First principles investigation of transition-metal doped group-IV semiconductors: Rx{_x}Y1−x_{1-x} (R=Cr, Mn, Fe; Y=Si, Ge)

A number of transition-metal (TM) doped group-IV semiconductors, R$_{x}$Y$_{1-x}$ (R=Cr, Mn and Fe; Y=Si, Ge), have been studied by the first principles calculations. The obtained results show that antiferromagnetic (AFM) order is energetically more favored than ferromagnetic (FM) order in Cr-doped Ge and Si with $x$=0.03125 and 0.0625. In 6.25% Fe-doped Ge, FM interaction dominates in all range of the R-R distances while for Fe-doped Ge at 3.125% and Fe-doped Si at both concentrations of 3.125% and 6.25%, only in a short R-R range can the FM states exist. In the Mn-doped case, the RKKY-like mechanism seems to be suitable for the Ge host matrix, while for the Mn-doped Si, the short-range AFM interaction competes with the long-range FM interaction. The different origin of the magnetic orders in these diluted magnetic semiconductors (DMSs) makes the microscopic mechanism of the ferromagnetism in the DMSs more complex and attractive.Comment: 14 pages, 2 figures, 6 table

Measurements of the effect of horizontal variability of atmospheric backscatter on dial measurements

The horizontal variability of atmospheric backscatter may have a substantial effect on how Differential Absorption Lidar (DIAL) data must be taken and analyzed. To minimize errors, lidar pulse pairs are taken with time separations which are short compared to the time scales associated with variations in atmospheric backscatter. To assess the atmospheric variability for time scales which are long compared to the lidar pulse repetition rate, the variance of the lidar return signal in a given channel can be computed. The variances of the on-line, off-line, and ration of the on-line to off-line signals at given altitudes obtained with the dual solid-state Alexandrite laser system were calculated. These evaluations were made for both down-looking aircraft and up-looking ground-based lidar data. Data were taken with 200 microsecond separation between on-line and off-line laser pulses, 30 m altitude resolution, 5 Hz repetition rate, and the signal were normalized for outgoing laser energy

Concepts, Developments and Advanced Applications of the PAX Toolkit

The Physics Analysis eXpert (PAX) is an open source toolkit for high energy physics analysis. The C++ class collection provided by PAX is deployed in a number of analyses with complex event topologies at Tevatron and LHC. In this article, we summarize basic concepts and class structure of the PAX kernel. We report about the most recent developments of the kernel and introduce two new PAX accessories. The PaxFactory, that provides a class collection to facilitate event hypothesis evolution, and VisualPax, a Graphical User Interface for PAX objects

Multi-subband effect in spin dephasing in semiconductor quantum wells

Multi-subband effect on spin precession and spin dephasing in $n$-type GaAs quantum wells is studied with electron-electron and electron-phonon scattering explicitly included. The effects of temperature, well width and applied electric field (in hot-electron regime) on the spin kinetics are thoroughly investigated. It is shown that due to the strong inter-subband scattering, the spin procession and the spin dephasing rate of electrons in different subbands are almost identical despite the large difference in the D'yakonov-Perel' (DP) terms of different subbands. It is also shown that for quantum wells with small well width at temperatures where only the lowest subband is occupied, the spin dephasing time increases with the temperature as well as the applied in-plane electric field until the contribution from the second subband is no longer negligible. For wide quantum wells the spin dephasing time tends to decrease with the temperature and the electric field.Comment: 6 pages, 4 figures in eps forma

Evolution of Ego-networks in Social Media with Link Recommendations

Ego-networks are fundamental structures in social graphs, yet the process of their evolution is still widely unexplored. In an online context, a key question is how link recommender systems may skew the growth of these networks, possibly restraining diversity. To shed light on this matter, we analyze the complete temporal evolution of 170M ego-networks extracted from Flickr and Tumblr, comparing links that are created spontaneously with those that have been algorithmically recommended. We find that the evolution of ego-networks is bursty, community-driven, and characterized by subsequent phases of explosive diameter increase, slight shrinking, and stabilization. Recommendations favor popular and well-connected nodes, limiting the diameter expansion. With a matching experiment aimed at detecting causal relationships from observational data, we find that the bias introduced by the recommendations fosters global diversity in the process of neighbor selection. Last, with two link prediction experiments, we show how insights from our analysis can be used to improve the effectiveness of social recommender systems.Comment: Proceedings of the 10th ACM International Conference on Web Search and Data Mining (WSDM 2017), Cambridge, UK. 10 pages, 16 figures, 1 tabl

Hot-electron effect in spin dephasing in nn-type GaAs quantum wells

We perform a study of the effect of the high in-plane electric field on the spin precession and spin dephasing due to the D'yakonov-Perel' mechanism in $n$-type GaAs (100) quantum wells by constructing and numerically solving the kinetic Bloch equations. We self-consistently include all of the scattering such as electron-phonon, electron-non-magnetic impurity as well as the electron-electron Coulomb scattering in our theory and systematically investigate how the spin precession and spin dephasing are affected by the high electric field under various conditions. The hot-electron distribution functions and the spin correlations are calculated rigorously in our theory. It is found that the D'yakonov-Perel' term in the electric field provides a non-vanishing effective magnetic field that alters the spin precession period. Moreover, spin dephasing is markedly affected by the electric field. The important contribution of the electron-electron scattering to the spin dephasing is also discussed.Comment: 11 pages, 11 figures, accepted for publication in Phys. Rev.