Relativistic Mean Field in A≈A\approx80 nuclei and low energy proton reactions

Relativistic Mean Field calculations have been performed for a number of nuclei in mass $A\approx$80 region. Ground state binding energy, charge radius and charge density values have been compared with experiment. Optical potential have been generated folding the nuclear density with the microscopic nuclear interaction DDM3Y. S-factors for low energy ($p,\gamma$) and ($p,n$) reactions have been calculated and compared with experiment.Comment: To appear in Physical Review

Alpha-particle optical potential proofs at astrophysically relevant energies

$(\alpha,\gamma)$ and $(\alpha$,n) reaction cross sections recently measured close to the reaction thresholds are rather well described by a previously developed regional optical potential. Thus, particular features of the $\alpha$-particle optical potential at energies below the Coulomb barrier, besides parameters describing $\alpha$-particle elastic scattering at higher energies are confirmed. Additional limitations of similar statistical model calculations for minor reaction channels are shown to be most likely due to an overlooked process or critical values of statistical model parameters around closed nuclear shells.Comment: 4 pages, 3 figure

Einstein crystal as a reference system in free energy estimation using adiabatic switching

CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOIn this paper we investigate the behavior of an Einstein crystal as a reference system in adiabatic switching procedures. We study the canonical massive Nose-Hoover chain (MNHC) dynamics [G.J. Martyna, M.L. Klein, and M. Tuckerman, J. Chem. Phys. 97, 2635 (1992)] and show it can be used to determine Helmholtz free energies within an adiabatic switching procedure. We calculate the Helmoltz free energy difference between two different Einstein crystals, each consisting of 100 identical independent harmonic oscillators with different characteristic frequencies by a MNHC molecular dynamics adiabatic switching procedure. The simulations were performed using two different switching functions. Applying the quantitative error analysis of Tsao, Sheu, and Mou [J. Chem. Phys. 101, 2302 (1994)], it is found that systematic errors during the switching process can be estimated quantitatively, allowing a correction of the converged results. The corrected results obtained by adiabatic switching deviate less than 1% from the analytical value. It is observed that quantitative correction of converged results can be avoided by choosing a groper switching function.In this paper we investigate the behavior of an Einstein crystal as a reference system in adiabatic switching procedures. We study the canonical massive Nose-Hoover chain (MNHC) dynamics [G.J. Martyna, M.L. Klein, and M. Tuckerman, J. Chem. Phys. 97, 2635 (1992)] and show it can be used to determine Helmholtz free energies within an adiabatic switching procedure. We calculate the Helmoltz free energy difference between two different Einstein crystals, each consisting of 100 identical independent harmonic oscillators with different characteristic frequencies by a MNHC molecular dynamics adiabatic switching procedure. The simulations were performed using two different switching functions. Applying the quantitative error analysis of Tsao, Sheu, and Mou [J. Chem. Phys. 101, 2302 (1994)], it is found that systematic errors during the switching process can be estimated quantitatively, allowing a correction of the converged results. The corrected results obtained by adiabatic switching deviate less than 1% from the analytical value. It is observed that quantitative correction of converged results can be avoided by choosing a groper switching function.531A465474CAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOCAPES - COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIORCNPQ - CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOSem informaçãoSem informaçãoThe simulations were performed on an IBM RISC/6000 computer of the CENAPAD-SP Computer Center. The authors gratefully acknowledge the financial support granted by the Brazilian funding agencies CAPES and CNPq

Spin-mediated dissipation and frequency shifts of a cantilever at milliKelvin temperatures

We measure the dissipation and frequency shift of a magnetically coupled cantilever in the vicinity of a silicon chip, down to $25$ mK. The dissipation and frequency shift originates from the interaction with the unpaired electrons, associated with the dangling bonds in the native oxide layer of the silicon, which form a two dimensional system of electron spins. We approach the sample with a $3.43$ $\mu$m-diameter magnetic particle attached to an ultrasoft cantilever, and measure the frequency shift and quality factor as a function of temperature and the distance. Using a recent theoretical analysis [J. M. de Voogd et al., arXiv:1508.07972 (2015)] of the dynamics of a system consisting of a spin and a magnetic resonator, we are able to fit the data and extract the relaxation time $T_1=0.39\pm0.08$ ms and spin density $\sigma=0.14\pm0.01$ spins per nm$^2$. Our analysis shows that at temperatures $\leq500$ mK magnetic dissipation is an important source of non-contact friction.Comment: 5 pages, 3 figure

Finite temperature molecular dynamics study of unstable stacking fault free energies in silicon

We calculate the free energies of unstable stacking fault (USF) configurations on the glide and shuffle slip planes in silicon as a function of temperature, using the recently developed Environment Dependent Interatomic Potential (EDIP). We employ the molecular dynamics (MD) adiabatic switching method with appropriate periodic boundary conditions and restrictions to atomic motion that guarantee stability and include volume relaxation of the USF configurations perpendicular to the slip plane. Our MD results using the EDIP model agree fairly well with earlier first-principles estimates for the transition from shuffle to glide plane dominance as a function of temperature. We use these results to make contact to brittle-ductile transition models.Comment: 6 pages revtex, 4 figs, 16 refs, to appear in Phys. Rev.

Pyrimidine biosynthesis is not an essential function for trypanosoma brucei bloodstream forms

<p>Background: African trypanosomes are capable of both pyrimidine biosynthesis and salvage of preformed pyrimidines from the host, but it is unknown whether either process is essential to the parasite.</p> <p>Methodology/Principal Findings: Pyrimidine requirements for growth were investigated using strictly pyrimidine-free media, with or without single added pyrimidine sources. Growth rates of wild-type bloodstream form Trypanosoma brucei brucei were unchanged in pyrimidine-free medium. The essentiality of the de novo pyrimidine biosynthesis pathway was studied by knocking out the PYR6-5 locus that produces a fusion product of orotate phosphoribosyltransferase (OPRT) and Orotidine Monophosphate Decarboxylase (OMPDCase). The pyrimidine auxotroph was dependent on a suitable extracellular pyrimidine source. Pyrimidine starvation was rapidly lethal and non-reversible, causing incomplete DNA content in new cells. The phenotype could be rescued by addition of uracil; supplementation with uridine, 2′deoxyuridine, and cytidine allowed a diminished growth rate and density. PYR6-5−/− trypanosomes were more sensitive to pyrimidine antimetabolites and displayed increased uracil transport rates and uridine phosphorylase activity. Pyrimidine auxotrophs were able to infect mice although the infection developed much more slowly than infection with the parental, prototrophic trypanosome line.</p> <p>Conclusions/Significance: Pyrimidine salvage was not an essential function for bloodstream T. b. brucei. However, trypanosomes lacking de novo pyrimidine biosynthesis are completely dependent on an extracellular pyrimidine source, strongly preferring uracil, and display reduced infectivity. As T. brucei are able to salvage sufficient pyrimidines from the host environment, the pyrimidine biosynthesis pathway is not a viable drug target, although any interruption of pyrimidine supply was lethal.</p&gt