Channeling Effect and Improvement of the Efficiency of Charged Particle Registration with Crystal Scintillators

The importance is emphasized of taking into account the channeling along the low index crystallographic axes and planes of a part of low-energy (1-10 keV) recoil ions in measurements of their parameters with crystal scintillators of the type of NaI(Tl) etc. Because the nucleus stopping power in channels is low as compared with electronic stopping power, the light yield of the scintillator must be, accordingly, higher in the given case than that for ions having higher energy (tens keV and more), which lose most part of their energy via nuclear collisions outside channels. Hence, in particular, it follows that the DAMA/NaI observations in Gran Sasso of the annual modulation of the signal frequency in a narrow range of scintillations with an amplitude of 2-6 keV electron equivalent may be due to incidence onto the Earth of exceedingly massive particles (of the type of Planckian objects) from elongated Earth-crossing heliocentric orbits at a velocity of 30-50 km/s. In NaI(Tl), these particles create the iodine recoil ions with just the energy of 2-6 keV.Comment: 10 page

Stochastic Green's function approach to disordered systems

Based on distributions of local Green's functions we present a stochastic approach to disordered systems. Specifically we address Anderson localisation and cluster effects in binary alloys. Taking Anderson localisation of Holstein polarons as an example we discuss how this stochastic approach can be used for the investigation of interacting disordered systems.Comment: 12 pages, 7 figures, conference proceedings: Progress in Nonequilibrium Green's Functions III, 22-26 August 2005, University of Kiel, German

Disorder-driven superconductor-normal metal phase transition in quasi-one-dimensional organic conductors

Effects of non-magnetic disorder on the critical temperature T_c and on diamagnetism of quasi-one-dimensional superconductors are reported. The energy of Josephson-coupling between wires is considered to be random, which is typical for dirty organic superconductors. We show that this randomness destroys phase coherence between wires and that T_c vanishes discontinuously at a critical disorder-strength. The parallel and transverse components of the penetration-depth are evaluated. They diverge at different critical temperatures T_c^{(1)} and T_c, which correspond to pair-breaking and phase-coherence breaking respectively. The interplay between disorder and quantum phase fluctuations is shown to result in quantum critical behavior at T=0, which manifests itself as a superconducting-normal metal phase transition of first-order at a critical disorder strength.Comment: 12 pages, 3 figure

Simulation of phosphorus implantation into silicon with a single-parameter electronic stopping power model

We simulate dopant profiles for phosphorus implantation into silicon using a new model for electronic stopping power. In this model, the electronic stopping power is factorized into a globally averaged effective charge Z1*, and a local charge density dependent electronic stopping power for a proton. There is only a single adjustable parameter in the model, namely the one electron radius rs0 which controls Z1*. By fine tuning this parameter, we obtain excellent agreement between simulated dopant profiles and the SIMS data over a wide range of energies for the channeling case. Our work provides a further example of implant species, in addition to boron and arsenic, to verify the validity of the electronic stopping power model and to illustrate its generality for studies of physical processes involving electronic stopping.Comment: 11 pages, 7 figures. See http://bifrost.lanl.gov/~reed

Two Dimensional Electron and Hole Gases at the Surface of Graphite

We report high-quality two-dimensional (2D) electron and hole gases induced at the surface of graphite by the electric field effect. The 2D carriers reside within a few near-surface atomic layers and exhibit mobilities up to 15,000 and 60,000 cm2/Vs at room and liquid-helium temperatures, respectively. The mobilities imply ballistic transport at micron scale. Pronounced Shubnikov-de Haas oscillations reveal the existence of two types of carries in both 2D electron and hole gases.Comment: related to cond-mat/0410631 where preliminary data for this experimental system were reporte

Small world of the miRNA science drives its publication dynamics

Many scientific articles became available in the digital form which allows for querying articles data, and specifically the automated metadata gathering, which includes the affiliation data. This in turn can be used in the quantitative characterization of the scientific field, such as organizations identification, and analysis of the co-authorship graph of those organizations to extract the underlying structure of science. In our work, we focus on the miRNA science field, building the organization co-authorship network to provide the higher-level analysis of scientific community evolution rather than analyzing author-level characteristics. To tackle the problem of the institution name writing variability, we proposed the k-mer/n-gram boolean feature vector sorting algorithm, KOFER in short. This approach utilizes the fact that the contents of the affiliation are rather consistent for the same organization, and to account for writing errors and other organization name variations within the affiliation metadata field, it converts the organization mention within the affiliation to the K-Mer (n-gram) Boolean presence vector. Those vectors for all affiliations in the dataset are further lexicographically sorted, forming groups of organization mentions. With that approach, we clustered the miRNA field affiliation dataset and extracted unique organization names, which allowed us to build the co-authorship graph on the organization level. Using this graph, we show that the growth of the miRNA field is governed by the small-world architecture of the scientific institution network and experiences power-law growth with exponent 2.64 ± 0.23 for organization number, in accordance with network diameter, proposing the growth model for emerging scientific fields. The first miRNA publication rate of an organization interacting with already publishing organization is estimated as 0.184 ± 0.002 year–1

Influence of Collision Cascade Statistics on Pattern Formation of Ion-Sputtered Surfaces

Theoretical continuum models that describe the formation of patterns on surfaces of targets undergoing ion-beam sputtering, are based on Sigmund's formula, which describes the spatial distribution of the energy deposited by the ion. For small angles of incidence and amorphous or polycrystalline materials, this description seems to be suitable, and leads to the classic BH morphological theory [R.M. Bradley and J.M.E. Harper, J. Vac. Sci. Technol. A 6, 2390 (1988)]. Here we study the sputtering of Cu crystals by means of numerical simulations under the binary-collision approximation. We observe significant deviations from Sigmund's energy distribution. In particular, the distribution that best fits our simulations has a minimum near the position where the ion penetrates the surface, and the decay of energy deposition with distance to ion trajectory is exponential rather than Gaussian. We provide a modified continuum theory which takes these effects into account and explores the implications of the modified energy distribution for the surface morphology. In marked contrast with BH's theory, the dependence of the sputtering yield with the angle of incidence is non-monotonous, with a maximum for non-grazing incidence angles.Comment: 12 pages, 13 figures, RevTe

Polaron and bipolaron transport in a charge segregated state of doped strongly correlated 2D semiconductor

The 2D lattice gas model with competing short and long range interactions is appliedused for calculation of the incoherent charge transport in the classical strongly-correlated charge segregated polaronic state. We show, by means of Monte-Carlo simulations, that at high temperature the transport is dominated by hopping of the dissociated correlated polarons, where with thetheir mobility is inversely proportional to the temperature. At the temperatures below the clustering transition temperature the bipolaron transport becomes dominant. The energy barrier for the bipolaron hopping is determined by the Coulomb effects and is found to be lower than the barrier for the single-polaron hopping. This leads to drastically different temperature dependencies of mobilities for polarons and bipolarons at low temperatures

Positive Magneto-Resistance in Quasi-1D Conductors

We present here a simple qualitative model that interpolates between the high and low temperature properties of quasi-1D conductors. At high temperatures we argue that transport is governed by inelastic scattering whereas at low temperatures the conductance decays exponentially with the electron dephasing length. The crossover between these regimes occurs at the temperature at which the elastic and inelastic scattering times become equal. This model is shown to be in quantitative agreement with the organic conductor $TTT_2I_{3-\delta}$. Within this model, we also show that on the insulating side, the positive magnetoresistance of the form $(H/T)^2$ observed in $TTT_2I_{3-\delta}$ and other quasi-1D conductors can be explained by the role spin-flip scattering plays in the electron dephasing rate.Comment: 4 pages, Latex, no figure