218 research outputs found
Multichannel quantum-defect theory for ultracold atom-ion collisions
We develop an analytical model for ultracold atom-ion collisions using the
multichannel quantum-defect formalism. The model is based on the analytical
solutions of the r^-4 long-range potential and on the application of a frame
transformation between asymptotic and molecular bases. This approach allows the
description of the atom-ion interaction in the ultracold domain in terms of
three parameters only: the singlet and triplet scattering lengths, assumed to
be independent of the relative motion angular momentum, and the lead dispersion
coefficient of the asymptotic potential. We also introduce corrections to the
scattering lengths that improve the accuracy of our quantum-defect model for
higher order partial waves, a particularly important result for an accurate
description of shape and Feshbach resonances at finite temperature. The theory
is applied to the system composed of a 40Ca+ ion and a Na atom, and compared to
numerical coupled-channel calculations carried out using ab initio potentials.
For this particular system, we investigate the spectrum of bound states, the
rate of charge-transfer processes, and the collision rates in the presence of
magnetic Feshbach resonances at zero and finite temperature.Comment: 39 pages, 21 figure
Characterization of nanometer scale compositionally inhomogeneous AlGaN active regions on bulk AlN substrates
The optical and structural properties of AlGaN active regions containing nanoscale compositional inhomogeneities (NCI) grown on low dislocation density bulk AlN substrates are reported. These substrates are found to improve the internal quantum efficiency and structural quality of NCI-AlGaN active regions for high Al content alloys, as well as the interfaces of the NCI with the surrounding wider bandgap matrix, as manifested in the absence of any significant long decay component of the low temperature radiative lifetime, which is well characterized by a single exponential photoluminescence decay with a 330 ps time constant. However, room temperature results indicate that non-radiative recombination associated with the high point defect density becomes a limiting factor in these films even at low dislocation densities for larger AlN mole fractions
Spin-photocurrent in p-SiGe quantum wells under terahertz laser irradiation
A detailed study of the circular photogalvanic effect (CPGE) in SiGe
structures is presented. It is shown that the CPGE becomes possible due to the
built-in asymmetry of quantum wells (QWs) in compositionally stepped samples
and in asymmetrically doped structures. The photocurrent arises due to optical
spin orientation of free carriers in QWs with spin splitting in k-space. It is
shown that the effect can be applied to probe the macroscopic in-plane symmetry
of low dimensional structures and allowing to conclude on Rashba or Dresselhaus
terms in the Hamiltonian
Local field distributions in spin glasses
Numerical results for the local field distributions of a family of Ising
spin-glass models are presented. In particular, the Edwards-Anderson model in
dimensions two, three, and four is considered, as well as spin glasses with
long-range power-law-modulated interactions that interpolate between a
nearest-neighbour Edwards-Anderson system in one dimension and the
infinite-range Sherrington-Kirkpatrick model. Remarkably, the local field
distributions only depend weakly on the range of the interactions and the
dimensionality, and show strong similarities except for near zero local field.Comment: 17 pages, 34 eps-figs included, extensive updates and new results, as
to appear in JPA, find related articles at
http://www.physics.emory.edu/faculty/boettche
Calculation of the effect of inertia on the dynamic viscosity of dilute emulsions in a pure straining motion
Near-Band-Edge Photoluminescence of Wurtzite-Type AlN
Temperature-dependentphotoluminescence(PL)measurements were performed for A-plane and C-plane bulk AlN single crystals and epitaxial layers on sapphire. A strong near-band-edge (NBE) emission and deep-level luminescence were observed. At low excitations, the emission spectra are dominated by free and bound excitonic transitions and their LO-phonon replicas. At high excitations, the broadening and redshift of the NBE band is attributed to dense electron–hole plasma formation. The PL spectra differences of bulk single crystals and epilayers is explained by the electron–hole plasma expansion peculiarities
The viscosity distribution around a rising bubble in shear-thinning non-newtonian fluids
A Virus-Encoded Cell–Cell Fusion Machine Dependent on Surrogate Adhesins
The reovirus fusion-associated small transmembrane (FAST) proteins function as virus-encoded cellular fusogens, mediating efficient cell–cell rather than virus–cell membrane fusion. With ectodomains of only ∼20–40 residues, it is unclear how such diminutive viral fusion proteins mediate the initial stages (i.e. membrane contact and close membrane apposition) of the fusion reaction that precede actual membrane merger. We now show that the FAST proteins lack specific receptor-binding activity, and in their natural biological context of promoting cell–cell fusion, rely on cadherins to promote close membrane apposition. The FAST proteins, however, are not specifically reliant on cadherin engagement to mediate membrane apposition as indicated by their ability to efficiently utilize other adhesins in the fusion reaction. Results further indicate that surrogate adhesion proteins that bridge membranes as close as 13 nm apart enhance FAST protein-induced cell–cell fusion, but active actin remodelling is required for maximal fusion activity. The FAST proteins are the first example of membrane fusion proteins that have specifically evolved to function as opportunistic fusogens, designed to exploit and convert naturally occurring adhesion sites into fusion sites. The capacity of surrogate, non-cognate adhesins and active actin remodelling to enhance the cell–cell fusion activity of the FAST proteins are features perfectly suited to the structural and functional evolution of these fusogens as the minimal fusion component of a virus-encoded cellular fusion machine. These results also provide a basis for reconciling the rudimentary structure of the FAST proteins with their capacity to fuse cellular membranes
New strategy for virus discovery: viruses identified in human feces in the last decade
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