Comparative Study of BCS-BEC Crossover Theories above TcT_c: the Nature of the Pseudogap in Ultra-Cold Atomic Fermi Gases

This paper presents a comparison of two finite-temperature BCS-Bose Einstein condensation (BEC) crossover theories above the transition temperature: Nozieres Schmitt-Rink (NSR) theory and finite $T$-extended BCS-Leggett theory. The comparison is cast in the form of numerical studies of the behavior of the fermionic spectral function both theoretically and as constrained by (primarily) radio frequency (RF) experiments. Both theories include pair fluctuations and exhibit pseudogap effects, although the nature of this pseudogap is very different. The pseudogap in finite $T$-extended BCS-Leggett theory is found to follow a BCS-like dispersion which, in turn, is associated with a broadened BCS-like self energy, rather more similar to what is observed in high temperature superconductors (albeit, for a d-wave case). The fermionic quasi-particle dispersion is different in NSR theory and the damping is considerably larger. We argue that the two theories are appropriate in different temperature regimes with the BCS-Leggett approach more suitable nearer to condensation. There should, in effect, be little difference at higher $T$ as the pseudogap becomes weaker and where the simplifying approximations used in the BCS-Leggett approach break down. On the basis of momentum-integrated radio frequency studies of unpolarized gases, it would be difficult to distinguish which theory is the better. A full comparison for polarized gases is not possible since there is claimed to be inconsistencies in the NSR approach (not found in the BCS-Leggett scheme). Future experiments along the lines of momentum resolved experiments look to be very promising in distinguishing the two theories.Comment: 16 pages, 11 figure

Minimal dark matter in type III seesaw

We explore the possibility of a new dark matter candidate in the supersymmetric type III seesaw mechanism where a neutral scalar component of the Y=0 triplet can be the lightest supersymmetric particle. Its thermal abundance can be in the right range if non-standard cosmology such as kination domination is assumed. The enhanced cross-section of the dark matter annihilation to W+W- can leave detectable astrophysical and cosmological signals whose current observational data puts a lower bound on the dark matter mass. The model predicts the existence of a charged scalar almost degenerate with the dark matter scalar and its lifetime lies between 5.5 cm and 6.3 m. It provides a novel opportunity of the dark mater mass measurement by identifying slowly-moving and highly-ionizing tracks in the LHC experiments. If the ordinary lightest supersymmetric particle is the usual Bino, its decay leads to clean signatures of same-sign di-lepton and di-charged-scalar associated with observable displaced vertices which are essentially background-free and can be fully reconstructed.Comment: 3 figures, 12 pages; An error in the antiproton limit corrected; the lower bound on the dark matter mass strengthened; references added; typos correcte

A Phenomenological Theory of The Pseudogap State

An ansatz is proposed for the coherent part of the single particle Green's function in a doped resonant valence bond (RVB) state by, analogy with the form derived by Konik and coworkers for a doped spin liquid formed by an array of 2-leg Hubbard ladders near half-filling. The parameters of the RVB state are taken from the renormalized mean field theory of Zhang and coworkers for underdoped cuprates. The ansatz shows good agreement with recent angle resolved photoemission (ARPES) on underdoped cuprates and resolves an apparent disagreement with the Luttinger Sum Rule. The transition in the normal state from a doped RVB spin liquid to a standard Landau Fermi liquid, that occurs in the renormalized mean field theory, appears as a quantum critical point characterized by a change in the analytic form of the Green's function. A d-wave superconducting dome surrounding this quantum critical point is introduced phenomenologically. Results are also presented for the Drude weight and tunneling density of states as functions of the hole density.Comment: 11 pages, 14 figure

Strain induced half-metal to semiconductor transition in GdN

We have investigated the electronic structure and magnetic properties of GdN as a function of unit cell volume. Based on the first-principles calculations of GdN, we observe that there is a transformation in conduction properties associated with the volume increase: first from halfmetallic to semi-metallic, then ultimately to semiconducting. We show that applying stress can alter the carrier concentration as well as mobility of the holes and electrons in the majority spin channel. In addition, we found that the exchange parameters depend strongly on lattice constant, thus the Curie temperature of this system can be enhanced by applying stress or doping impurities.Comment: 9 pages, 3 figure

Effect of pmma-mwnts loading on Co2 separation performance of thin film nanocomposite membrane

Nanocomposite membrane, especially the thin film nanocomposite (TFN) fabricated via interfacial polymerization (IP) is a relatively new class of membrane which features good separation performance and practical processing. This study investigated on the effects of multi-walled carbon nanotubes (MWNTs) loading on the gas separation performance of the resultant TFNs. TFNs were tested with pure CO2, N2 and CH4 gases at feed pressure of 2 bar. The findings from this study suggested that the optimum fillers loading was around 0.25 g/L in the coating solution which gives TFN with CO2 permeance of 53.5 gas permeation unit (GPU) (12% higher than base membrane without filler), CO2/N2 selectivity of 61 and CO2/CH4 selectivity of 35. The enhancement in CO2 permeance without sacrificing the membrane selectvities was attributed to the good dispersion and compatibility of the MWNTs with the polymer matrix while the nanotubes serve as rapid diffusion channels to facilitate transport of gases. TFN embedded with polymethyl methacrylate (PMMA)-MWNTs showed potential for low pressure carbon capture and storage application

Molecular dynamics study of the fragmentation of silicon doped fullerenes

Tight binding molecular dynamics simulations, with a non orthogonal basis set, are performed to study the fragmentation of carbon fullerenes doped with up to six silicon atoms. Both substitutional and adsorbed cases are considered. The fragmentation process is simulated starting from the equilibrium configuration in each case and imposing a high initial temperature to the atoms. Kinetic energy quickly converts into potential energy, so that the system oscillates for some picoseconds and eventually breaks up. The most probable first event for substituted fullerenes is the ejection of a C2 molecule, another very frequent event being that one Si atom goes to an adsorbed position. Adsorbed Si clusters tend to desorb as a whole when they have four or more atoms, while the smaller ones tend to dissociate and sometimes interchange positions with the C atoms. These results are compared with experimental information from mass abundance spectroscopy and the products of photofragmentation.Comment: Seven two-column pages, six postscript figures. To be published in Physical Review

BRYNTRN: A baryon transport model

The development of an interaction data base and a numerical solution to the transport of baryons through an arbitrary shield material based on a straight ahead approximation of the Boltzmann equation are described. The code is most accurate for continuous energy boundary values, but gives reasonable results for discrete spectra at the boundary using even a relatively coarse energy grid (30 points) and large spatial increments (1 cm in H2O). The resulting computer code is self-contained, efficient and ready to use. The code requires only a very small fraction of the computer resources required for Monte Carlo codes

Mechanical ventilation modulates Toll-like receptor-3-induced lung inflammation via a MyD88-dependent, TLR4-independent pathway: a controlled animal study

<p>Abstract</p> <p>Background</p> <p>Mechanical ventilation augments lung inflammation resulting from exposure to microbial products. The objective of this study was to test the hypothesis that ventilator-associated immune modulation requires MyD88-dependent signaling. Because MyD88 is a critical adapter protein utilized for pro-inflammatory signaling by all Toll-like receptors (TLRs), with the exception of TLR3, as well as by the IL-1 and IL-18 receptors, MyD88 dependence would implicate generation of an endogenous soluble ligand recognized by one or more of these receptors during mechanical ventilation and would provide an opportunity for a potential future therapeutic intervention.</p> <p>Methods</p> <p>We compared the effect of mechanical ventilation on lung inflammation and permeability between poly(I:C) exposed mice with or without expression of MyD88. Poly(I:C) is a synthetic ligand for TLR3, the only MyD88-independent TLR, allowing isolation of the effect of MyD88 deletion on ventilator-augmentation of lung inflammation. Lung inflammation was assessed by cytokine concentration in lung tissue homogenate and polymorphonuclear cell (PMN) number in bronchoalveolar lavage fluid (BALF). Lung permeability was assessed by total protein, IgM, and intravenously injected FITC-dextran concentrations in BALF.</p> <p>Results</p> <p>We found that MyD88 was required for mechanical ventilation augmentation of TLR3-induced lung inflammation and permeability. Because TLR4 is the most commonly reported receptor for endogenous ligands generated during tissue injury, we performed a second experiment comparing wildtype and TLR4-/- mice. We found that mechanical ventilation increased TLR3-mediated inflammation and permeability independent of TLR4.</p> <p>Conclusion</p> <p>These data support the hypothesis that mechanical ventilation with moderate tidal volumes generates an endogenous ligand(s) recognized by MyD88-dependent receptor(s) other than TLR4, and that this mechanism can contribute to the development of ventilator-associated lung inflammation and injury. Identification of these ligands and/or receptors could lead to new pharmacological treatments for ARDS.</p