182 research outputs found
Spin susceptibility and fluctuation corrections in the BCS-BEC crossover regime of an ultracold Fermi gas
We investigate magnetic properties and effects of pairing fluctuations in the
BCS (Bardeen-Cooper-Schrieffer)-BEC (Bose-Einstein condensation) crossover
regime of an ultracold Fermi gas. Recently, Liu and Hu, and Parish, pointed out
that the strong-coupling theory developed by Nozi\`eres and Schmitt-Rink (NSR),
which has been extensively used to successfully clarify various physical
properties of cold Fermi gases, unphysically gives negative spin susceptibility
in the BCS-BEC crossover region. The same problem is found to also exist in the
ordinary non-self-consistent T-matrix approximation. In this paper, we clarify
that this serious problem comes from incomplete treatment in term of pseudogap
phenomena originating from strong pairing fluctuations, as well as effects of
spin fluctuations on the spin susceptibility. Including these two key issues,
we construct an extended T-matrix theory which can overcome this problem. The
resulting positive spin susceptibility agrees well with the recent experiment
on a 6Li Fermi gas done by Sanner and co-workers. We also apply our theory to a
polarized Fermi gas to examine the superfluid phase transition temperature Tc,
as a function of the polarization rate. Since the spin susceptibility is an
important physical quantity, especially in singlet Fermi superfluids, our
results would be useful in considering how singlet pairs appear above and below
Tc in the BCS-BEC crossover regime of cold Fermi gases.Comment: 21 pages, 9 figure
Theoretical Aspects of Charge Ordering in Molecular Conductors
Theoretical studies on charge ordering phenomena in quarter-filled molecular
(organic) conductors are reviewed. Extended Hubbard models including not only
the on-site but also the inter-site Coulomb repulsion are constructed in a
straightforward way from the crystal structures, which serve for individual
study on each material as well as for their systematic understandings. In
general the inter-site Coulomb interaction stabilizes Wigner crystal-type
charge ordered states, where the charge localizes in an arranged manner
avoiding each other, and can drive the system insulating. The variety in the
lattice structures, represented by anisotropic networks in not only the
electron hopping but also in the inter-site Coulomb repulsion, brings about
diverse problems in low-dimensional strongly correlated systems. Competitions
and/or co-existences between the charge ordered state and other states are
discussed, such as metal, superconductor, and the dimer-type Mott insulating
state which is another typical insulating state in molecular conductors.
Interplay with magnetism, e.g., antiferromagnetic state and spin gapped state
for example due to the spin-Peierls transition, is considered as well. Distinct
situations are pointed out: influences of the coupling to the lattice degree of
freedom and effects of geometrical frustration which exists in many molecular
crystals. Some related topics, such as charge order in transition metal oxides
and its role in new molecular conductors, are briefly remarked.Comment: 21 pages, 19 figures, to be published in J. Phys. Soc. Jpn. special
issue on "Organic Conductors"; figs. 4 and 11 replaced with smaller sized
fil
Extreme temperature and precipitation response to solar dimming and stratospheric aerosol geoengineering
We examine extreme temperature and precipitation under two
potential geoengineering methods forming part of the Geoengineering Model
Intercomparison Project (GeoMIP). The solar dimming experiment G1 is designed
to completely offset the global mean radiative forcing due to a
CO2-quadrupling experiment (abrupt4  ×  CO2), while in
GeoMIP experiment G4, the radiative forcing due to the representative
concentration pathway 4.5 (RCP4.5) scenario is partly offset by a simulated
layer of aerosols in the stratosphere. Both G1 and G4 geoengineering
simulations lead to lower minimum temperatures (TNn) at higher latitudes and
on land, primarily through feedback effects involving high-latitude processes
such as snow cover, sea ice and soil moisture. There is larger cooling of TNn
and maximum temperatures (TXx) over land compared with oceans, and the
land–sea cooling contrast is larger for TXx than TNn. Maximum 5-day
precipitation (Rx5day) increases over subtropical oceans, whereas warm spells
(WSDI) decrease markedly in the tropics, and the number of consecutive dry
days (CDDs) decreases in most deserts. The precipitation during the tropical
cyclone (hurricane) seasons becomes less intense, whilst the remainder of the
year becomes wetter. Stratospheric aerosol injection is more effective than
solar dimming in moderating extreme precipitation (and flooding). Despite the
magnitude of the radiative forcing applied in G1 being  ∼  7.7 times
larger than in G4 and despite differences in the aerosol chemistry and
transport schemes amongst the models, the two types of geoengineering show
similar spatial patterns in normalized differences in extreme temperatures
changes. Large differences mainly occur at northern high latitudes, where
stratospheric aerosol injection more effectively reduces TNn and TXx. While
the pattern of normalized differences in extreme precipitation is more
complex than that of extreme temperatures, generally stratospheric aerosol
injection is more effective in reducing tropical Rx5day, while solar dimming
is more effective over extra-tropical regions.</p
Diffusion and perfusion weighted magnetic resonance imaging for tumor volume definition in radiotherapy of brain tumors
Abstract
Accurate target volume delineation is crucial for the radiotherapy of tumors. Diffusion and perfusion magnetic resonance imaging (MRI) can provide functional information about brain tumors, and they are able to detect tumor volume and physiological changes beyond the lesions shown on conventional MRI. This review examines recent studies that utilized diffusion and perfusion MRI for tumor volume definition in radiotherapy of brain tumors, and it presents the opportunities and challenges in the integration of multimodal functional MRI into clinical practice. The results indicate that specialized and robust post-processing algorithms and tools are needed for the precise alignment of targets on the images, and comprehensive validations with more clinical data are important for the improvement of the correlation between histopathologic results and MRI parameter images
Pre-Column Derivatization HPLC Procedure for the Quantitation of Aluminium Chlorohydrate in Antiperspirant Creams Using Quercetin as Chromogenic Reagent
Closing the Loop: Modelling of Heart Failure Progression from Health to End-Stage Using a Meta-Analysis of Left Ventricular Pressure-Volume Loops
Introduction The American Heart Association (AHA)/American College of Cardiology (ACC) guidelines for the classification of heart failure (HF) are descriptive but lack precise and objective measures which would assist in categorising such patients. Our aim was two fold, firstly to demonstrate quantitatively the progression of HF through each stage using a meta-analysis of existing left ventricular (LV) pressure-volume (PV) loop data and secondly use the LV PV loop data to create stage specific HF models. Methods and Results A literature search yielded 31 papers with PV data, representing over 200 patients in different stages of HF. The raw pressure and volume data were extracted from the papers using a digitising software package and the means were calculated. The data demonstrated that, as HF progressed, stroke volume (SV), ejection fraction (EF%) decreased while LV volumes increased. A 2-element lumped parameter model was employed to model the mean loops and the error was calculated between the loops, demonstrating close fit between the loops. The only parameter that was consistently and statistically different across all the stages was the elastance (Emax). Conclusions For the first time, the authors have created a visual and quantitative representation of the AHA/ACC stages of LVSD-HF, from normal to end-stage. The study demonstrates that robust, load-independent and reproducible parameters, such as elastance, can be used to categorise and model HF, complementing the existing classification. The modelled PV loops establish previously unknown physiological parameters for each AHA/ACC stage of LVSD-HF, such as LV elastance and highlight that it this parameter alone, in lumped parameter models, that determines the severity of HF. Such information will enable cardiovascular modellers with an interest in HF, to create more accurate models of the heart as it fails
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