814 research outputs found
Mapping Itinerant Electrons around Kondo Impurities
We investigate single Fe and Co atoms buried below a Cu(100) surface using
low temperature scanning tunneling spectroscopy. By mapping the local density
of states of the itinerant electrons at the surface, the Kondo resonance near
the Fermi energy is analyzed. Probing bulk impurities in this well-defined
scattering geometry allows separating the physics of the Kondo system and the
measuring process. The line shape of the Kondo signature shows an oscillatory
behavior as a function of depth of the impurity as well as a function of
lateral distance. The oscillation period along the different directions reveals
that the spectral function of the itinerant electrons is anisotropic.Comment: 5 pages, 4 figures, accepted by Physical Review Letter
Theory of Optical Orientation in n-Type Semiconductors
Time resolved measurements of magnetization in n-GaAs have revealed a rich
array of spin decoherence processes, and have shown that fairly long lifetimes
(\sim 100 ns) can be achieved under certain circumstances. In time-resolved
Faraday rotation and time-resolved Kerr rotation the evolution of the
magnetization can be followed as a function of temperature, applied field,
doping level and excitation level. We present a theory for the spin relaxation
in n-GaAs based on a set of rate equations for two interacting thermalized
subsystems of spins: localized states on donor sites and itinerant states in
the conduction band. The conduction band spins relax by scattering from defects
or phonons through the D'yakonov-Perel' mechanism, while the localized spins
relax by interacting with phonons (when in an applied field) or through the
Dzyaloshinskii-Moriya interaction. In this model, numerous features of the
data, including puzzling temperature and doping dependences of the relaxation
time, find an explanation.Comment: 4 pages, 2 figures; revised version has a more complete discussion of
the Elliott-Yafet and spin-phonon decay mechanism
On the Connection of Anisotropic Conductivity to Tip Induced Space Charge Layers in Scanning Tunneling Spectroscopy of p-doped GaAs
The electronic properties of shallow acceptors in p-doped GaAs{110} are
investigated with scanning tunneling microscopy at low temperature. Shallow
acceptors are known to exhibit distinct triangular contrasts in STM images for
certain bias voltages. Spatially resolved I(V)-spectroscopy is performed to
identify their energetic origin and behavior. A crucial parameter - the STM
tip's work function - is determined experimentally. The voltage dependent
potential configuration and band bending situation is derived. Ways to validate
the calculations with the experiment are discussed. Differential conductivity
maps reveal that the triangular contrasts are only observed with a depletion
layer present under the STM tip. The tunnel process leading to the anisotropic
contrasts calls for electrons to tunnel through vacuum gap and a finite region
in the semiconductor.Comment: 11 pages, 8 figure
Intracerebral Implantation of Hydrogel-Coupled Adhesion Peptides: Tissue Reaction
Arg-Gly-Asp peptides (RGD) were synthesized
and chemically coupled to the bulk of N-(2-hydroxypropyl) methacrylamide-based polymer
hydrogels. Fourier Transform Infrared Spectroscopy
(FFIR) and amino acid analysis confirmed
the peptide coupling to the polymer. Activated
and control (unmodified) polymer matrices were
stereotaxically implanted in the striata of rat
brains, and two months later the brains were
processed for immunohistochemistry using antibodies
for glial acidic fibrillary protein (GFAP),
laminin and neurofilaments. RGD-containing
polymer matrices promoted stronger adhesion to
the host tissue than the unmodified polymer
matrices. In addition, the RGD-grafted polymer
implants promoted and supported the growth
and spread of GFAP-positive glial tissue onto
and into the hydrogels. Neurofilament-positive
fibers were also seen running along the surface
of the polymer and, in some instances, penetrating
the matrix. These findings are discussed
in the context of using bioactive polymers as a
new approach for promoting tissue repair and
axonal regeneration of damaged structures of
the central nervous system
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The 20 February 2010 Madeira flash-floods: synoptic analysis and extreme rainfall assessment
This study aims to characterise the rainfall exceptionality and the meteorological context of the 20 February 2010 flash-floods in Madeira (Portugal). Daily and hourly precipitation records from the available rain-gauge station networks are evaluated in order to reconstitute the temporal evolution of the rainstorm, as its geographic incidence, contributing to understand the flash-flood dynamics and the type and spatial distribution of the associated impacts. The exceptionality of the rainstorm is further confirmed by the return period associated with the daily precipitation registered at the two long-term record stations, with 146.9 mm observed in the city of Funchal and 333.8 mm on the mountain top, corresponding to an estimated return period of approximately 290 yr and 90 yr, respectively. Furthermore, the synoptic associated situation responsible for the flash-floods is analysed using different sources of information, e.g., weather charts, reanalysis data, Meteosat images and radiosounding data, with the focus on two main issues: (1) the dynamical conditions that promoted such anomalous humidity availability over the Madeira region on 20 February 2010 and (2) the uplift mechanism that induced deep convection activity
Local Density of States at Metal-Semiconductor Interfaces: An Atomic Scale Study
We investigate low temperature grown, abrupt, epitaxial, nonintermixed, defect-free n-type and p-type Fe/GaAs(110) interfaces by cross-sectional scanning tunneling microscopy and spectroscopy with atomic resolution. The probed local density of states shows that a model of the ideal metal-semiconductor interface requires a combination of metal-induced gap states and bond polarization at the interface which is nicely corroborated by density functional calculations. A three-dimensional finite element model of the space charge region yields a precise value for the Schottky barrier height
Electronic disorder of P- and B-doped Si at the metal-insulator transition investigated by scanning tunnelling microscopy and electronic transport
The (111)-2 × 1 surface of in situ cleaved heavily P- or B-doped Si is investigated by scanning tunnelling microscopy and spectroscopy at room temperature and at low temperature. P atoms have been identified on different sites of the Si(111)-2 × 1 surface by their characteristic voltage-dependent contrast for positive as well as negative buckling of the π-bonded chains. The distributions of dopants per surface area and of nearest-neighbour distances are found to be in agreement with a random arrangement of dopants in Si up to doping levels well above the metal–insulator transition. In addition, P atoms have been identified by their depth-dependent contrast down to the third layer beneath the surface with a volume density in agreement with the bulk doping density. The random electronic disorder supports the view of an Anderson transition driven by disorder close to the critical concentration or critical uniaxial stress
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Evaluation of moisture sources for the Central European summer flood of May/June 2013 based on regional climate model simulations
Heavy precipitation affected Central Europe in May/June 2013, triggering damaging floods both on the Danube and the Elbe rivers. Based on a modelling approach with COSMO-CLM, moisture fluxes, backward trajectories, cyclone tracks and precipitation fields are evaluated for the relevant time period 30 May–2 June 2013. We identify potential moisture sources and quantify their contribution to the flood event focusing on the Danube basin through sensitivity experiments: Control simulations are performed with undisturbed ERA-Interim boundary conditions, while multiple sensitivity experiments are driven with modified evaporation characteristics over selected marine and land areas. Two relevant cyclones are identified both in reanalysis and in our simulations, which moved counter-clockwise in a retrograde path from Southeastern Europe over Eastern Europe towards the northern slopes of the Alps. The control simulations represent the synoptic evolution of the event reasonably well. The evolution of the precipitation event in the control simulations shows some differences in terms of its spatial and temporal characteristics compared to observations. The main precipitation event can be separated into two phases concerning the moisture sources. Our modelling results provide evidence that the two main sources contributing to the event were the continental evapotranspiration (moisture recycling; both phases) and the North Atlantic Ocean (first phase only). The Mediterranean Sea played only a minor role as a moisture source. This study confirms the importance of continental moisture recycling for heavy precipitation events over Central Europe during the summer half year
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Objective climatology of cyclones in the Mediterranean region: a consensus view among methods with different system identification and tracking criteria
The Mediterranean storm track constitutes a well-defined branch of the North Hemisphere storm track and is characterised by small but intense features and frequent cyclogenesis. The goal of this study is to assess the level of consensus among cyclone detection and tracking methods (CDTMs), to identify robust features and to explore sources of disagreement. A set of 14 CDTMs has been applied for computing the climatology of cyclones crossing the Mediterranean region using the ERA-Interim dataset for the period 1979–2008 as common testbed. Results show large differences in actual cyclone numbers identified by different methods, but a good level of consensus on the interpretation of results regarding location, annual cycle and trends of cyclone tracks. Cyclogenesis areas such as the north-western Mediterranean, North Africa, north shore of the Levantine basin, as well as the seasonality of their maxima are robust features on which methods show a substantial agreement. Differences among methods are greatly reduced if cyclone numbers are transformed to a dimensionless index, which, in spite of disagreement on mean values and interannual variances of cyclone numbers, reveals a consensus on variability, sign and significance of trends. Further, excluding ‘weak’ and ‘slow’ cyclones from the computation of cyclone statistics improves the agreement among CDTMs. Results show significant negative trends of cyclone frequency in spring and positive trends in summer, whose contrasting effects compensate each other at annual scale, so that there is no significant long-term trend in total cyclone numbers in the Mediterranean basin in the 1979–2008 period
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