2,617 research outputs found
Pulse Propagation in Resonant Tunneling
We consider the analytically solvable model of a Gaussian pulse tunneling
through a transmission resonance with a Breit-Wigner characteristic. The
solution allows for the identification of two opposite pulse propagation
regimes: if the resonance is broad compared to the energetic width of the
incident Gaussian pulse a weakly deformed and slightly delayed transmitted
Gaussian pulse is found. In the opposite limit of a narrow resonance the dying
out of the transmitted pulse is dominated by the slow exponential decay
characteristic of a quasi-bound state with a long life time (decaying state).
We discuss the limitation of the achievable pulse transfer rate resulting from
the slow decay. Finally, it is demonstrated that for narrow resonances a small
second component is superimposed to the exponential decay which leads to
characteristic interference oscillations.Comment: 6 pages, 4 figure
Manifestation of the Hofstadter butterfly in far-infrared absorption
The far-infrared absorption of a two-dimensional electron gas with a
square-lattice modulation in a perpendicular constant magnetic field is
calculated self-consistently within the Hartree approximation. For strong
modulation and short period we obtain intra- and intersubband magnetoplasmon
modes reflecting the subbands of the Hofstadter butterfly in two or more Landau
bands. The character of the absorption and the correlation of the peaks to the
number of flux quanta through each unit cell of the periodic potential depends
strongly on the location of the chemical potential with respect to the
subbands, or what is the same, on the density of electrons in the system.Comment: RevTeX file + 4 postscript figures, to be published Phys. Rev. B
Rapid Com
Imputating missing values in diary records of sun-exposure study
In a sun-exposure study, questionnaires concerning sunhabits were collected from 195 subjects. This paper focuses on the general problem of missing data values, which occurs when some, or even all of the questions have not been answered in a questionnaire. Here, only missing values of low concentration are investigated. We consider and compare two different models for imputating missing values: the Gaussian model and the non-parametric K-Nearest Neighbor model
Properties of Interfaces in the two and three dimensional Ising Model
To investigate order-order interfaces, we perform multimagnetical Monte Carlo
simulations of the and Ising model. Following Binder we extract the
interfacial free energy from the infinite volume limit of the magnetic
probability density. Stringent tests of the numerical methods are performed by
reproducing with high precision exact results. In the physically more
interesting case we estimate the amplitude of the critical
interfacial tension to be . This
result is in good agreement with a previous MC calculation by Mon, as well as
with experimental results for related amplitude ratios. In addition, we study
in some details the shape of the magnetic probability density for temperatures
below the Curie point.Comment: 25 pages; sorry no figures include
Thomas-Fermi-Dirac-von Weizsacker hydrodynamics in laterally modulated electronic systems
We have studied the collective plasma excitations of a two-dimensional
electron gas with an arbitrary lateral charge-density modulation. The dynamics
is formulated using a previously developed hydrodynamic theory based on the
Thomas-Fermi-Dirac-von Weizsacker approximation. In this approach, both the
equilibrium and dynamical properties of the periodically modulated electron gas
are treated in a consistent fashion. We pay particular attention to the
evolution of the collective excitations as the system undergoes the transition
from the ideal two-dimensional limit to the highly-localized one-dimensional
limit. We also calculate the power absorption in the long-wavelength limit to
illustrate the effect of the modulation on the modes probed by far-infrared
(FIR) transmission spectroscopy.Comment: 27 page Revtex file, 15 Postscript figure
Ultraviolet radiation exposure to the face in patients with xeroderma pigmentosum and healthy controls:applying a novel methodology to define photoprotection behaviour*
BACKGROUND: In xeroderma pigmentosum (XP), the main means of preventing skin and eye cancers is extreme protection against ultraviolet radiation (UVR). Protection is most important for the face. OBJECTIVES: We aimed to assess how well patients with XP adhere to medical advice to protect against UVR by objectively estimating the mean daily dose of UVR to the face. METHODS: We objectively estimated the UVR dose to the face in 36 patients with XP and 25 healthy individuals over 3âweeks in the summer. We used a new methodology which combined UVR dose measurements from a wristâworn dosimeter with an activity diary record of face photoprotection behaviour for each 15âmin period spent outside. A protection factor was associated with each behaviour, and the data were analysed using a negative binomial mixedâeffects model. RESULTS: The mean daily UVR dose (weighted for DNA damage capacity) to the face in the patients with XP was 0·13 standard erythemal doses (SEDs) (mean in healthy individuals = 0·51 SED). There was wide variation between patients (range < 0·01â0·48 SED/day). Selfâcaring adult patients had a very similar UVR dose to the face as caredâfor patients (0·13 vs. 0·12 SED/day), despite photoprotecting much more poorly when outside, because the selfâcaring adults were outside in daylight much less. CONCLUSIONS: Photoprotection behaviour varies widely within the XP group indicating that nonadherence to photoprotection advice is a significant issue. The timing and duration of going outside are as important as photoprotective measures taken when outside, to determine the UVR exposure to the face. This new methodology will be of value in identifying the sources of UVR exposure in other conditions in which facial UVR exposure is a key outcome, particularly in patients with multiple nonmelanoma skin cancers
Coulomb effects on the quantum transport of a two-dimensional electron system in periodic electric and magnetic fields
The magnetoresistivity tensor of an interacting two-dimensional electron
system with a lateral and unidirectional electric or magnetic modulation, in a
perpendicular quantizing magnetic field, is calculated within the Kubo
formalism. The influence of the spin splitting of the Landau bands and of the
density of states (DOS) on the internal structure of the Shubnikov-de Haas
oscillations is analyzed. The Coulomb electron - electron interaction is
responsible for strong screening and exchange effects and is taken into account
in a screened Hartree-Fock approximation, in which the exchange contribution is
calculated self-consistently with the DOS at the Fermi level. This
approximation describes both the exchange enhancement of the spin splitting and
the formation of compressible edge strips, unlike the simpler Hartree and
Hartree-Fock approximations, which yield either the one or the other.Comment: 20 pages, revtex, 7 ps figures, to appear in Phys. Rev.
Magnetoresistance through a single molecule
The use of single molecules to design electronic devices is an extremely
challenging and fundamentally different approach to further downsizing
electronic circuits. Two-terminal molecular devices such as diodes were first
predicted [1] and, more recently, measured experimentally [2]. The addition of
a gate then enabled the study of molecular transistors [3-5]. In general terms,
in order to increase data processing capabilities, one may not only consider
the electron's charge but also its spin [6,7]. This concept has been pioneered
in giant magnetoresistance (GMR) junctions that consist of thin metallic films
[8,9]. Spin transport across molecules, i.e. Molecular Spintronics remains,
however, a challenging endeavor. As an important first step in this field, we
have performed an experimental and theoretical study on spin transport across a
molecular GMR junction consisting of two ferromagnetic electrodes bridged by a
single hydrogen phthalocyanine (H2Pc) molecule. We observe that even though
H2Pc in itself is nonmagnetic, incorporating it into a molecular junction can
enhance the magnetoresistance by one order of magnitude to 52%.Comment: To appear in Nature Nanotechnology. Present version is the first
submission to Nature Nanotechnology, from May 18th, 201
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