Tip-induced reduction of the resonant tunneling current on semiconductor surfaces

We report scanning tunneling microscope measurements showing a substantial decrease of the current, almost to zero, on the Si(111)-(7x7) reconstruction in the near-to-contact region under low bias conditions. First principles simulations for the tip-sample interaction and transport calculations show that this effect is driven by the substantial local modification of the atomic and electronic structure of the surface. The chemical reactivity of the adatom dangling bond states that dominate the electronic density of states close to the Fermi level and their spatial localization result in a strong modification of the electronic current.     

The mean spherical model in a random external field and the replica method

We provide a quick elementary solution of the mean spherical model in a random external field. This also allows an immediate proof of the self-averaging property of the free energy. We calculate the free energy by means of the replica method, i.e., for any (not necessarily integer) replica numbern, and show that when a phase transition occurs the limits andn 0 are not interchangeable.     

An inflationary bianchi V model with shear and rotation

A Bianchi V model, in the presence of a fluid with shear and rotation, is investigated. Asymptotic solutions are worked out, showing that shear and rotation may give rise to a conflict with present day observations, a problem that can be solved with the help of an inflationary solution.     

Computation of Almost Split Sequences with Applications to Relatively Projective and Prinjective Modules

Let be an Artin algebra, let mod be the category of finitely generated -modules, and let Amod be a contravariantly finite and extension closed subcategory. For an indecomposable and not Ext-projective module CA, we compute the almost split sequence 0ABC0 in A from the almost split sequence 0DTrCEC0 in mod. Since the computation is particularly simple if the minimal right A-approximation of DTrC is indecomposable for all indecomposable and not Ext-projective CA, we manufacture subcategories A with the desired property using orthogonal subcategories. The method of orthogonal subcategories is applied to compute almost split sequences for relatively projective and prinjective modules.     

A row-action method for convex programming

We present a new method for minimizing a strictly convex function subject to general convex constraints. Constraints are used one at a time, no changes are made in the constraint functions (thus the row-action nature of the algorithm) and at each iteration a subproblem is solved consisting of minimization of the objective function subject to one or two linear equations. Convergence of the algorithm is established and the method is compared with other row-action algorithms for several relevant particular cases.Corresponding author. Research of this author was partially supported by CNPq grant No. 301280/86.     

K-invariants in the universal enveloping algebra of the desitter group

Let G be a non-compact connected semisimple Lie group with finite center and let G^K denote the centralizer of a maximal compact subgroup K of G inG, the universal enveloping algebra over of the Lie algebra of G. In [4] Lepowsky defines an injective anti-homo morphism P:G ^KK ^MA, where M is the centralizer in K of a Cartan subalgebraa of the symmetric pair (G,K),K andA are the universal enveloping algebras over corresponding to K anda, respectively, andK ^M is the centralizer of M inK. The subalgebra P(G ^K) ofK ^MA has considerable significance in the infinite dimensional representation theory of G. In this paper we explicitly compute P(G ^K) when G=S0_o(4,1), and show how this result leads to the determination of all irreducible representations of G and its universal covering group Spin(4,1).Partially supported by CONICET (Argentina) grants.     

Comparison of vacancy and antisite defects in GaAs and InGaAs through hybrid functionals

The formation energies and charge transition levels of vacancy and antisite defects in GaAs and In(0.5)Ga(0.5)As are calculated through hybrid density functionals. In As-rich conditions, the As antisite is the most stable defect in both GaAs and InGaAs, except for n-type GaAs for which the Ga vacancy is favored. The Ga antisite shows the lowest formation energy in Ga-rich conditions. The As antisite provides a consistent interpretation of the defect densities measured at mid-gap for both GaAs/oxide and InGaAs/oxide interfaces.     

Activation of muscarinic acetylcholine receptors elicits pigment granule dispersion in retinal pigment epithelium isolated from bluegill

Background

In fish, melanin pigment granules in the retinal pigment epithelium disperse into apical projections as part of the suite of responses the eye makes to bright light conditions. This pigment granule dispersion serves to reduce photobleaching and occurs in response to neurochemicals secreted by the retina. Previous work has shown that acetylcholine may be involved in inducing light-adaptive pigment dispersion. Acetylcholine receptors are of two main types, nicotinic and muscarinic. Muscarinic receptors are in the G-protein coupled receptor superfamily, and five different muscarinic receptors have been molecularly cloned in human. These receptors are coupled to adenylyl cyclase, calcium mobilization and ion channel activation. To determine the receptor pathway involved in eliciting pigment granule migration, we isolated retinal pigment epithelium from bluegill and subjected it to a battery of cholinergic agents.

Results

The general cholinergic agonist carbachol induces pigment granule dispersion in isolated retinal pigment epithelium. Carbachol-induced pigment granule dispersion is blocked by the muscarinic antagonist atropine, by the M₁ antagonist pirenzepine, and by the M₃ antagonist 4-DAMP. Pigment granule dispersion was also induced by the M₁ agonist 4-[N-(4-chlorophenyl) carbamoyloxy]-4-pent-2-ammonium iodide. In contrast the M₂ antagonist AF-DX 116 and the M₄ antagonist tropicamide failed to block carbachol-induced dispersion, and the M₂ agonist arecaidine but-2-ynyl ester tosylate failed to elicit dispersion.

Conclusions

Our results suggest that carbachol-mediated pigment granule dispersion occurs through the activation of M_odd muscarinic receptors, which in other systems couple to phosphoinositide hydrolysis and elevation of intracellular calcium. This conclusion must be corroborated by molecular studies, but suggests Ca²⁺-dependent pathways may be involved in light-adaptive pigment dispersion.