Convergence of Particle-Hole Expansions for the Description of Nuclear Correlations

The convergence properties of a multiparticle-multihole (mp-mh) configuration mixing approach whose purpose is to describe ground state correlations in nuclei without particle number and Pauli violations is investigated in the case of an exactly solvable pairing hamiltonian. Two different truncation schemes are tested by looking at quantities as correlation energies and single-particle occupation probabilities. Results show that pairing correlations present in usual superfluid nuclei can be accurately described using up to 6 particle-6 hole excitations, a convergence fast enought for envisaging extensions to fully microscopic calculations.Comment: 8 pages, 4 figure

The Effect of N-Terminal Cyclization on the Function of the HIV Entry Inhibitor 5P12-RANTES.

Despite effective treatment for those living with Human Immunodeficiency Virus (HIV), there are still two million new infections each year. Protein-based HIV entry inhibitors, being highly effective and specific, could be used to protect people from initial infection. One of the most promising of these for clinical use is 5P12-RANTES, a variant of the chemokine RANTES/CCL5. The N-terminal amino acid of 5P12-RANTES is glutamine (Gln; called Q0), a residue that is prone to spontaneous cyclization when at the N-terminus of a protein. It is not known how this cyclization affects the potency of the inhibitor or whether cyclization is necessary for the function of the protein, although the N-terminal region of RANTES has been shown to be critical for receptor interactions, with even small changes having a large effect. We have studied the kinetics of cyclization of 5P12-RANTES as well as N-terminal variations of the protein that either produce an identical cyclized terminus (Glu0) or that cannot similarly cyclize (Asn0, Phe0, Ile0, and Leu0). We find that the half life for N-terminal cyclization of Gln is roughly 20 h at pH 7.3 at 37 °C. However, our results show that cyclization is not necessary for the potency of this protein and that several replacement terminal amino acids produce nearly-equally potent HIV inhibitors while remaining CC chemokine receptor 5 (CCR5) antagonists. This work has ramifications for the production of active 5P12-RANTES for use in the clinic, while also opening the possibility of developing other inhibitors by varying the N-terminus of the protein

Anisotropic Magneto-Thermopower: the Contribution of Interband Relaxation

Spin injection in metallic normal/ferromagnetic junctions is investigated taking into account the anisotropic magnetoresistance (AMR) occurring in the ferromagnetic layer. It is shown, on the basis of a generalized two channel model, that there is an interface resistance contribution due to anisotropic scattering, beyond spin accumulation and giant magnetoresistance (GMR). The corresponding expression of the thermopower is derived and compared with the expression for the thermopower produced by the GMR. First measurements of anisotropic magnetothermopower are presented in electrodeposited Ni nanowires contacted with Ni, Au and Cu. The results of this study show that while the giant magnetoresistance and corresponding thermopower demonstrates the role of spin-flip scattering, the observed anisotropic magnetothermopower indicates interband s-d relaxation mechanisms.Comment: 20 pages, 4 figure

Approximating the minimum directed tree cover

Given a directed graph $G$ with non negative cost on the arcs, a directed tree cover of $G$ is a rooted directed tree such that either head or tail (or both of them) of every arc in $G$ is touched by $T$. The minimum directed tree cover problem (DTCP) is to find a directed tree cover of minimum cost. The problem is known to be $NP$-hard. In this paper, we show that the weighted Set Cover Problem (SCP) is a special case of DTCP. Hence, one can expect at best to approximate DTCP with the same ratio as for SCP. We show that this expectation can be satisfied in some way by designing a purely combinatorial approximation algorithm for the DTCP and proving that the approximation ratio of the algorithm is $\max\{2, \ln(D^+)\}$ with $D^+$ is the maximum outgoing degree of the nodes in $G$.Comment: 13 page

Origin of conductivity cross over in entangled multi-walled carbon nanotube network filled by iron

A realistic transport model showing the interplay of the hopping transport between the outer shells of iron filled entangled multi-walled carbon nanotubes (MWNT) and the diffusive transport through the inner part of the tubes, as a function of the filling percentage, is developed. This model is based on low-temperature electrical resistivity and magneto-resistance (MR) measurements. The conductivity at low temperatures showed a crossover from Efros-Shklovski (E-S) variable range hopping (VRH) to Mott VRH in 3 dimensions (3D) between the neighboring tubes as the iron weight percentage is increased from 11% to 19% in the MWNTs. The MR in the hopping regime is strongly dependent on temperature as well as magnetic field and shows both positive and negative signs, which are discussed in terms of wave function shrinkage and quantum interference effects, respectively. A further increase of the iron percentage from 19% to 31% gives a conductivity crossover from Mott VRH to 3D weak localization (WL). This change is ascribed to the formation of long iron nanowires at the core of the nanotubes, which yields a long dephasing length (e.g. 30 nm) at the lowest measured temperature. Although the overall transport in this network is described by a 3D WL model, the weak temperature dependence of inelastic scattering length expressed as L_phi ~T^-0.3 suggests the possibility for the presence of one-dimensional channels in the network due to the formation of long Fe nanowires inside the tubes, which might introduce an alignment in the random structure.Comment: 29 pages,10 figures, 2 tables, submitted to Phys. Rev.

Noise and nonlinearities in high-throughput data

High-throughput data analyses are becoming common in biology, communications, economics and sociology. The vast amounts of data are usually represented in the form of matrices and can be considered as knowledge networks. Spectra-based approaches have proved useful in extracting hidden information within such networks and for estimating missing data, but these methods are based essentially on linear assumptions. The physical models of matching, when applicable, often suggest non-linear mechanisms, that may sometimes be identified as noise. The use of non-linear models in data analysis, however, may require the introduction of many parameters, which lowers the statistical weight of the model. According to the quality of data, a simpler linear analysis may be more convenient than more complex approaches. In this paper, we show how a simple non-parametric Bayesian model may be used to explore the role of non-linearities and noise in synthetic and experimental data sets.Comment: 12 pages, 3 figure

On "the authentic damping mechanism" of the phonon damping model

Some general features of the phonon damping model are presented. It is concluded that the fits performed within this model have no physical content

External Control of a Metal-Insulator Transition in GaMnAs Wires

Quantum transport in disordered ferromagnetic (III,Mn)V semiconductors is studied theoretically. Mesoscopic wires exhibit an Anderson disorder-induced metal-insulator transition that can be controlled by a weak external magnetic field. This metal-insulator transition should also occur in other materials with large anisotropic magneto resistance effects. The transition can be useful for studies of zero-temperature quantum critical phase transitions and fundamental material properties.Comment: Major revised final versio