17,607 research outputs found
Collective excitations and low temperature transport properties of bismuth
We examine the influence of collective excitations on the transport
properties (resistivity, magneto- optical conductivity) for semimetals,
focusing on the case of bismuth. We show, using an RPA approximation, that the
properties of the system are drastically affected by the presence of an
acoustic plasmon mode, consequence of the presence of two types of carriers
(electrons and holes) in this system. We found a crossover temperature T*
separating two different regimes of transport. At high temperatures T > T* we
show that Baber scattering explains quantitatively the DC resistivity
experiments, while at low temperatures T < T* interactions of the carriers with
this collective mode lead to a T^5 behavior of the resistivity. We examine
other consequences of the presence of this mode, and in particular predict a
two plasmon edge feature in the magneto-optical conductivity. We compare our
results with the experimental findings on bismuth. We discuss the limitations
and extensions of our results beyond the RPA approximation, and examine the
case of other semimetals such as graphite or 1T-TiSe_2
Management of germ cell tumors in children: Approaches to cure
The introduction of cisplatinum chemotherapy and current advances in the surgical treatment have resulted in a dramatic improvement of the prognosis of children with malignant germ cell tumors (GCT). Cisplatinum chemotherapy generally results in sufficient systemic tumor control, but local relapses may still occur in patients who did not receive adequate local treatment. Therefore, the therapeutic consideration must take into account age, primary site of the tumor, and its histology. In gonadal tumors, there is a high chance of primary complete resection since these tumors tend to be encapsulated, and particularly testicular GCT are often detected at a low tumor stage. In contrast, a primary complete resection may be impossible in large nongonadal tumors such as sacrococcygeal or mediastinal GCT. In these tumors, a neoadjuvant or pre-operative chemotherapy after clinical diagnosis by imaging and evaluation of tumor markers significantly facilitates complete resection on delayed surgery. In addition, the impact of chemotherapy on local tumor control may be enhanced by locoregional hyperthermia. In most intracranial GCT complete resection is impossible and may be associated with significant morbidity. Nevertheless, biopsy is essential for diagnosis in nonsecreting tumors. In intracranial GCT, radiotherapy significantly contributes to local tumor control, and doses are stratified according to histology. These general considerations have been integrated into national and international cooperative treatment protocols. In most current protocols, treatment is stratified according to an initial risk assessment that includes the parameters age, site, histology, stage, completeness of resection and the tumor markers alpha(1)-fetoprotein (AFP) and human choriogonadotropin (beta-HCG). With such modern protocols overall cure rates above 80% can be achieved. Moreover, the previously high-risk groups may now expect a favorable prognosis with this risk-adapted treatment, whereas an increasing number of low-risk patients are treated expectantly or with significantly reduced chemotherapy. As current biologic studies reveal distinct genetic patterns in childhood GCT, it can be expected that further combined clinical and genetic studies will be valuable for risk assessment of childhood GCT
The uniting of Europe and the foundation of EU studies: revisiting the neofunctionalism of Ernst B. Haas
This article suggests that the neofunctionalist theoretical legacy left by Ernst B. Haas is somewhat richer and more prescient than many contemporary discussants allow. The article develops an argument for routine and detailed re-reading of the corpus of neofunctionalist work (and that of Haas in particular), not only to disabuse contemporary students and scholars of the normally static and stylized reading that discussion of the theory provokes, but also to suggest that the conceptual repertoire of neofunctionalism is able to speak directly to current EU studies and comparative regionalism. Neofunctionalism is situated in its social scientific context before the theory's supposed erroneous reliance on the concept of 'spillover' is discussed critically. A case is then made for viewing Haas's neofunctionalism as a dynamic theory that not only corresponded to established social scientific norms, but did so in ways that were consistent with disciplinary openness and pluralism
Probing the Light Pseudoscalar Window
Very light pseudoscalars can arise from the symmetry-breaking sector in many
extensions of the Standard Model. If their mass is below 200 MeV, they can be
long-lived and have interesting phenomenology. We discuss the experimental
constraints on several models with light pseudoscalars, including one in which
the pseudoscalar is naturally fermiophobic. Taking into account the stringent
bounds from rare K and B decays, we find allowed parameter space in each model
that may be accessible in direct production experiments. In particular, we
study the photoproduction of light pseudoscalars at Jefferson Lab and conclude
that a beam dump experiment could explore some of the allowed parameter space
of these models.Comment: 22 pages, 4 figure
Reexamining Black-Body Shifts for Hydrogenlike Ions
We investigate black-body induced energy shifts for low-lying levels of
atomic systems, with a special emphasis on transitions used in current and
planned high-precision experiments on atomic hydrogen and ionized helium.
Fine-structure and Lamb-shift induced black-body shifts are found to increase
with the square of the nuclear charge number, whereas black-body shifts due to
virtual transitions decrease with increasing nuclear charge as the fourth power
of the nuclear charge. We also investigate the decay width acquired by the
ground state of atomic hydrogen, due to interaction with black-body photons.
The corresponding width is due to an instability against excitation to higher
excited atomic levels, and due to black-body induced ionization. These effects
limit the lifetime of even the most fundamental, a priori absolutely stable,
"asymptotic" state of atomic theory, namely the ground state of atomic
hydrogen.Comment: 11 pages; LaTe
Density of bulk trap states in organic semiconductor crystals: discrete levels induced by oxygen in rubrene
The density of trap states in the bandgap of semiconducting organic single
crystals has been measured quantitatively and with high energy resolution by
means of the experimental method of temperature-dependent
space-charge-limited-current spectroscopy (TD-SCLC). This spectroscopy has been
applied to study bulk rubrene single crystals, which are shown by this
technique to be of high chemical and structural quality. A density of deep trap
states as low as ~ 10^{15} cm^{-3} is measured in the purest crystals, and the
exponentially varying shallow trap density near the band edge could be
identified (1 decade in the density of states per ~25 meV). Furthermore, we
have induced and spectroscopically identified an oxygen related sharp hole bulk
trap state at 0.27 eV above the valence band.Comment: published in Phys. Rev. B, high quality figures:
http://www.cpfs.mpg.de/~krellner
Initial Characteristics of Kepler Short Cadence Data
The Kepler Mission offers two options for observations -- either Long Cadence
(LC) used for the bulk of core mission science, or Short Cadence (SC) which is
used for applications such as asteroseismology of solar-like stars and transit
timing measurements of exoplanets where the 1-minute sampling is critical. We
discuss the characteristics of SC data obtained in the 33.5-day long Quarter 1
(Q1) observations with Kepler which completed on 15 June 2009. The truly
excellent time series precisions are nearly Poisson limited at 11th magnitude
providing per-point measurement errors of 200 parts-per-million per minute. For
extremely saturated stars near 7th magnitude precisions of 40 ppm are reached,
while for background limited measurements at 17th magnitude precisions of 7
mmag are maintained. We note the presence of two additive artifacts, one that
generates regularly spaced peaks in frequency, and one that involves additive
offsets in the time domain inversely proportional to stellar brightness. The
difference between LC and SC sampling is illustrated for transit observations
of TrES-2.Comment: 5 pages, 4 figures, ApJ Letters in pres
Design and manufacture of a head and neck phantom for the assessment of IMRT delivery with gel dosimetry
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