2,931 research outputs found
Spin injection and electric field effect in degenerate semiconductors
We analyze spin-transport in semiconductors in the regime characterized by
(intermediate to degenerate), where is the Fermi
temperature. Such a regime is of great importance since it includes the lightly
doped semiconductor structures used in most experiments; we demonstrate that,
at the same time, it corresponds to the regime in which carrier-carrier
interactions assume a relevant role. Starting from a general formulation of the
drift-diffusion equations, which includes many-body correlation effects, we
perform detailed calculations of the spin injection characteristics of various
heterostructures, and analyze the combined effects of carrier density
variation, applied electric field and Coulomb interaction. We show the
existence of a degenerate regime, peculiar to semiconductors, which strongly
differs, as spin-transport is concerned, from the degenerate regime of metals.Comment: Version accepted for publication in Phys. Rev.
A semi-Markov model with memory for price changes
We study the high frequency price dynamics of traded stocks by a model of
returns using a semi-Markov approach. More precisely we assume that the
intraday returns are described by a discrete time homogeneous semi-Markov which
depends also on a memory index. The index is introduced to take into account
periods of high and low volatility in the market. First of all we derive the
equations governing the process and then theoretical results have been compared
with empirical findings from real data. In particular we analyzed high
frequency data from the Italian stock market from first of January 2007 until
end of December 2010
Spin Coulomb drag in the two-dimensional electron liquid
We calculate the spin-drag transresistivity
in a two-dimensional electron gas at temperature in the random phase
approximation. In the low-temperature regime we show that, at variance with the
three-dimensional low-temperature result [], the spin transresistivity of a two-dimensional {\it spin unpolarized}
electron gas has the form . In the
spin-polarized case the familiar form is
recovered, but the constant of proportionality diverges logarithmically as
the spin-polarization tends to zero. In the high-temperature regime we obtain
(where
is the effective Rydberg energy) {\it independent} of the density.
Again, this differs from the three-dimensional result, which has a logarithmic
dependence on the density. Two important differences between the spin-drag
transresistivity and the ordinary Coulomb drag transresistivity are pointed
out: (i) The singularity at low temperature is smaller, in the Coulomb
drag case, by a factor where is the Fermi wave vector and
is the separation between the layers. (ii) The collective mode contribution
to the spin-drag transresistivity is negligible at all temperatures. Moreover
the spin drag effect is, for comparable parameters, larger than the ordinary
Coulomb drag effect.Comment: 6 figures; various changes; version accepted for publicatio
Weighted-indexed semi-Markov models for modeling financial returns
In this paper we propose a new stochastic model based on a generalization of
semi-Markov chains to study the high frequency price dynamics of traded stocks.
We assume that the financial returns are described by a weighted indexed
semi-Markov chain model. We show, through Monte Carlo simulations, that the
model is able to reproduce important stylized facts of financial time series as
the first passage time distributions and the persistence of volatility. The
model is applied to data from Italian and German stock market from first of
January 2007 until end of December 2010.Comment: arXiv admin note: substantial text overlap with arXiv:1109.425
Massive Cosmologies
We explore the cosmological solutions of a recently proposed extension of
General Relativity with a Lorentz-invariant mass term. We show that the same
constraint that removes the Boulware-Deser ghost in this theory also prohibits
the existence of homogeneous and isotropic cosmological solutions.
Nevertheless, within domains of the size of inverse graviton mass we find
approximately homogeneous and isotropic solutions that can well describe the
past and present of the Universe. At energy densities above a certain crossover
value, these solutions approximate the standard FRW evolution with great
accuracy. As the Universe evolves and density drops below the crossover value
the inhomogeneities become more and more pronounced. In the low density regime
each domain of the size of the inverse graviton mass has essentially non-FRW
cosmology. This scenario imposes an upper bound on the graviton mass, which we
roughly estimate to be an order of magnitude below the present-day value of the
Hubble parameter. The bound becomes especially restrictive if one utilizes an
exact self-accelerated solution that this theory offers. Although the above are
robust predictions of massive gravity with an explicit mass term, we point out
that if the mass parameter emerges from some additional scalar field
condensation, the constraint no longer forbids the homogeneous and isotropic
cosmologies. In the latter case, there will exist an extra light scalar field
at cosmological scales, which is screened by the Vainshtein mechanism at
shorter distances.Comment: 21 page
Coulomb interaction effects in spin-polarized transport
We study the effect of the electron-electron interaction on the transport of
spin polarized currents in metals and doped semiconductors in the diffusive
regime. In addition to well-known screening effects, we identify two additional
effects, which depend on many-body correlations and exchange and reduce the
spin diffusion constant. The first is the "spin Coulomb drag" - an intrinsic
friction mechanism which operates whenever the average velocities of up-spin
and down-spin electrons differ. The second arises from the decrease in the
longitudinal spin stiffness of an interacting electron gas relative to a
noninteracting one. Both effects are studied in detail for both degenerate and
non-degenerate carriers in metals and semiconductors, and various limiting
cases are worked out analytically. The behavior of the spin diffusion constant
at and below a ferromagnetic transition temperature is also discussed.Comment: 9 figure
Intrinsic electric field effects on few-particle interactions in coupled GaN quantum dots
We study the multi-exciton optical spectrum of vertically coupled GaN/AlN
quantum dots with a realistic three-dimensional direct-diagonalization approach
for the description of few-particle Coulomb-correlated states. We present a
detailed analysis of the fundamental properties of few-particle/exciton
interactions peculiar of nitride materials. The giant intrinsic electric fields
and the high electron/hole effective masses give rise to different effects
compared to GaAs-based quantum dots: intrinsic exciton-exciton coupling,
non-molecular character of coupled dot exciton wavefunction, strong dependence
of the oscillator strength on the dot height, large ground state energy shift
for dots separated by different barriers. Some of these effects make GaN/AlN
quantum dots interesting candidates in quantum information processing.Comment: 23 pages, 8 figures, 1 tabl
Speedy Techniques to Evaluate Seismic Site Effects in Particular Geomorphologic Conditions: Faults, Cavities, Landslides and Topographic Irregularities
The ground motion that can be recorded at the free surface of a terrain is the final result of a series of phenomena that can be grouped into three fundamental typologies: the source mechanism, the seismic wave propagation till the bedrock interface below the investigated site and the site effects (Fig. 1). The first two features define the kind of seismic input whereas the third represents all modifications that can occur as a consequence of the interaction between seismic waves and local characteristics of the investigated site. The physical and mechanical properties of terrains as well as their morphologic and stratigraphic features appreciably affect the characteristics of the ground motion observed at the surface. The whole process of modifications undergone by a given seismic input in terms of amplitude, frequency content and duration, as a consequence of local characteristics, is generally termed the âlocal seismic responseâ. It is indeed well known that the spectral composition of a seismic event is modified first during the source-bedrock path (attenuation function), and second, when the seismic input interacts with the soft terrains layered between the bedrock and the free surface (Fig. 1a). This latter effect, significantly changes the spectral content so that it is extremely important for estimating the final input to which all structures built in the study area will be subjected.peer-reviewe
Public Higher Education Funding, Budget Drivers, and Related Issues: The State Community College Director Perspective
This article presents results from the 2012 National Survey of Access and Finance Issues conducted by the National Council of State Directors of Community Colleges (NCSDCC), an affiliated council of the American Association of Community Colleges, and includes a comparison of survey results from previous years dating back to 2003, with the exception of 2005 and 2006 when the survey was not conducted
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