Low-energy electron diffraction study of potassium adsorbed on single-crystal graphite and highly oriented pyrolytic graphite

Potassium adsorption on graphite has been a model system for the understanding of the interaction of alkali metals with surfaces. The geometries of the s232d structure of potassium on both single-crystal graphite (SCG) and highly oriented pyrolytic graphite (HOPG) were investigated for various preparation conditions for graphite temperatures between 55 and 140 K. In all cases, the geometry was found to consist of K atoms in the hollow sites on top of the surface. The K-graphite average perpendicular spacing is 2.79±0.03 Å, corresponding to an average C-K distance of 3.13±0.03 Å, and the spacing between graphite planes is consistent with the bulk spacing of 3.35 Å. No evidence was observed for a sublayer of potassium. The results of dynamical LEED studies for the clean SCG and HOPG surfaces indicate that the surface structures of both are consistent with the truncated bulk structure of graphite

SPITZER: Accretion in Low Mass Stars and Brown Dwarfs in the Lambda Orionis Cluster

We present multi-wavelength optical and infrared photometry of 170 previously known low mass stars and brown dwarfs of the 5 Myr Collinder 69 cluster (Lambda Orionis). The new photometry supports cluster membership for most of them, with less than 15% of the previous candidates identified as probable non-members. The near infrared photometry allows us to identify stars with IR excesses, and we find that the Class II population is very large, around 25% for stars (in the spectral range M0 - M6.5) and 40% for brown dwarfs, down to 0.04 Msun, despite the fact that the H(alpha) equivalent width is low for a significant fraction of them. In addition, there are a number of substellar objects, classified as Class III, that have optically thin disks. The Class II members are distributed in an inhomogeneous way, lying preferentially in a filament running toward the south-east. The IR excesses for the Collinder 69 members range from pure Class II (flat or nearly flat spectra longward of 1 micron), to transition disks with no near-IR excess but excesses beginning within the IRAC wavelength range, to two stars with excess only detected at 24 micron. Collinder 69 thus appears to be at an age where it provides a natural laboratory for the study of primordial disks and their dissipation.Comment: ApJ, in pres

Lattice sites of ion-implanted Li in diamond

Published in: Appl. Phys. Lett. 66 (1995) 2733-2735 citations recorded in [Science Citation Index] Abstract: Radioactive Li ions were implanted into natural IIa diamonds at temperatures between 100 K and 900 K. Emission channelling patterns of a-particles emitted in the nuclear decay of 8Li (t1/2 = 838 ms) were measured and, from a comparison with calculated emission channelling and blocking effects from Monte Carlo simulations, the lattice sites taken up by the Li ions were quantitatively determined. A fraction of 40(5)% of the implanted Li ions were found to be located on tetrahedral interstitial lattice sites, and 17(5)% on substitutional sites. The fractions of implanted Li on the two lattice sites showed no change with temperature, indicating that Li diffusion does not take place within the time window of our measurements.

Doping Evolution of the Underlying Fermi Surface in La2-xSrxCuO4

We have performed a systematic doping dependent study of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) (0.03$\leq x \leq$0.3) by angle-resolved photoemission spectroscopy. In the entire doping range, the underlying ``Fermi surface" determined from the low energy spectral weight approximately satisfies Luttinger's theorem, even down to the lightly-doped region. This is in strong contrast to the result on Ca$_{2-x}$Na$_x$CuO$_2$Cl$_2$ (Na-CCOC), which shows a strong deviation from Luttinger's theorem. The differences between LSCO and Na-CCOC are correlated with the different behaviors of the chemical potential shift and spectral weight transfer induced by hole doping.Comment: 4 pages, 4 figure

Nickel: A very fast diffuser in silicon

Nickel is increasingly used in both IC and photovoltaic device fabrication, yet it has the potential to create highly recombination-active precipitates in silicon. For nearly three decades, the accepted nickel diffusivity in silicon has been DNi(T)=2.3×10exp−3 exp(−0.47 eV/kBT) cm2/s, a surprisingly low value given reports of rapid nickel diffusion in industrial applications. In this paper, we employ modern experimental methods to measure the higher nickel diffusivity DNi(T)=(1.69±0.74)×10exp−4 exp(−0.15±0.04 eV/kBT)  cm2/s. The measured activation energy is close to that predicted by first-principles theory using the nudged-elastic-band method. Our measured diffusivity of nickel is higher than previously published values at temperatures below 1150 °C, and orders of magnitude higher when extrapolated to room temperature.Peer reviewe

Towards a first-principles theory of surface thermodynamics and kinetics

Understanding of the complex behavior of particles at surfaces requires detailed knowledge of both macroscopic and microscopic processes that take place; also certain processes depend critically on temperature and gas pressure. To link these processes we combine state-of-the-art microscopic, and macroscopic phenomenological, theories. We apply our theory to the O/Ru(0001) system and calculate thermal desorption spectra, heat of adsorption, and the surface phase diagram. The agreement with experiment provides validity for our approach which thus identifies the way for a predictive simulation of surface thermodynamics and kinetics.Comment: 4 pages including 3 figures. Related publications can be found at http://www.fhi-berlin.mpg.de/th/paper.htm

A LEED structural analysis of the Co(100) surface

The structure of the clean Co(1010) surface has been analysed by LEED. Application of a recently developed computational scheme reveals the prevalence of the termination A in which the two topmost layers exhibit a narrow spacing of 0.62 Å, corresponding to a 12.8(±0.5)% contraction with respect to the bulk value, while the spacing between the second and third layer is slightly expanded by 0.8(±0.2)%

Topical Review on "Beta-beams"

Neutrino physics is traversing an exciting period, after the important discovery that neutrinos are massive particles, that has implications from high-energy physics to cosmology. A new method for the production of intense and pure neutrino beams has been proposed recently: the ``beta-beam''. It exploits boosted radioactive ions decaying through beta-decay. This novel concept has been the starting point for a new possible future facility. Its main goal is to address the crucial issue of the existence of CP violation in the lepton sector. Here we review the status and the recent developments with beta-beams. We discuss the original, the medium and high-energy scenarios as well as mono-chromatic neutrino beams produced through ion electron-capture. The issue of the degeneracies is mentioned. An overview of low energy beta-beams is also presented. These beams can be used to perform experiments of interest for nuclear structure, for the study of fundamental interactions and for nuclear astrophysics.Comment: Topical Review for Journal of Physics G: Nuclear and Particle Physics, published version, minor corrections, references adde

Neutrino Beams From Electron Capture at High Gamma

We investigate the potential of a flavor pure high gamma electron capture electron neutrino beam directed towards a large water cherenkov detector with 500 kt fiducial mass. The energy of the neutrinos is reconstructed by the position measurement within the detector and superb energy resolution capabilities could be achieved. We estimate the requirements for such a scenario to be competitive to a neutrino/anti-neutrino running at a neutrino factory with less accurate energy resolution. Although the requirements turn out to be extreme, in principle such a scenario could achieve as good abilities to resolve correlations and degeneracies in the search for sin^2(2 theta_13) and delta_CP as a standard neutrino factory experiment.Comment: 21 pages, 7 figures, revised version, to appear in JHEP, Fig.7 extended, minnor changes, results unchange