6,578 research outputs found
The Josephson effect throughout the BCS-BEC crossover
We study the stationary Josephson effect for neutral fermions across the
BCS-BEC crossover, by solving numerically the Bogoliubov-de Gennes equations at
zero temperature. The Josephson current is found to be considerably enhanced
for all barriers at about unitarity. For vanishing barrier, the Josephson
critical current approaches the Landau limiting value which, depending on the
coupling, is determined by either pair-breaking or sound-mode excitations. In
the coupling range from the BCS limit to unitarity, a procedure is proposed to
extract the pairing gap from the Landau limiting current.Comment: 4 pages, 3 figures; improved version to appear in Phys. Rev. Let
Adubação foliar em tomateiro estaqueado. (Lycopersicum esculentum, mill). Santa Cruz - Kada.
Experimento realizado em solo latossol vermelho escuro-fase arenosa intergra de Terra roxa estruturada.Resumo
Effective theory for the Goldstone field in the BCS-BEC crossover at T=0
We perform a detailed study of the effective Lagrangian for the Goldstone
mode of a superfluid Fermi gas at zero temperature in the whole BCS-BEC
crossover. By using a derivative expansion of the response functions, we derive
the most general form of this Lagrangian at the next to leading order in the
momentum expansion in terms of four coefficient functions. This involves the
elimination of all the higher order time derivatives by careful use of the
leading order field equations. In the infinite scattering length limit where
conformal invariance is realized, we show that the effective Lagrangian must
contain an unnoticed invariant combination of higher spatial gradients of the
Goldstone mode, while explicit couplings to spatial gradients of the trapping
potential are absent. Across the whole crossover, we determine all the
coefficient functions at the one-loop level, taking into account the dependence
of the gap parameter on the chemical potential in the mean-field approximation.
These results are analytically expressed in terms of elliptic integrals of the
first and second kind. We discuss the form of these coefficients in the extreme
BCS and BEC regimes and around the unitary limit, and compare with recent work
by other authors.Comment: 27 pages. 4 references added, typos corrected, expanded Section III
Weakly Interacting, Dilute Bose Gases in 2D
This article surveys a number of theoretical problems and open questions in
the field of two-dimensional dilute Bose gases with weak repulsive
interactions. In contrast to three dimensions, in two dimensions the formation
of long-range order is prohibited by the Bogoliubov-Hohenberg theorem, and
Bose-Einstein condensation is not expected to be realized. Nevertheless, first
experimental indications supporting the formation of the condensate in low
dimensional systems have been recently obtained. This unexpected behaviour
appears to be due to the non-uniformity, introduced into a system by the
external trapping potential. Theoretical predictions, made for homogeneous
systems, require therefore careful reexamination.
We survey a number of popular theoretical treatments of the dilute weakly
interacting Bose gas and discuss their regions of applicability. The
possibility of Bose-Einstein condensation in a two-dimensional gas, the
validity of perturbative t-matrix approximation and diluteness condition are
issues that we discuss in detail.Comment: Survey, 25 pages RMP style, revised version, refs added, some changes
made, accepted for publication in Rev. Mod. Phy
Momentum distribution of a trapped Fermi gas with large scattering length
Using a scattering length parametrization of the BCS-BEC crossover as well as
the local density approximation for the density profile, we calculate the
momentum distribution of a harmonically trapped atomic Fermi gas at zero
temperature. Various interaction regimes are considered, including the BCS
phase, the unitarity limit and the molecular regime. We show that the relevant
parameter which characterizes the crossover is given by the dimensionless
combination , where is the number of atoms, is the
scattering length and is the oscillator length. The width of the
momentum distribution is shown to depend in a crucial way on the value and sign
of this parameter. Our predictions can be relevant for experiments on ultracold
atomic Fermi gases near a Feshbach resonance.Comment: 6 pages, 2 figures. Submitted to Phys. Rev. A. Added reference
Melanoma metastasis mimicking gastric cancer: a challenge that starts from diagnosis
The gastrointestinal tract is an uncommon site of metastasis in melanoma. However, when the primary melanoma cannot be found, the diagnosis of gastric melanoma by endoscopic biopsy is problematic mainly because some tumors are amelanotic and do not contain melanin granules detectable by microscopy. A 56-year-old Caucasian man with melanoma was referred to us following an initial histopathological diagnosis via gastroscopy of poorly differentiated primary gastric carcinoma. A computerized tomography (CT) scan showed metastatic disease and on the basis of this information we started palliative chemotherapy. However, the atypical presentation of the disease with subcutaneous metastases prompted us to make a more in-depth evaluation. Immunohistochemical evaluation modified the diagnosis to melanoma. After only one cycle of chemotherapy, treatment was changed to dabrafenib + trametinib, which was better tolerated and initially induced a partial response. The patient is currently in good clinical condition 20 months after diagnosis. Our case report highlights the difficulty in diagnosing melanoma of the gastrointestinal tract and indicates the need for pathologists and clinicians to consider such a possibility when they are faced with a diagnosis of poorly differentiated gastric cancer and unusual sites of metastasis
Search for neutrino oscillations on a long base-line at the CHOOZ nuclear power station
This final article about the CHOOZ experiment presents a complete description
of the electron antineutrino source and detector, the calibration methods and
stability checks, the event reconstruction procedures and the Monte Carlo
simulation. The data analysis, systematic effects and the methods used to reach
our conclusions are fully discussed. Some new remarks are presented on the
deduction of the confidence limits and on the correct treatment of systematic
errors.Comment: 41 pages, 59 figures, Latex file, accepted for publication by
Eur.Phys.J.
FM22 (Frozen Mitochondria bioassay): an animal alternative bioassay for toxicity measures for water soluble samples.
Several methods have been proposed, using responses of whole organisms. The problem, however, is not only strictly scientific, but also involves cost, resources and time. For example, assay with organisms require expensive testing facilities and long operational times are necessary for toxicity measurements. In order to evaluate potential compound toxicity (acute and sub-chronic), we standardized a bioassay using mitochondria of beef hearth, and their applicability and sensitivity was verified.
In respect to other based on mitochondria tests, this bioassay (called FM22) showed unquestionable advantages: i) to freeze mitochondria at -22 °C instead of the classical -80 °C, ii) to perform a very big quantity of biological test using always the same mitochondria pool (avoiding differences from age, sex, or health status depending on different organisms); iii) to identify quickly a tested compounds IC50, easily comparable. FM22 end point is the inhibition of mitochondrial respiratory chain and this event is quantified by oxygen monitoring. The oxygen consumption was measured by a Clark electrode that was interfaced to a PC to collect test analysis data (1200 in 20 Macro, identifiedmin run). A piecewise regression, through an Excel the break point in the oxygen consumption and calculated the toxicity. Blank tests were carried out to verify the oxygen consumption linear fitting. Toxicity tests were performed using pure/mix organic and inorganic compounds, elutriates from sea- and fresh-water sediment, sewage, dissolved burned compound sub-products. The FM22 test was a good predictor of toxicity for water and soluble samples; the bioassay is easy, low cost and rapid, then usable for routine tests or like a part of a battery of ecotoxicological tests
Data acquisition software for the CMS strip tracker
The CMS silicon strip tracker, providing a sensitive area of approximately 200 m2 and comprising 10 million readout channels, has recently been completed at the tracker integration facility at CERN. The strip tracker community is currently working to develop and integrate the online and offline software frameworks, known as XDAQ and CMSSW respectively, for the purposes of data acquisition and detector commissioning and monitoring. Recent developments have seen the integration of many new services and tools within the online data acquisition system, such as event building, online distributed analysis, an online monitoring framework, and data storage management. We review the various software components that comprise the strip tracker data acquisition system, the software architectures used for stand-alone and global data-taking modes. Our experiences in commissioning and operating one of the largest ever silicon micro-strip tracking systems are also reviewed
Grey solitons in a strongly interacting superfluid Fermi Gas
The Bardeen-Cooper-Schrieffer to Bose-Einstein condensate (BCS to BEC)
crossover problem is solved for stationary grey solitons via the Boguliubov-de
Gennes equations at zero temperature. These \emph{crossover solitons} exhibit a
localized notch in the gap and a characteristic phase difference across the
notch for all interaction strengths, from BEC to BCS regimes. However, they do
not follow the well-known Josephson-like sinusoidal relationship between
velocity and phase difference except in the far BEC limit: at unitary the
velocity has a nearly linear dependence on phase difference over an extended
range. For fixed phase difference the soliton is of nearly constant depth from
the BEC limit to unitarity and then grows progressively shallower into the BCS
limit, and on the BCS side Friedel oscillations are apparent in both gap
amplitude and phase. The crossover soliton appears fundamentally in the gap; we
show, however, that the density closely follows the gap, and the soliton is
therefore observable. We develop an approximate power law relationship to
express this fact: the density of grey crossover solitons varies as the square
of the gap amplitude in the BEC limit and a power of about 1.5 at unitarity.Comment: 10 pages, 6 figures, part of New Journal of Physics focus issue
"Strongly Correlated Quantum Fluids: From Ultracold Quantum Gases to QCD
Plasmas," in pres
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