9,001 research outputs found

### Criticality in the collapse of spherically symmetric massless scalar fields in semi-classical loop quantum gravity

In a recent paper we showed that the collapse to a black hole in
one-parameter families of initial data for massless, minimally coupled scalar
fields in spherically symmetric semi-classical loop quantum gravity exhibited a
universal mass scaling similar to the one in classical general relativity. In
particular, no evidence of a mass gap appeared as had been suggested by
previous studies. The lack of a mass gap indicated the possible existence of a
self-similar critical solution as in general relativity. Here we provide
further evidence for its existence. Using an adaptive mesh refinement code, we
show that "echoes" arise as a result of the discrete self-similarity in
space-time. We also show the existence of "wiggles" in the mass scaling
relation, as in the classical theory. The results from the semi-classical
theory agree well with those of classical general relativity unless one takes
unrealistically large values for the polymerization parameter.Comment: 7 pages, RevTe

### Fingerprinting the magnetic behavior of antiferromagnetic nanostructures using remanent magnetization curves

Antiferromagnetic (AF) nanostructures from Co3O4, CoO and Cr2O3 were prepared
by the nanocasting method and were characterized magnetometrically. The field
and temperature dependent magnetization data suggests that the nanostructures
consist of a core-shell structure. The core behaves as a regular
antiferromagnet and the shell as a two-dimensional diluted antiferromagnet in a
field (2d DAFF) as previously shown on Co3O4 nanowires [Benitez et al., Phys.
Rev. Lett. 101, 097206 (2008)]. Here we present a more general picture on three
different material systems, i.e. Co3O4, CoO and Cr2O3. In particular we
consider the thermoremanent (TRM) and the isothermoremanent (IRM) magnetization
curves as "fingerprints" in order to identify the irreversible magnetization
contribution originating from the shells. The TRM/IRM fingerprints are compared
to those of superparamagnetic systems, superspin glasses and 3d DAFFs. We
demonstrate that TRM/IRM vs. H plots are generally useful fingerprints to
identify irreversible magnetization contributions encountered in particular in
nanomagnets.Comment: submitted to PR

### On the 2-point function of the O(N) model

The self-energy of the critical 3-dimensional O(N) model is calculated. The
analysis is performed in the context of the Non-Perturbative Renormalization
Group, by exploiting an approximation which takes into account contributions of
an infinite number of vertices. A very simple calculation yields the 2-point
function in the whole range of momenta, from the UV Gaussian regime to the
scaling one. Results are in good agreement with best estimates in the
literature for any value of N in all momenta regimes. This encourages the use
of this simple approximation procedure to calculate correlation functions at
finite momenta in other physical situations

### CAFE: Calar Alto Fiber-fed Echelle spectrograph

We present here CAFE, the Calar Alto Fiber-fed Echelle spectrograph, a new
instrument built at the Centro Astronomico Hispano Alem\'an (CAHA). CAFE is a
single fiber, high-resolution ($R\sim$70000) spectrograph, covering the
wavelength range between 3650-9800\AA. It was built on the basis of the common
design for Echelle spectrographs. Its main aim is to measure radial velocities
of stellar objects up to $V\sim$13-14 mag with a precision as good as a few
tens of $m s^{-1}$. To achieve this goal the design was simplified at maximum,
removing all possible movable components, the central wavelength is fixed, so
the wavelentgth coverage; no filter wheel, one slit and so on, with a
particular care taken in the thermal and mechanical stability. The instrument
is fully operational and publically accessible at the 2.2m telescope of the
Calar Alto Observatory.
In this article we describe (i) the design, summarizing its manufacturing
phase; (ii) characterize the main properties of the instrument; (iii) describe
the reduction pipeline; and (iv) show the results from the first light and
commissioning runs. The preliminar results indicate that the instrument fulfill
the specifications and it can achieve the foreseen goals. In particular, they
show that the instrument is more efficient than anticipated, reaching a
$S/N\sim$20 for a stellar object as faint as $V\sim$14.5 mag in $\sim$2700s
integration time. The instrument is a wonderful machine for exoplanetary
research (by studying large samples of possible systems cotaining massive
planets), galactic dynamics (high precise radial velocities in moving groups or
stellar associations) or astrochemistry.Comment: 12 pages, 23 figures; Acepted for publishing in A&A, 201

### Rotational dynamics induced by low energy binary collisions of quantum droplets

A theoretical analysis of the rotational dynamics induced by off axis binary
collisions of quantum droplets constituted by ultracold atoms is reported. We
focus on quantum droplets formed by degenerate dilute Bose gases made up from
binary mixtures of alkaline atoms under feasible experimental conditions. The
stability of the ground state is known to be longer for the chosen
heteronuclear gases than for the homonuclear ones. In both cases, we find out
that the dynamics seems to privilege a high similarity of the density of each
atomic species. However, the evolution of the phase of the corresponding order
parameter differs significantly for heteronuclear admixtures. We evaluate the
fidelity as a figure of merit for the overlap between the order parameters of
each atomic species. Dynamical evidence of the differences between the phase of
the order parameters are predicted to manifest in their corresponding linear
and angular momenta. We numerically verify that the total angular and linear
momenta are conserved both during the collision. Some direct correlations
between the Weber number and the impact parameter with the distribution of the
dynamical variables are established.Comment: 12 pages, 6 figure

### General framework of the non-perturbative renormalization group for non-equilibrium steady states

This paper is devoted to presenting in detail the non-perturbative
renormalization group (NPRG) formalism to investigate out-of-equilibrium
systems and critical dynamics in statistical physics. The general NPRG
framework for studying non-equilibrium steady states in stochastic models is
expounded and fundamental technicalities are stressed, mainly regarding the
role of causality and of Ito's discretization. We analyze the consequences of
Ito's prescription in the NPRG framework and eventually provide an adequate
regularization to encode them automatically. Besides, we show how to build a
supersymmetric NPRG formalism with emphasis on time-reversal symmetric
problems, whose supersymmetric structure allows for a particularly simple
implementation of NPRG in which causality issues are transparent. We illustrate
the two approaches on the example of Model A within the derivative expansion
approximation at order two, and check that they yield identical results.Comment: 28 pages, 1 figure, minor corrections prior to publicatio

### Solutions of renormalization group flow equations with full momentum dependence

We demonstrate the power of a recently-proposed approximation scheme for the
non-perturbative renormalization group that gives access to correlation
functions over their full momentum range. We solve numerically the
leading-order flow equations obtained within this scheme, and compute the
two-point functions of the O(N) theories at criticality, in two and three
dimensions. Excellent results are obtained for both universal and non-universal
quantities at modest numerical cost.Comment: 4 pages, 1 figur

### Primordial Earth mantle heterogeneity caused by the Moon-forming giant impact

The giant impact hypothesis for Moon formation successfully explains the dynamic properties of the Earth–Moon system but remains challenged by the similarity of isotopic fingerprints of the terrestrial and lunar mantles. Moreover, recent geochemical evidence suggests that the Earth's mantle preserves ancient (or "primordial") heterogeneity that pre-dates the Moon-forming giant impact. Using a new hydrodynamical method, we here show that Moon-forming giant impacts lead to a stratified starting condition for the evolution of the terrestrial mantle. The upper layer of the Earth is compositionally similar to the disk, out of which the Moon evolves, whereas the lower layer preserves proto-Earth characteristics. As long as this predicted compositional stratification can at least partially be preserved over the subsequent billions of years of Earth mantle convection, a compositional similarity between the Moon and the accessible Earth's mantle is a natural outcome of realistic and high-probability Moon-forming impact scenarios. The preservation of primordial heterogeneity in the modern Earth not only reconciles geochemical constraints but is also consistent with recent geophysical observations. Furthermore, for significant preservation of a proto-Earth reservoir, the bulk major-element composition of the Earth–Moon system may be systematically shifted toward chondritic values

### Folding of a donor–acceptor polyrotaxane by using noncovalent bonding interactions

Mechanically interlocked compounds, such as bistable catenanes and bistable rotaxanes, have been used to bring about actuation in nanoelectromechanical systems (NEMS) and molecular electronic devices (MEDs). The elaboration of the structural features of such rotaxanes into macromolecular materials might allow the utilization of molecular motion to impact their bulk properties. We report here the synthesis and characterization of polymers that contain π electron-donating 1,5-dioxynaphthalene (DNP) units encircled by cyclobis(paraquat-p-phenylene) (CBPQT4+), a π electron-accepting tetracationic cyclophane, synthesized by using the copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC). The polyrotaxanes adopt a well defined “folded” secondary structure by virtue of the judicious design of two DNP-containing monomers with different binding affinities for CBPQT4+. This efficient approach to the preparation of polyrotaxanes, taken alongside the initial investigations of their chemical properties, sets the stage for the preparation of a previously undescribed class of macromolecular architectures

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