28,146 research outputs found
Natural and laser-induced cavitation in corn stems: On the mechanisms of acoustic emissions
Water in plant xylem is often superheated, and therefore in a meta-stable
state. Under certain conditions, it may suddenly turn from the liquid to the
vapor state. This cavitation process produces acoustic emissions. We report the
measurement of ultrasonic acoustic emissions (UAE) produced by natural and
induced cavitation in corn stems. We induced cavitation and UAE in vivo, in
well controlled and reproducible experiments, by irradiating the bare stem of
the plants with a continuous-wave laser beam. By tracing the source of UAE, we
were able to detect absorption and frequency filtering of the UAE propagating
through the stem. This technique allows the unique possibility of studying
localized embolism of plant conduits, and thus to test hypotheses on the
hydraulic architecture of plants. Based on our results, we postulate that the
source of UAE is a transient "cavity oscillation" triggered by the disruptive
effect of cavitation inception.Comment: 8 pages, 5 figure
X-ray burst induced spectral variability in 4U 1728-34
Aims. INTEGRAL has been monitoring the Galactic center region for more than a
decade. Over this time INTEGRAL has detected hundreds of type-I X-ray bursts
from the neutron star low-mass X-ray binary 4U 1728-34, a.k.a. "the slow
burster". Our aim is to study the connection between the persistent X-ray
spectra and the X-ray burst spectra in a broad spectral range. Methods. We
performed spectral modeling of the persistent emission and the X-ray burst
emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI
instruments. Results. We constructed a hardness intensity diagram to track
spectral state variations. In the soft state the energy spectra are
characterized by two thermal components - likely from the accretion disc and
the boundary/spreading layer - together with a weak hard X-ray tail that we
detect in 4U 1728-34 for the first time in the 40 to 80 keV range. In the hard
state the source is detected up to 200 keV and the spectrum can be described by
a thermal Comptonization model plus an additional component: either a powerlaw
tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect
emission up to 80 keV during the X-ray bursts. We find that during the bursts
the emission above 40 keV decreases by a factor of about three with respect to
the persistent emission level. Conclusions. Our results suggest that the
enhanced X-ray burst emission changes the spectral properties of the accretion
disc in the hard state. The likely cause is an X-ray burst induced cooling of
the electrons in the inner hot flow near the neutron star.Comment: 7 pages, 5 figures, Accepted for publication in A&
In-gap impurity states as the hallmark of the Quantum Spin Hall phase
We study the different response to an impurity of the two topologically
different phases shown by a two dimensional insulator with time reversal
symmetry, namely, the Quantum Spin Hall and the normal phase. We consider the
case of graphene as a toy model that features the two phases driven,
respectively, by intrinsic spin-orbit coupling and inversion symmetry breaking.
We find that strictly normalizable in-gap impurity states only occur in the
Quantum Spin Hall phase and carry dissipationless current whose quirality is
determined by the spin and pseudospin of the residing electron. Our results
imply that topological order can be unveiled by local probes of defect states.Comment: 5 pages, 3 figure
Single exciton spectroscopy of semimagnetic quantum dots
A photo-excited II-VI semiconductor nanocrystal doped with a few Mn spins is
considered. The effects of spin-exciton interactions and the resulting
multi-spin correlations on the photoluminescence are calculated by numerical
diagonalization of the Hamiltonian, including exchange interaction between
electrons, holes and Mn spins, as well as spin-orbit interaction. The results
provide a unified description of recent experiments of photoluminesnce of dots
with one and many Mn atoms as well as optically induced ferromagnetism in
semimagnetic nanocrystals.Comment: 5 pages, 3 figure
Single-photon exchange interaction in a semiconductor microcavity
We consider the effective coupling of localized spins in a semiconductor
quantum dot embedded in a microcavity. The lowest cavity mode and the quantum
dot exciton are coupled and close in energy, forming a polariton. The fermions
forming the exciton interact with localized spins via exchange. Exact
diagonalization of a Hamiltonian in which photons, spins and excitons are
treated quantum mechanically shows that {\it a single polariton} induces a
sizable indirect exchange interaction between otherwise independent spins. The
origin, symmetry properties and the intensity of that interaction depend both
on the dot-cavity coupling and detuning. In the case of a (Cd,Mn)Te quantum
dot, Mn-Mn ferromagnetic coupling mediated by a single photon survives above 1
K whereas the exciton mediated coupling survives at 15 K.Comment: 4 pages, 3 figure
De retibus socialibus et legibus momenti
Online Social Networks (OSNs) are a cutting edge topic. Almost everybody
--users, marketers, brands, companies, and researchers-- is approaching OSNs to
better understand them and take advantage of their benefits. Maybe one of the
key concepts underlying OSNs is that of influence which is highly related,
although not entirely identical, to those of popularity and centrality.
Influence is, according to Merriam-Webster, "the capacity of causing an effect
in indirect or intangible ways". Hence, in the context of OSNs, it has been
proposed to analyze the clicks received by promoted URLs in order to check for
any positive correlation between the number of visits and different "influence"
scores. Such an evaluation methodology is used in this paper to compare a
number of those techniques with a new method firstly described here. That new
method is a simple and rather elegant solution which tackles with influence in
OSNs by applying a physical metaphor.Comment: Changes made for third revision: Brief description of the dataset
employed added to Introduction. Minor changes to the description of
preparation of the bit.ly datasets. Minor changes to the captions of Tables 1
and 3. Brief addition in the Conclusions section (future line of work added).
Added references 16 and 18. Some typos and grammar polishe
An extended Agassi model: algebraic structure, phase diagram, and large size limit
The Agassi model is a schematic two-level model that involves pairing and
monopole-monopole interactions. It is, therefore, an extension of the well
known Lipkin-Meshkov-Glick (LMG) model. In this paper we review the algebraic
formulation of an extension of the Agassi model as well as its bosonic
realization through the Schwinger representation. Moreover, a mean-field
approximation for the model is presented and its phase diagram discussed.
Finally, a analysis, with proportional to the degeneracy of each
level, is worked out to obtain the thermodynamic limit of the ground state
energy and some order parameters from the exact Hamiltonian diagonalization for
finite.Comment: Accepted in Physica Scripta. Focus on SSNET 201
Phase diagram of an extended Agassi model
Background: The Agassi model is an extension of the Lipkin-Meshkov-Glick
model that incorporates the pairing interaction. It is a schematic model that
describes the interplay between particle-hole and pair correlations. It was
proposed in the 1960's by D. Agassi as a model to simulate the properties of
the quadrupole plus pairing model.
Purpose: The aim of this work is to extend a previous study by Davis and
Heiss generalizing the Agassi model and analyze in detail the phase diagram of
the model as well as the different regions with coexistence of several phases.
Method: We solve the model Hamiltonian through the Hartree-Fock-Bogoliubov
(HFB) approximation, introducing two variational parameters that play the role
of order parameters. We also compare the HFB calculations with the exact ones.
Results: We obtain the phase diagram of the model and classify the order of
the different quantum phase transitions appearing in the diagram. The phase
diagram presents broad regions where several phases, up to three, coexist.
Moreover, there is also a line and a point where four and five phases are
degenerated, respectively.
Conclusions: The phase diagram of the extended Agassi model presents a rich
variety of phases. Phase coexistence is present in extended areas of the
parameter space. The model could be an important tool for benchmarking novel
many-body approximations.Comment: Accepted for publication in PR
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