Diffusion and Interdiffusion in Binary Metallic Melts

We discuss the dependence of self- and interdiffusion coefficients on temperature and composition for two prototypical binary metallic melts, Al-Ni and Zr-Ni, in molecular-dynamics (MD) computer simulations and the mode-coupling theory of the glass transition (MCT). Dynamical processes that are mainly entropic in origin slow down mass transport (as expressed through self diffusion) in the mixture as compared to the ideal-mixing contribution. Interdiffusion of chemical species is a competition of slow kinetic modes with a strong thermodynamic driving force that is caused by non-entropic interactions. The combination of both dynamic and thermodynamic effects causes qualitative differences in the concentration dependence of self-diffusion and interdiffusion coefficients. At high temperatures, the thermodynamic enhancement of interdiffusion prevails, while at low temperatures, kinetic effects dominate the concentration dependence, rationalized within MCT as the approach to its ideal-glass transition temperature $T_c$. The Darken equation relating self- and interdiffusion qualitatively reproduces the concentration-dependence in both Zr-Ni and Al-Ni, but quantitatively, the kinetic contributions to interdiffusion can be slower than the lower bound suggested by the Darken equation. As temperature is decreased, the agreement with Darken's equation improves, due to a strong coupling of all kinetic modes that is a generic feature predicted by MCT.Comment: 16 pages, 12 figure

Studies on changes of estimated breeding values of U.S. Holstein bulls for final score from the first to second crop of daughters

The purpose of this study was to find ways of reducing changes of sire predicted transmitting ability for type’s final scores (PTATs) from the first to second crop of daughters. The PTATs were estimated from two datasets: D01 (scores recorded up to 2001) and D05 (scores recorded up to 2005). The PTAT changes were calculated as the difference between the evaluations based on D01 and D05. The PTATs were adjusted to a common genetic base of all evaluated cows born in 1995. The single-trait (ST) animal model included the fixed effects of the herd–year–season–classifier, age by year group at classification, stage of lactation at classification, registry status of animals, and additive genetic and permanent environment random effects. Unknown parent groups (UPGs) were defined based on every other birth year starting from 1972. Modifications to the ST model included the usage of a single record per cow, separate UPGs for first and second crop daughters, separate UPGs for sires and dams, and deepened pedigrees for dams with missing phenotypic records. Also, the multiple-trait (MT) model treated records of registered and grade cows as correlated traits. The mean PTAT change, for all of the sires, was close to zero in all of the models analyzed. The estimated mean PTAT change for 145 sires with 40 to 100 first crop and ≥200 second crop daughters was −0.33, −0.20, −0.13, −0.28, and −0.12 with ST, only first records, only last records, updated pedigrees, and allowing separate parent groups (PGs) for sires and dams after updating the pedigrees, respectively. The percentages of sires showing PTAT decline were reduced from 74.5 (with ST) to 57.3 by using only the last records of cows, and to 56.4 by allowing separate UPGs for sires and dams after updating the pedigrees. Though updating of the pedigrees alone was not effective, separate UPGs for sires together with additional pedigree was helpful in reducing the bias

Near-infrared spectroscopy of EX Lupi in outburst

EX Lup is the prototype of the EXor class of young eruptive stars: objects showing repetitive brightenings due to increased accretion from the circumstellar disk to the star. In this paper, we report on medium-resolution near-infrared spectroscopy of EX\,Lup taken during its extreme outburst in 2008, as well as numerical modeling with the aim of determining the physical conditions around the star. We detect emission lines from atomic hydrogen, helium, and metals, as well as first overtone bandhead emission from carbon monoxide. Our results indicate that the emission lines are originating from gas located in a dust-free region within ~ 0.2 AU of the star. The profile of the CO bandhead indicates that the CO gas has a temperature of 2500 K, and is located in the inner edge of the disk or in the outer parts of funnel flows. The atomic metals are probably co-located with the CO. Some metallic lines are fluorescently excited, suggesting direct exposure to ultraviolet photons. The Brackett series indicates emission from hot (10000 K) and optically thin gas. The hydrogen lines display a strong spectro-astrometric signal, suggesting that the hydrogen emission is probably not coming from an equatorial boundary layer; a funnel flow or disk wind origin is more likely. This picture is broadly consistent with the standard magnetospheric accretion model usually assumed for normally accreting T Tauri stars. Our results also set constraints on the eruption mechanism, supporting a model where material piles up around the corotation radius and episodically falls onto the star.Comment: 12 pages, 8 figures, 1 table, accepted for publication in Ap

Single photon generation by pulsed excitation of a single dipole

The fluorescence of a single dipole excited by an intense light pulse can lead to the generation of another light pulse containing a single photon. The influence of the duration and energy of the excitation pulse on the number of photons in the fluorescence pulse is studied. The case of a two-level dipole with strongly damped coherences is considered. The presence of a metastable state leading to shelving is also investigated.Comment: 17 pages, 4 figures, submitted to PR

Angle-resolved photoemission study and first principles calculation of the electronic structure of GaTe

The electronic band structure of GaTe has been calculated by numerical atomic orbitals density-functional theory, in the local density approximation. In addition, the valence-band dispersion along various directions of the GaTe Brillouin zone has been determined experimentally by angle-resolved photoelectron spectroscopy. Along these directions, the calculated valence-band structure is in good concordance with the valence-band dispersion obtained by these measurements. It has been established that GaTe is a direct-gap semiconductor with the band gap located at the Z point, that is, at Brillouin zone border in the direction perpendicular to the layers. The valence-band maximum shows a marked \textit{p}-like behavior, with a pronounced anion contribution. The conduction band minimum arises from states with a comparable \textit{s}- \textit{p}-cation and \textit{p}-anion orbital contribution. Spin-orbit interaction appears to specially alter dispersion and binding energy of states of the topmost valence bands lying at $\Gamma$. By spin-orbit, it is favored hybridization of the topmost \textit{p}$_z$-valence band with deeper and flatter \textit{p$_x$}-\textit{p$_y$} bands and the valence-band minimum at $\Gamma$ is raised towards the Fermi level since it appears to be determined by the shifted up \textit{p$_x$}-\textit{p$_y$} bands.Comment: 7 text pages, 6 eps figures, submitted to PR

Shot noise suppression in multimode ballistic Fermi conductors

We have derived a general formula describing current noise in multimode ballistic channels connecting source and drain electrodes with Fermi electron gas. In particular (at $eV\gg k_{B}T$), the expression describes the nonequilibrium ''shot'' noise, which may be suppressed by both Fermi correlations and space charge screening. The general formula has been applied to an approximate model of a 2D nanoscale, ballistic MOSFET. At large negative gate voltages, when the density of electrons in the channel is small, shot noise spectral density $S_{I}(0)$ approaches the Schottky value $2eI$, where $I$ is the average current. However, at positive gate voltages, when the maximum potential energy in the channel is below the Fermi level of the electron source, the noise can be at least an order of magnitude smaller than the Schottky value, mostly due to Fermi effects.Comment: 4 page

How citation boosts promote scientific paradigm shifts and Nobel Prizes

Nobel Prizes are commonly seen to be among the most prestigious achievements of our times. Based on mining several million citations, we quantitatively analyze the processes driving paradigm shifts in science. We find that groundbreaking discoveries of Nobel Prize Laureates and other famous scientists are not only acknowledged by many citations of their landmark papers. Surprisingly, they also boost the citation rates of their previous publications. Given that innovations must outcompete the rich-gets-richer effect for scientific citations, it turns out that they can make their way only through citation cascades. A quantitative analysis reveals how and why they happen. Science appears to behave like a self-organized critical system, in which citation cascades of all sizes occur, from continuous scientific progress all the way up to scientific revolutions, which change the way we see our world. Measuring the "boosting effect" of landmark papers, our analysis reveals how new ideas and new players can make their way and finally triumph in a world dominated by established paradigms. The underlying "boost factor" is also useful to discover scientific breakthroughs and talents much earlier than through classical citation analysis, which by now has become a widespread method to measure scientific excellence, influencing scientific careers and the distribution of research funds. Our findings reveal patterns of collective social behavior, which are also interesting from an attention economics perspective. Understanding the origin of scientific authority may therefore ultimately help to explain, how social influence comes about and why the value of goods depends so strongly on the attention they attract.Comment: 6 pages, 6 figure

NACO Polarimetric Differential Imaging of TW Hya: A Sharp Look at the Closest T Tauri Disk

We present high-contrast imaging data on the disk of the classical T Tauri star TW Hya. The images were obtained through the polarimetric differential imaging technique with the adaptive optics system NACO. Our commissioning data show the presence of polarized disk emission between 0.1" and 1.4" from the star. We derive the first Ks-band radial polarized intensity distribution. We show that the polarized intensity compares well to shorter wavelengths surface brightness observations and confirm the previously reported gradual slope change around 0.8". These results show the potential of the new polarimetric differential imaging technique at 8m-class telescopes to map the inner regions of protoplanetary disks.Comment: Accepted for publication in Astronomy & Astrophysics; 7 pages, 7 figure

High-resolution polarimetry of Parsamian 21: revealing the structure of an edge-on FU Ori disc

We present the first high spatial resolution near-infrared direct and polarimetric observations of Parsamian 21, obtained with the VLT/NACO instrument. We complemented these measurements with archival infrared observations, such as HST/WFPC2 imaging, HST/NICMOS polarimetry, Spitzer IRAC and MIPS photometry, Spitzer IRS spectroscopy as well as ISO photometry. Our main conclusions are the following: (1) we argue that Parsamian 21 is probably an FU Orionis-type object; (2) Parsamian 21 is not associated with any rich cluster of young stars; (3) our measurements reveal a circumstellar envelope, a polar cavity and an edge-on disc; the disc seems to be geometrically flat and extends from approximately 48 to 360 AU from the star; (4) the SED can be reproduced with a simple model of a circumstellar disc and an envelope; (5) within the framework of an evolutionary sequence of FUors proposed by Green et al. (2006) and Quanz et al. (2007), Parsamian 21 can be classified as an intermediate-aged object.Comment: Accepted for publication in the MNRAS. 16 pages, 18 figures and 5 table

Supercurrents through gated superconductor-normal-metal-superconductor contacts: the Josephson-transistor

We analyze the transport through a narrow ballistic superconductor-normal- metal-superconductor Josephson contact with non-ideal transmission at the superconductor-normal-metal interfaces, e.g., due to insulating layers, effective mass steps, or band misfits (SIN interfaces). The electronic spectrum in the normal wire is determined through the combination of Andreev- and normal reflection at the SIN interfaces. Strong normal scattering at the SIN interfaces introduces electron- and hole-like resonances in the normal region which show up in the quasi-particle spectrum. These resonances have strong implications for the critical supercurrent $I_c$ which we find to be determined by the lowest quasi-particle level: tuning the potential $\mu_{x0}$ to the points where electron- and hole-like resonances cross, we find sharp peaks in $I_{\rm c}$, resulting in a transitor effect. We compare the performance of this Resonant Josephson-Transistor (RJT) with that of a Superconducting Single Electron Transistor (SSET).Comment: to appear in PRB, 11 pages, 9 figure