14,103 research outputs found
Quantum theory of intersubband polarons
We present a microscopic quantum theory of intersubband polarons,
quasiparticles originated from the coupling between intersubband transitions
and longitudinal optical phonons. To this aim we develop a second quantized
theory taking into account both the Fr\"ohlich interaction between phonons and
intersubband transitions and the Coulomb interaction between the intersubband
transitions themselves. Our results show that the coupling between the phonons
and the intersubband transitions is extremely intense, thanks both to the
collective nature of the intersubband excitations and to the natural tight
confinement of optical phonons. Not only the coupling is strong enough to
spectroscopically resolve the resonant splitting between the modes (strong
coupling regime), but it can become comparable to the bare frequency of the
excitations (ultrastrong coupling regime). We thus predict the possibility to
exploit intersubband polarons both for applied optoelectronic research, where a
precise control of the phonon resonances is needed, and also to observe
fundamental quantum vacuum physics, typical of the ultrastrong coupling regime
Mechanics of tubular meshes formed by elastic helical fibers
Tubular structures made of elastic helical fibers are widely found in nature and in technology. The complex and highly nonlinear mechanical properties of such assemblies have been understood either through minimal models or through complex simulations describing each individual fiber and their interactions. Here, inspired by Chebyshev’s geometric model of nets, we propose and experimentally validate a modeling framework that treats tubular braided meshes as continuum surfaces corresponding to the virtual envelope defined by the fibers. The key idea is to relate surface geometry and fiber kinematics, enabling us to follow large deformations. This theory is amenable to efficient computations and, in axisymmetric cases, the problem reduces to finding two scalar fields defined over 1D segments. We validate our model against experiments of axial compression, revealing the existence of a plateau with vanishing stiffness in the axial force–displacement curve, a feature that could prove particularly useful in applications where an applied compressive force needs to be held constant even against settlements of the compressed object
Leptogenesis in models with keV sterile neutrino dark matter
We analyze leptogenesis in gauge extensions of the Standard Model with keV
sterile neutrino dark matter. We find that both the observed dark matter
abundance and the correct baryon asymmetry of the Universe can simultaneously
emerge in these models. Both the dark matter abundance and the leptogenesis are
controlled by the out of equilibrium decays of the same heavy right handed
neutrino.Comment: 6 pages, 1 figur
Social Support and Self-Efficacy on Turnover Intentions: The Mediating Role of Conflict and Commitment
Turnover intentions are a phenomenon that affects the life of organizations and causes highly negative consequences. Based on previous studies, it is possible to consider antecedents to turnover in terms of both individual and social perceived resources, which previous research does not usually examine simultaneously. The aim of this study was to explore the role of both resources (individual and social) on turnover intentions. Thus, we hypothesized that perceived social support and self-efficacy have an impact on turnover intentions and that this relationship is mediated by interpersonal conflict and Affective Commitment. A total of 392 Italian employees completed a self-report questionnaire. A structural equation model was tested. The results showed that interpersonal conflict and Affective Commitment fully mediated the relationship between social support, self-efficacy and turnover intentions. Practical implications are discussed
Glycolaldehyde in Perseus young solar analogs
Aims: In this paper we focus on the occurrence of glycolaldehyde (HCOCH2OH)
in young solar analogs by performing the first homogeneous and unbiased study
of this molecule in the Class 0 protostars of the nearby Perseus star forming
region. Methods: We obtained sub-arcsec angular resolution maps at 1.3mm and
1.4mm of glycolaldehyde emission lines using the IRAM Plateau de Bure (PdB)
interferometer in the framework of the CALYPSO IRAM large program. Results:
Glycolaldehyde has been detected towards 3 Class 0 and 1 Class I protostars out
of the 13 continuum sources targeted in Perseus: NGC1333-IRAS2A1,
NGC1333-IRAS4A2, NGC1333-IRAS4B1, and SVS13-A. The NGC1333 star forming region
looks particularly glycolaldehyde rich, with a rate of occurrence up to 60%.
The glycolaldehyde spatial distribution overlaps with the continuum one,
tracing the inner 100 au around the protostar. A large number of lines (up to
18), with upper-level energies Eu from 37 K up to 375 K has been detected. We
derived column densities > 10^15 cm^-2 and rotational temperatures Trot between
115 K and 236 K, imaging for the first time hot-corinos around NGC1333-IRAS4B1
and SVS13-A. Conclusions: In multiple systems glycolaldehyde emission is
detected only in one component. The case of the SVS13-A+B and IRAS4-A1+A2
systems support that the detection of glycolaldehyde (at least in the present
Perseus sample) indicates older protostars (i.e. SVS13-A and IRAS4-A2), evolved
enough to develop the hot-corino region (i.e. 100 K in the inner 100 au).
However, only two systems do not allow us to firmly conclude whether the
primary factor leading to the detection of glycolaldehyde emission is the
environments hosting the protostars, evolution (e.g. low value of Lsubmm/Lint),
or accretion luminosity (high Lint).Comment: A&A, in pres
Scaling of stiffness energy for 3d +/-J Ising spin glasses
Large numbers of ground states of 3d EA Ising spin glasses are calculated for
sizes up to 10^3 using a combination of a genetic algorithm and Cluster-Exact
Approximation. A detailed analysis shows that true ground states are obtained.
The ground state stiffness (or domain wall) energy D is calculated. A D ~ L^t
behavior with t=0.19(2) is found which strongly indicates that the 3d model has
an equilibrium spin-glass-paramagnet transition for non-zero T_c.Comment: 4 pages, 4 figure
Deuterium and N fractionation in NH during the formation of a Sun-like star
Although chemical models predict that the deuterium fractionation in
NH is a good evolutionary tracer in the star formation process, the
fractionation of nitrogen is still a poorly understood process. Recent models
have questioned the similar evolutionary trend expected for the two
fractionation mechanisms in NH, based on a classical scenario in which
ion-neutral reactions occurring in cold gas should have caused an enhancement
of the abundance of ND, NNH, and NNH. In the
framework of the ASAI IRAM-30m large program, we have investigated the
fractionation of deuterium and N in NH in the best known
representatives of the different evolutionary stages of the Sun-like star
formation process. The goal is to ultimately confirm (or deny) the classical
"ion-neutral reactions" scenario that predicts a similar trend for D and
N fractionation. We do not find any evolutionary trend of the
N/N ratio from both the NNH and NNH
isotopologues. Therefore, our findings confirm that, during the formation of a
Sun-like star, the core evolution is irrelevant in the fractionation of
N. The independence of the N/N ratio with time, found also
in high-mass star-forming cores, indicates that the enrichment in N
revealed in comets and protoplanetary disks is unlikely to happen at core
scales. Nevertheless, we have firmly confirmed the evolutionary trend expected
for the H/D ratio, with the NH/ND ratio decreasing before the
pre-stellar core phase, and increasing monotonically during the protostellar
phase. We have also confirmed clearly that the two fractionation mechanisms are
not related.Comment: 9 pages, 2 figures, accepted for publication in MNRA
Cloning transformations in spin networks without external control
In this paper we present an approach to quantum cloning with unmodulated spin
networks. The cloner is realized by a proper design of the network and a choice
of the coupling between the qubits. We show that in the case of phase covariant
cloner the XY coupling gives the best results. In the 1->2 cloning we find that
the value for the fidelity of the optimal cloner is achieved, and values
comparable to the optimal ones in the general N->M case can be attained. If a
suitable set of network symmetries are satisfied, the output fidelity of the
clones does not depend on the specific choice of the graph. We show that spin
network cloning is robust against the presence of static imperfections.
Moreover, in the presence of noise, it outperforms the conventional approach.
In this case the fidelity exceeds the corresponding value obtained by quantum
gates even for a very small amount of noise. Furthermore we show how to use
this method to clone qutrits and qudits. By means of the Heisenberg coupling it
is also possible to implement the universal cloner although in this case the
fidelity is 10% off that of the optimal cloner.Comment: 12 pages, 13 figures, published versio
The critical exponents of the two-dimensional Ising spin glass revisited: Exact Ground State Calculations and Monte Carlo Simulations
The critical exponents for of the two-dimensional Ising spin glass
model with Gaussian couplings are determined with the help of exact ground
states for system sizes up to and by a Monte Carlo study of a
pseudo-ferromagnetic order parameter. We obtain: for the stiffness exponent
, for the magnetic exponent
and for the chaos exponent . From Monte Carlo simulations we
get the thermal exponent . The scaling prediction is
fulfilled within the error bars, whereas there is a disagreement with the
relation .Comment: 8 pages RevTeX, 7 eps-figures include
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