfer even in equilibrium,
and there is a quantitative relation between this ``hidden heat transfer'' and
the directed motion of the particle. This relation is clearly understood in
terms of the ``principle of heat transfer enhancement'', which is expected to
apply to a large class of highly nonequilibrium systems.Comment: 4 pages, 4 figures; revise
Equilibrium for fragmentation with immigration
This paper introduces stochastic processes that describe the evolution of
systems of particles in which particles immigrate according to a Poisson
measure and split according to a self-similar fragmentation. Criteria for
existence and absence of stationary distributions are established and
uniqueness is proved. Also, convergence rates to the stationary distribution
are given. Linear equations which are the deterministic counterparts of
fragmentation with immigration processes are next considered. As in the
stochastic case, existence and uniqueness of solutions, as well as existence
and uniqueness of stationary solutions, are investigated.Comment: Published at http://dx.doi.org/10.1214/105051605000000340 in the
Annals of Applied Probability (http://www.imstat.org/aap/) by the Institute
of Mathematical Statistics (http://www.imstat.org
The Black Hole Binary Nova Scorpii 1994 (GRO J1655-40): An improved chemical analysis
The chemical analysis of secondary stars of low mass X-ray binaries provides
an opportunity to study the formation processes of compact objects, either
black holes or neutron stars. Following the discovery of overabundances of
-elements in the HIRES/Keck spectrum of the secondary star of Nova
Scorpii 1994 (Israelian et al. 1999), we obtained UVES/VLT high-resolution
spectroscopy with the aim of performing a detailed abundance analysis of this
secondary star. Using a -minimization procedure and a grid of synthetic
spectra, we derive the stellar parameters and atmospheric abundances of O, Mg,
Al, Ca, Ti, Fe and Ni, using a new UVES spectrum and the HIRES spectrum.The
abundances of Al, Ca, Ti, Fe and Ni seem to be consistent with solar values,
whereas Na, and especially O, Mg, Si and S are significantly enhanced in
comparison with Galactic trends of these elements. A comparison with
spherically and non-spherically symmetric supernova explosion models may
provide stringent constraints to the model parameters as mass-cut and the
explosion energy, in particular from the relative abundances of Si, S, Ca, Ti,
Fe and Ni. Most probably the black hole in this system formed in a hypernova
explosion of a 30--35 \Msun progenitor star with a mass-cut in the range 2--3.5
\Msun. However, these models produce abundances of Al and Na almost ten times
higher than the observed values.Comment: New Accepted version for publication in Astronomy and Astrophysics
Table 2: Correcte
Experimental measurement of efficiency and transport coherence of a cold atom Brownian motor in optical lattices
The rectification of noise into directed movement or useful energy is
utilized by many different systems. The peculiar nature of the energy source
and conceptual differences between such Brownian motor systems makes a
characterization of the performance far from straightforward. In this work,
where the Brownian motor consists of atoms interacting with dissipative optical
lattices, we adopt existing theory and present experimental measurements for
both the efficiency and the transport coherence. We achieve up to 0.3% for the
efficiency and 0.01 for the P\'eclet number
Symmetries shape the current in ratchets induced by a bi-harmonic force
Equations describing the evolution of particles, solitons, or localized
structures, driven by a zero-average, periodic, external force, and invariant
under time reversal and a half-period time shift, exhibit a ratchet current
when the driving force breaks these symmetries. The bi-harmonic force
does
it for almost any choice of and , provided and are
two co-prime integers such that is odd. It has been widely observed, in
experiments in Josephson-junctions, photonic crystals, etc., as well as in
simulations, that the ratchet current induced by this force has the shape
for
small amplitudes, where depends on the damping ( if
there is no damping, and for overdamped systems). We rigorously
prove that this precise shape can be obtained solely from the broken symmetries
of the system and is independent of the details of the equation describing the
system.Comment: 4 page
Thermal ratchet effects in ferrofluids
Rotational Brownian motion of colloidal magnetic particles in ferrofluids
under the influence of an oscillating external magnetic field is investigated.
It is shown that for a suitable time dependence of the magnetic field, a noise
induced rotation of the ferromagnetic particles due to rectification of thermal
fluctuations takes place. Via viscous coupling, the associated angular momentum
is transferred from the magnetic nano-particles to the carrier liquid and can
then be measured as macroscopic torque on the fluid sample. A thorough
theoretical analysis of the effect in terms of symmetry considerations,
analytical approximations, and numerical solutions is given which is in
accordance with recent experimental findings.Comment: 18 pages, 6 figure
Infrared phonon dynamics of multiferroic BiFeO3 single crystal
We discuss the first infrared reflectivity measurement on a BiFeO3 single
crystal between 5 K and room temperature. The 9 predicted ab-plane E phonon
modes are fully and unambiguously determined. The frequencies of the 4 A1
c-axis phonons are found. These results settle issues between theory and data
on ceramics. Our findings show that the softening of the lowest frequency E
mode is responsible for the temperature dependence of the dielectric constant,
indicating that the ferroelectric transition in BiFeO3 is soft-mode driven.Comment: 5 pages (figures included
On the formation/dissolution of equilibrium droplets
We consider liquid-vapor systems in finite volume at parameter
values corresponding to phase coexistence and study droplet formation due to a
fixed excess of particles above the ambient gas density. We identify
a dimensionless parameter and a
\textrm{universal} value \Deltac=\Deltac(d), and show that a droplet of the
dense phase occurs whenever \Delta>\Deltac, while, for \Delta<\Deltac, the
excess is entirely absorbed into the gaseous background. When the droplet first
forms, it comprises a non-trivial, \textrm{universal} fraction of excess
particles. Similar reasoning applies to generic two-phase systems at phase
coexistence including solid/gas--where the ``droplet'' is crystalline--and
polymorphic systems. A sketch of a rigorous proof for the 2D Ising lattice gas
is presented; generalizations are discussed heuristically.Comment: An announcement of a forthcoming rigorous work on the 2D Ising model;
to appear in Europhys. Let
Using elicitation interview within a phenomenographic framework: developing the breadth of research designs associated with game based approaches.
This paper describes how elicitation interview technique was used within a phenomenographic research design to explore physical education teachers’ experiences of teaching games using a Game Based Approach (GBA). Participants taught in one of two different international contexts, Australia or England, and all had some experience of using a GBA to teach games. The focus of the paper is the presentation and discussion of the unique research design used to generate understanding about GBA teaching experiences as well as extending the examination of GBAs from different philosophical viewpoints. Authors’ reflections on the utilised research design are presented with concluding discussion identifying further research opportunities relating to GBAs in teaching and coaching contexts
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