274 research outputs found
Scattering Polarization in the Presence of Magnetic and Electric Fields
The polarization of radiation by scattering on an atom embedded in combined
external quadrupole electric and uniform magnetic fields is studied
theoretically. Limiting cases of scattering under Zeeman effect and Hanle
effect in weak magnetic fields are discussed. The theory is general enough to
handle scattering in intermediate magnetic fields (Hanle-Zeeman effect) and for
arbitrary orientation of magnetic field. The quadrupolar electric field
produces asymmetric line shifts and causes interesting level-crossing phenomena
either in the absence of an ambient magnetic field or in its presence. It is
shown that the quadrupolar electric field produces an additional depolarization
in the profiles and rotation of the plane of polarization in the
profile over and above that arising from magnetic field itself. This
characteristic may have a diagnostic potential to detect steady state and time
varying electric fields that surround radiating atoms in Solar atmospheric
layers.Comment: 41 pages, 6 figure
The Hanle Effect in 1D, 2D and 3D
This paper addresses the problem of scattering line polarization and the
Hanle effect in one-dimensional (1D), two-dimensional (2D) and
three-dimensional (3D) media for the case of a two-level model atom without
lower-level polarization and assuming complete frequency redistribution. The
theoretical framework chosen for its formulation is the QED theory of Landi
Degl'Innocenti (1983), which specifies the excitation state of the atoms in
terms of the irreducible tensor components of the atomic density matrix. The
self-consistent values of these density-matrix elements is to be determined by
solving jointly the kinetic and radiative transfer equations for the Stokes
parameters. We show how to achieve this by generalizing to Non-LTE polarization
transfer the Jacobi-based ALI method of Olson et al. (1986) and the iterative
schemes based on Gauss-Seidel iteration of Trujillo Bueno and Fabiani Bendicho
(1995). These methods essentially maintain the simplicity of the
Lambda-iteration method, but their convergence rate is extremely high. Finally,
some 1D and 2D model calculations are presented that illustrate the effect of
horizontal atmospheric inhomogeneities on magnetic and non-magnetic resonance
line polarization signals.Comment: 14 pages and 5 figure
Competition and stability in the credit industry: banking vs. factoring industries
Over the last decade, most credit-industries registered a decline in lending volumes, while factoring industries instead registered a substantial growth in terms of turnover. Surprisingly, only a handful of papers so far investigate factoring companies. Do factoring firms display the same stability levels of banks? Is the competition similar in factoring and banking industries? Is the relationship between competition and stability the same in these industries? Focusing on Italy (one of the largest factoring and banking markets in Europe) and using a unique dataset, we show three main results: factoring companies are (on average) more stable than banks; 2) the stability of factoring companies increase when competition declines (competition-fragility view); 3) the competition-fragility view is weaker in the factoring industry than in the banking industry. Our findings indicate that competition in the Italian credit industry was greater in factoring than in banking
Competition and risk-taking in investment banking
How does competition affect the investment banking business and the risks individual institutions are exposed to? Using a large sample of investment banks operating in seven developed economies over 1997-2014, we apply a panel VAR model to examine the relationships between competition and risk without assuming any a priori restrictions. Our main finding is that investment banks’ higher risk exposure, measured as a long-term capital-at-risk and return volatility, was facilitated by greater competitive pressures for both boutique investment banks and full service investment banks. Overall, we find some evidence that more competition leads to more fragility before and during the recent financial crisis
Mixed Quantum/Classical Calculations of Total and Differential Elastic and Rotationally Inelastic Scattering Cross Sections for Light and Heavy Reduced Masses in a Broad Range of Collision Energies
The mixed quantum/classical theory (MQCT) for rotationally inelastic scattering developed recently [A. Semenov and D. Babikov, J. Chem. Phys.139, 174108 (2013)] is benchmarked against the full quantum calculations for two molecular systems: He + H2 and Na + N2. This allows testing new method in the cases of light and reasonably heavy reduced masses, for small and large rotational quanta, in a broad range of collision energies and rotational excitations. The resultant collision cross sections vary through ten-orders of magnitude range of values. Both inelastic and elastic channels are considered, as well as differential (over scattering angle) cross sections. In many cases results of the mixed quantum/classical method are hard to distinguish from the full quantum results. In less favorable cases (light masses, larger quanta, and small collision energies) some deviations are observed but, even in the worst cases, they are within 25% or so. The method is computationally cheap and particularly accurate at higher energies, heavier masses, and larger densities of states. At these conditions MQCT represents a useful alternative to the standard full-quantum scattering theory
Constraining the Variation of G by Cosmic Microwave Background Anisotropies
We use the Cosmic Microwave Background Anisotropies (CMBA) power spectra to
constrain the cosmological variation of gravitational constant G. It is found
that the sensitivity of CMBA to the variation of G is enhanced when G is
required to converge to its present value. The variations of G from the CMB
decoupling epoch z ~ 1000 to the present time are modelled by a step function
and a linear function of scale factor respectively, and the corresponding
95% confidence intervals for G/G_0 are [0.95, 1.05] and [0.89, 1.13], G_0 being
the present value. The CMBA constraint is unique in the sense that it entails
the range of redshift from z \approx 1000 to 0.Comment: 7 pages, 8 figures, discussion added, references adde
Recent progress in terahertz metamaterial modulators
The terahertz (0.1–10 THz) range represents a fast-evolving research and industrial field. The great interest for this portion of the electromagnetic spectrum, which lies between the photonics and the electronics ranges, stems from the unique and disruptive sectors where this radiation finds applications in, such as spectroscopy, quantum electronics, sensing and wireless communications beyond 5G. Engineering the propagation of terahertz light has always proved to be an intrinsically difficult task and for a long time it has been the bottleneck hindering the full exploitation of the terahertz spectrum. Amongst the different approaches that have been proposed so far for terahertz signal manipulation, the implementation of metamaterials has proved to be the most successful one, owing to the relative ease of realisation, high efficiency and spectral versatility. In this review, we present the latest developments in terahertz modulators based on metamaterials, while highlighting a few selected key applications in sensing, wireless communications and quantum electronics, which have particularly benefitted from these developments
Magnetic Field Measurement with Ground State Alignment
Observational studies of magnetic fields are crucial. We introduce a process
"ground state alignment" as a new way to determine the magnetic field direction
in diffuse medium. The alignment is due to anisotropic radiation impinging on
the atom/ion. The consequence of the process is the polarization of spectral
lines resulting from scattering and absorption from aligned atomic/ionic
species with fine or hyperfine structure. The magnetic field induces precession
and realign the atom/ion and therefore the polarization of the emitted or
absorbed radiation reflects the direction of the magnetic field. The atoms get
aligned at their low levels and, as the life-time of the atoms/ions we deal
with is long, the alignment induced by anisotropic radiation is susceptible to
extremely weak magnetic fields (G). In fact,
the effects of atomic/ionic alignment were studied in the laboratory decades
ago, mostly in relation to the maser research. Recently, the atomic effect has
been already detected in observations from circumstellar medium and this is a
harbinger of future extensive magnetic field studies. A unique feature of the
atomic realignment is that they can reveal the 3D orientation of magnetic
field. In this article, we shall review the basic physical processes involved
in atomic realignment. We shall also discuss its applications to
interplanetary, circumstellar and interstellar magnetic fields. In addition,
our research reveals that the polarization of the radiation arising from the
transitions between fine and hyperfine states of the ground level can provide a
unique diagnostics of magnetic fields in the Epoch of Reionization.Comment: 30 pages, 12 figures, chapter in Lecture Notes in Physics "Magnetic
Fields in Diffuse Media". arXiv admin note: substantial text overlap with
arXiv:1203.557
Near-Field Spectroscopy of Individual Asymmetric Split-Ring Terahertz Resonators
Metamaterial resonators have become an efficient and versatile platform in the terahertz frequency range, finding applications in integrated optical devices, such as active modulators and detectors, and in fundamental research, e.g., ultrastrong light–matter investigations. Despite their growing use, characterization of modes supported by these subwavelength elements has proven to be challenging and it still relies on indirect observation of the collective far-field transmission/reflection properties of resonator arrays. Here, we present a broadband time-domain spectroscopic investigation of individual metamaterial resonators via a THz aperture scanning near-field microscope (a-SNOM). The time-domain a-SNOM allows the mapping and quantitative analysis of strongly confined modes supported by the resonators. In particular, a cross-polarized configuration presented here allows an investigation of weakly radiative modes. These results hold great potential to advance future metamaterial-based optoelectronic platforms for fundamental research in THz photonics.
Fusion rate enhancement due to energy spread of colliding nuclei
Experimental results for sub-barrier nuclear fusion reactions show cross
section enhancements with respect to bare nuclei which are generally larger
than those expected according to electron screening calculations. We point out
that energy spread of target or projectile nuclei is a mechanism which
generally provides fusion enhancement. We present a general formula for
calculating the enhancement factor and we provide quantitative estimate for
effects due to thermal motion, vibrations inside atomic, molecular or crystal
system, and due to finite beam energy width. All these effects are marginal at
the energies which are presently measurable, however they have to be considered
in future experiments at still lower energies. This study allows to exclude
several effects as possible explanation of the observed anomalous fusion
enhancements, which remain a mistery.Comment: 17 pages with 3 ps figure included. Revtex styl
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